Key message Degradation of nitrogen-rich purines is tightly and oppositely regulated under drought and low nitrogen supply in bread wheat. Allantoin is a key target metabolite for improving nitrogen homeostasis under stress. Abstract The metabolite allantoin is an intermediate of the catabolism of purines (components of nucleotides) and is known for its housekeeping role in nitrogen (N) recycling and also for its function in N transport and storage in nodulated legumes. Allantoin was also shown to differentially accumulate upon abiotic stress in a range of plant species but little is known about its role in cereals. To address this, purine catabolic pathway genes were identified in hexaploid bread wheat and their chromosomal location was experimentally validated. A comparative study of two Australian bread wheat genotypes revealed a highly significant increase of allantoin (up to 29-fold) under drought. In contrast, allantoin significantly decreased (up to 22-fold) in response to N deficiency. The observed changes were accompanied by transcriptional adjustment of key purine catabolic genes, suggesting that the recycling of purine-derived N is tightly regulated under stress. We propose opposite fates of allantoin in plants under stress: the accumulation of allantoin under drought circumvents its degradation to ammonium + + (NH ) thereby preventing N losses. On the other hand, under N deficiency, increasing the NH liberated via allantoin 4 4 catabolism contributes towards the maintenance of N homeostasis. Keywords Allantoin · Drought · Nitrogen deficiency · Nutrient recycling · Purine catabolism · Triticum aestivum Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1110 3-019-00831 -z) contains Introduction supplementary material, which is available to authorized users. Nitrogen (N) is a macronutrient required in large quantities * Sigrid Heuer for plant development and growth with N deficiency causing email@example.com chlorosis in older leaves and significant yield losses. Under School of Agriculture Food and Wine, The University N deficiency and natural senescence plants translocate avail - of Adelaide, Urrbrae, SA 5064, Australia able N from source tissues to sink tissues, such as young School of Agriculture and Food, The University leaves (Masclaux-Daubresse et al. 2010) and developing of Melbourne, Parkville, VIC 3010, Australia grains, accounting for 60–92% of total grain N (Barbottin Metabolomics Australia, The University of Melbourne, et al. 2005). The glutamine synthetase–glutamate synthase Parkville, VIC 3052, Australia (GS-GOGAT) cycle plays an important role in this process Max Plank Institute of Molecular Plant Physiology, since it recycles N liberated from the catabolism of N-rich 14476 Potsdam, Golm, Germany macromolecules, such as protein and nucleic acids, into low- Rothamsted Research, Plant Science Department, Harpenden, molecular-weight organic compounds for long-distance N Hertfordshire AL5 2JQ, UK transport (Lea and Miflin 2010). Present Address: Strube Research GmbH & Co. KG, Purines are the most abundant N heterocyclic com- 38387 Söllingen, Germany pounds in nature and are found in nucleic acids (DNA, Present Address: Graduate School of Biological Sciences, RNA) and many other cellular components, such as ATP, Nara Institute of Science and Technology, Ikoma, GTP or NADH (Werner and Witte 2011). Plants undergo the Nara 630-0192, Japan Vol.:(0123456789) 1 3 478 Plant Molecular Biology (2019) 99:477–497 complete breakdown of the purine ring via a catabolic path- N deficiency, however, early reports showed that AtALN way enabling the recycling of both carbon (C) and N (Fig. was strongly up-regulated when Arabidopsis seedlings were S2). Overall, the oxidation of one molecule of xanthine to grown under N starvation (Yang and Han 2004). one molecule of glyoxylate liberates three molecules of CO Given the potentially important role of allantoin in N and four molecules of ammonium (NH ), which are likely metabolism and stress tolerance but the limited information to be reassimilated by the GS-GOGAT cycle into amino available in cereals, the aim of the present study was to char- acids. The pathway starts with the conversion of xanthine acterise the purine catabolic pathway in bread wheat. For to urate catalysed by xanthine dehydrogenase (XDH) (Tri- this, we selected two wheat genotypes (RAC875 and Mace) plett et al. 1982; Werner and Witte 2011). Urate is further that are adapted to Australian environments. The gene loci processed by urate oxidase (UOX) producing 5-hydroxyi- were identified based on the reference genome of the cultivar sourate (5-HIU), 5-HIU is then converted to allantoin via the Chinese Spring and their chromosomal location was experi- 2-oxo-4-hydroxy-4-carboxy-5-ureido-imidazoline (OHCU) mentally verified. Quantification of allantoin in different tis- intermediate by allantoin synthase (AS) (Hanks et al. 1981; sues throughout plant development revealed accumulation of Ramazzina et al. 2006; Kim et al. 2007; Lamberto et al. allantoin under drought and reduced allantoin levels under 2010; Pessoa et al. 2010). Allantoin is metabolised to allan- N limitation and this was associated with differential regula- toate by allantoinase (ALN) and then to ureidoglycine by tion of the genes in the purine catabolic pathway. Monitor- allantoate amidohydrolase (AAH) (Yang and Han 2004; ing of N-metabolite pools in seeds revealed that allantoin Todd and Polacco 2006; Werner et al. 2008). The last two levels progressively increase in developing grains and that enzymatic steps are catalysed by ureidoglycine aminohy- genotypic differences exist for accumulation of allantoin and drolase (UGAH), which converts ureidoglycine to ureido- other important N-containing metabolites. glycolate (Serventi et al. 2010). Ureidoglycolate amidohy- drolase (UAH) converts ureidoglycolate to hydroxyglycine and, lastly, hydroxyglycine decays to glyoxylate by a non- Materials and methods enzymatic reaction (Werner et al. 2010). In addition to its housekeeping role in N recycling, the Plant material purine catabolic pathway has an important function in cer- tain dinitrogen (N )-fixing legumes (Schubert 1986; Sinclair Two semi-dwarf South Australian wheat genotypes RAC875 and Serraj 1995; Alamillo et al. 2010; Coleto et al. 2014). and Mace were evaluated in this study. RAC875 (RAC655/3/ Allantoin and allantoate (also known as ureides) are the Sr21/4*LANCE//4*BAYONET) is high-yielding in the main products of atmospheric N fixation in root nodules, drought and heat-prone South Australian environments which are then translocated to the shoot (Herridge et al. (Izanloo et al. 2008; Bennett et al. 2012). Mace (WYALK- 1978; Pate et al. 1980). In recent years, the ureide allantoin ATCHEM/STYLET//WYALKATCHEM) was bred and has gained attention by the scientific community as several released by Australian Grain Technologies (AGT) in 2008 metabolomics studies reported this metabolite to accumulate and preliminary studies suggest that Mace has high N-use in a broad range of plant species under drought (Bowne et al. efficiency across different South Australian environments 2011; Oliver et al. 2011; Silvente et al. 2012; Degenkolbe (Mahjourimajd et al. 2016). et al. 2013; Yobi et al. 2013; Casartelli et al. 2018), high salt (Kanani et al. 2010; Wu et al. 2012; Nam et al. 2015; Wang Plant growth et al. 2016), cold (Kaplan et al. 2004) and sulfate starvation (Nikiforova et al. 2005). In contrast, allantoin was found The experiment was conducted in a controlled environ- to be reduced under prolonged N deficiency in maize and ment with day/night cycle of 12 h/12 h at a flux density at −2 −1 rice (Amiour et al. 2012; Coneva et al. 2014). Further, the canopy level of 300 µmol m s , 20 °C/15 °C day/night work of Watanabe et al. (2014) revealed that Arabidopsis temperature and 82% average humidity. Potting mixture AtALN mutants, which constitutively accumulated allantoin, was composed of river sand and coco-peat and prepared were more tolerant to desiccation stress. It was demonstrated according to Melino et al. (2015). Granular urea was pro- that allantoin mediates abscisic acid (ABA) signalling by vided as basal N application with rates of 150 and 75 mg N −1 stimulating the activity of genes and enzymes belonging to kg for high and low N treatments, respectively. At stem the ABA-producing pathways. In addition, allantoin was elongation (39 days after sowing) a third of the basal urea shown to accumulate in Arabidopsis leaves grown under rates for each treatment were applied by soil drenching. drought and salt stress in coordination with transcriptional A soil water retention curve was constructed by measur- changes of a number of purine catabolic genes (Irani and ing the pre-dawn leaf water potential of 3-week old seed- Todd 2016; Lescano et al. 2016). To date, there are few stud- lings under progressive drought stress with a Scholander- ies on the regulation of the purine catabolic pathway under type pressure chamber (Soil Moisture Equipment Corp., 1 3 Plant Molecular Biology (2019) 99:477–497 479 Santa Barbara, USA) (Fig. S1b). Overall, the experiment additionally included in the gene name to allow distinction was comprised of two cultivars (RAC875 and Mace), of the three homeologous sequences (Table S1). three treatments (high nitrogen well-watered (HN-WW), low nitrogen well-watered (LN-WW) and high nitrogen drought (HN-D)), six