Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.)

Current achievements and future directions in genetic engineering of European plum (Prunus... Transgenic Res (2018) 27:225–240 https://doi.org/10.1007/s11248-018-0072-3 REVIEW Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.) . . . . Cesar Petri Nuria Alburquerque Mohamed Faize Ralph Scorza Chris Dardick Received: 21 November 2017 / Accepted: 6 April 2018 / Published online: 12 April 2018 The Author(s) 2018 Abstract In most woody fruit species, transforma- continuous flowering, etc. This review focuses on tion and regeneration are difficult. However, European the main advances in genetic transformation of plum (Prunus domestica) has been shown to be European plum achieved to date, and the lines of amenable to genetic improvement technologies from work that are converting genetic engineering into a classical hybridization, to genetic engineering, to contemporary breeding tool for this species. rapid cycle crop breeding (‘FasTrack’ breeding). Since the first report on European plum transformation Keywords Biotechnology  Woody plants with marker genes in the early 90 s, numerous Rosaceae  Stone fruit  Plant breeding manuscripts have been published reporting the gener- ation of new clones with agronomically interesting traits, such as pests, diseases and/or abiotic stress resistance, shorter juvenile period, dwarfing, Introduction Plums (Prunus domestica and P. salicina) are second C. Petri (&) ´ only to peach and nectarine in world stone fruit Departamento de Produccion Vegetal, Instituto de Biotecnologıa Vegetal, UPCT, Campus Muralla del Mar, production reaching around 11 million tons a year and, 30202 Cartagena, Murcia, Spain according to FAO data, the gross product value in e-mail: cesar.petri@upct.es 2014 reached more than 9500 million USD (FAO- N. Alburquerque STAT 2017). The most important commercial culti- Departamento de Mejora Vegetal, CEBAS-CSIC, Campus vars belong to the hexaploid European plum (Prunus de Espinardo, 30100 Espinardo, Murcia, Spain domestica L.) and the diploid Japanese plum (P. salicina L.). In addition, different plum species (e.g. P. M. Faize insititia, P. cerasifera, P. domestica, and interspecific Laboratory of Plant Biotechnology, Ecology and Ecosystem Valorization, Faculty of Sciences, University hybrids) are widely used as rootstocks for plum and Chouaib Doukkali, 24000 El Jadida, Morocco other stone fruits. However, P. domestica has been the most important plum species historically. R. Scorza Hybridization has been used to develop most of Ag Biotech and Plant Breeding Consulting Services, Ralph Scorza LLC, Shepherdstown, WV 25443, USA plums cultivars/clones, and along with the selection of clonal variants, it remains as the dominant technology. C. Dardick Sometimes, seedlings are the result of non-controlled USDA-ARS, Appalachian Fruit Research Station, 2217 pollination, but normally, hybridization crosses are Wiltshire Road, Kearneysville, WV 25430, USA 123 226 Transgenic Res (2018) 27:225–240 controlled and parental individual/s are chosen and converting genetic engineering into a contemporary self- or cross-pollinated. breeding tool for this species. Conventional breeding of plum is constrained by their long reproductive cycle with long juvenile European plum breeding objectives periods, complex reproductive biology and high degree of heterozygosity. Another drawback is the The objectives are clearly different in scion or large land area necessary for planting seedling fruit rootstocks breeding programs. tree populations and the associated expenses of field Plum scion breeding programs are established by operations. Frequently, in order to obtain a new the market requirements and consumer demand for the offspring that meets the desired agronomic and fruit as well as the regional climatic conditions, soils, commercial characteristics it is necessary to perform and pest/disease pressures. European plums may be several rounds of introgressive backcrossing (Petri and eaten fresh, canned, dried or distilled into brandy. Scorza 2008). Since European plum average genera- Each use requires different selection criteria in the tion time is about 3–7 years, generally, 15–20 years breeding program. Main breeding goals include resis- are required from first fruiting to cultivar release. tance to biotic and/or abiotic stress, chilling require- Two are the main potential advantages of transfor- ments, tree size, productivity and fruit quality traits mation for genetic improvement. Firstly, genetic (Callahan 2008; Neumu¨ller 2011). Some traits, such as high productivity and fruit quality, are shared goals for engineering would allow the discrete modification of an established genotype; cultivar or rootstock. This any fruit tree species. However European plum process may require less time, labor and field space breeding programs have some peculiarities. Since without the need of sexual crosses. Once a useful new most of the European plums are cultivated in coun- transgenic clone is obtained, vegetative propagation tries/regions with severe winters, cold hardiness and through graftage or rooting of cuttings or microprop- late blooming are major breeding objectives. Related agation provides unlimited production of the desired to plum affecting diseases breeding efforts have been clone, same way as any other conventional scion/root- focused on: brown rot, caused by the fungus Monilinia stock. This would be an ideal situation, but unfortu- spp.; bacterial canker, caused by Pseudomonas nately, transformation protocols for most important syringae van Hall; bacterial spot, caused by Xan- genotypes in the majority of fruit tree species are not thomonas campestris pv. Pruni; plum leaf scald, currently available. Secondly, and perhaps most caused by the bacteria Xylella fastidiosa, and Sharka, significantly, transformation may provide for genetic the most important disease affecting stone fruits, improvements that would otherwise be impossible caused by the plum pox virus (PPV). using traditional breeding. A clear example are the Plum rootstocks are selected based on traits such as transgenic papaya varieties ‘Rainbow’ and ‘SunUp’ rootstock-scion compatibility, scion vigor control, resistant to papaya ringspot virus (PRSV), the most tolerance to different soil conditions (salinity, pH, devastating disease threatening papaya production drought, etc.), lack of root suckers and resistance to worldwide (Gonsalves et al. 2009). There is not soil diseases and insects (Gainza et al. 2015). Iron natural source of PRSV resistance in the papaya chlorosis that often occurs in calcareous soils is one of germplasm, and genetic transformation allowed the the most limiting factors in the production of Prunus. production of PRSV resistant cultivars fast enough to One of the major pests in stone fruit orchards fight against an emergent disease in Hawaii during the worldwide are Root-knot nematode (RKN), therefore 90 s. obtaining resistant rootstocks is a main goal. More- In most woody fruit species, transformation and over, in poorly drained and dense clayish soils, Prunus adventitious regeneration are difficult, with low effi- rootstocks are at risk of being affected by diverse soil ciency and often limited to a few genotypes or to seed- related diseases such as crown gall (Agrobacterium derived tissues (Petri and Burgos 2005). European tumefaciens), crown rot (Phytophthora spp.), bacterial plum has been the most successful species among canker (P. syringae pv. syringae), oak root rot fungus, Prunus to transform. This review focuses on the main Armillaria mellea and Armillaria tabescens (Gainza advances in genetic transformation of European plum et al. 2015). achieved to date, and the lines of work that are 123 Transgenic Res (2018) 27:225–240 227 As we will show in this review, the improvement of has been the case in European plum, where numerous some of these traits has been the objective of genetic successful results have been published with different engineering in Prunus domestica. genotypes using seed-derived tissues as the explant The search of molecular markers associated to source (Table 1). Since embryo tissues are not agronomical interesting traits in Prunus has focused somatic, transformation of seed-derived material is the efforts of many scientists worldwide. Marker- not an ideal system for improving plum scion culti- assisted selection (MAS) saves time and money in vars. Nevertheless, these procedures are very useful to fruit tree crops breeding programs allowing the generate of new engineered rootstock varieties and to elimination of undesirable plants from progeny pop- introduce novel genes into plum germplasm. ulations as early as at the seedling stage. There are not Rhizobium radiobacter-mediated (AKA Agrobac- yet molecular markers for agronomic traits available terium tumefaciens) transformation has been the in Prunus domestica, which could be applied in principal technique applied to European plum breeding programs, due to the highly polymorphic (Table 1). In 1991, an initial transformation/regener- hexaploid genome of this species (Neumu¨ller 2011). ation protocol was described (Mante et al. 1991). This However, in rootstock breeding programs MAS is procedure used embryonic hypocotyl slices from being routinely used for selection of root-knot nema- mature seeds as the source of explants (Fig. 1a), and tode (RKN) resistance since years ago (Claverie et al. selection of transgenic plantlets was performed with kanamycin (km), using of nptII as the 2004; Dirlewanger et al. 2004; Lecouls et al. 2004). selectable marker gene (Mante et al. 1991). The Genetic transformation of European plum protocol was later enhanced and currently has allowed transformation efficiencies up to 42% and enabled the In most woody fruit species transformation and production of self-rooted transgenic plants in the regeneration of commercial cultivars is not routine greenhouse in approximately 6 months (Fig. 1) (Gon- and this is the main technical barrier to the application zalez Padilla et al. 2003; Petri et al. 2008). of biotechnology to fruit trees. The technological Both the original and improved protocol have been bottleneck is the difficulty or inability to regenerate employed successfully for the introduction of agro- shoots in vitro from clonal explants. Selection strate- nomically useful genes into European plum (Albur- gies (selection pressure, timing of selection, selective querque et al. 2017; Callahan and Scorza 2007; Diaz- agent, selective-marker gene, etc.) to identify and to Vivancos et al. 2013; Faize et al. 2013; Garcı´a- isolate the transgenic cells also constitutes a key factor Almodo´var et al. 2015; Gonzalez Padilla et al. 2003; for the success in the regeneration of transgenic Guseman et al. 2017; Hily et al. 2007; Hollender et al. shoots. 2016; Kalariya et al. 2011; Monticelli et al. 2012; There are few documents reporting regeneration of Nagel et al. 2008; Petri et al. 2008, 2011; Scorza et al. transgenic European plum plants from transformed 1994, 1995; Srinivasan et al. 2012; Wang et al. 2013b). somatic cells (Table 1), although in most cases, only Normally in plant transformation, the transferred marker genes were introduced into the plant genome foreign DNA sequence(s) are stably incorporated into (Mikhailov and Dolgov 2007; Yancheva et al. 2002; relatively few cells. Selectable marker genes are co- Sidorova et al. 2017), with few reports of modification introduced with the gene(s) of interest and their of agronomically important traits (Escalettes et al. function is the identification and/or selection of the 1994; Dolgov et al. 2010). Procedures developed for transformed cells that are then induced to form shoots one cultivar are often not suitable for other cultivars. and whole transgenic plants. Once transgenic shoots Essentially to date, ‘Startovaya’ remains as the only are generated and stablished, selectable marker genes European plum cultivar amenable for transformation have no further purpose. At this point, their presence with the procedures developed by Dr. Dolgov’s only creates complications with regulatory agencies laboratory (Table 1). and potential consumers. In this context, the high- While regeneration of shoots from clonal explants throughput transformation system developed in Euro- is problematic, the use of seed-derived tissues seems to pean plum, allowed the regeneration of transgenic reduce the genotype effect as these explants are plums without the use of selectable marker genes generally more likely to produce shoots in vitro. This (Petri et al. 2011). 