To unravel the molecular mechanisms of drought responses in tomato, gene expression profiles of two drought-tolerant lines identified from a population of Solanum pennellii introgression lines, and the recurrent parent S. lycopersicum cv. M82, a drought-sensitive cultivar, were investigated under drought stress using tomato microarrays. Around 400 genes identified were responsive to drought stress only in the drought-tolerant lines. These changes in genes expression are most likely caused by the two inserted chromosome segments of S. pennellii, which possibly contain drought-tolerance quantitative trait loci (QTLs). Among these genes are a number of transcription factors and signalling proteins which could be global regulators involved in the tomato responses to drought stress. Genes involved in organism growth and development processes were also specifically regulated by drought stress, including those controlling cell wall structure, wax biosynthesis, and plant height. Moreover, key enzymes in the pathways of gluconeogenesis (fructose-bisphosphate aldolase), purine and pyrimidine nucleotide biosynthesis (adenylate kinase), tryptophan degradation (aldehyde oxidase), starch degradation (β-amylase), methionine biosynthesis (cystathionine β-lyase), and the removal of superoxide radicals (catalase) were also specifically affected by drought stress. These results indicated that tomato plants could adapt to water-deficit conditions through decreasing energy dissipation, increasing ATP energy provision, and reducing oxidative damage. The drought-responsive genes identified in this study could provide further information for understanding the mechanisms of drought tolerance in tomato.
Plant miRNA regulates multiple developmental and physiological processes, including drought responses. We found that the accumulation of Sly-miR169 in tomato (Solanum lycopersicum) was induced by drought stress. Consequently, Sly-miR169 targets, namely, three nuclear factor Y subunit genes (SlNF-YA1/2/3) and one multidrug resistance-associated protein gene (SlMRP1), were significantly down-regulated by drought stress. Constitutive over-expression of a miR169 family member, Sly-miR169c, in tomato plant can efficiently down-regulate the transcripts of the target genes. Compared with non-transgenic plants, transgenic plants over-expressing Sly-miR169c displayed reduced stomatal opening, decreased transpiration rate, lowered leaf water loss, and enhanced drought tolerance. Our study is the first to provide evidence that the Sly-miR169c negatively regulates stomatal movement in tomato drought responses.
Universal stress protein (USP) appears to play an active role in the abiotic stress response, but their functions remain largely unknown in plants. A USP gene (SpUSP) was cloned from wild tomato (Solanum pennellii) and functionally characterized in cultivated tomato in the present study. The SpUSP transcript is abundantly accumulated in leaf stomata and its expression varied with the circadian rhythm. SpUSP was remarkably induced by dehydration, salt stress, oxidative stress, and the phytohormone abscisic acid (ABA) etc. This protein was predominantly localized in the nucleus and cell membrane. Overexpressing SpUSP increased drought tolerance of tomato in the seedling and adult stages. Under drought stress, the ABA content significantly increased in the SpUSP-overexpressing plants, which induced stomatal closure and reduced water loss, leading to the enhancement of drought tolerance. Based on the microarray data, a large number of chlorophyll a/b-binding proteins and photosystem-related genes were up-regulated in the SpUSP-overexpressing plants under drought conditions, which possibly enhanced the stomatal sensivitity to ABA and maintained the photosynthetic function. SpUSP overexpression also alleviated the oxidative damage accompanied by oxidative stress-responsive gene activation and osmolyte accumulation. Annexin (SGN-U314161) was found to interacte with SpUSP in the yeast two-hybrid method. This interaction was further confirmed by the bimolecular fluorescence complementation assay. The present study demonstrated that the annexin-interacting SpUSP plays important roles in the drought tolerance of tomato by influencing ABA-induced stomatal movement, increasing photosynthesis, and alleviating oxidative stress.
Plants have evolved and adapted to different environments. Dwarfism is an adaptive trait of plants that helps them avoid high-energy costs under unfavourable conditions. The role of gibberellin (GA) in plant development has been well established. Several plant dehydration-responsive element-binding proteins (DREBs) have been identified and reported to be induced under abiotic and biotic stress conditions. A tomato DREB gene named SlDREB, which is a transcription factor and was cloned from cultivated tomato M82, was found to play a negative role in tomato plant architecture and enhances drought tolerance. Tissue expression profiles indicated that SlDREB was expressed mainly in the stem and leaf and could be induced by abscisic acid (ABA) but suppressed by GA and ethylene. SlDREB altered plant morphology by restricting leaf expansion and internode elongation when overexpressed, and the resulting dwarfism of tomato plants could be recovered by application of exogenous gibberellic acid (GA3). Transcriptional analysis of transgenic plants revealed that overexpression of SlDREB caused the dwarf phenotype by downregulating key genes involved in GA biosynthesis such as ent-copalyl diphosphate synthase (SlCPS) and GA 20-oxidases (SlGA20ox1, -2, and -4), thereby decreasing endogenous GA levels in transgenic plants. A yeast activity assay demonstrated that SlDREB specifically bound to dehydration-responsive element/C-repeat (DRE/CRT) elements of the SlCPS promoter region. Taken together, these data demonstrated that SlDREB can downregulate the expression of key genes required for GA biosynthesis and that it acts as a positive regulator in drought stress responses by restricting leaf expansion and internode elongation.
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