Showing papers in "Tropical Plant Biology in 2015"

Sulfur Metabolism and Stress Defense Responses in Plants

Abstract: Sulfur management is an important issue in crop plant nutrition Sulfur has a role in fundamental processes such as electron transport, structure and regulation It is also associated with photosynthetic oxygen production, abiotic and biotic stress resistance and secondary metabolism Sulfate uptake, reductive assimilation and integration into cysteine and methionine are the central processes that direct oxidized and reduced forms of organically bound S into their various functions Sulfur-containing defense compounds that are crucial for plant survival during biotic and abiotic stress include elemental sulfur, hydrogen sulfide, glutathione, phytochelatins, S-rich proteins and various secondary metabolites Formation of these compounds in plants is closely related to the supply, demand, uptake and assimilation of S This review will highlight the role of S during the stress response in plants and the relationship between S metabolism and primary S nutrition

Anatomical Assessment of Root Formation and Tuberization in Cassava (Manihot esculenta Crantz)

Abstract: 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.

RNA-Seq Transcriptome Analysis in Date Palm Suggests Multi-Dimensional Responses to Salinity Stress

Abstract: The ability of date palm tree to survive under adverse abiotic conditions renders it as a valuable genomic resource for identifying tolerance genes. While mechanisms for salt tolerance have been heavily investigated in model as well as in some agronomic crops, no such studies have been undertaken in date palm. The aim of this study is to identify stress tolerance-related genes through transcriptomic analysis to support further functional studies. Young roots of Deglet Beida cultivar have been subjected to salt stress treatment, and used for RNA-Seq expression profiling and transmission electron microscopy (TEM) analysis. A total of 1939 genes are found to be differentially expressed between mock-treated roots and salt-stressed roots using log2FC ≥15≤−15. Many of these regulatory genes belong to DNA/RNA, protein, membrane, and signaling functional categories, suggesting that these genes play functional roles in tolerance to salt stress. Furthermore, RNA-Seq analysis has revealed activation of abscisic acid signaling pathways through SNF1-related protein kinases 2. Additionally, certain key genes involved in sodium uptake and transport are down-regulated, suggesting a potential mechanism for slowing down up-take and transport of salt solutes within plant tissues. TEM analysis has revealed that stressed roots exhibit plasmolysis in cortical cells of the distal region, while epidermal cells do not. Interestingly, root-tip regions of stressed roots do not exhibit plasmolysis, and this is likely due to higher solute contents present in these sink cells. These findings provide new information on multi-dimensional responses of date palm to salinity stress.

Expression Profile of Sugarcane Transcription Factor Genes Involved in Lignin Biosynthesis

Abstract: 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.

Physiological and Biochemical Changes During Heat Stress Induced Browning of Detached Backhousia myrtifolia (Cinnamon Myrtle) Tissues

Abstract: 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.

MicroRNA Networks in Plant-Microorganism Interactions

Abstract: 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.

Cell Viability, Mitotic Index and Callus Morphology of Byrsonima verbascifolia (Malpighiaceae)

Abstract: The combined analysis of cell viability, mitotic index and callus morphology of species of commercial interest may favor the identification of cell masses with embryogenic characteristics at early stages of cultivation, allowing the development or improvement of protocols for in vitro plant regeneration through indirect embryogenesis. Byrsonima verbascifolia L. (DC.) (Malpighiaceae) is a Brazilian medicinal species native to the Cerrado, which has a low seed germination rate due to tegument dormancy. This study aimed to identify cells with embryogenic characteristics during growth of callus of B. verbascifolia, by assessing cell viability, mitotic index and cell morphology. Callus was induced from leaf explants and inoculated on MS medium with 4.52 μM 2,4-D and 4.44 μM benzylaminopurine (BAP). Friable calli were obtained at four distinct growth phases: lag (until the 30th), exponential (from 31st to the 70th day), linear (from 71st to the 100th day) and decline (from 101st to the 120th day). Calli with 20 and 30 days showed cells with embryogenic characteristics, that is, isodiametric shape, juxtaposed, with small vacuoles, thin cell wall and presence of amyloplasts. After 120 days, calli were mostly elongated cells with lower cell viability and mitotic index.

Cytogenetic and ITS- psbA-trnH Sequence Analysis for Phylogenetic Inference in Mucuna sp. of India

Abstract: This paper reports new chromosomal information and earliest proof on efficacy of nrITS and cp-psbA-trnH gene sequences for barcoding and phylogenetic studies in Mucuna sp. First cytological evidence on M. sempervirens and M. bracteata revealed symmetrical karyotype in both the species. Meiotic anomalies evident from the formation of chromosomal laggards and bridges in M. gigantea and M. atropurpurea and unreduced pollens in M. bracteata suggest evolutionary events that might have shaped annual species in this genus. Molecular investigations using nrITS and cp-psbA-trnH sequences revealed them to be phylogenetically informative. Vis-a-vis individual genes, combined sequences of ITS and psbA-trnH sequences offered reliable data for species delineation. The results summarized here are expected to galvanize molecular taxonomic studies and open-up newer means for answering phylogenetic questions in this genus.