123 228 Transgenic Res (2018) 27:225–240 Table 1 Transformation of Prunus domestica Cultivar/clone Technique Genes Explant TE (%) References Damas de Tolouse Rhizobium rhizogenes T-DNA (ipt) Shoots 0.0 Escalettes et al. (1994) T-DNA (ipt), PPV-CP Marianna (GF8-1) R. radiobacter nptII, gus Leaves – nptII, gus, PPV-CP, hpt B70146 R. radiobacter nptII, gus, PRV-CP Hypocotyls 3.0 Scorza et al. (1995) Quetsche R. radiobacter nptII, gfp Leaves 0.8 Yancheva et al. (2002) Kyustendilska sinya 2.7 Bluebyrd R. radiobacter nptII, gus Hypocotyls 0.4 Gonzalez Padilla et al. (2003) nptII, PDV-CP 1.4 nptII,PNRSV-CP 0.7 nptII, gus, TomRSV-CP 4.2 nptII, gus, antisense ACCO 2.0 nptII, PPV-CP hairpin 42.0 Petri et al. (2008) nptII, PDS hairpin 15.0 nptII, GAFP – Kalariya et al. (2011) PPV-CP hairpin (marker-free) 2.5 Petri et al. (2011) nptII, MdKN1 – Srinivasan et al. (2011) nptII MdKN2 – nptII, gus, KNOX1 – nptII, PtFT1 105.7 Srinivasan et al. (2012) nptII, PpeGID1c hairpin – Hollender et al. (2016) Startovaya R. radiobacter nptII, gfp Leaves 0.2 Mikhailov and Dolgov (2007) hpt, gfp 2.2 hpt, PPV-CP hairpin 1.1 Dolgov et al. (2010) pmi, gfp 1.4 Sidorova et al. (2017) Stanley R. radiobacter nptII, gus Hypocotyls 3.3 Mante et al. (1991) nptII, gus, PPV-CP 1.2 Scorza et al. (1994) nptII, PPV-CP hairpin – Hily et al. (2007) nptII, gus, GAFP – Nagel et al. (2008) hpt, gus 5.0 Tian et al. (2009) hpt, ihp-elF4E – Wang et al. (2013b) hpt, ihp-elF(iso)4E – nptII, UTR/P1 PPV hairpin – Garcı´a-Almodo´var et al. (2015) – Monticelli et al. (2012) nptII, PpeDRO1 – Guseman et al. (2017) Claudia verde R. radiobacter nptII, cytsod, cytapx Hypocotyls 39.0 Faize et al. (2013) – Diaz-Vivancos et al. (2013, 2016) pmi, gus 2.0 Wang et al. (2013a) nptII, PpSAP1 – Lloret et al. (2017) nptII, iaa-ipt hairpin 7.7 Alburquerque et al. (2017) Transformation efficiency. When authors reported several TE, depending on different factors, the best results are displayed in the table. When not indicated, could not be deduced from the information provided by the authors Further procedures based on the plum hypocotyls as regeneration of transgenic European plum plants using the source of explants with alternative hygromycin (hpt selective marker gene) or mannose selectable marker genes, other than nptII, have been (pmi selective marker gene) as the selective agents published. These reports demonstrated the successful (Sidorova et al. 2017; Tian et al. 2009; Wang et al. 123 Transgenic Res (2018) 27:225–240 229 Fig. 1 Regeneration of transgenic plums. a Source of explants: regeneration from hypocotyl slices in selective medium. mature-seed hypocotyl slices. Epicotyl (E) and radicle (R) are c Transgenic plants cultured in a greenhouse not used. Vertical bar represents 1 mm. b Adventitious 2013a, b). These systems may allow multiple genetic virus-derived sequences in plants. A notable example transformations of plum, and therefore, stacking using this approach is the transgenic cultivar ‘Hon- several transgenic events in a single clone. eysweet’ developed at the USDA-AFRS (Kear- Similar methods, with hypocotyls slices as neysville, WV, USA) (Scorza et al. 2013, 2016). In explants, have been applied successfully to Japanese Abel et al. (1986), demonstrated that transgenic plum (Prunus salicina) and apricot (Prunus armeni- expression of a viral coat protein (CP) gene would aca), showing the feasibility of the technique in prevent virus replication via inhibition of virus another species, and the generation of transgenic disassembly. This mechanism was dubbed CP-medi- plantlets from explants of different cultivars has been ated protection and was subsequently used to engineer reported (Petri et al. 2015; Urtubia et al. 2008). papaya, where the expression of papaya ringspot virus Nevertheless, only marker genes were introduced into (PRSV) CP gene led to the generation of PRSV the plant genomes at that time. resistant papaya clones (Gonsalves 1998). Since PRSV-CP had significant homology to the PPV-CP Agronomical traits genetically engineered in plum gene, some experiments with the PRSV-CP were performed in transgenic plums. Protection against the Plum pox virus resistance virus was effective for several years in greenhouse tests, but after 32 months symptoms were evident and Plum pox virus (PPV) is the etiological agent of sharka virus was detected throughout the plants (Scorza et al. disease. It causes chlorotic ring spots, vein clearing 1995). The PPV-CP gene was then isolated, and leaf distortion. Symptoms recorded from fruits sequenced, and cloned (Ravelonandro et al. 1992) include severe fruit malformations, reduced fruit and used for Agrobacterium-mediated transformation quality (Usenik et al. 2015), and premature abscission, of plum (Scorza et al. 1994). The regenerated trans- reducing both yield and marketability (Sochor et al. genic plum lines were tested for resistance during 2012). Although eradication and quarantine programs 2 years under greenhouse conditions. The one trans- are in place in most stone fruits producing countries, genic clone (C5) that appeared highly resistant in the PPV is still widespread in all of them. The develop- greenhouse tests did not express PPV-CP (Ravelo- ment of resistant clones appears as the most appropri- nandro et al. 1997; Scorza et al. 2001). This clone ate methodology to control of sharka disease (Sochor ‘‘C5’’, renamed as ‘HoneySweet’, showed low levels et al. 2012). of transgene mRNA suggesting that resistance was not Ilardi and Tavazza (2015) reviewed the different due to the expression of CP. In 1993, a new biotechnological approaches applied to obtain PPV mechanism of virus resistance was reported whereby resistance in Prunus. Most of the transgenic strategies cells of plants transformed with a viral CP gene were used are based on the heterologous expression of resistant due to cellular degradation of viral RNA 123 230 Transgenic Res (2018) 27:225–240 (Lindbo et al. 1993). This mechanism was later property restrictions for fruit production and for use as dubbed RNA interference (RNAi) (Fire et al. 1998). a source of PPV resistance in the USA (Scorza et al. In ‘HoneySweet’, transgene methylation was observed 2016). Outside of the USA ‘HoneySweet’ has not along with the production of small interfering RNA received approval for cultivation yet, but the clone is (siRNA) specific to the PPV-CP transgene; indicating being made freely available to researchers upon the that resistance was through RNAi (Hily et al. certification that appropriate foreign regulatory 2004, 2005; Scorza et al. 2001). Constructs with approvals have been obtained (Scorza et al. 2016). self-complementary sequences separated by an intron Since the development of ‘HoneySweet’, additional produce ‘‘hairpin’’ RNA structures that efficiently ihpRNA PPV-CP constructs were designed (Hily et al. cause the RNAi response. In the case of ‘Honeysweet’ 2007; Petri et al. 2008; Scorza et al. 2010) and resistance was not produced by an ihpRNA vector, but additional PPV-CP silenced plum lines were generated rather RNAi developed as a result of peculiarities of and evaluated. Authors reported at that time that some the insertion event that produced a hairpin of the PPV- of these new clones resulted resistant to PPV (Ravelo- CP transgene (Scorza et al. 2001). nandro et al. 2013; Scorza et al. 2013). The predicted rearranged PPV-CP (hairpin) In 2010, Dolgov and collaborators reported the sequence was further confirmed (Scorza et al. 2010), generation of five additional independent transgenic and the hairpin insert was cloned from C5 and PPV-CP ihpRNA plum lines. The novelty of this report lies in the source of the explants. In this case, the expressed into ‘Bluebyrd’, demonstrating that the PPV-CP hairpin sequence from ‘HoneySweet’ plum authors regenerated transgenic plantlets from clonal provides PPV resistance (Scorza et al. 2010). Exten- material (leaf segments) of the plum cultivar ‘Starto- sive testing and risk assessment, over 20 years, has vaya’. Authors reported at that time that the transgenic shown that the resistance is highly effective, stable, lines were under evaluation for PPV resistance durable, and environmentally safe (Scorza et al. (Dolgov et al. 2010). To our knowledge there is not 2013, 2016). ‘HoneySweet’ has successfully gone further information published about these clones through the US regulatory process, that required afterwards. critical safety evaluation by three agencies, the Hairpin RNA constructs targeting PPV sequences Animal and Plant Health Inspection Service (APHIS), other than the PPV-CP have also been tested. A set of Environmental Protection Agency (EPA) and the Food hairpin constructs targeting diverse regions on the 5 and Drug Administration (FDA), being the first woody end of the PPV genome were designed (Di Nicola- perennial tree crop to have done so (Scorza et al. Negri et al. 2005). These constructs were based on the 2013). All the regulatory decision documents are sequences of the most economically important viral available on line at the Center for Environmental Risk isolates (PPV-D and PPV-M) and selecting highly Assessment (CERA 2017). An important aspect in the conserved genomic regions. Among the vectors pro- deregulation of ‘Honeysweet’ is that new hybrids duced, promising results were obtained in Nicotiana derived from its crosses will not require further benthamiana with the h-UTR/P1 RNAi, which regulatory approval. encodes an ihpRNA containing the 50 nt untranslated ‘HoneySweet’ is self-incompatible and sexually region and a portion of P1 gene of the Italian PPV-M incompatible with most other Prunus species due to its ISPaVe44 isolate (Di Nicola-Negri et al. 2005, 2010). hexaploidy. However, it has shown compatibility with Later, transgenic P. domestica (cv. ‘Stanley’) seedling several P. domestica plum varieties (Scorza et al. lines expressing the h-UTR/P1 construct were pro- 2016). PPV-CP insert appears in heterozygosis, and duced (Garc´ıa-Almodo´var et al. 2015; Monticelli et al. therefore, PPV resistance segregates as a single 2012). Transgenic clones were micrografted onto PPV dominant locus where approximately 50% of the infected ‘GF305’ and the presence of the virus in the progeny displays resistance to sharka disease (Scorza grafted material was evaluated by RT-PCR. Seven out ´ ´ et al. 2016). ‘Honeysweet’ is currently being used as a of ten clones (Garcıa-Almodovar et al. 2015) and two source of PPV resistance in European plum breeding out of two clones (Monticelli et al. 2012) displayed programs (Scorza et al. 2013). resistance to the PPV-D isolate. Although, the clone has been patented (US Additional RNAi strategies in European plum have PP15154 P2), it is freely available with no intellectual been reported targeting simultaneously different 123 Transgenic Res (2018) 27:225–240 231 Fruit softening delayed plums genome conserved regions among PPV isolates (Wang et al. 2015). Fruit are harvested at a time determined by the Ravelonandro et al. (2013) using computational target predictions produced artificial miRNA PPV handling properties that will yield the highest quality fruit that can withstand storage and transport. Reduced silencing constructs. When engineered in Nicotiana benthamiana more than 70% of tested clones were or delayed ethylene production in the fruit might result in a firmer fruit that could remain in the tree longer to resistant demonstrating the potential application of amiRNAs for the development of PPV resistant plums. develop more tree-ripened flavors, yet resist damage Other strategies to induce PPV resistance in plum incurred during harvesting, processing and shipping have been based on targeting host factors. Plant (Callahan and Scorza 2007). viruses are obligate intracellular parasites and they Plum hypocotyls (cv. ‘Blueblyrd’) were trans- formed with an antisense construct of a peach ACC require interaction