sampling time points and six bio- Synteny analysis logical replicates for a total of 216 experimental units. Individual seeds were germinated in black square plastic Synteny analysis were conducted based on available pots (11 × 11 cm area, 14 cm height) containing 1.5 kg of genome sequences of Sorghum bicolor, Oryza sativa soil mix arranged in a randomised complete block design. and Brachypodium distachyon using the Phytozome 11 WW conditions were maintained by daily watering of ‘Ancestry’ tool (phytozome.jgi.doe.gov). Generally, five the pots to 20% soil water content (SWC), calculated as genes upstream and downstream of the respective purine SWC = [(m − m ):m ] × 100. Drought was catabolic pathway gene were included in the orthology wet soil dry soil dry soil induced by withholding water until signs of leaf rolling analysis. These genes in bread wheat were identified by appeared (approximately 6.5% SWC; Fig. S1a). Samples TBLASTN searches of Brachypodium genes against the collected during the reproductive stages were harvested TGACv1 wheat genome assembly (http://pre.plants.ensem according to the anthesis date of individual plants and bl.org/Triticum_aes tivum/Inf o/Index). Genes w ere consid- water was first withheld 2 weeks after anthesis. Whole ered syntenic if they were present on the same TGACv1 spikes were collected from the main stem of each plant scaffold where the wheat purine catabolic genes were and to minimise the differences in development of the annotated. grains along the spike, only the middle part of the spike was employed for further analyses. Rachis was removed and the vegetative part of the spike (hereafter referred as Wheat nulli‑tetrasomic lines spikelet) was separated from the developing grains. Details on the sample collection throughout the experiment are To verify the chromosomal location of the wheat purine given in Fig. S1a. Samples for molecular analysis were catabolic gene Chinese Spring nulli-tetrasomic (NT) lines snap-frozen in liquid nitrogen and stored at − 80 °C until were used in which individual wheat chromosomes were further use. replaced (nullisomic) by an extra pair (tetrasomic) of their homeologs (Sears 1954). PCR was performed using 150 ng of genomic DNA of the NT lines as template using standard Identification of the purine catabolic genes in wheat reaction conditions of ThermoPol (BioLabs, USA) using the and other grass genomes primers provided in Table S1. The purine catabolic genes of Oryza sativa, Zea mays, Sorghum bicolor and Brachypodium distachyon were Allantoin quantification identified with a BLASTP search using Arabidopsis thali- ana protein sequences as a query in Phytozome (https :// Metabolite extraction phyto zome.jgi.doe.gov). B. distachyon protein sequences were then used for a TBLASTN search on the barley WGS Metabolites were extracted from 10 mg of homogenised, Morex Assembly version 3 (https://ics.hutt on.ac.uk/mor ex freeze-dried tissue with 500 µl of 100% (v/v) methanol 13 15 Genes /blast _page.html) and the barley predicted protein containing 12.5 µM C N-allantoin (internal standard), 13 15 sequences were used for a TBLASTN search against except for mature grains samples for which 25 µM C N- the Chinese Spring TGACv1 genome assembly (http:// allantoin was used. Samples were vortexed and incubated pr e.plant s.ensem bl.or g/T r iti cum_aes ti vum/Inf o/Inde x ) in an Eppendorf Thermomixer at 1400 rpm and 30 °C for (Clavijo et al. 2017). In cases where full- length sequences 15 min followed by a 15 min centrifugation at 13,000 rpm were not found in Chinese Spring TGACv1, we searched (4 °C). The supernatant was transferred to a new tube. 500 µl the Chinese Spring IWGSC chromosome survey sequence of milli-Q water was added to the remaining pellet, vortexed (css) version 3 (IWGSC 2014—http://www.wheat genom and centrifuged for 15 min at 15,000 rpm. The supernatant e.org) and the sequences were merged with TGACv1 was combined with the previous one, vortexed for 30 s and sequences using Geneious version 10.0.2 (http://www. centrifuged for 15 min at 15,000 rpm. The resulting superna- ge n e io u s . c o m) (see Table S1 for details). Each purine cata- tant was transferred to a new tube and 300 µl of 100% (v/v) bolic gene was assigned with the name of the Arabidop- chloroform was added, vortexed and centrifuged for 5 min at sis orthologous according to Watanabe et al. (2014). For 15,000 rpm. 800 µl of the top (polar) phase was transferred wheat, the sub-genome localisation (e.g., Chr 1AL) was into a new tube for allantoin analysis. 1 3 480 Plant Molecular Biology (2019) 99:477–497 Sample derivatization detection for the allantoin MRM, was 0.294 µM based on a signal to noise ratio of 3. 200 µl of the upper polar phase was aliquoted in a glass Data was processed using Agilent MassHunter QQQ insert and dried under vacuum (RVC 2-33 CD plus, John Quantitative Analysis software (B.07.00). Allantoin was Morris Scientific Australia) set at ambient temperature. quantified by single point calibration based on the relative All samples were re-constituted in 10 µl of methoxy- response (the response area of allantoin (MRM 398 → 171) −1 13 15 amine hydrochloride (30 mg ml pyridine) and deriva- divided by the response area of C N-allantoin (MRM 13 15 tised at 45 °C for 60 min at 500 rpm before adding 20 µl 400 → 173)) and C N-allantoin concentration. Sample dry of N-methyl-N-(ter t-butyldimethylsilyl)trif luoroaceta- weight and extraction volume were taken into consideration mide (MTBSTFA) with 1% (w/v) trimethyl chlorosilane when calculating the final allantoin concentration. (TMCS) and incubated at 45 °C for 45 min. Metabolome‑wide analysis GC–MS instrument conditions Metabolites were extracted from 60 mg dry weight (DW) 1 µl of derivatised sample was injected into a GC-QqQ- of freeze-dried wheat spikelet and developing grain sam- MS system comprised of a Gerstel 2.5.2 Autosampler, a ples using 100% methanol containing C-sorbitol as inter- 7890A Agilent gas chromatograph and a 7000 Agilent nal standard. Chloroform followed by water were added to triple-quadrupole MS (Agilent, Santa Clara, USA) with the mixture. Polar phase aliquots were freeze-dried using a an electron impact (EI) ion source. The GC was operated speed vacuum. Aliquots for GC-time of flight (TOF)-MS in constant flow mode with helium as carrier gas. The MS analysis were reconstituted in N-Methyl-N-(trimethylsilyl) was adjusted according to the manufacturer’s recommen- trifluoroacetamide (MSTFA) and methoxyamine hydro- dations using tris-(perfluorobutyl)-amine (CF43). A J&W chloride as derivatizing agents. Samples were analysed as Scientific VF-5MS column (30 m long with 10 m guard described previously by Lisec et al. (2006) and Erban et al. column, 0.25 mm inner diameter, 0.25 µm film thickness) (2007). Aliquots for ion chromatography (IC) were recon- was used. The injection temperature was set at 250 °C, the stituted in milli-Q water and analysed according to Moschen MS transfer line at 290 °C, the ion source was adjusted to et al. (2016). Aliquots for free spermidine analysis were 230 °C and the quadrupole at 150 °C. Helium was used reconstituted in 0.2 N perchloric acid solution, dansylated −1 as carrier gas at flow rate of 1 ml min . Nitrogen (UHP and quantified by HPLC according to Do et al. (2013). 5.0) was used as collision cell gas at flow rate of 1.5 ml −1 min . Helium (UHP 5.0) was used as quenching gas at a −1 flow rate of 2.25 ml min . Gain factor for the triple axis Amino acid analysis detector was set at 2. Derivatised sample was injected into the column at 100 °C followed by 1 min hold, followed by Amino acids were extracted from 50 mg DW of freeze-dried −1 a ramp of 25 °C min to 325 °C. wheat samples with 1 ml of 10 mM sodium acetate contain- −1 ing 250 nmol ml Norvaline (internal standard). Amino acids were quantified on a Waters Acquity™ UPLC system Method optimization using the Waters AccQ-Tag Ultra Chemistry Kit follow- ing the manufacturer’s instructions (Waters Corp., USA). 13 15 Allantoin and C N-allantoin standards were purchased Chromatograms were analysed with Empower® 3 Software from Sigma Aldrich (Australia). Retention time, a corre- (Waters Corp., USA). sponding unique precursor ion and product ions were iden- tified on the GC-QqQ-MS instrument for each standard. Collision energy was optimised between 0 and 20 V for Total N analysis the identified precursor to product ion transitions (Multiple Reaction Monitoring; MRM). A product ion was selected as Total N concentration was determined using the Elementar the Target ion (T) and the other subsequent MRM transition rapid N exceed (Elementar Analysensysteme GmbHana- was set as the Qualifier ion (Q) (see Table S2 for details). lyser, Germany) using 100 mg DW of homogenised wheat Linearity of the method was tested using serial dilutions grain samples. Aspartic acid (250 mg) was used as standard of the calibration standard and showed a linear calibration for calibration: the theoretical aspartic acid N% (10.52) was 13 15 range between 0.98 µM to 250 µM C N-allantoin. Cor- divided by the N% measured by the instrument generating relation coefficient (R ) of the calibration curve was 0.99 a N factor. The N factor was then used to correct the N% for the target allantoin MRM (398 → 171). The limit of measured for each sample. 