Tropical Plant Biology

Chaweewan, Yeetoh; Taylor, Nigel

2015 Tropical Plant Biology

doi: 10.1007/s12042-014-9145-5

Formation of storage root organs in cassava (Manihot esculenta) is poorly understood, but considered to occur when a subset of fibrous roots receive unknown signals to undergo secondary thickening. Large amounts of secondary xylem parenchyma are then produced in which starch is synthesized and stored. Anatomical studies were undertaken to examine rhizogenesis from greenhouse-grown cassava stem cuttings. Root formation was observed from the stem cut end (basal) and from close to the buried nodes (nodal) 5–10 days after planting. Transverse sectioning of the stem provided evidence that the basal roots were initiated from the cambium, while the nodal-derived roots developed from tissues deeper within the stem, at the boundary of the xylem and pith. Basal root anatomy remained constant with age, with minimal development of metaxylem. No tuberization was seen to occur from the basal roots. In contrast, nodal roots produced significant amounts of metaxylem and subsequently secondary xylem to form a large central stele. Further development established the storage organ in which secondary xylem parenchyma, tracheids and vessels were produced from the cambium. As a result, the nodal-derived roots were seen as precursors of the storage organs. It is proposed that nodal-derived and basal-derived fibrous roots are fundamentally different organs, that they originate through different rhizogenic processes and are committed to their respective developmental fates from the earliest stages of their initiation. These anatomical investigations offer new insight into root tuberization in cassava and should guide better focused studies into the underlying molecular and developmental control mechanisms.

Guo, Yan; Li, Pan; Yuyama, Nana; Tan, Lubin; Fu, Yongcai; Zhu, Zuofeng; Liu, Fengxia; Sun, Chuanqing; Cai, Hongwei

2015 Tropical Plant Biology

doi: 10.1007/s12042-015-9146-z

Seed dormancy is an important trait during the domestication of major crops. Dormant seeds are unable to germinate under conditions normally suitable for non-dormant seeds. Quantitative trait loci (QTLs) and genes affecting seed dormancy in sorghum remain largely unknown. To identify the genomic regions controlling seed dormancy in sorghum, we produced two F2 segregating populations from two crosses between a deep dormant weedy line B140 (as the female parent) and two weak dormant grain lines CK60B and MS138B (as the male parents). A genetic linkage map of the B140/CK60B population was constructed with 216 simple sequence repeat (SSR) markers spanning 1710.3 cM. One QTL on chromosome 4 and two QTLs on chromosome 7 were identified in the B140/CK60B population. They accounted for 17.9 to 24.9% of the phenotypic variance using a simple interval mapping method. The QTL on chromosome 4 was verified in another F2 mapping population (MS138B/B140). To understand the mechanism of dormancy, we conducted an expression analysis of four sorghum genes encoding homologs of dormancy genes of other plant species: Vp1 (maize), DOG1 (Arabidopsis), qSD12 and Sdr4 (rice). We demonstrated that there were clear differences in the expression level of all these four genes between the two parental lines with different seed dormancy level, and their expression were also tissue-specific with the expression levels of qSD12 low throughout all growth stages and tissues in B140. Our findings will help to define the genetic mechanism of seed dormancy in sorghum, and the identified QTLs may be useful biomarker for selection in sorghum improvement programs.

Brito, Michael; Nobile, Paula; Bottcher, Alexandra; Santos, Adriana; Creste, Silvana; Landell, Marcos; Vincentz, Michel; Vicentini, Renato; Mazzafera, Paulo

2015 Tropical Plant Biology

doi: 10.1007/s12042-015-9147-y

Cell wall recalcitrance, which is conferred in part by lignin, is the main bottleneck in lignocellulosic ethanol production. Transcription factors (TFs) have been suggested as targets to reduce or modify lignin. Here we analysed the expression profile of nine sugarcane TFs, their relationships with genes of the monolignol biosynthesis pathway, and their effects on lignin content and composition. Our assays compared two sugarcane genotypes with different lignin contents. To identify differences between tissue types and between the top and bottom of the plant, the culm was divided into intermediary and mature internodes, and the internodes were separated into pith and rind. The expression profiles obtained for the nine TFs were rather complex, showing that not only the genotype but also the tissue type and developmental stage influenced the results. Pearson correlation analysis indicated that ShMYB58/63 was positively correlated with the syringyl/guaiacyl ratio. In addition, a Bayesian network showed predicted interactions between the TFs and genes for lignin biosynthesis that were previously reported in the literature, as well as novel interactions such as those between ShMYB58/63 and ShF5H. These findings suggest that in sugarcane culm, the differential lignin deposition between tissue types (rind and pith) and at different developmental stages is under transcriptional regulation.

Sommano, Sarana

2015 Tropical Plant Biology

doi: 10.1007/s12042-015-9148-x

Postharvest discolouration is found in leaf and floral tissues of Backhousia myrtifolia (Cinnamon myrtle). Towards discerning the biochemical mechanisms, heat-induced browning was investigated. Differential browning behaviour was observed for green versus yellowed leaves. Initial pre-treatment chlorophyll contents (Chl a and b) and chlorophyll fluorescence (CF) were measured for both coloured leaves. After heat treatment, both, coloured leaf and floral tissue, were analysed for electrolyte leakage (EL), malondialdehyde (MDA) content, polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia lyase (PAL) enzyme activities, total phenolic content, diphenylpicryl-hydrazyl (DPPH) antioxidant activity and surface colour. They were also rated for their browning score (BS). Low chlorophyll fluorescence ratios (F v /F m values) of 0.68 for both leaf types suggested that this sub-tropical plant species experienced cold stress during winter period in which the study was conducted. Compared to detached green leaves, detached yellowed leaves showed more browning after heat treatment. Yellowed leaves had significantly greater EL levels, higher pre-treatment PPO and POX activities, and greater pre- and post-treatment PAL activities than green leaves. PPO, POD and PAL enzymes are typically involved in browning mechanisms in plant tissues. Their higher levels in yellowed leaves at least partly accounted for their greater browning than for green leaves.

Thiebaut, Flávia; Grativol, Clícia; Hemerly, Adriana; Ferreira, Paulo

2015 Tropical Plant Biology

doi: 10.1007/s12042-015-9149-9

MicroRNA constitutes an important class of small RNAs that negative regulates post-transcriptionally protein-coding genes. MiRNA-guided gene regulation has been reported as essential for developmental processes and for plant proper responses to biotic and abiotic stresses. When plants are exposed to microorganisms, they resort to various strategies to either establish a beneficial association, or to fight against pathogenic infection. These strategies include changes in metabolic pathways and modifications in gene expression states, which can be achieved by the action of miRNA-guided complexes. Plants growing in tropical regions are exposed to numerous biotic factors and can show large differences in miRNA regulation when exposed to either pathogenic or beneficial microorganisms. Recent insights in this field have begun to shed light on the role played by miRNA in plant-microbe associations. Aiming to understand how plants sense the diverse microorganisms, we review here the current knowledge of the roles played by miRNAs during plant-microbe interactions, focusing in results of studies carried out with tropical plants.