with host factors for different steps in their cycle, such as translation, replication and/or oxidase (ACCO) gene (the enzyme responsible for the last step in ethylene synthesis) under the control of the movement. Consequently, knockout or knockdown of host genes essential for viral functions or mutations in CaMV35S constitutive promoter (Gonzalez Padilla et al. 2003). The antisense DNA strategy is similar to essential host genes that impair their capacity to bind viral proteins can result in the loss of virus infectivity. the RNAi approach. In this case, antisense mRNA anneals to the endogenous target mRNA. Conse- In nature, such virus resistant traits usually occur as recessive characters. Some of these genes encode for quently, dsRNA is formed and post-transcriptional eukaryotic translation initiation factors such as 4E gene silencing specific to the target sequence is (eIF4E), 4G (eIF4G) or their isoforms, eIF(iso)4E and triggered. Data analyses revealed that, in some trans- eIF(iso)4G (Wang and Krishnaswamy 2012). The role genic lines, ethylene production and softening was delayed (Callahan and Scorza 2007). of these translation initiation factors appears to be of particular importance for Potyvirus infection, but Soil pathogens resistance natural or induced resistance conferred by muta- tions/knock-out of these factors is not limited to the Several soil-borne organisms cause significant loses family Potyviridae, and can also target different (?) strand RNA viruses, such as carmoviruses, cucu- on Prunus production worldwide. Control of soil moviruses, sobemoviruses and waikaviruses (Sanfa- pathogens, such as fungi (Phytophtora cinnamomi, c¸on 2015). Armillaria mellea), bacteria (Agrobacterium tumefa- The physical interaction between PPV-VPg and ciens) or nematodes (Meloidogyne sp.), is difficult plum eIF(iso)4E was confirmed by Wang et al. once they are established in an orchard. (2013b) using a Y2H system. Moreover, a ihpRNA The Gastrodia antifungal protein (GAFP), a mono- cot mannose-binding lectin isolated from the Asiatic construct targeting the plum eIF(iso)4E was designed and introduced into plum. More than 80% of trans- orchid Gastrodia elata (Hu et al. 1988), was expressed constitutively in Nicotiana tabacum and transgenic genic silenced eIF(iso)4E plums displayed resistance when challenged with the strain PPV-D (Wang et al. plants showed increased resistance against different soil pathogens (Cox et al. 2006). Later, gafp gene was 2013b). If this transcription factor strategy is con- firmed to be effective in field trials, it could be very engineered in transgenic plums (driven by the consti- interesting as it would lend itself to the engineering of tutive CaMV35S promoter or the polyubiquitin pro- disease resistance via gene editing techniques as well moter bul409) and three transgenic lines exhibited as provide a complementary mechanism to RNAi increased tolerance to Phytophthora root rot (PRR), strategies. Additionally, transient eIF(iso)4E silencing caused by P. cinnamomi, and to infection by in peach plants through virus induced gene silencing Meloidogyne incognita. (Kalariya et al. 2011; Nagel et al. 2008). Authors stated these results as promising (VIGS) lead to PPV resistance up to 25 days post- inoculation (Cui and Wang 2017). since the pathogen pressure in the experiments was much higher than the usual under natural field conditions, and long-term field trials will be necessary 123 232 Transgenic Res (2018) 27:225–240 to confirm these results (Kalariya et al. 2011; Nagel with ROS toxicity, plants have developed anti-oxidant et al. 2008). mechanisms, by partially suppressing ROS produc- Other studies have been focused on resistance to tion, or through their scavenging by enzymatic Agrobacterium tumefaciens through a biotechnologi- defenses such as superoxide dismutase (SOD), ascor- cal approach. Bacterial infection produces tumors in bate–glutathione (ASC–GSH) cycle enzymes, cata- the plant known as crown galls, a disease that affects lase (CAT) and peroxidases (POX) (Asada 1999; many perennial fruit, nut and ornamental crops, Noctor and Foyer 1998). causing large annual losses to growers and nurseries Genes encoding cytosolic antioxidants ascorbate world-wide (Alburquerque et al. 2012). peroxidase (cytapx) from spinach (Spinacia oleracea) An RNAi approach, with a chimeric self-comple- and Cu/Zn-superoxide dismutase (cytsod) from pea mentary construct, was designed to silence simulta- (Pisum sativum) were genetically engineered and neously the bacterium ipt and iaaM oncogenes. Its constitutively expressed in European plum. Several expression in Nicotiana tabacum induced resistance to transgenic plantlets showed an enhanced tolerance to crown gall disease (Alburquerque et al. 2012). salt stress when challenged to 100 mM of NaCl. The Recently, this construct has been introduced and enzymatic study showed that the increased tolerance expressed in European plum, and several transgenic was related to modulation of enzymatic antioxidants lines, derived from ‘Claudia verde’ hypocotyls, as well as enhancement of non-enzymatic antioxidants showed a significant reduction in the development of such as glutathione and ascorbate (Diaz-Vivancos the crown gall disease after infection with the C58 and et al. 2013). Furthermore, one transgenic line with A281 Agrobacterium strains (Alburquerque et al. elevated ascorbate peroxidase activity was tolerant to 2017). severe water stress, correlated with a tighter control of water-use efficiency and enhanced photosynthetic Abiotic stress resistance performance (Diaz-Vivancos et al. 2016). The clones developed by these researchers could be very useful as In Prunus spp., as an adaptation against the effects of rootstocks in arid and semi-arid regions affected by cold and water stress, meristems go through dormancy salinity and/or drought. during the cold period of autumn and winter. After Other studies showed that overexpression of peach transcriptomic analyses of differentially expressed DEEPER ROOTING 1 (PpeDRO1) in Prunus domes- transcripts during the dormancy process in reproduc- tica led to deeper-rooting phenotypes (Guseman et al. tive buds of peach (Prunus persica [L.] Batsch), 2017). Their data suggested a potential application for researchers identified a gene coding for a protein DRO1-related genes to alter root architecture for similar to Stress Associated Proteins (SAP) containing drought avoidance and improved resource use, and the two specific Zn-finger domains (PpSAP) (Leida et al. transgenic clones obtained in this study might be 2010). Authors observed up-regulation in PpSAP useful as rootstocks under water stress conditions expression in dormant buds and down-regulation (Guseman et al. 2017). occurred, along with dormancy release, and they stated that PpSAP expression seemed to be regulated Modified size and shape trees with the developmental stage of buds under apparently variable environmental circumstances (Leida et al. Plant size and architecture are currently main goals in 2010). PpSAP over-expression in transgenic plum many fruit trees genetic breeding programs, and some plants led to alterations in leaf shape and increased researchers have focused their studies in genetic tolerance to leaf desiccation, suggesting that this gene factors controlling tree size and shape. In the man- might be useful in manipulating abiotic stress toler- agement of an orchard many of the regular practices ance in plants (Lloret et al. 2017). are associated with plant size and/or architecture, such Exposure to abiotic stress, such as salinity, deficit as grafting, pruning, spraying, harvesting, etc. There- irrigation and osmotic stress is harmful for plants fore, tree size is crucial for a proper orchard manage- because of the induced damage caused by reactive ment, optimizing productivity, labor and, oxygen species (ROS), such as hydrogen peroxide consequently, benefits. (H O ) and superoxide radicals (O ). To manage 2 2 2 123 Transgenic Res (2018) 27:225–240 233 Transgenic Prunus domestica plums silenced for generation cycle of plum from 3 to 7 years to less than the gibberellic acid receptor GID1c displayed a range 1 year. of dwarf phenotypes, suggesting that a reduction in This unique phenotype displayed by the FT-plums GID1c levels could be utilized to develop semi-dwarf is currently being applied in a novel breeding strategy. trees (Hollender et al. 2016). To these authors, At the USDA-AFRS facility (Kearneysville, WV, knocking down GID1c expression, through genetic USA), the FT-plums are being used in crosses for what engineering or gene editing, or by selecting trees with they have termed ‘‘FasTrack’’ breeding (Fig. 2) naturally reduced GID1c expression in a breeding (Scorza et al. 2014). Moreover, ‘FasTrack’ breeding program, maybe a useful strategy to obtained new is carried out in a greenhouse, therefore, the environ- dwarf or semi-dwarf plum cultivars or rootstocks mental limitations of winter chilling can be overcome, (Hollender et al. 2016). and flowering and fruit production is year-round. The system may allow for the rapid incorporation of Early-continuous flowering trees and fast breeding important traits into plums. of plums The FT phenotype is dominant, therefore in the breeding process hemizygous early flowering progeny The tree fruit industry is facing highly dynamic can be recurrently selected with the desired genotype situations such as climate change, reductions in and used for the next cross. Extreme dwarf plum plants are eliminated in each generation since they are not available labor, increasing environmental concerns leading to restrictions in the use of agrichemicals, productive of flowers or fruit and therefore are not changing consumer preferences and the spread of useful for breeding (R. Scorza, personal communica- exotic pathogens and insect pests. To meet these tion). When considerable improvements in the breed- challenges, breeding new adapted fruit cultivars is ing process are clearly obvious, only seedlings that do critical. For tree fruit crops, the main factors slowing not contain the early flowering gene are selected the rate of new cultivar development is the long (Fig. 2). At this point, new plantlets are not genetically juvenile period, that is, the time between seed planting modified and can be planted in the field for their and fruiting (3–7 years in the case of European plum), evaluation of agronomic and commercial traits. the large land areas necessary for planting seedling Afterwards, new clones could be used directly as fruit tree populations, and the associated expenses of new varieties or as elite lines for further breeding. field operations. Hybridization is dependent upon More information about the innovative breeding flowering which occurs only once each year, and is program is available at http://ucanr.edu/sites/ dependent upon sufficient chill in the winter, warmth fastrack/ (accessed February, 2018). in the spring. Moreover, flowering and fruit set are In European plum ‘FasTrack’ breeding is in significantly affected by many environmental factors progress. ‘Prune d’Agen’ (also known as ‘Prune such as severe low winter temperatures, spring frosts, d’Ente or ‘French Prune’) along with its numerous high spring temperatures, and rain during pollination clonal selections, is perhaps the most economically season. Manipulation of cultural conditions can important Prunus domestica in the world. It accounts shorten the period of juvenility, nevertheless promot- for most of the world’s trade in dried plums (prunes). ing flowering through biotechnology has arisen in the The cultivar is very important in California, where it last years as a feasible solution to overcome these accounts for 99% of the dried plum production, limitations (van Nocker and Gardiner 2014). however it is susceptible to sharka disease. Dried Plum trees transformed with Poplar Flowering Plum industry members are especially concerned locus T1 (PtFT1) showed altered architecture, dor- about this fact. Consequently, the ‘FasTrack’ system mancy requirement, and continuous flowering (Srini- is currently being applied to move the PPV resistance vasan et al. 2012). In the greenhouse conditions, trait from plum type ‘Honeysweet’ into ‘French’ type transgenic plants over expressing PtFT1 flowered and plums (http://ucanr.edu/sites/fastrack/DriedPlum/; produced fruits continuously in few months (from 1 to accessed February, 2018). Additionally, the FT tech- 10 months, depending on the transgenic line) (Srini- nology has been incorporated in a breeding program vasan et al. 2012). This means a reduction in the that attempt the obtainment of new stoneless plum cultivars (Callahan et al. 2015). 