1 3 Plant Molecular Biology (2019) 99:477–497 481 manufacturers’ instructions. cDNA quality was verified by RNA‑Seq reads mapping for the TaXDH2 paralogous genes PCR amplification of the actin gene (Table S3). Publicly available bread wheat (cv. Chinese Spring) RNA- Primer design and quantitative reverse Seq data was used for generating the per-base expression transcription PCR (qRT‑PCR) analysis values. This data set covers 15 duplicated samples corre- sponding to five organs (root, leaf, stem, spike, grain), each Quantitative real-time PCR was performed with KAPA at three developmental stages ( http://ur gi.versa illes .inr a. SYBR Fast qPCR kit Master Mix, and amplification was fr/files /RNASe qWhea t/) (Choulet et al. 2014). The RNA- real-time monitored on a QuantStudio™ 6 Flex Real-Time Seq reads were quality-, adapter- and length-trimmed using PCR System (Applied Biosystems, USA). Gene-specific Trimmomatic (Bolger et al. 2014), version 0.30 with a cus- primers targeted to amplify all three homeologs simulta- tom list of adapter sequences and the following settings: neously were designed with AlleleID® software (Premiere ‘ILLUMINACLIP:adapters.fa:1:6:6 LEADING:3 TRAIL- Biosoft) (Table S3) and the specificity of each pair was ING:3 SLIDINGWINDOW:4:6 MINLEN:60’. The reads verified by melting curve analysis and sequencing of the were aligned to the scaffolds (version 3) from the Chinese products. Change in gene expression were calculated using Spring whole genome assembly (version 0.4) using STAR qBASE + software and reported as calibrated normalised (version 2.5.1b) (Dobin et al. 2013), with the following set- relative quantities (CNRQ) that represents the relative gene tings: --outFilterMultimapScoreRange 0; --outFilterMul- expression level between different samples for a given target timapNmax 5; --outFilterMismatchNoverLmax 0 --out- gene: FilterMatchNminOverLread 1; --outSJfilterOverhangMin 35,20,20,20; --outSJfilterCountTotalMin 10,3,3,3; --outSJfil- terCountUniqueMin 5,1,1,1 --alignEndsType EndToEnd; --alignSoftClipAtReferenceEnds No; --alignIntronMax 10,000; --alignMatesGapMax 10,000. The remaining set- tings were left at their defaults. The resulting BAM files were merged using samtools merge (version 1.2) (Li et al. 2009). The manually annotated coordinates of the genes define the regions for which the depth of the aligned reads NRQ is then divided by a calibration factor (CF) (Hel- was computed from the aligned BAM file using the depth lemans et al. 2007). Four reference genes (Table S3) were module of samtools. quantified by qRT-PCR: TaActin, TaGAPdH, TaCyclo- philin and TaEFA2. CNRQ values were calculated using RNA preparation the most stable genes within a specific tissue (selected by qBASE + software). cDNA libraries were prepared from snap-frozen samples of the youngest fully emerged leaf (YFEL), flag leaf, stem, spikelet and developing grain under control (HN-WW) Data analysis and treatment (HN-D and LN-WW) conditions. Samples were ground to a fine powder using a 2010 Geno/Grinder Statistical analyses were performed using GraphPad Prism version 7.00 for Windows (GraphPad Software, La Jolla (SPEX SamplePrep, USA) and total RNA was extracted from frozen samples with a phenol and guanidine thiocy- California USA. h t t p : / / w w w . g r a p h p a d . c o m). All data are reported as mean ± SEM. Significant differences between anate buffer according to Chomczynski (1993). To extract RNA from developing grains which have a high polysac- means of two groups of data were tested by Student’s t-test. Significant differences between means of more than two charide content, an extraction buffer (1% (w/v) sarcosyl, 150 mM NaCl, pH 9) and a guanidine hydrochloride-based groups were tested by two-way ANOVA. For metabolome- wide analysis, metabolites levels were log transformed to buffer for purification according to Singh et al. (2003) was employed. Genomic DNA was removed using the TURBO improve the normality of the data set and then scaled by subtracting the median metabolite value in each metabo- DNA-free™ Kit (Ambion®, Thermo Fisher Scientic fi , USA) following the manufacturers’ instructions. High RNA quality lite distribution. Hierarchical clustering by Pearson’s cor- relation distance and PCA analyses were performed with was confirmed in a 2% (w/v) agarose gel visualized under UV light and RNA concentrations were quantified using the support of ClustVis web tool (Metsalu and Vilo 2015). Transcriptional data was reported as log2 ratio of the fold- a ND-1000 spectrophotometer (NanoDrop Technologies, USA). 1 µg of RNA was used for cDNA synthesis using the change between treatment (drought or low N) and control conditions. SuperScript® III kit (Thermo Fisher Scientific, USA) as per 1 3 482 Plant Molecular Biology (2019) 99:477–497 Fig. 1 Distribution of the purine catabolic genes across the bread wheat allohexaploid genome (Triticum aestivum cv. Chinese Spring). a The 24 purine catabolic genes are represented by grey bars and their putative location on chromosomes was estimated based on the related Munich Information Centre for Protein Sequences (MIPS) gene annotation on Ensem- blPlants (plants.ensembl.org). The figure was adapted from Gill et al. (2004). b Homoelog- specific primers were used for PCR amplification of the genes using DNA derived from nulli-tetrasomic lines of the cv. Chinese Spring. Labels shown in the agarose gel indicate the nullisomic chromosome, e.g. “N1A” indicates that chro- mosomal group 1A is absent (see Table S1 for details). The caption indicates the primer set employed to amplify the NT DNA (details provided in Table S1). For example, (i) XDH1-1AL,-1BL and − 1DL primer sets were used to amplify nullisomic lines for chromosome subgroup 1. The absence of an amplicon in a specific NT line (whilst ampli- cons are derived in the two remaining NT lines) indicates localisation of the gene on the respective absent chromosome short arm of chromosome group 6 (TaXDH2-6AS/TaXDH2- Results 6BS/TaXDH2-6DS), respectively. The TaXDH1 and TaXDH2 paralogs share 89.6–90.5% sequence identity within the Identification of the wheat purine catabolic genes predicted coding region (CDS) and the TaXDH1 home- ologs showed a higher CDS identity (98.6–98.8%) than the Based on comparative sequence analyses, a total of 24 wheat TaXDH2 homeologs (93.5–95.3%) (Table S4). The TaALN genes that are homologous to the purine catabolic genes of genes (TaALN-2AL/TaALN-2BL/TaALN-2DL) and TaU- Arabidopsis and rice were identified (Fig. 1a). Generally, GAH genes (TaUGAH-2AS/TaUGAH-2BS/TaUGAH-2DS) for each rice gene three wheat homeologous sequences were localised on chromosome group 2, whilst the TaUOX were identified, except for XDH that had paralogous copies genes were localised on chromosome group 3 (TaUOX- on two different chromosome groups. Specifically, the two 3AL/TaUOX-3BL/TaUOX-3DL). The TaAS genes were iden- allelic variants were located on the long arm of chromosome tified on chromosome group 4 and the TaUAH genes were group 1 (TaXDH1-1AL/TaXDH1-1BL/TaXDH1-1DL) and 1 3 Plant Molecular Biology (2019) 99:477–497 483 localised on chromosome group 5 (TaUAH-5AS/TaUAH- with either high N (HN) or low N (LN). During the course 5BS/TaUAH-5DS). The TaAAH genes were found located of the experiment, a subset of plants grown under HN were on chromosome group 7 (TaAAH-7AL/TaAAH-7BL/TaAAH- subjected to drought (D) at tillering stage and during grain 7DL) and were the least conserved among the analysed filling, (Fig. S1a). Therefore, the experiment comprised of genes with 95–92.9% sequence identity within the predicted three treatments: control (high N, well-watered; HN-WW), CDS (Table S4). low N (low N, well-watered; LN-WW) and drought (high To experimentally validate the chromosomal localization N, drought; HN-D). Analysis of plants at maturity revealed of the wheat orthologues genes predicted by the TAGCv1 that the low N treatment reduced above ground biomass by and IWGSC css assembly, nulli-tetrasomic (NT) lines of the 20.8% and 23.3% in RAC875 and Mace, respectively, whilst wheat cultivar Chinese Spring were used (Fig. 1b; Sears grain yield was reduced by 24.4% and 25.2% in RAC875 and 1954; see M&M for details) and homeolog-specific primers Mace as compared to the controls (HN-WW), respectively (Table S1) designed for PCR analysis of genomic DNA from (Fig. 3). When plants were grown under drought conditions, the NT lines. The absence of an amplicon in the respective above-ground biomass was reduced by approximately 15% NT line confirms that the target gene is physically located in both genotypes as compared to their respective controls. on that nullisomic chromosome pair. Under drought conditions, Mace plants were also higher To further corroborate the orthology between the identi- yielding (grain yield) than RAC875 plants (p < 0.02 by Stu- fied loci we analysed the synteny of the genomic regions dent’s t-test); this corresponded to a 16.0% and 26.6% yield between bread wheat, Brachypodium, rice and sorghum loss for Mace and RAC875, respectively, relative to control (for gene IDs see Supplementary Tables S1 and S5). The conditions (Fig. 3). fragmented nature of the bread wheat TGACv1 assembly (Clavijo et al. 2017) may have reduced the resolution of the Allantoin accumulation under low N and drought analysis, however, wheat showed a high degree of synteny stress with the three diploid genomes included in the analysis and the position of several purine catabolic genes and their To assess whether N and water