Seed-Specific Stable Expression of the α-AI1 Inhibitor in Coffee Grains and the In Vivo Implications for the Development of the Coffee Berry Borer

Abstract: Genetic transformation of coffee (Coffea spp.), the second most traded commodity worldwide, is an alternative approach to introducing features that cannot be introgressed by traditional crossings. The transgenic stability, heritability and quantitative and spatial expression patterns of the seed-specific promoter phytohemagglutinin (PHA-L) from Phaseolus vulgaris were characterized in genetically modified C. arabica expressing the α-amylase inhibitor-1 (α-AI1) gene. The α-AI1 inhibitor shows considerable activity toward digestive enzymes of the coffee berry borer (CBB) Hypothenemus hampei. This insect pest expends its life cycle almost entirely in coffee berries. Transgene containment in the fruit is important to meeting food and environmental safety requirements for releasing genetically modified (GM) crops. PCR analysis of T2 coffee plants showed a Mendelian single-copy segregation pattern. Ectopic transgene expression was only detected in coffee grains, as demonstrated by reverse transcription-PCR analysis of different plant tissues. An intense immunocytochemical signal associated with α-AI1 protein expression was localized to endospermic cells. In addition, a delay in the larval development of CBB was observed after challenging transgenic coffee seeds with the insect. These results indicate that the PHA-L promoter might be a useful tool in coffee for the seed-specific expression of genes related to coffee bean productivity, quality and pest protection. The biotechnological applicability of the α-AI1 gene for controlling CBB is also discussed. This work is the first report showing a seed-specific transgene expression in coffee plants.

Breeding and Pollination Biology of Combretum constrictum (Benth) Laws. (Combretaceae)

Abstract: Combretum species possess horticultural attributes. We investigated the phenology, floral morphology, breeding and pollination biology of Combretum constrictum. Visual observation for morphology, day and night monitoring for floral visitors, pollen viability test by the lactophenol cotton blue method, and different pollination experiments were performed on the species. C. constrictum possesses terminal racemes and flowers all year round, with the peak blooming period occurring between August and November. Fruit set predominantly occurs from August to December. The average lifespan of an inflorescence, as observed in the species, is 32 days, while the lifespan of a single flower is 7–12 days. Anthesis is diurnal, proceeds centripetally and lasts approximately 5 h. The average viability of the pollen was 97.05 % on Day 1, decreasing to 11.64 % on Day 4. Visitors from ten families {Dolichoderinae, Hymenoptera (Formicidae), Muscidae, Nectariniidae, Drosophilidae, Tettigoniidae, Acrididae, Hesperiidae, Achatinidae and Myrmicinae} were encountered on flowers of the species during the study period. Five of these visitors, including Cinnyris venustus, Meranoplus bicolor, Camponotus consobrinus and Technomyrmex sp., are the main pollinators of the species. These pollinators predominantly visited the flowers for a few hours until the opening of the flower buds and continued visiting the flowers until 2 days after the opening of the last flower bud on the inflorescence. The flowers are unscented but are brilliant red-pink, suggesting that the visitors/pollinators are attracted to the flowers due to their colour. After pollination, the number of immature fruits in this species ranged from 6 to 51, and averaged 25.80. The percentage of fruit set and mature fruits per inflorescence varied from one treatment to another (self-pollinated, 2.60 ± 1.10; hand cross-pollinated, 4.55 ± 1.43; and visitor-pollinated, 8.75 ± 2.65). These findings indicate that C. constrictum is self-compatible, but the presence of pollinators increases the number of fruits and probably enhances the fertility of the fruits. The species exhibits entomophily (insect pollination) and ornithophily (bird pollination) syndromes.

Identification of Quantitative Trait Locus for Seed Dormancy and Expression Analysis of Four Dormancy-Related Genes in Sorghum

Abstract: 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.

Genome wide polymorphisms and yield heterosis in rice (Oryza sativa subsp. indica)

Abstract: The linear relationship between genetic distance and heterosis within maize heterotic groups has inspired efforts to predict heterosis using molecular markers in other species. In this study, three cytoplasmic male sterile and three restorer lines from a hybrid rice breeding program were crossed in all possible (9) combinations and the F1 offspring grown in replicated trials in three locations. Whole genome sequence analyses were used to identify hybrid pair genome-wide polymorphisms and the extent to which these polymorphisms were associated with heterosis estimated. The number of total hybrid pair SNP ranged from 150,823 to 331,876 and mid-parent heterosis from −13.6 % to 18.6 %. Hybrid pair total SNP, total INDEL, gene SNP and gene INDEL counts were generally correlated at the whole genome and chromosome level. There was a close correlation between chromosome SNP and INDEL frequencies while gene presence-absence analysis found little difference between cross combinations. The relationship between whole genome SNP and heterosis was best explained by a second order polynomial of negative sign, suggesting a positive heterotic response may be achieved by maintaining a balance between inbreeding and outbreeding depression, and that genome wide polymorphisms may have a role in predicting heterosis in rice.