123 234 Transgenic Res (2018) 27:225–240 Fig. 2 An example of FasTrack breeding technology. The of FT-resistant individuals with the desired type plum. Step 3 scheme shows a procedure to move a disease resistant trait to an Undertake two backcrosses with the original desired genotype elite clone or commercial cultivar. Step 1 Undertake initial cross using MAS to select the desirable traits. Step 4 Select the of an early flowering FT-plum with a resistant genotype (R). resistant progeny (R) with the desired original type traits (in the Among the FT-plums progeny, select the resistant individuals red ellipse). These trees are non-transgenic and they can be field using marker assisted selection (MAS). Step 2 Undertake a cross planted in evaluation plots. (Color figure online) Similar rapid cycle crop breeding approaches, are and are not found in conventional fruit trees. In currently being applied to other perennial tree fruits addition, beneficial effects of dried plums on bone such as apple and citrus (Flachowsky et al. 2011;Le health have been demonstrated (Deyhim et al. 2005; Roux et al. 2012; Rodriguez et al. 2014). Franklin et al. 2006; Halloran et al. 2010; Rendina Furthermore, the FT-plums did not show chilling et al. 2013; Smith et al. 2014a, b), and therefore, FT- requirement and their phenotype was different from plums could be used as a countermeasure to micro- the wild type, losing apical dominance and showing a gravity-induced bone loss of the crewmembers during bushy or vine-like genotype (Srinivasan et al. 2012). long term space missions (Graham et al. 2015). For these authors, these new characteristics may facilitate the cultivation of European plum in tropical Future prospects in genetic engineering of plum climates and the design of novel production methods for this crop, such as greenhouse intense production Gene editing systems. In fact, FT-plums have been proposed as a crop for In recent years site-directed nucleases (SDNs), such as spaceflights and extraterrestrial colonization (Graham CRISPR/Cas9, have emerged as an attractive technol- et al. 2015). The small plant size and lack of any ogy for production of mutated crops/cultivars (Pacher significant chilling requirement for flowering and and Puchta 2017). Successful applications of CRISPR/ fruiting are characteristics required for space travel Cas9 system to modify gene expression of several 123 Transgenic Res (2018) 27:225–240 235 species, including perennial plants have been CRISPR/Cas9 components transiently in European reviewed by Bortesi and Fischer (2015) and Samanta plum, as well as, DNA-based virus vectors, such as et al. (2016). In Populus, Fan et al. (2015) generated geminivirus, which have been successfully used for homozygous knock-out mutation in predicted loci in plant genome editing (Lozano-Dura´n 2016). T0 generation with CRISPR/Cas9 system. Once again To date in plum, eIF(iso4E) appears as an remark- in Populus, Zhou et al. (2015) created knockout able target for SDNs since its down regulation mutation in two genes involved in lignin and flavonoid displayed resistance to PPV infection in European biosynthesis achieving with 100% efficacy with this plum and peach (Cui and Wang 2017; Wang et al. system. 2013b). The CRISPR/Cas9 technology was employed Currently, the successful use of these strategies in to introduce sequence-specific mutations at the woody perennial species is mainly limited to Populus, eIF(iso)4E locus in Arabidopsis thaliana and com- but there are many interesting traits that could plete resistance to Turnip mosaic virus (TuMV) was potentially be modified by genome editing, including successfully engineered (Pyott et al. 2016). Trans- virus or insect resistance, herbicide tolerance, gene-free T2 generation was obtained by segregating improved fruit quality traits, etc. However, there are the induced mutation from the CRISPR/Cas9 some hindrances to the routine use of these new transgene. techniques in fruit trees, including their long life Other plant translation factors have been identified as new possible targets for PPV resistance. In cycles and the obligate vegetative propagation. The combination of genome editing tools and the early Arabidopsis thaliana, DNA-binding protein phos- flowering approach via transformation could allow the phatase 1 (AtDBP1) and a protein named GRF6, segregation of the induced mutation, the SDN used in which interacts with both AtDBP1 and the mitogen- the process, and the early flowering transgene in a activated protein kinase 11, have been described as relative short period. The new cultivars generated will PPV susceptibility factors (Carrasco et al. 2014; not be transgenic, since the genetic engineering will Castello et al. 2010). If Prunus orthologues of these only be used for inducing mutation and speeding up genes are identified at one point, they could be easily the breeding process. This might facilitate their silenced following an RNAi approach in Prunus deregulation and commercialization, making the pro- domestica and later evaluated to confirm the results cess similar to any conventional new hybrid, and observed in Arabidopsis thaliana. In addition, Prunus eliminating, or at least reducing, public concerns about persica DEAD-box RNA helicase-like (PpDDXL) has biotech-crops. Good news in this direction are that been reported to interact with PPV-VPg and, PPV was USDA-APHIS does not consider breeding stock and not able to infect a knock-out mutant of AtRH8, an cultivars produced from ‘FasTrack’ breeding as GM orthologue gene in A. thaliana (Huang et al. 2010). plants (https://www.aphis.usda.gov/biotechnology/ This protein seems to be a suitable candidate for the downloads/reg_loi/Drs%20Scorza%20and%20Callah application of genome editing approaches since it is an%20Final.pdf). not involved in plant growth and development. In the application of sequence-specific nucleases, PPV resistance has also been shown to be affected editing plant genomes without the stable incorporation by a cluster of six meprin and TRAF-C homology of recombinant DNA into the target genome may help domain (MATHd) (Zuriaga et al. 2013). Mutations in the deregulation and commercialization process of and deletions in the alleles were associated with the new genetically modified cultivars or varieties. There- resistant phenotype. The mutated allele showed dom- fore, DNA-free and/or virus-based genetic engineer- inance and resistance was displayed in heterozygosity ing tools may be of additional benefit in regions where (Zuriaga et al. 2013), which makes this locus an legislation is less science-based. Viral vectors are excellent candidate as a target for future SDN being applied to express transiently the Cas9 protein approaches. and/or specific gRNAs (Reviewed by Pacher and Puchta 2017). In peach, a PRSV based-vector has been Genotype-independent procedures successfully used to silence endogenous genes through virus induced gene silencing (Cui and Wang 2017). As mentioned above, genotype is a key factor for This sort of vector could be used to deliver the transformation and procedures developed for specific 123 236 Transgenic Res (2018) 27:225–240 genotypes are often not suitable for other cultivars. number of transgenic plums expressing resistance to This fact is a big limiting factor in the application of virus, nematode, bacterial, and fungal diseases have genetic engineering to plum established commercial been produced along with resistance to abiotic stresses cultivars/clones, allowing discrete modifications in a and with altered fruit ripening have been produced in desired genotype. Therefore, researchers have tried to laboratories world-wide. Practical application of overcome this problem with different strategies. genetic engineering in P. domestica has been the Adventitious organogenesis and embryogenesis are development, U.S. regulatory approval, and release of usually very sensitive processes, strongly affected by a transgenic cultivar (‘HoneySweet’) with resistance minor changes. The use of regeneration-promoting to PPV, the most serious stone fruit virus disease. This genes could aid to reduce the genotype effect. Ectopic release demonstrates the practical value of the appli- expression of the corn Knotted-like homeodomain cation of these biotechnologies for plum, and for tree (KNOX1) gene, which is involved in meristem and fruit improvement in general. Further, the P. domes- shoot apical meristem formation (Doerner 2003), tica genetic engineering system along with the ‘Fas- enhanced the frequency of adventitious shoot regen- Track’ technology provides a platform for the rapid eration from leaves of the ‘Bluebyrd’ plum cultivar functional analysis of genes that not only affect from 0%, in the untransformed control explants, up to vegetative characteristics, but also those affecting 96% (Srinivasan et al. 2011). Also, the ipt gene, flowering, fruiting, and seed development. In turn, this genetic knowledge provides the raw material for that oncogene involved in cytokines biosynthesis, increased regeneration of transformed plants from can be applied to produce further improvements to apricot in vitro leaf explants of the cultivar ‘Helena’ plum and other fruit crops that address the current and (Lopez-Noguera et al. 2009). Further studies with future challenges of fruit production. In the future, additional cultivars need to be performed to probe the integration of classical and biotechnological technolo- utility of these genes. gies in breeding programs is crucial to obtain new Transformation of meristematic tissues has been improved plum trees, with good agronomical traits and also proposed as a possible solution to reduce the fruit quality according to growers and consumers genotype effect, allowing genetic manipulation of demands. Nowadays, the greatest barrier to achieve somatic cells of established cultivars. However, dif- the transition to efficient, and effective breeding ferent authors have tried unsuccessfully to transform programs for plums, and all fruit tree crops, that will woody perennial plants meristems, where most of the enable breeders to meet the demands of growers and transgenic regenerated shoots resulted as chimeras consumers and enable the production of healthy, (Petri and Burgos 2005; Wang et al. 2009; Faize et al. abundant crops in an era of changing climate is the 2010). Further studies would be necessary for the lack of clear, efficient, science-based regulatory establishment of a dissociation methodology and regimes that will allow for the application of modern regeneration of non-chimeric transgenic plants in methodologies in applied breeding. order to make these methodologies applicable in Acknowledgements C.P. thanks the UPCT and the Ministerio woody perennial plants biotechnology. Espan˜ol de Economı´a y Competitividad for his Ramo´n and Cajal contract. This research was partially funded by Fundacio´n SENECA (Project Ref: 1925/PI/2014). Conclusions Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// The European plum has been shown to be amenable to creativecommons.org/licenses/by/4.0/), which permits unre- genetic improvement technologies from classical stricted use, distribution, and reproduction in any medium, hybridization, to genetic engineering, to rapid cycle provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com- crop breeding (‘FasTrack’ breeding). P. domestica mons license, and indicate if changes were made. shows a high regeneration capacity from seed-derived explants and these regeneration rates allow the production of marker-free transgenic plants. Trans- formation from clonal material has also been demon- strated, although only in one cultivar (‘Startovaya’). 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Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.)