stress alter allantoin levels neighbouring genes was highly conserved (Fig. 2). The only in the two wheat genotypes, allantoin concentration was exception was XDH, for which we could identify only one quantified in the youngest fully emerged leaf (YFEL) during syntenic gene (Bradi1g15910, depicted in blue) among all vegetative growth and in flag leaf, stem, spikelet and devel- analysed genomes. This gene is located upstream of XDH in oping grain samples during reproductive growth in plants sorghum, downstream of XDH in rice and Brachypodium, grown under HN-WW, LN-WW and HN-D (Fig. 4). Nitro- and on chromosome group 1 in wheat. Interestingly, we gen deficiency significantly reduced allantoin concentration identified two Brachypodium genes (Bradi1g15820 and Bra- at different time points across different tissues and in both di1g15826, depicted in brown and grey, respectively) with genotypes when compared to control conditions (Fig. 4a). In orthologous sequences on wheat chromosome group 6, sup- both genotypes grown under LN-WW conditions, allantoin −1 porting the evidence that a duplication of the genomic region levels were generally below 50 nmol g DW in YFEL, flag −1 harbouring XDH may have occurred during wheat genome leaves and stems, but much higher (50 to 100 nmol g DW) evolution. Analysis of the predicted open reading frames in spikelets and developing grains. The highest reduction of (ORFs) of the TaXDH genes revealed five premature stop allantoin in both genotypes was measured in the stem under codons in TaXDH2-6AS and several mutations in TaXDH2- LN-WW as compared to the HN-WW control, with a 22-fold 6BS, both likely to result in non-functional proteins. In con- and 10-fold reduction in RAC875 and Mace, respectively trast, the TaXDH2-6DS ORF appeared to translate into a (Fig. 4a). Interestingly, under control conditions (HN-WW) functional protein (data not shown). This was supported by Mace flag leaves significantly accumulated much higher −1 in-silico analysis of publicly available Chinese Spring RNA- levels of allantoin at 19 DAA (305 nmol g DW) as com- −1 seq transcriptomics data which revealed that TaXDH2-6DS pared to RAC875 (34 nmol g DW), whilst a significant was the only homeolog expressed (Fig. S3a). Synteny analy- but relatively smaller difference was recorded under low N −1 sis further revealed the presence of two copies of UAH in the at 17 DAA (36 and 12 nmol g DW for Mace and RAC875, Brachypodium genome of which only one gene (BdUAH1) respectively) (Fig. 4a). Similarly, allantoin concentration in showed a syntenic relationship with the other genomes. Mace developing grains was significantly higher compared to RAC875 at 22DAA under both control (HN-WW) and low The effects of low N and drought on plant N (LN-WW) conditions. performance In contrast to the reduced accumulation of allantoin under N deficiency, allantoin significantly positively accu- RAC875 and Mace plants were initially grown under well- mulated under drought in all tissues assessed, with the watered (WW) conditions divided in two subsets supplied exception of the stem where, despite a clear positive trend, 1 3 484 Plant Molecular Biology (2019) 99:477–497 1 3 Chr5AS Chr4AS ALNS Chr7AL AAH UAH Chr6AS XDH2 Chr1AL Breadwheat XDH1 Chr4BL Chr5BS ALNS Chr7BL Chr 6BS XDH2 Chr1BL XDH1 AAH UAH Chr 5DS Chr1DL XDH1 Chr4DL ALNS Chr 7DL UAH Chr6DS XDH2 AAH Brachypodium Bd Chr2 UAH2 Chr4 Chr1 UAH1 XDH ALNS AAH Rice Chr12 Chr3 XDH ALNS AAH UAH Chr6 Sorghum Chr10 Chr8 UAH Chr1 XDH ALNS AAH ALN Breadwheat Chr3AL Chr2AL Chr2AL UOX Chr2AS UGAH Chr2BL ALN Chr2BS Chr3BL UOX UGAH Chr2DL ALN Chr3DL Chr2DS UOX UGAH Brachypodim Chr1 Chr5 UGAH Chr2 UOX ALN Rice Chr9 Chr4 ALN Chr7 UGAH Chr1 UOX Sorghum Chr2 UGAH Chr3 UOX Chr6 ALN Fig. 2 Syntenic relations of the genomic regions harbouring purine catabolic genes in sorghum, rice, Brachypodium and wheat. The four species are shown in four distinct rows underneath each other and genes are represented as coloured shapes with abbreviations of the respective purine catabolic genes indicated. For each gene, the presence of the same coloured shape in different genomes denotes the presence of gene orthologs. Solid lines represent chromosomes and the chromosome number is indicated. This figure is not drawn to scale and therefore genome distances cannot be determined from the length of lines. Dashed lines indicate that genes are located on the same chromosome but not adjacent to the genes of interest (GOI). In the bread wheat genome, genes that were located on the same TGACv1 scaffold as the purine catabolic genes are represented on the same solid line. Genes that were identified on a different scaffold but assembled on the same chromosome are separated from the GOI by a dashed line Plant Molecular Biology (2019) 99:477–497 485 (A)(B) RAC875 Biomass Grain yield aa Mace aa bb 10 bb HN-WWL N-WW HN-D HN-WWL N-WW HN-D Fig. 3 Analysis of agronomic traits of plants at maturity. a Above and letters indicate significant differences between genotypes and −1 −1 ground biomass (g DW plant ) and b grain yield (g plant ) were treatments at p < 0.05. The analysis revealed a significant treatment measured from the remaining tillers of wheat RAC875 and Mace effect (p < 0.0001) for both traits and a significant genotype × treat- plants used for molecular analyses. Plants were grown under control ment interaction (p < 0.05) for grain yield. Asterisk denotes signifi- (HN-WW), low N (LN-WW) and drought (HN-D) conditions and cant difference (p < 0.02) between grain yield of RAC875 and Mace each value represent the mean ± SEM of 16–18 biological replicates. under drought according to Student’s t-test Two-way ANOVA analysis was performed with Tukey’s correction (A) (B) Fig. 4 Allantoin concentration in bread wheat genotypes RAC875 corresponding to 3, 5 and 8 days of drought (days after last water- and Mace under N deficiency and drought stress. Allantoin concen- ing), respectively (see Fig. S1 for details). Data are mean ± SEM of −1 tration (nmol g DW) was quantified by GC-QqQ-MS under a low N four to six biological replicates and letters indicate significant differ - and b drought conditions with respect to control conditions in young- ences between treatments and time points within a genotype (lower est fully emerged leaf (YFEL), flag leaf, stem, spikelet and develop- case: RAC875; upper case: Mace) by two-way ANOVA with Tukey’s ing grain. YFEL samples were collected during tillering stages at correction at p < 0.05. Asterisks indicate significant allantoin differ - 23, 24 and 27 days after sowing, corresponding to 2, 3 and 6 days ences between the genotypes RAC875 and Mace for a given treat- of drought stress (days after last watering), respectively. Flag leaf, ment and time point by Student’s t-test, corrected for multiple com- stem, spikelet and developing grain samples were collected dur- parison using the Bonferroni-Dunn method (*p < 0.05; **p < 0.01; ing grain filling stages at 17, 19 and 22 days after anthesis (DAA), ***p < 0.001; ****p < 0.0001 by Student’s t-test) 1 3 -1 g(DW) plant -1 g plant 486 Plant Molecular Biology (2019) 99:477–497 allantoin accumulation was not significant (Fig. 4b). Dur- most severe (Fig. 4b). Interestingly, flag leaves of RAC875 ing the tillering stage, under severe drought stress (day 8; progressively accumulated allantoin during the course of −1 6.3% SWC) allantoin significantly accumulated 3.6- and the drought treatment, increasing from 136 nmol g DW −1 2.5-fold in the YFEL of RAC875 and Mace, respectively. at day 5 to 1197 nmol g DW at day 8, representing an The higher magnitude of allantoin accumulation under almost 30-fold increase relative to HN-WW. On the other drought in RAC875 were due to significantly higher levels hand, Mace accumulated 1614 nmol of allantoin per gram of allantoin in the YFEL of RAC875 under drought com- DW after only 5 days of drought treatment and levels pared to Mace (Fig. 4b). Similarly, during grain filling, remained constant until day 8. However, the magnitude of allantoin significantly accumulated in Mace flag leaves allantoin accumulation under HN-D in Mace was only 2.5- at day 5 and 8 of the drought treatment (8.6% and 6.6% fold as compared to HN-WW at day 8 (Fig. 4b). The large SWC, respectively), whilst it significantly accumulated in difference in the magnitude of allantoin accumulation RAC875 flag leaves only at day 8 (Fig. 4b). In spikelet between RAC875 and Mace flag leaf can be associated and developing grains, allantoin significantly increased with the higher allantoin levels in Mace already present only at day 8 in both genotypes, when drought stress was under control conditions at day 5 (19DAA). YFEL Flag leaf Stem Spikelet Developing grain LowN (Z22) (Z79) (Z79) (Z79) (Z79) Log2 CNRQ (LN/HN) 23 DAS24DAS 27 DAS17DAA 19 DAA22DAA 17 DAA19DAA 22 DAA17DAA 19 DAA22DAA 17 DAA19DAA 22 DAA TaXDH1 0.12 0.33 0.38 0.94 2.03 -0.050.420.400.540.010.290.090.52-0.05 0.79 TaXDH2 -0.07-0.08 0.40 -0.491.390.860.190.570.450.060.400.211.940.170.81 TaUOX 0.22 0.27 0.26 0.52 1.94 0.44 0.08 0.44 0.28 0.09 0.18 0.02 0.63 -0.170.12 TaALNS 0.11 0.28 0.05 0.82 1.65 0.45 -0.080.300.240.010.17-0.07 0.30 0.39 0.33 TaALN 1.78 1.00 1.12 0.59 1.42 1.14 1.91 1.16 0.45 1.33 0.82 0.60 0.60 0.26 0.25 TaAAH 0.60 1.90 0.85 1.50 2.37 2.74 1.24 1.08 1.39 0.79 0.54 0.71 0.50 -0.010.15 TaUGAH -0.02-0.09 -0.150.210.610.020.090.480.32-0.09 0.22 0.06 0.84 0.26 -0.25 TaUAH -0.030.160.090.271.720.450.360.260.31-0.08 0.30 -0.140.44-0.11 0.12 YFEL Flag leaf Stem Spikelet Developing grain Drought (Z22) (Z79) (Z79) (Z79) (Z79) Day2 Day3 Day6 Day3 Day5 Day8 Day3 Day5 Day8 Day3 Day5 Day8 Day3 Day5 Day8 Log2 CNRQ (D/WW) TaXDH1 0.16 0.46 0.63 -0.320.362.620.01-0.06 0.89 -0.110.922.060.06-0.14 0.14 TaXDH2 -0.44-0.90 -0.22-1.48 -1.04-0.74 -0.23-0.29 -1.37-0.52 -0.14-0.14 0.96 0.06 0.09 TaUOX 0.16 0.19 0.66 -0.220.781.06-0.12 -0.060.14-0.14 0.81 1.49 0.31 -0.27-0.54 TaALNS 0.20 0.41 0.11 -0.070.53-1.04 -0.14-0.13 -0.45-0.01 0.51 0.50 -0.040.16-0.85 TaALN -0.78-1.57 -1.53-1.18 -1.88-0.07 0.60 -1.10-2.07 0.49 -0.26-0.09 0.03 -0.79-2.86 TaAAH -0.020.800.210.08-0.49 1.38 0.14 -0.55-1.21 0.01 0.20 0.43 0.16 -0.64-1.19 TaUGAH -0.30-0.14 0.31 -0.44-0.64 -2.89-0.19 -0.44-1.36 -0.210.42-0.08 0.66 0.19 -1.01 TaUAH -0.120.380.85-0.37 0.24 1.71 0.08 0.07 0.58 -0.060.861.70-0.01 -0.27-0.72 log2 -3 Fig. 5 qRT-PCR analysis of the purine catabolic genes in different grain. Data is expressed as log2 calibrated normalised relative quan- tissues of the bread wheat genotype RAC875 under stress. RNA was tity (CNRQ) of gene transcription under N deficiency or drought extracted from the youngest fully emerged leaf (YFEL), flag leaf, divided by CNRQ under control. Colour denotes significant differ - stem, spikelet and developing grain of plants grown under control ences between treatment and the respective control conditions (low (HN-WW), low N (LN) and drought (D) conditions collected at dif- N/high N; well-watered/drought) as determined by Student’s t-test ferent time points (see Fig. S1a for details). Transcript abundance of using the Two-stage linear step-up procedure of Benjamini, Krieger the genes of interest were determined by quantitative real-time PCR and Yekutieli, with Q = 5% within a given time point. Red indicates (qRT-PCR). Calibrated normalised relative quantity (CNRQ) was cal- up-regulation and blue down-regulation with respect to the control. culated using the most stable reference genes across tissues: TaActin Time points highlighted in green indicate that significant changes in and TaGAPdH for YFEL and spikelet; TaCyclophilin and TaGAPdH allantoin concentration were detected between treatment and control for flag leaf and stem; TaCyclophilin and TaEFA2 for developing conditions according to two-way ANOVA with Tukey’s test (p < 0.05) 1 3 Plant Molecular Biology (2019) 99:477–497 487 the tissue analysed (e.g. were down-regulated in stem and Transcription of purine catabolic genes under low N and drought stress up-regulated in the spikelet). Analysis of the transcriptional profile of purine cata - To assess stress responsiveness of wheat purine catabolic bolic genes in the YFEL of Mace are largely in agreement with those in RAC875. N deficiency increased transcrip- genes, transcript abundance was quantified in all of the RAC875 samples collected during the course of the experi- tion of TaXDH1, TaALN and TaAAH in Mace YFEL; whilst TaALN was down-regulated andTaXDH1 up-regulated under ment (Fig. 5) and on YFEL (Z22) Mace samples (Fig. S4). The data for each gene and treatment is presented in Fig. 5 drought (Fig. S4). as log2 ratio of the calibrated normalised relative quantities (CNRQ) (Hellemans et al. 2007) between treatment (LN- Metabolite signature of spikelet and developing grain under drought and low N conditions WW or HN-D) and control conditions (HN-WW). For this analysis qRT-PCR primers were designed to amplify all To assess the involvement of allantoin in N and C metabo- three homeologs of each purine catabolic gene. The only exception was the design of primers to specifically amplify lism in wheat under drought and N limitation during grain filling, the metabolic signatures from the spikelet and the TaXDH2-6DS, which is the only TaXDH2 homeolog that is expressed (Fig. S3a). This likely explains why the abun- developing grain of RAC875 and Mace were assessed and visualised by hierarchical clustering (Figs. 6, 7) and PCA dance of TaXDH2-6DS transcripts in RAC875 was approxi- mately 20 times lower than in TaXDH1 (Fig. S3b). (Supplementary Fig. S5 and S6). This analysis revealed that allantoin accumulation under Analysis of the transcriptional regulation of purine catab- olism under LN-WW revealed that purine catabolic genes drought conditions (HN-D) at day 8 showed the same pat- tern with additional metabolites forming a distinct cluster in were up-regulated in RAC875 in most tissues and devel- opmental stages analysed (Fig. 5). In particular, the genes both the spikelet (D.S.ii; Fig. 6a) and the developing grain (D.Dg.ii; Fig. 6b) of RAC875 and Mace. In both tissues, TaALN and TaAAH, putatively coding for the key enzymes involved in allantoin and allantoate degradation, respec- these clusters contained mostly amino acids including the well-known drought responsive amino acids proline and tively, were highly responsive to the LN-WW treatment. The differential expression of purine catabolic genes in RAC875 4-amino-butanoic acid (GABA) and, in the developing grain, the polyamine putrescine. Generally, metabolites belonging flag leaves, stem and spikelets at 17DAA corresponded to a significant reduction of allantoin measured in the same to these clusters appeared to accumulate with higher mag- nitude in RAC875 compared to Mace. A genotype-specific tissues as presented in Fig. 4a (time point highlighted in green in Fig. 5). At 19DAA, all purine catabolic genes were response was also apparent in two additional clusters in developing grain with metabolites specifically accumulated significantly up-regulated in flag leaves and, with a lower magnitude, also in stems and spikelets (Fig. 5). (D.Dg.i) and reduced (D.Dg.iv) in RAC875 under drought, respectively (Fig. 6b). In contrast to the transcriptional response under low N conditions, RAC875 droughted plants displayed a differ - Clusters containing metabolites with an apparent reduc- tion under drought in both genotypes were also identified, ential regulation of specific sets of purine catabolic genes (Fig. 5). Generally, TaALN was down-regulated in all ana- in both the spikelet (D.S.i; Fig. 6a) and the developing grain (D.Dg.iii; Fig. 6b).These clusters contained sucrose, lysed tissues of droughted plants (HN-D) relative to plants grown under HN-WW, except in the flag leaves and spikelets myo-inositol and quinic acid. In addition, in spikelets, this cluster contained other organic acids (e.g., isocitric acid, where no significant changes in transcription of TaALN were detected. A decrease in transcript abundance of TaALN in 3-phosphate-glyceric acid) as well as the tri-saccharides raf- finose and 1-kestose. The cluster in developing grain further the YFEL occurred already after exposure to mild drought conditions (day 3), suggesting that this gene is particularly contained glucose-6-phosphate and fructose-6-phosphate, as well as tryptophan and glutamate. drought responsive in this tissue (Fig. 5). TaALN down-reg- ulation at day 6 in YFEL corresponded with accumulation Analysis of the metabolite signature under low N condi- tion revealed contrasting patterns compared to the drought of allantoin as presented in Fig. 4b (time point highlighted in green in Fig. 5). The TaXDH1 and TaUOX genes, coding treatment. Generally, the drought treatment at day 8 clearly separated metabolites between the two treatments (HN- for the enzymes that synthesise allantoin, were up-regulated under HN-D across the analysed tissues, except in devel- WW and HN-D; Fig. S5). However, the low N treatment appeared to influence the metabolite levels to a lower oping grain (Fig. 5). Interestingly, TaXDH2 showed the opposite transcriptional regulation to its paralog, TaXDH1. extent as supported by PCA analysis (Fig. S6). In par- ticular, in spikelet the major dividing component of the In fact, TaXDH2 was down-regulated under HN-D in the YFEL, the flag leaf and the stem (Fig. 5). TaAAH and TaU- data points was PC2 that separated according to the geno- type (Fig. S6a). On the other hand, PCA of developing GAH presented different drought responses depending on 1 3 488 Plant Molecular Biology (2019) 99:477–497 (A) (B) 1 3 Plant Molecular Biology (2019) 99:477–497 489 ◂Fig. 6 Metabolite response of RAC875 and Mace spikelet and Allantoin accumulation in the grain developing grain at day 8 of the drought treatment. Metabolite lev- els measured with GC-MS, ion chromatography and HPLC were log To assess the contribution of allantoin to the overall N pools transformed and scaled by subtracting the median metabolite value in the grain under control (HN-WW) and stress conditions in each metabolite distribution. Hierarchical clustering and heatmap of metabolite levels of RAC875 and Mace spikelet a and developing (HN-D and LN-WW), the plants minus the sampled tiller grain b at day 8 of the drought treatment was performed with the sup- (Supplementary Fig. S1) were grown until maturity, cor- port of ClustVis web tool (Metsalu and Vilo 2015). Pearson’s correla- responding overall to 16–18 plants per treatment. Total N, tion distance of scaled data was used for the hierarchical clustering. allantoin and free amino acid concentrations were then deter- Each column represents a biological replicate. Representative cluster containing allantoin (measured with GC-QqQ method) is noted with mined in grain harvested from the tallest remaining tiller a green line, other representative clusters are noted with a blue