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Abstract

Transgenic Res (2018) 27:225–240 https://doi.org/10.1007/s11248-018-0072-3 REVIEW Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.) . . . . Cesar Petri Nuria Alburquerque Mohamed Faize Ralph Scorza Chris Dardick Received: 21 November 2017 / Accepted: 6 April 2018 / Published online: 12 April 2018 The Author(s) 2018 Abstract In most woody fruit species, transforma- continuous flowering, etc. This review focuses on tion and regeneration are difficult. However, European the main advances in genetic transformation of plum (Prunus domestica) has been shown to be European plum achieved to date, and the lines of amenable to genetic improvement technologies from work that are converting genetic engineering into a classical hybridization, to genetic engineering, to contemporary breeding tool for this species. rapid cycle crop breeding (‘FasTrack’ breeding). Since the first report on European plum transformation Keywords Biotechnology  Woody plants with marker genes in the early 90 s, numerous Rosaceae  Stone fruit  Plant breeding manuscripts have been published reporting the gener- ation of new clones with agronomically interesting traits, such as pests, diseases and/or abiotic stress resistance, shorter juvenile period, dwarfing, Introduction Plums (Prunus domestica and P. salicina) are second C. Petri (&) ´ only to peach and nectarine in world stone fruit Departamento de Produccion Vegetal, Instituto de Biotecnologıa Vegetal, UPCT, Campus Muralla del Mar, production reaching around 11 million tons a year and, 30202 Cartagena, Murcia, Spain according to FAO data, the gross product value in e-mail: cesar.petri@upct.es 2014 reached more than 9500 million USD (FAO- N. Alburquerque STAT 2017). The most important commercial culti- Departamento de Mejora Vegetal, CEBAS-CSIC, Campus vars belong to the hexaploid European plum (Prunus de Espinardo, 30100 Espinardo, Murcia, Spain domestica L.) and the diploid Japanese plum (P. salicina L.). In addition, different plum species (e.g. P. M. Faize insititia, P. cerasifera, P. domestica, and interspecific Laboratory of Plant Biotechnology, Ecology and Ecosystem Valorization, Faculty of Sciences, University hybrids) are widely used as rootstocks for plum and Chouaib Doukkali, 24000 El Jadida, Morocco other stone fruits. However, P. domestica has been the most important plum species historically. R. Scorza Hybridization has been used to develop most of Ag Biotech and Plant Breeding Consulting Services, Ralph Scorza LLC, Shepherdstown, WV 25443, USA plums cultivars/clones, and along with the selection of clonal variants, it remains as the dominant technology. C. Dardick Sometimes, seedlings are the result of non-controlled USDA-ARS, Appalachian Fruit Research Station, 2217 pollination, but normally, hybridization crosses are Wiltshire Road, Kearneysville, WV 25430, USA 123 226 Transgenic Res (2018) 27:225–240 controlled and parental individual/s are chosen and converting genetic engineering into a contemporary self- or cross-pollinated. breeding tool for this species. Conventional breeding of plum is constrained by their long reproductive cycle with long juvenile European plum breeding objectives periods, complex reproductive biology and high degree of heterozygosity. Another drawback is the The objectives are clearly different in scion or large land area necessary for planting seedling fruit rootstocks breeding programs. tree populations and the associated expenses of field Plum scion breeding programs are established by operations. Frequently, in order to obtain a new the market requirements and consumer demand for the offspring that meets the desired agronomic and fruit as well as the regional climatic conditions, soils, commercial characteristics it is necessary to perform and pest/disease pressures. European plums may be several rounds of introgressive backcrossing (Petri and eaten fresh, canned, dried or distilled into brandy. Scorza 2008). Since European plum average genera- Each use requires different selection criteria in the tion time is about 3–7 years, generally, 15–20 years breeding program. Main breeding goals include resis- are required from first fruiting to cultivar release. tance to biotic and/or abiotic stress, chilling require- Two are the main potential advantages of transfor- ments, tree size, productivity and fruit quality traits mation for genetic improvement. Firstly, genetic (Callahan 2008; Neumu¨ller 2011). Some traits, such as high productivity and fruit quality, are shared goals for engineering would allow the discrete modification of an established genotype; cultivar or rootstock. This any fruit tree species. However European plum process may require less time, labor and field space breeding programs have some peculiarities. Since without the need of sexual crosses. Once a useful new most of the European plums are cultivated in coun- transgenic clone is obtained, vegetative propagation tries/regions with severe winters, cold hardiness and through graftage or rooting of cuttings or microprop- late blooming are major breeding objectives. Related agation provides unlimited production of the desired to plum affecting diseases breeding efforts have been clone, same way as any other conventional scion/root- focused on: brown rot, caused by the fungus Monilinia stock. This would be an ideal situation, but unfortu- spp.; bacterial canker, caused by Pseudomonas nately, transformation protocols for most important syringae van Hall; bacterial spot, caused by Xan- genotypes in the majority of fruit tree species are not thomonas campestris pv. Pruni; plum leaf scald, currently available. Secondly, and perhaps most caused by the bacteria Xylella fastidiosa, and Sharka, significantly, transformation may provide for genetic the most important disease affecting stone fruits, improvements that would otherwise be impossible caused by the plum pox virus (PPV). using traditional breeding. A clear example are the Plum rootstocks are selected based on traits such as transgenic papaya varieties ‘Rainbow’ and ‘SunUp’ rootstock-scion compatibility, scion vigor control, resistant to papaya ringspot virus (PRSV), the most tolerance to different soil conditions (salinity, pH, devastating disease threatening papaya production drought, etc.), lack of root suckers and resistance to worldwide (Gonsalves et al. 2009). There is not soil diseases and insects (Gainza et al. 2015). Iron natural source of PRSV resistance in the papaya chlorosis that often occurs in calcareous soils is one of germplasm, and genetic transformation allowed the the most limiting factors in the production of Prunus. production of PRSV resistant cultivars fast enough to One of the major pests in stone fruit orchards fight against an emergent disease in Hawaii during the worldwide are Root-knot nematode (RKN), therefore 90 s. obtaining resistant rootstocks is a main goal. More- In most woody fruit species, transformation and over, in poorly drained and dense clayish soils, Prunus adventitious regeneration are difficult, with low effi- rootstocks are at risk of being affected by diverse soil ciency and often limited to a few genotypes or to seed- related diseases such as crown gall (Agrobacterium derived tissues (Petri and Burgos 2005). European tumefaciens), crown rot (Phytophthora spp.), bacterial plum has been the most successful species among canker (P. syringae pv. syringae), oak root rot fungus, Prunus to transform. This review focuses on the main Armillaria mellea and Armillaria tabescens (Gainza advances in genetic transformation of European plum et al. 2015). achieved to date, and the lines of work that are 123 Transgenic Res (2018) 27:225–240 227 As we will show in this review, the improvement of has been the case in European plum, where numerous some of these traits has been the objective of genetic successful results have been published with different engineering in Prunus domestica. genotypes using seed-derived tissues as the explant The search of molecular markers associated to source (Table 1). Since embryo tissues are not agronomical interesting traits in Prunus has focused somatic, transformation of seed-derived material is the efforts of many scientists worldwide. Marker- not an ideal system for improving plum scion culti- assisted selection (MAS) saves time and money in vars. Nevertheless, these procedures are very useful to fruit tree crops breeding programs allowing the generate of new engineered rootstock varieties and to elimination of undesirable plants from progeny pop- introduce novel genes into plum germplasm. ulations as early as at the seedling stage. There are not Rhizobium radiobacter-mediated (AKA Agrobac- yet molecular markers for agronomic traits available terium tumefaciens) transformation has been the in Prunus domestica, which could be applied in principal technique applied to European plum breeding programs, due to the highly polymorphic (Table 1). In 1991, an initial transformation/regener- hexaploid genome of this species (Neumu¨ller 2011). ation protocol was described (Mante et al. 1991). This However, in rootstock breeding programs MAS is procedure used embryonic hypocotyl slices from being routinely used for selection of root-knot nema- mature seeds as the source of explants (Fig. 1a), and tode (RKN) resistance since years ago (Claverie et al. selection of transgenic plantlets was performed with kanamycin (km), using of nptII as the 2004; Dirlewanger et al. 2004; Lecouls et al. 2004). selectable marker gene (Mante et al. 1991). The Genetic transformation of European plum protocol was later enhanced and currently has allowed transformation efficiencies up to 42% and enabled the In most woody fruit species transformation and production of self-rooted transgenic plants in the regeneration of commercial cultivars is not routine greenhouse in approximately 6 months (Fig. 1) (Gon- and this is the main technical barrier to the application zalez Padilla et al. 2003; Petri et al. 2008). of biotechnology to fruit trees. The technological Both the original and improved protocol have been bottleneck is the difficulty or inability to regenerate employed successfully for the introduction of agro- shoots in vitro from clonal explants. Selection strate- nomically useful genes into European plum (Albur- gies (selection pressure, timing of selection, selective querque et al. 2017; Callahan and Scorza 2007; Diaz- agent, selective-marker gene, etc.) to identify and to Vivancos et al. 2013; Faize et al. 2013; Garcı´a- isolate the transgenic cells also constitutes a key factor Almodo´var et al. 2015; Gonzalez Padilla et al. 2003; for the success in the regeneration of transgenic Guseman et al. 2017; Hily et al. 2007; Hollender et al. shoots. 2016; Kalariya et al. 2011; Monticelli et al. 2012; There are few documents reporting regeneration of Nagel et al. 2008; Petri et al. 2008, 2011; Scorza et al. transgenic European plum plants from transformed 1994, 1995; Srinivasan et al. 2012; Wang et al. 2013b). somatic cells (Table 1), although in most cases, only Normally in plant transformation, the transferred marker genes were introduced into the plant genome foreign DNA sequence(s) are stably incorporated into (Mikhailov and Dolgov 2007; Yancheva et al. 2002; relatively few cells. Selectable marker genes are co- Sidorova et al. 2017), with few reports of modification introduced with the gene(s) of interest and their of agronomically important traits (Escalettes et al. function is the identification and/or selection of the 1994; Dolgov et al. 2010). Procedures developed for transformed cells that are then induced to form shoots one cultivar are often not suitable for other cultivars. and whole transgenic plants. Once transgenic shoots Essentially to date, ‘Startovaya’ remains as the only are generated and stablished, selectable marker genes European plum cultivar amenable for transformation have no further purpose. At this point, their presence with the procedures developed by Dr. Dolgov’s only creates complications with regulatory agencies laboratory (Table 1). and potential consumers. In this context, the high- While regeneration of shoots from clonal explants throughput transformation system developed in Euro- is problematic, the use of seed-derived tissues seems to pean plum, allowed the regeneration of transgenic reduce the genotype effect as these explants are plums without the use of selectable marker genes generally more likely to produce shoots in vitro. This (Petri et al. 2011). 123 228 Transgenic Res (2018) 27:225–240 Table 1 Transformation of Prunus domestica Cultivar/clone Technique Genes Explant TE (%) References Damas de Tolouse Rhizobium rhizogenes T-DNA (ipt) Shoots 0.0 Escalettes et al. (1994) T-DNA (ipt), PPV-CP Marianna (GF8-1) R. radiobacter nptII, gus Leaves – nptII, gus, PPV-CP, hpt B70146 R. radiobacter nptII, gus, PRV-CP Hypocotyls 3.0 Scorza et al. (1995) Quetsche R. radiobacter nptII, gfp Leaves 0.8 Yancheva et al. (2002) Kyustendilska sinya 2.7 Bluebyrd R. radiobacter nptII, gus Hypocotyls 0.4 Gonzalez Padilla et al. (2003) nptII, PDV-CP 1.4 nptII,PNRSV-CP 0.7 nptII, gus, TomRSV-CP 4.2 nptII, gus, antisense ACCO 2.0 nptII, PPV-CP hairpin 42.0 Petri et al. (2008) nptII, PDS hairpin 15.0 nptII, GAFP – Kalariya et al. (2011) PPV-CP hairpin (marker-free) 2.5 Petri et al. (2011) nptII, MdKN1 – Srinivasan et al. (2011) nptII MdKN2 – nptII, gus, KNOX1 – nptII, PtFT1 105.7 Srinivasan et al. (2012) nptII, PpeGID1c hairpin – Hollender et al. (2016) Startovaya R. radiobacter nptII, gfp Leaves 0.2 Mikhailov and Dolgov (2007) hpt, gfp 2.2 hpt, PPV-CP hairpin 1.1 Dolgov et al. (2010) pmi, gfp 1.4 Sidorova et al. (2017) Stanley R. radiobacter nptII, gus Hypocotyls 3.3 Mante et al. (1991) nptII, gus, PPV-CP 1.2 Scorza et al. (1994) nptII, PPV-CP hairpin – Hily et al. (2007) nptII, gus, GAFP – Nagel et al. (2008) hpt, gus 5.0 Tian et al. (2009) hpt, ihp-elF4E – Wang et al. (2013b) hpt, ihp-elF(iso)4E – nptII, UTR/P1 PPV hairpin – Garcı´a-Almodo´var et al. (2015) – Monticelli et al. (2012) nptII, PpeDRO1 – Guseman et al. (2017) Claudia verde R. radiobacter nptII, cytsod, cytapx Hypocotyls 39.0 Faize et al. (2013) – Diaz-Vivancos et al. (2013, 2016) pmi, gus 2.0 Wang et al. (2013a) nptII, PpSAP1 – Lloret et al. (2017) nptII, iaa-ipt hairpin 7.7 Alburquerque et al. (2017) Transformation efficiency. When authors reported several TE, depending on different factors, the best results are displayed in the table. When not indicated, could not be deduced from the information provided by the authors Further procedures based on the plum hypocotyls as regeneration of transgenic European plum plants using the source of explants with alternative hygromycin (hpt selective marker gene) or mannose selectable marker genes, other than nptII, have been (pmi selective marker gene) as the selective agents published. These reports demonstrated the successful (Sidorova et al. 2017; Tian et al. 2009; Wang et al. 123 Transgenic Res (2018) 27:225–240 229 Fig. 1 Regeneration of transgenic plums. a Source of explants: regeneration from hypocotyl slices in selective medium. mature-seed hypocotyl slices. Epicotyl (E) and radicle (R) are c Transgenic plants cultured in a greenhouse not used. Vertical bar represents 1 mm. b Adventitious 2013a, b). These systems may allow multiple genetic virus-derived sequences in plants. A notable example transformations of plum, and therefore, stacking using this approach is the transgenic cultivar ‘Hon- several transgenic events in a single clone. eysweet’ developed at the USDA-AFRS (Kear- Similar methods, with hypocotyls slices as neysville, WV, USA) (Scorza et al. 2013, 2016). In explants, have been applied successfully to Japanese Abel et al. (1986), demonstrated that transgenic plum (Prunus salicina) and apricot (Prunus armeni- expression of a viral coat protein (CP) gene would aca), showing the feasibility of the technique in prevent virus replication via inhibition of virus another species, and the generation of transgenic disassembly. This mechanism was dubbed CP-medi- plantlets from explants of different cultivars has been ated protection and was subsequently used to engineer reported (Petri et al. 2015; Urtubia et al. 2008). papaya, where the expression of papaya ringspot virus Nevertheless, only marker genes were introduced into (PRSV) CP gene led to the generation of PRSV the plant genomes at that time. resistant papaya clones (Gonsalves 1998). Since PRSV-CP had significant homology to the PPV-CP Agronomical traits genetically engineered in plum gene, some experiments with the PRSV-CP were performed in transgenic plums. Protection against the Plum pox virus resistance virus was effective for several years in greenhouse tests, but after 32 months symptoms were evident and Plum pox virus (PPV) is the etiological agent of sharka virus was detected throughout the plants (Scorza et al. disease. It causes chlorotic ring spots, vein clearing 1995). The PPV-CP gene was then isolated, and leaf distortion. Symptoms recorded from fruits sequenced, and cloned (Ravelonandro et al. 1992) include severe fruit malformations, reduced fruit and used for Agrobacterium-mediated transformation quality (Usenik et al. 2015), and premature abscission, of plum (Scorza et al. 1994). The regenerated trans- reducing both yield and marketability (Sochor et al. genic plum lines were tested for resistance during 2012). Although eradication and quarantine programs 2 years under greenhouse conditions. The one trans- are in place in most stone fruits producing countries, genic clone (C5) that appeared highly resistant in the PPV is still widespread in all of them. The develop- greenhouse tests did not express PPV-CP (Ravelo- ment of resistant clones appears as the most appropri- nandro et al. 1997; Scorza et al. 2001). This clone ate methodology to control of sharka disease (Sochor ‘‘C5’’, renamed as ‘HoneySweet’, showed low levels et al. 2012). of transgene mRNA suggesting that resistance was not Ilardi and Tavazza (2015) reviewed the different due to the expression of CP. In 1993, a new biotechnological approaches applied to obtain PPV mechanism of virus resistance was reported whereby resistance in Prunus. Most of the transgenic strategies cells of plants transformed with a viral CP gene were used are based on the heterologous expression of resistant due to cellular degradation of viral RNA 123 230 Transgenic Res (2018) 27:225–240 (Lindbo et al. 1993). This mechanism was later property restrictions for fruit production and for use as dubbed RNA interference (RNAi) (Fire et al. 1998). a source of PPV resistance in the USA (Scorza et al. In ‘HoneySweet’, transgene methylation was observed 2016). Outside of the USA ‘HoneySweet’ has not along with the production of small interfering RNA received approval for cultivation yet, but the clone is (siRNA) specific to the PPV-CP transgene; indicating being made freely available to researchers upon the that resistance was through RNAi (Hily et al. certification that appropriate foreign regulatory 2004, 2005; Scorza et al. 2001). Constructs with approvals have been obtained (Scorza et al. 2016). self-complementary sequences separated by an intron Since the development of ‘HoneySweet’, additional produce ‘‘hairpin’’ RNA structures that efficiently ihpRNA PPV-CP constructs were designed (Hily et al. cause the RNAi response. In the case of ‘Honeysweet’ 2007; Petri et al. 2008; Scorza et al. 2010) and resistance was not produced by an ihpRNA vector, but additional PPV-CP silenced plum lines were generated rather RNAi developed as a result of peculiarities of and evaluated. Authors reported at that time that some the insertion event that produced a hairpin of the PPV- of these new clones resulted resistant to PPV (Ravelo- CP transgene (Scorza et al. 2001). nandro et al. 2013; Scorza et al. 2013). The predicted rearranged PPV-CP (hairpin) In 2010, Dolgov and collaborators reported the sequence was further confirmed (Scorza et al. 2010), generation of five additional independent transgenic and the hairpin insert was cloned from C5 and PPV-CP ihpRNA plum lines. The novelty of this report lies in the source of the explants. In this case, the expressed into ‘Bluebyrd’, demonstrating that the PPV-CP hairpin sequence from ‘HoneySweet’ plum authors regenerated transgenic plantlets from clonal provides PPV resistance (Scorza et al. 2010). Exten- material (leaf segments) of the plum cultivar ‘Starto- sive testing and risk assessment, over 20 years, has vaya’. Authors reported at that time that the transgenic shown that the resistance is highly effective, stable, lines were under evaluation for PPV resistance durable, and environmentally safe (Scorza et al. (Dolgov et al. 2010). To our knowledge there is not 2013, 2016). ‘HoneySweet’ has successfully gone further information published about these clones through the US regulatory process, that required afterwards. critical safety evaluation by three agencies, the Hairpin RNA constructs targeting PPV sequences Animal and Plant Health Inspection Service (APHIS), other than the PPV-CP have also been tested. A set of Environmental Protection Agency (EPA) and the Food hairpin constructs targeting diverse regions on the 5 and Drug Administration (FDA), being the first woody end of the PPV genome were designed (Di Nicola- perennial tree crop to have done so (Scorza et al. Negri et al. 2005). These constructs were based on the 2013). All the regulatory decision documents are sequences of the most economically important viral available on line at the Center for Environmental Risk isolates (PPV-D and PPV-M) and selecting highly Assessment (CERA 2017). An important aspect in the conserved genomic regions. Among the vectors pro- deregulation of ‘Honeysweet’ is that new hybrids duced, promising results were obtained in Nicotiana derived from its crosses will not require further benthamiana with the h-UTR/P1 RNAi, which regulatory approval. encodes an ihpRNA containing the 50 nt untranslated ‘HoneySweet’ is self-incompatible and sexually region and a portion of P1 gene of the Italian PPV-M incompatible with most other Prunus species due to its ISPaVe44 isolate (Di Nicola-Negri et al. 2005, 2010). hexaploidy. However, it has shown compatibility with Later, transgenic P. domestica (cv. ‘Stanley’) seedling several P. domestica plum varieties (Scorza et al. lines expressing the h-UTR/P1 construct were pro- 2016). PPV-CP insert appears in heterozygosis, and duced (Garc´ıa-Almodo´var et al. 2015; Monticelli et al. therefore, PPV resistance segregates as a single 2012). Transgenic clones were micrografted onto PPV dominant locus where approximately 50% of the infected ‘GF305’ and the presence of the virus in the progeny displays resistance to sharka disease (Scorza grafted material was evaluated by RT-PCR. Seven out ´ ´ et al. 2016). ‘Honeysweet’ is currently being used as a of ten clones (Garcıa-Almodovar et al. 2015) and two source of PPV resistance in European plum breeding out of two clones (Monticelli et al. 2012) displayed programs (Scorza et al. 2013). resistance to the PPV-D isolate. Although, the clone has been patented (US Additional RNAi strategies in European plum have PP15154 P2), it is freely available with no intellectual been reported targeting simultaneously different 123 Transgenic Res (2018) 27:225–240 231 Fruit softening delayed plums genome conserved regions among PPV isolates (Wang et al. 2015). Fruit are harvested at a time determined by the Ravelonandro et al. (2013) using computational target predictions produced artificial miRNA PPV handling properties that will yield the highest quality fruit that can withstand storage and transport. Reduced silencing constructs. When engineered in Nicotiana benthamiana more than 70% of tested clones were or delayed ethylene production in the fruit might result in a firmer fruit that could remain in the tree longer to resistant demonstrating the potential application of amiRNAs for the development of PPV resistant plums. develop more tree-ripened flavors, yet resist damage Other strategies to induce PPV resistance in plum incurred during harvesting, processing and shipping have been based on targeting host factors. Plant (Callahan and Scorza 2007). viruses are obligate intracellular parasites and they Plum hypocotyls (cv. ‘Blueblyrd’) were trans- formed with an antisense construct of a peach ACC require interaction with host factors for different steps in their cycle, such as translation, replication