line (Fig. 8). Analysis of grain N% highlighted significant differ - ences between treatments (two-way ANOVA, p < 0.0001), in grain revealed no major differences between genotypes particular, it showed a 20% reduction in grain N% of plants and treatment, although a weak separation according to grown under LN-WW with no evident genotypic differences the time point, especially for 22 DAA, was detected along between RAC875 and Mace (Fig. 8a). Allantoin concentra- PC1 (Fig. S6b). tion was significantly responsive to the applied treatments Hierarchical clustering of the spikelet data under dif- and genotypes (two-way ANOVA, Genotype × Treatment, ferent N conditions placed allantoin into a representative p = 0.0028) (Fig. 8b). Particularly, allantoin was reduced cluster (Ln.S.i) which contained a large number of metab- under LN-WW by 44% and 49% in RAC875 and Mace olites, mostly carbon-rich compounds (Fig. 7a). Specifi- plants, respectively. In contrast, under HN-D, allantoin in cally, this allantoin cluster contained eleven organic acids RAC875 grains increased by 39% relatively to HN-WW. (including citric acid, malic acid, succinic acid, glyceric Interestingly, the largest difference in allantoin concentration acid and shikimic acid) and ten sugars (including fructose, between genotypes was observed under HN-WW, as previ- sucrose, myo-inositol, raffinose, 1-kestose). These metabo- ously observed for the flag leaf (Fig. 4). In fact, Mace grains lites appeared to accumulate to higher levels in spikelet of accumulated 66% more allantoin than RAC875, whilst there Mace as compared to RAC875, which showed a small or were no significant differences between the genotypes under no treatment effect (Fig. 7a). LN-WW and HN-D, as mentioned above (Fig. 8b). Similarly, in the developing grain allantoin grouped in Analysis of the free amino acids content of grain from a representative cluster (Ln.Dg.iii) which was composed plants grown under HN-WW revealed that allantoin accu- of allantoin and three carbon-rich metabolites, including mulated to levels comparable to other amino acids, such raffinose, galactinol and 1-kestose. This is in contrast to as glutamine, aspartate, asparagine and arginine (Fig. 8c). the metabolite profile in the drought treatment, where When considering that each allantoin molecule contains allantoin grouped with amino acids. These amino acids, four N atoms (1:1 nitrogen to carbon ratio, N:C), the over- under low N conditions, are scattered between two repre- all N stored in allantoin in grain was even larger than the sentative clusters in spikelets (Ln.S.ii and Ln.S.iii; Fig. 7a) N present in glutamate and aspartate, which were the most or, in developing grain, largely not assigned to any repre- abundant free amino acids identified but have only one N sentative cluster (Fig. 7b). Although, in the developing atom (1:4 N:C) (Fig. 8c). However, the N-rich amino acid grain, this large area clustered according to the time points arginine (2:3 N:C) retained approximately double the N (DAA). content of allantoin in both RAC875 and Mace grains. The However, the accumulation of allantoin, galactinol and analysis also showed significant genotypic differences in cer - raffinose in developing grain under N starvation appears tain amino acids between RAC875 and Mace. In particular, to be developmental rather than representing a specific Mace had 70% and 31% higher concentration of arginine response to N starvation as similar metabolic changes and alanine than RAC875, whilst RAC875 had 179%, 43% occurred also under control conditions at 22DAA (Fig. 8b). and 35% higher concentration of tryptophan, aspartate and In agreement with the quantitative data on allantoin shown serine than Mace, respectively (Fig. 8c). in Fig. 4a, Mace appeared to accumulate higher levels of allantoin and the other metabolites in cluster Ln.Dg.iii sug- gesting genotypic differences within wheat. Discussion In summary, the metabolite profiling showed that, under drought, allantoin clustered predominantly with N-rich com- The aim of this study was to assess the role of the purine pounds (amino acids), whereas under N-starvation allan- intermediate allantoin and the purine catabolic genes toin clustered with C-rich compounds (sugars and organic under water and nutrient stress in bread wheat. For this, we acids) and therefore allantoin represented the only N-rich selected two Australian genotypes, specifically RAC875, compound. a breeding line that has been characterised as tolerant to 1 3 490 Plant Molecular Biology (2019) 99:477–497 (A) (B) 1 3 Plant Molecular Biology (2019) 99:477–497 491 ◂Fig. 7 Metabolite response of RAC875 and Mace spikelet and devel- Wheat highly accumulates allantoin under drought oping grain under High N and Low N treatments during grain filling. stress Metabolite levels measured with GC–MS, ion chromatography and HPLC were log transformed and scaled by subtracting the median There was a significant accumulation of allantoin in all metabolite value in each metabolite distribution. Hierarchical cluster- ing and heatmap of metabolite levels of RAC875 and Mace spikelet analysed tissues and genotypes of wheat plants exposed to a and developing grain b at 17, 19 and 22 days after anthesis (DAA) drought (Fig. 4b). This was accompanied by a significant was performed with the support of ClustVis web tool (Metsalu and reduction in transcript levels of TaALN, i.e., the first step Vilo 2015). Pearson’s correlation distance of scaled data was used in allantoin degradation (Fig. 5). for the hierarchical clustering. Dashed lines represent different time points within a given genotype and treatment. Each column repre- Allantoin was previously reported to accumulate in sents a biological replicate. Representative cluster containing allan- wheat and rice in response to abiotic stresses, includ- toin (measured with GC-QqQ method) is noted with a green line, ing drought (Bowne et al. 2011; Degenkolbe et al. 2013; other representative clusters are noted with a blue line Casartelli et al. 2018), however, these studies did not investigate the regulation of the purine catabolic genes drought (Izanloo et al. 2008; Bennett et al. 2012; Bonneau under those conditions. et al. 2013) and the locally adapted variety Mace, which Analysis of the expression of purine catabolic genes in is suggested to have a higher N-use efficiency based on Arabidopsis (Irani and Todd 2016) recently showed that leaf preliminary studies (Mahjourimajd et al. 2016). accumulation of allantoin upon drought stress was associated with transcriptional up-regulation of the purine catabolic genes leading to allantoin synthesis (AtXDH1, AtXDH2, The wheat purine catabolic genes show high AtUOX, AtAS) whilst expression of the allantoin-degrading synteny but also differences compared with other gene AtALN was only marginally increased. Similarly, Yes- grasses bergenova et al. (2005) showed that, in tomato, LeXDH1 and LeXDH2 were up-regulated under drought in leaf and The genes of the purine catabolic pathway, identified in root tissues. In Arabidopsis, the allantoin pathway was also several grass genomes including hexaploid bread wheat, implicated with salt stress, showing reduced AtALN and showed a high degree of synteny among three grass AtAAH expression and increased AtXDH1 expression (Irani genomes, suggesting that the identified wheat loci are and Todd 2016; Hesberg et al. 2004), as well as an accu- true gene orthologues (Fig. 2). However, the poor synteny mulation of allantoin accompanied by AtUOX and AtALNS displayed by XDH and its adjacent genes even in diploid up-regulation and AtALN down-regulatoin (Lescano et al. genomes (Brachypodium, rice and sorghum) suggests that 2016). XDH is located in an unstable genomic region prone to Our data agree with the above studies confirming that rearrangements. The majority of the grass genomes had allantoin significantly accumulates under drought (Fig. 4b) only one copy of the purine catabolic genes (Table S5), and that this is paralleled by the up-regulation of genes puta- corresponding to three homeologs in wheat (Table S1), tively encoding enzymes for allantoin synthesis (TaXDH1 with the only exceptions of the fore-mentioned XDH, and and TaUOX) and/or down-regulation of TaALN (Fig. 5). UAH, for which there was no synteny for the Brachypo- Although we have not quantified xanthine, the gene expres- dium orthologs BdUAH2. sion data suggest that under drought, an increased amount For XDH we have identified a second copy on chromo- of xanthine is likely to feed into this pathway which, in com- some 6 of which, based on in-silico expression analysis bination with a reduced allantoin degradation, results in the (Fig. S3a), only the homeolog on chromosome 6, TaXDH2- accumulation of allantoin under drought which was observed 6DS is a functional gene explaining its low expression in our study. level compared with TaXDH1 (Fig. S3b; Fig. 5). Inter- In contrast, studies in common bean (Phaseolus vul- estingly, TaXDH2 shows an opposite transcriptional garis) showed up-regulation of both allantoin-synthetising response to drought (reduced transcript level) compared genes (PvUOX) and allantoin-degrading genes (PvALN and with TaXDH1 suggesting different roles and/or functional PvAAH) in leaves under drought and this corresponded to divergence. A differential response to drought and other increased levels of allantoin and allantoate (Alamillo