and/or oxidase (ACCO) gene (the enzyme responsible for the last step in ethylene synthesis) under the control of the movement. Consequently, knockout or knockdown of host genes essential for viral functions or mutations in CaMV35S constitutive promoter (Gonzalez Padilla et al. 2003). The antisense DNA strategy is similar to essential host genes that impair their capacity to bind viral proteins can result in the loss of virus infectivity. the RNAi approach. In this case, antisense mRNA anneals to the endogenous target mRNA. Conse- In nature, such virus resistant traits usually occur as recessive characters. Some of these genes encode for quently, dsRNA is formed and post-transcriptional eukaryotic translation initiation factors such as 4E gene silencing specific to the target sequence is (eIF4E), 4G (eIF4G) or their isoforms, eIF(iso)4E and triggered. Data analyses revealed that, in some trans- eIF(iso)4G (Wang and Krishnaswamy 2012). The role genic lines, ethylene production and softening was delayed (Callahan and Scorza 2007). of these translation initiation factors appears to be of particular importance for Potyvirus infection, but Soil pathogens resistance natural or induced resistance conferred by muta- tions/knock-out of these factors is not limited to the Several soil-borne organisms cause significant loses family Potyviridae, and can also target different (?) strand RNA viruses, such as carmoviruses, cucu- on Prunus production worldwide. Control of soil moviruses, sobemoviruses and waikaviruses (Sanfa- pathogens, such as fungi (Phytophtora cinnamomi, c¸on 2015). Armillaria mellea), bacteria (Agrobacterium tumefa- The physical interaction between PPV-VPg and ciens) or nematodes (Meloidogyne sp.), is difficult plum eIF(iso)4E was confirmed by Wang et al. once they are established in an orchard. (2013b) using a Y2H system. Moreover, a ihpRNA The Gastrodia antifungal protein (GAFP), a mono- cot mannose-binding lectin isolated from the Asiatic construct targeting the plum eIF(iso)4E was designed and introduced into plum. More than 80% of trans- orchid Gastrodia elata (Hu et al. 1988), was expressed constitutively in Nicotiana tabacum and transgenic genic silenced eIF(iso)4E plums displayed resistance when challenged with the strain PPV-D (Wang et al. plants showed increased resistance against different soil pathogens (Cox et al. 2006). Later, gafp gene was 2013b). If this transcription factor strategy is con- firmed to be effective in field trials, it could be very engineered in transgenic plums (driven by the consti- interesting as it would lend itself to the engineering of tutive CaMV35S promoter or the polyubiquitin pro- disease resistance via gene editing techniques as well moter bul409) and three transgenic lines exhibited as provide a complementary mechanism to RNAi increased tolerance to Phytophthora root rot (PRR), strategies. Additionally, transient eIF(iso)4E silencing caused by P. cinnamomi, and to infection by in peach plants through virus induced gene silencing Meloidogyne incognita. (Kalariya et al. 2011; Nagel et al. 2008). Authors stated these results as promising (VIGS) lead to PPV resistance up to 25 days post- inoculation (Cui and Wang 2017). since the pathogen pressure in the experiments was much higher than the usual under natural field conditions, and long-term field trials will be necessary 123 232 Transgenic Res (2018) 27:225–240 to confirm these results (Kalariya et al. 2011; Nagel with ROS toxicity, plants have developed anti-oxidant et al. 2008). mechanisms, by partially suppressing ROS produc- Other studies have been focused on resistance to tion, or through their scavenging by enzymatic Agrobacterium tumefaciens through a biotechnologi- defenses such as superoxide dismutase (SOD), ascor- cal approach. Bacterial infection produces tumors in bate–glutathione (ASC–GSH) cycle enzymes, cata- the plant known as crown galls, a disease that affects lase (CAT) and peroxidases (POX) (Asada 1999; many perennial fruit, nut and ornamental crops, Noctor and Foyer 1998). causing large annual losses to growers and nurseries Genes encoding cytosolic antioxidants ascorbate world-wide (Alburquerque et al. 2012). peroxidase (cytapx) from spinach (Spinacia oleracea) An RNAi approach, with a chimeric self-comple- and Cu/Zn-superoxide dismutase (cytsod) from pea mentary construct, was designed to silence simulta- (Pisum sativum) were genetically engineered and neously the bacterium ipt and iaaM oncogenes. Its constitutively expressed in European plum. Several expression in Nicotiana tabacum induced resistance to transgenic plantlets showed an enhanced tolerance to crown gall disease (Alburquerque et al. 2012). salt stress when challenged to 100 mM of NaCl. The Recently, this construct has been introduced and enzymatic study showed that the increased tolerance expressed in European plum, and several transgenic was related to modulation of enzymatic antioxidants lines, derived from ‘Claudia verde’ hypocotyls, as well as enhancement of non-enzymatic antioxidants showed a significant reduction in the development of such as glutathione and ascorbate (Diaz-Vivancos the crown gall disease after infection with the C58 and et al. 2013). Furthermore, one transgenic line with A281 Agrobacterium strains (Alburquerque et al. elevated ascorbate peroxidase activity was tolerant to 2017). severe water stress, correlated with a tighter control of water-use efficiency and enhanced photosynthetic Abiotic stress resistance performance (Diaz-Vivancos et al. 2016). The clones developed by these researchers could be very useful as In Prunus spp., as an adaptation against the effects of rootstocks in arid and semi-arid regions affected by cold and water stress, meristems go through dormancy salinity and/or drought. during the cold period of autumn and winter. After Other studies showed that overexpression of peach transcriptomic analyses of differentially expressed DEEPER ROOTING 1 (PpeDRO1) in Prunus domes- transcripts during the dormancy process in reproduc- tica led to deeper-rooting phenotypes (Guseman et al. tive buds of peach (Prunus persica [L.] Batsch), 2017). Their data suggested a potential application for researchers identified a gene coding for a protein DRO1-related genes to alter root architecture for similar to Stress Associated Proteins (SAP) containing drought avoidance and improved resource use, and the two specific Zn-finger domains (PpSAP) (Leida et al. transgenic clones obtained in this study might be 2010). Authors observed up-regulation in PpSAP useful as rootstocks under water stress conditions expression in dormant buds and down-regulation (Guseman et al. 2017). occurred, along with dormancy release, and they stated that PpSAP expression seemed to be regulated Modified size and shape trees with the developmental stage of buds under apparently variable environmental circumstances (Leida et al. Plant size and architecture are currently main goals in 2010). PpSAP over-expression in transgenic plum many fruit trees genetic breeding programs, and some plants led to alterations in leaf shape and increased researchers have focused their studies in genetic tolerance to leaf desiccation, suggesting that this gene factors controlling tree size and shape. In the man- might be useful in manipulating abiotic stress toler- agement of an orchard many of the regular practices ance in plants (Lloret et al. 2017). are associated with plant size and/or architecture, such Exposure to abiotic stress, such as salinity, deficit as grafting, pruning, spraying, harvesting, etc. There- irrigation and osmotic stress is harmful for plants fore, tree size is crucial for a proper orchard manage- because of the induced damage caused by reactive ment, optimizing productivity, labor and, oxygen species (ROS), such as hydrogen peroxide consequently, benefits. (H O ) and superoxide radicals (O ). To manage 2 2 2 123 Transgenic Res (2018) 27:225–240 233 Transgenic Prunus domestica plums silenced for generation cycle of plum from 3 to 7 years to less than the gibberellic acid receptor GID1c displayed a range 1 year. of dwarf phenotypes, suggesting that a reduction in This unique phenotype displayed by the FT-plums GID1c levels could be utilized to develop semi-dwarf is currently being applied in a novel breeding strategy. trees (Hollender et al. 2016). To these authors, At the USDA-AFRS facility (Kearneysville, WV, knocking down GID1c expression, through genetic USA), the FT-plums are being used in crosses for what engineering or gene editing, or by selecting trees with they have termed ‘‘FasTrack’’ breeding (Fig. 2) naturally reduced GID1c expression in a breeding (Scorza et al. 2014). Moreover, ‘FasTrack’ breeding program, maybe a useful strategy to obtained new is carried out in a greenhouse, therefore, the environ- dwarf or semi-dwarf plum cultivars or rootstocks mental limitations of winter chilling can be overcome, (Hollender et al. 2016). and flowering and fruit production is year-round. The system may allow for the rapid incorporation of Early-continuous flowering trees and fast breeding important traits into plums. of plums The FT phenotype is dominant, therefore in the breeding process hemizygous early flowering progeny The tree fruit industry is facing highly dynamic can be recurrently selected with the desired genotype situations such as climate change, reductions in and used for the next cross. Extreme dwarf plum plants are eliminated in each generation since they are not available labor, increasing environmental concerns leading to restrictions in the use of agrichemicals, productive of flowers or fruit and therefore are not changing consumer preferences and the spread of useful for breeding (R. Scorza, personal communica- exotic pathogens and insect pests. To meet these tion). When considerable improvements in the breed- challenges, breeding new adapted fruit cultivars is ing process are clearly obvious, only seedlings that do critical. For tree fruit crops, the main factors slowing not contain the early flowering gene are selected the rate of new cultivar development is the long (Fig. 2). At this point, new plantlets are not genetically juvenile period, that is, the time between seed planting modified and can be planted in the field for their and fruiting (3–7 years in the case of European plum), evaluation of agronomic and commercial traits. the large land areas necessary for planting seedling Afterwards, new clones could be used directly as fruit tree populations, and the associated expenses of new varieties or as elite lines for further breeding. field operations. Hybridization is dependent upon More information about the innovative breeding flowering which occurs only once each year, and is program is available at http://ucanr.edu/sites/ dependent upon sufficient chill in the winter, warmth fastrack/ (accessed February, 2018). in the spring. Moreover, flowering and fruit set are In European plum ‘FasTrack’ breeding is in significantly affected by many environmental factors progress. ‘Prune d’Agen’ (also known as ‘Prune such as severe low winter temperatures, spring frosts, d’Ente or ‘French Prune’) along with its numerous high spring temperatures, and rain during pollination clonal selections, is perhaps the most economically season. Manipulation of cultural conditions can important Prunus domestica in the world. It accounts shorten the period of juvenility, nevertheless promot- for most of the world’s trade in dried plums (prunes). ing flowering through biotechnology has arisen in the The cultivar is very important in California, where it last years as a feasible solution to overcome these accounts for 99% of the dried plum production, limitations (van Nocker and Gardiner 2014). however it is susceptible to sharka disease. Dried Plum trees transformed with Poplar Flowering Plum industry members are especially concerned locus T1 (PtFT1) showed altered architecture, dor- about this fact. Consequently, the ‘FasTrack’ system mancy requirement, and continuous flowering (Srini- is currently being applied to move the PPV resistance vasan et al. 2012). In the greenhouse conditions, trait from plum type ‘Honeysweet’ into ‘French’ type transgenic plants over expressing PtFT1 flowered and plums (http://ucanr.edu/sites/fastrack/DriedPlum/; produced fruits continuously in few months (from 1 to accessed February, 2018). Additionally, the FT tech- 10 months, depending on the transgenic line) (Srini- nology has been incorporated in a breeding program vasan et al. 2012). This means a reduction in the that attempt the obtainment of new stoneless plum cultivars (Callahan et al. 2015). 123 234 Transgenic Res (2018) 27:225–240 Fig. 2 An example of FasTrack breeding technology. The of FT-resistant individuals with the desired type plum. Step 3 scheme shows a procedure to move a disease resistant trait to an Undertake two backcrosses with the original desired genotype elite clone or commercial cultivar. Step 1 Undertake initial cross using MAS to select the desirable traits. Step 4 Select the of an early flowering FT-plum with a resistant genotype (R). resistant progeny (R) with the desired original type traits (in the Among the FT-plums progeny, select the resistant individuals red ellipse). These trees are non-transgenic and they can be field using marker assisted selection (MAS). Step 2 Undertake a cross planted in evaluation plots. (Color figure online) Similar rapid cycle crop breeding approaches, are and are not found in conventional fruit trees. In currently being applied to other perennial tree fruits addition, beneficial effects of dried plums on bone such as apple and citrus (Flachowsky et al. 2011;Le health have been demonstrated (Deyhim et al. 2005; Roux et al. 2012; Rodriguez et al. 2014). Franklin et al. 2006; Halloran et al. 2010; Rendina Furthermore, the FT-plums did not show chilling et al. 2013; Smith et al. 2014a, b), and therefore, FT- requirement and their phenotype was different from plums could be used as a countermeasure to micro- the wild type, losing apical dominance and showing a gravity-induced bone loss of the crewmembers during bushy or vine-like genotype (Srinivasan et al. 2012). long term space missions (Graham et al. 2015). For these authors, these new characteristics may facilitate the cultivation of European plum in tropical Future prospects in genetic engineering of plum climates and the design of novel production methods for this crop, such as greenhouse intense production Gene editing systems. In fact, FT-plums have been proposed as a crop for In recent years site-directed nucleases (SDNs), such as spaceflights and extraterrestrial colonization (Graham CRISPR/Cas9, have emerged as an attractive technol- et al. 2015). The small plant size and lack of any ogy for production of mutated crops/cultivars (Pacher significant chilling requirement for flowering and and Puchta 2017). Successful applications of CRISPR/ fruiting are characteristics required for space travel Cas9 system to modify gene expression of several 123 Transgenic Res (2018) 27:225–240 235 species, including perennial plants have been CRISPR/Cas9 components transiently in European reviewed by Bortesi and Fischer (2015) and Samanta plum, as well as, DNA-based virus vectors, such as et al. (2016). In Populus, Fan et al. (2015) generated geminivirus, which have been successfully used for homozygous knock-out mutation in predicted loci in plant genome editing (Lozano-Dura´n 2016). T0 generation with CRISPR/Cas9 system. Once again To date in plum, eIF(iso4E) appears as an remark- in Populus, Zhou et al. (2015) created knockout able target for SDNs since its down regulation mutation in two genes involved in lignin and flavonoid displayed resistance to PPV infection in European biosynthesis achieving with 100% efficacy with this plum and peach (Cui and Wang 2017; Wang et al. system. 2013b). The CRISPR/Cas9 technology was employed Currently, the successful use of these strategies in to introduce sequence-specific mutations at the woody perennial species is mainly limited to Populus, eIF(iso)4E locus in Arabidopsis thaliana and com- but there are many interesting traits that could plete resistance to Turnip mosaic virus (TuMV) was potentially be modified by genome editing, including successfully engineered (Pyott et al. 2016). Trans- virus or insect resistance, herbicide tolerance, gene-free T2 generation was obtained by segregating improved fruit quality traits, etc. However, there are the induced mutation from the CRISPR/Cas9 some hindrances to the routine use of these new transgene. techniques in fruit trees, including their long life Other plant translation factors have been identified as new possible targets for PPV resistance. In cycles and the obligate vegetative propagation. The combination of genome editing tools and the early Arabidopsis thaliana, DNA-binding protein phos- flowering approach via transformation could allow the phatase 1 (AtDBP1) and a protein named GRF6, segregation of the induced mutation, the SDN used in which interacts with both AtDBP1 and the mitogen- the process, and the early flowering transgene in a activated protein kinase 11, have been described as relative short period. The new cultivars generated will PPV susceptibility factors (Carrasco et al. 2014; not be transgenic, since the genetic engineering will Castello et al. 2010). If Prunus orthologues of these only be used for inducing mutation and speeding up genes are identified at one point, they could be easily the breeding process. This might facilitate their silenced following an RNAi approach in Prunus deregulation and commercialization, making the pro- domestica and later evaluated to confirm the results cess similar to any conventional new hybrid, and observed in Arabidopsis thaliana. In addition, Prunus eliminating, or at least reducing, public concerns about persica DEAD-box RNA helicase-like (PpDDXL) has biotech-crops. Good news in this direction are that been reported to interact with PPV-VPg and, PPV was USDA-APHIS does not consider breeding stock and not able to infect a knock-out mutant of AtRH8, an cultivars produced from ‘FasTrack’ breeding as GM orthologue gene in A. thaliana (Huang et al. 2010). plants (https://www.aphis.usda.gov/biotechnology/ This protein seems to be a suitable candidate for the downloads/reg_loi/Drs%20Scorza%20and%20Callah application of genome editing approaches since it is an%20Final.pdf). not involved in plant growth and development. In the application of sequence-specific nucleases, PPV resistance has also been shown to be affected editing plant genomes without the stable incorporation by a cluster of six meprin and TRAF-C homology of recombinant DNA into the target genome may help domain (MATHd) (Zuriaga et al. 2013). Mutations in the deregulation and commercialization process of and deletions in the alleles were associated with the new genetically modified cultivars or varieties. There- resistant phenotype. The mutated allele showed dom- fore, DNA-free and/or virus-based genetic engineer- inance and resistance was displayed in heterozygosity ing tools may be of additional benefit in regions where (Zuriaga et al. 2013), which makes this locus an legislation is less science-based. Viral vectors are excellent candidate as a target for future SDN being applied to express transiently the Cas9 protein approaches. and/or specific gRNAs (Reviewed by Pacher and Puchta 2017). In peach, a PRSV based-vector has been Genotype-independent procedures successfully used to silence endogenous genes through virus induced gene silencing (Cui and Wang 2017). As mentioned above, genotype is a key factor for This sort of vector could be used to deliver the transformation and procedures developed for specific 123 236 Transgenic Res (2018) 27:225–240 genotypes are often not suitable for other cultivars. number of transgenic plums expressing resistance to This fact is a big limiting factor in the application of virus, nematode, bacterial, and fungal diseases have genetic engineering to plum established commercial been produced along with resistance to abiotic stresses cultivars/clones, allowing discrete modifications in a and with altered fruit ripening have been produced in desired genotype. Therefore, researchers have tried to laboratories world-wide. Practical application of overcome this problem with different strategies. genetic engineering in P. domestica has been the Adventitious organogenesis and embryogenesis are development, U.S. regulatory approval, and release of usually very sensitive processes, strongly affected by a transgenic cultivar (‘HoneySweet’) with resistance minor changes. The use of regeneration-promoting to PPV, the most serious stone fruit virus disease. This genes could aid to reduce the genotype effect. Ectopic release demonstrates the practical value of the appli- expression of the corn Knotted-like homeodomain cation of these biotechnologies for plum, and for tree (KNOX1) gene, which is involved in meristem and fruit improvement in general. Further, the P. domes- shoot apical meristem formation (Doerner 2003), tica genetic engineering system along with the ‘Fas- enhanced the frequency of adventitious shoot regen- Track’ technology provides a platform for the rapid eration from leaves of the ‘Bluebyrd’ plum cultivar functional analysis of genes that not only affect from 0%, in the untransformed control explants, up to vegetative characteristics, but also those affecting 96% (Srinivasan et al. 2011). Also, the ipt gene, flowering, fruiting, and seed development. In turn, this genetic knowledge provides the raw material for that oncogene involved in cytokines biosynthesis, increased regeneration of transformed plants from can be applied to produce further improvements to apricot in vitro leaf explants of the cultivar ‘Helena’ plum and other fruit crops that address the current and (Lopez-Noguera et al. 2009). Further studies with future challenges of fruit production. In the future, additional cultivars need to be performed to probe the integration of classical and biotechnological technolo- utility of these genes. gies in breeding programs is crucial to obtain new Transformation of meristematic tissues has been improved plum trees, with good agronomical traits and also proposed as a possible solution to reduce the fruit quality according to growers and consumers genotype effect, allowing genetic manipulation of demands. Nowadays, the greatest barrier to achieve somatic cells of established cultivars. However, dif- the transition to efficient, and effective breeding ferent authors have tried unsuccessfully to transform programs for plums, and all fruit tree crops, that will woody perennial plants meristems, where most of the enable breeders to meet the demands of growers and transgenic regenerated shoots resulted as chimeras consumers and enable the production of healthy, (Petri and Burgos 2005; Wang et al. 2009; Faize et al. abundant crops in an era of changing climate is the 2010). Further studies would be necessary for the lack of clear, efficient, science-based regulatory establishment of a dissociation methodology and regimes that will allow for the application of modern regeneration of non-chimeric transgenic plants in methodologies in applied breeding. order to make these methodologies applicable in Acknowledgements C.P. thanks the UPCT and the Ministerio woody perennial plants biotechnology. Espan˜ol de Economı´a y Competitividad for his Ramo´n and Cajal contract. This research was partially funded by Fundacio´n SENECA (Project Ref: 1925/PI/2014). Conclusions Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// The European plum has been shown to be amenable to creativecommons.org/licenses/by/4.0/), which permits unre- genetic improvement technologies from classical stricted use, distribution, and reproduction in any medium, hybridization, to genetic engineering, to rapid cycle provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com- crop breeding (‘FasTrack’ breeding). P. domestica mons license, and indicate if changes were made. shows a high regeneration capacity from seed-derived explants and these regeneration rates allow the production of marker-free transgenic plants. Trans- formation from clonal material has also been demon- strated, although only in one cultivar (‘Startovaya’). 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Transgenic ResearchSpringer Journals

Published: Apr 12, 2018

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