et al. treatments (salt, cold, ABA) has also been shown in Arabi- 2010; Coleto et al. 2014). Interestingly, Coleto et al. (2014) dopsis, which carries a tandem duplication of AtXDH1 and reported that these ureides were more concentrated in the AtXDH2 on a single chromosome (Hesberg et al. 2004). tissues of drought-sensitive genotypes. This is in contrast to It will therefore be interesting to investigate the role and our data and other studies showing that allantoin specifically enzymatic function of TaXDH2 in more detail in relation accumulates in drought tolerant genotypes of wheat, rice and to drought and other stresses. resurrection plants (Bowne et al. 2011; Oliver et al. 2011; Degenkolbe et al. 2013; Yobi et al. 2013; Casartelli et al. 1 3 492 Plant Molecular Biology (2019) 99:477–497 Fig. 8 Total N and N-containing metabolites in mature grain of bread RAC875 and Mace grown under control conditions (HN-WW). wheat genotypes RAC875 and Mace. Comparison of a N concen- Amino acid concentration is reported in black, whilst N atoms present −1 −1 tration (%) and b allantoin concentration (nmol g DW) in mature in each amino acid in grey, values are expressed as nmol g DW. grain harvested from the tallest remaining tiller of RAC875 and Mace Allantoin and corresponding N concentration are reported in lighter plants grown under control conditions (HN-WW), drought (HN-D) colours to allow comparison. Asterisks denote significant differences and low N (LN-WW) conditions. Letters indicate significant differ - between RAC875 (R) and Mace (M) by Student’s t-test (*p < 0.05; ences between genotypes and treatments by two-way ANOVA with **p < 0.01; ****p < 0.0001). All data are mean ± SEM of 16–18 bio- Tukey’s test (p < 0.05); c free amino acids levels in mature grain of logical replicates 2018). This suggests that purine catabolism is regulated dif- supplementation, stimulated ABA and jasmonic acid (JA) ferently under drought stress in ureidic legumes. metabolism, key components of abiotic stress responses. Evidence of the functional role of purine catabolism However, a previous study suggested that allantoin does not under abiotic stress were revealed using reverse genet- possess any in-vitro antioxidant activity (Wang et al. 2012), ics approaches. Arabidopsis XDH mutants (xdh) showed prompting to speculate that allantoin and purine catabolism that disrupting the first step of purine catabolism caused play a role in stress sensing and regulation rather than a hypersensitivity to water stress (Watanabe et al. 2010) and direct ROS scavenging function. impaired recovery from extended dark exposure (Brychkova Metabolomic profiling of the spikelet (defined here as et al. 2008), which in both cases led to excessive reactive vegetative part of the spike) and the developing grain after oxygen species (ROS) accumulation compared to WT plants. 8 days of drought stress showed that allantoin clustered with In contrast, the constitutive accumulation of the intermedi- a set of highly drought-responsive metabolites (Fig. 6a, b). ate allantoin in Arabidopsis ALN knock-out mutants (aln) These clusters were composed mainly of amino acids, with led to enhanced performance under dehydration, drought alanine, valine, leucine, and proline common to the allantoin and salt stress, which also corresponded to reduced ROS clusters in spike samples (D.S.ii) and developing grain (D. levels (Watanabe et al. 2014; Irani and Todd 2016). These Dg.ii). Proline is amongst the best characterized drought- studies further showed that increasing allantoin concentra- related amino acids and has been shown to have a range tion, either constitutively (aln mutants) or by exogenous of functions under stress, e.g., osmolyte, regulator of redox 1 3 Plant Molecular Biology (2019) 99:477–497 493 potential, molecular chaperone, ROS scavenger and signal- purines ling molecule (Yoshiba et al. 1995; Hare and Cress 1997; Verbruggen and Hermans 2008; Szabados and Savoure xanthine 2010; Mohanty and Matysik 2001; Khedr et al. 2003). The XDH specific accumulation of allantoin under drought in a wide UOX range of different plant species and the suggested role in ALNS stimulating the ABA and JA pathway (see above) justifies further investigations into the importance of allantoin in the allantoin mitigation or tolerance to drought. N limitaon Drought ALN Preventing N losses under drought and allantoin AAH glutamine as a source of N under nutrient deprivation UGAH GS-GOGAT GS-GOGAT UAH Under drought, RNA and DNA as well as protein degrada- + + 4NH 4NH 4 4 tion and nutrient remobilisation caused by premature leaf glyoxylate Toxic build-up senescence (Munné-Bosch and Alegre 2004) and increased photorespiration (Mattsson et al. 1997; Wingler et al. 1999; NH volalizaon Kumagai et al. 2011b) are sources of high tissue NH and related emission of volatile ammonia (Mattsson and Sch- Fig. 9 Schematic model for the dual-role of allantoin under stress in joerring 2002). In principle, free NH can be recycled by relation to N homeostasis. Under low N conditions, increased expres- the GS-GOGAT cycle (Fig. S2), however, this pathway is sion of ureide-degrading genes ALN and AAH indicate that NH is negatively responsive to drought (Nagy et al. 2013; Singh liberated from purines providing an internal source of organic N that can be re-assimilated by the GS-GOGAT cycle. As a result, allantoin and Gosh 2013) and might therefore not be sufficiently effec - concentration in plant tissues is reduced. In contrast, during drought tive in capturing NH . Increased ammonia emission due to stress, allantoin accumulates due to down-regulation of the ALN gene inhibition of GS with MSO has been demonstrated (Matts- preventing the liberation of NH that could lead to cellular toxic- son and Schjoerring 1996) and has been directly linked ity and part of it might be lost as volatile ammonia due to drought- impaired GS-GOGAT activity with enhanced photorespiration under high light and high O stress (Kumagai et al. 2011b), as well as with senescence (Parton et al. 1988) and heat stress (Mattson et al. 1997). drought-tolerant specific GS responses have also been shown Ammonia emission can be considered an efficient, though very wasteful mechanism, to prevent the build-up of high tis- in tomato (Sanchez-Rodriguez et al. 2011) and wheat (Nagy et al. 2013).We suggest that the selection of high allantoin sue concentrations, which is toxic to plants (for a review see Britto and Kronzucker 2002). The accumulation of allantoin and maintenance of GS activity (reduced N losses) under drought stress could be a specific target for breeding (Fig. 9). (e.g. 30-fold accumulation in RAC875 flag leaves, Fig. 4b) would therefore be beneficial to plants because it prevents The importance of allantoin and purine catabolism in N metabolism have long been established in ureidic legumes, accumulation of NH to toxic levels and also, to some extent, to retain organic N in the plant that could be lost to which employ allantoin and allantoate as major forms for long-distance transport of N (Schubert 1986; Sinclair and the atmosphere in the form of volatile ammonia. Although allantoin represents just a fraction of the total organic N in a Serraj 1995). However, recently an important role in main- taning N homeostasis in non-ureidic plant species is becom- wheat plant, allantoin accumulation under drought appears to occur simultaneously with the accumulation of several ing evident. Soltabayeva et al. (2018) reported that Arabi- dopsis Atxdh1, Ataln and Ataah mutants displayed an early other small metabolites containing N, notably amino acids such as proline (Fig. 6). This response seems to be shared senescence phenotype when grown under low N O con- ditions and that the activity of nitrate reductase (NR) was by other plant species (see Introduction for references), in fact we previously reported high accumulation of allan- increased in leaves of Atxdh1 mutants suggesting a higher demand for nitrate than wild type plants. Similarly, Naka- toin and several amino acids when rice plants were sub- jected to drought stress (Casartelli et al. 2018). Therefore, gawa et al. (2007) reported an early onset of senescence in RNAi-induced xdh mutants which also displayed reduced this could underlie a global strategy that plants adopt to improve N balance under stress, when GS-GOGAT enzy- chlorophyll content and increased cytosolic GS1 protein, which is known to be induced during senescence (Bernard matic activity is reduced. Differences in the regulation of GS (Singh and Gosh 2013) and ammonia emission (Kumagai et al. 2008). In our study, the quantification of allantoin in the et al. 2011) between drought tolerant and intolerant gen- otypes have been shown in rice and genetic diversity and wheat plants grown under N-limited conditions revealed 1 3 494 Plant Molecular Biology (2019) 99:477–497 a significant reduction of allantoin in all tissues analysed the majority of metabolites that showed an opposite trend (Fig. 4a) which is in contrast to the observed accumulation (Fig. 7). of allantoin under drought (Fig. 4b) as discussed above. A Although we calculated that allantoin accounted for only decrease of allantoin under N deprivation was also reported about 0.3% of the total N measured in wheat grains, this in other cereal species, specifically in maize and rice show - represents a significant portion of the soluble N pool, com- ing reduced allantoin in leaves and roots, respectively (Ami- parable to the most concentrated and N-rich amino acids, our et al. 2012; Coneva et al. 2014). In addition, high induc- such as asparagine and arginine (Fig. 8c). Previous studies tion of OsALN expression under low N conditions in rice have shown that allantoin stored in wheat grains is quickly was recently shown (Lee et al. 2018), this was previously utilised, from as early as 1 day after germination (Montalbini also observed in Arabidopsis and the leguminous tree Rob- 1992), suggesting that it can be used as a readily available inia pseudoacacia (Yang and Han 2004). In agreement with N substrate. this, the reduction of allantoin under low N reported in this The data reported here further show that genotypic dif- study (Fig. 4a) was accompanied by increased transcription ferences exist among bread wheat genotypes, in fact, Mace of purine catabolic genes, and this was particularly evident accumulated 66% more grain allantoin than RAC875, in genes encoding for the ureide degrading enzymes TaALN despite similar % grain N (Fig. 8a, b). This difference was and TaAAH (Fig. 5). also particularely evident in flag leaves of Mace, which had Downstream compounds of the purine catabolic path- up to 10-fold higher allantoin content compared to RAC875 way were recently analysed in more detail under N depriva- under both, control and low N conditions (Fig. 4a). Mace tion using the same wheat genotypes, RAC875 and Mace is widely grown in Australia because of its superior yield (Melino et al. 2018) showing, in addition to reduced levels of under drought and nitrogen-use efficiency (Mahjourimajd allantoin, a significant reduction of allantoate in leaves and et al. 2016). High allantoin concentrations in the leaves an accumulation of glyoxylic acid, the end product of purine and grain might indeed be a positive contributing factor to degradation. The assumption that the N (and C) remobilised high NUE in Mace. This is in agreement with findings in from allantoin supports plant growth is supported by studies rice where differences in allantoin levels in genotypes with showing that Arabidopsis and rice seedlings could grow with contrasting drought tolerance were already apparent under ureides as a sole N source (Desimone et al. 2002; Brychkova control conditions (Casartelli et al. 2018). In addition to et al. 2008; Lee et al. 2018), although growth was delayed greater accumulation of allantoin, mature grains of Mace in comparison to plants supplied with inorganic N. This is also accumulated 70% more arginine than RAC875 (Fig. 8c). in contrast to wheat seedlings that, resupplied with allantoin Arginine is the proteinogenic amino acid with the highest and xanthine as a sole source of N after N O starvation, N:C ratio (3:2), known to represent a major N form for stor- grew and photosynthesised as well as those re-supplied with age and transport. NO (Melino et al. 2018). Our data thus indicate that the two analysed wheat geno- Taken together, these findings suggest that the enhanced types prioritize different N compounds for transport and activity of the purine catabolic pathway provides an internal grain loading. Previous studies in Mace suggest that this source of organic N used to maintain homeostasis under cultivar has high N use efficiency (Mahjourimajd et al. 2016) low N conditions whereas the accumulation of allantoin and this study and Kastury et al. (2018) additionally dem- under drought releases pressure from the GS-GOGAT cycle onstrated that Mace is higher yielding under drought than thereby preventing accumulation of toxic levels of NH and RAC875. The possibility that the superior performance of possibly N losses due to volatilization (Fig. 9). Mace may be related to the preferential use of metabolites with high N:C ratio, which are more energy effective forms Allantoin is a relevant component of grain nitrogen for transporting and storing N, is an intriguing hypothesis in wheat that requires further validation. In addition, allantoin and arginine may also play a regulatory role in plant growth and In wheat, N translocated to the grain during plant senes- development as they participate in ABA, JA, and nitric oxide cence is an important determinant of grain quality and there (NO) signalling, respectively (Watanabe et al. 2014; Winter is currently little information on the contribution of allan- et al. 2015; Takagi et al. 2016). toin and/or other purine catabolites to grain N in non-ureide plants. The data reported here show progressive accumula- tion of allantoin during grain filling (Fig. 4) up to more than Summary −1 1400 nmol g DW at maturity in Mace (Fig. 8b). This was confirmed by metabolomics analyses of developing grains In summary, our data are in support of the converging showing that allantoin levels correlated with other metab- evidence that allantoin has a relevant role in non-ureide olites specifically accumulating at 22DAA, in contrast to plants under water stress and in N homeostasis. In addition 1 3 Plant Molecular Biology (2019) 99:477–497 495 to the proposed role of allantoin in drought-induced ABA References responses, accumulation of this N-rich molecule under Alamillo JM, Diaz-Leal JL, Sanchez-Moran MV, Pineda M (2010) drought would contribute to optimise N (and C) balance in + Molecular analysis of ureide accumulation under drought stress in the plant by preventing toxic build-up of NH and possible Phaseolus vulgaris L. PlantCell. Environ 33:1828–1837 N losses through volatile ammonia due to a reduced activ- Amiour N, Imbaud S, Clément G et al (2012) The use of metabolomics ity of the GS-GOGAT cycle (Fig. 9). 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Bioinformatics 30:2114–2120 Acknowledgements We thank Akiko Enju, Pia Müller and Jessey Bonneau J, Taylor J, Parent B, Bennett D, Reynolds M, Feuillet C, George (University of Adelaide) for supporting with sample collec- Langridge P, Mather D (2013) Multi-environment analysis and tion; Juan Carlos Sanchez-Ferrero (University of Adelaide) for sup- improved mapping of a yield-related QTL on chromosome 3B port with bioinformatic analyses; Adam Lukaszewski (University of of wheat. Theo Appl Genetics 126:747–761 Riverside, California, US) for providing the Chinese Spring NT seeds Bowne JB, Erwin TA, Juttner J, Schnurbusch T, Langridge P, Bacic stock and Margaret Pallotta (University of Adelaide) for providing A, Roessner U (2011) Drought responses of leaf tissues from DNA of the NT lines; Sanjiv Satija, Larissa Chirkova and Yuan Li wheat cultivars of differing drought tolerance at the metabolite (University of Adelaide) for technical support with molecular analy- level. Mol Plant 5:418–429 ses; Julian Taylor (University of Adelaide) for support in experimental Britto DT, Kronzucker HJ (2002) NH toxicity in higher plants: a design; Siria Natera, Gina Barossa and Veronica Liu (Metabolomics critical review. Plant Physol 159:567–584 Australia) for support with the allantoin quantification. We are grateful Brychkova G, Fluhr R, Sagi M (2008) Formation of xanthine and the to Elmien Heyneke, Ines Fehrle and Astrid Basner (Max Plank Insti- use of purine metabolites as a nitrogen source in Arabidopsis tute, Potsdam-Golm) for their support in performing the metabolomics plants. Plant Signal Behav 3:999–1001 analysis. This work was supported by an Australian Research Coun- Casartelli A, Riewe D, Hubberten HM, Altmann T, Hoefgen R, cil linkage Grant LP1400100239 in partnership with and additional Heuer S (2018) Exploring traditional aus-type rice for metabo- funding from DuPont-Pioneer (USA). Rothamsted Research receives lites conferring drought tolerance. Rice 11:9 strategic funding from the Biotechnological and Biological Sciences Chomczynski P (1993) A reagent for the single-step simultaneous Research Council of the United Kingdom. We acknowledge support isolation of RNA, DNA and proteins from cell and tissue sam- through the Designing Future Wheat (DFW) Strategic Programme (BB/ ples. Biotechniques 15:532–534 P016855/1). Choulet F, Alberti A, Theil S, Glover N, Barbe V, Daron J, Pin- gault L, Sourdille P, Couloux A, Paux E (2014) Structural and functional partitioning of bread wheat chromosome 3B. Science Author Contributions SH was leading the project. MO, VJM and RH 345:1249721 conceived the ideas, edited the manuscript and co-supervised A.C. Clavijo BJ, Venturini L, Schudoma C et al (2017) An improved who conducted the experiments as part of his PhD thesis. AC wrote the assembly and annotation of the allohexaploid wheat genome manuscript with inputs from SH, MO, VJM and RH. The quantitative identifies complete families of agronomic genes and provides allantoin method was developed by UR, NSJ and HM. The identifica- genomic evidence for chromosomal translocations. Genome Res tion of the wheat genes and bioinformatics analyses was supported by 27:885–896 UB and RS. Inputs on the quantitative gene expression analyses was Coleto I, Pineda M, Rodino AP, De Ron AM, Alamillo JM (2014) provided by MR. Access to analytical pipelines, technical support and Comparison of inhibition of N fixation and ureide accumulation data analysis was provided by AE, MW, and EZ. under water deficit in four common bean genotypes of contrasting drought tolerance. 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Plant Molecular Biology – Springer Journals
Published: Feb 5, 2019
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