Abstract
BackgroundHIGH-LEVEL EXPRESSION OF SUGAR INDUCIBLE GENE2 (HSI2), also known as VAL1, is a B3 domain transcriptional repressor that acts redundantly with its closest relative, HSI2-LIKE1 (HSL1), to suppress the seed maturation program following germination. Mutant hsi2 hsl1 seedlings are arrested early in development and differentially express a number of abiotic stress-related genes. To test the potential requirement for HSI2 during abiotic stress, hsi2 single mutants and plants overexpressing HSI2 were subjected to simulated drought stress by withholding watering, and characterized through physiological, metabolic and gene expression studies.ResultsThe hsi2 mutants demonstrated reduced wilting and maintained higher relative water content than wild-type after withholding watering, while the overexpressing lines displayed the opposite phenotype. The hsi2 mutant displayed lower constitutive and ABA-induced stomatal conductance than wild-type and accumulated lower levels of ABA metabolites and several osmolytes and osmoprotectants following water withdrawal. Microarray comparisons between wild-type and the hsi2 mutant revealed that steady-state levels of numerous stress-induced genes were up-regulated in the mutant in the absence of stress but down-regulated at visible wilting. Plants with altered levels of HSI2 responded to exogenous application of ABA and a long-lived ABA analog, but the hsi2 mutant did not show altered expression of several ABA-responsive or ABA signalling genes 4 hr after application.ConclusionsThese results implicate HSI2 as a negative regulator of drought stress response in Arabidopsis, acting, at least in part, by regulating transpirational water loss. Metabolic and global transcript profiling comparisons of the hsi2 mutant and wild-type plants do not support a model whereby the greater drought tolerance observed in the hsi2 mutant is conferred by the accumulation of known osmolytes and osmoprotectants. Instead, data are consistent with mutants experiencing a relatively milder dehydration stress following water withdrawal.
Highlights
HIGH-LEVEL EXPRESSION OF SUGAR INDUCIBLE GENE2 (HSI2), known as VIVIPAROUS Absicic acid insensitive3 (ABI3)-LIKE1 (VAL1), is a B3 domain transcriptional repressor that acts redundantly with its closest relative, HSI2-LIKE1 (HSL1), to suppress the seed maturation program following germination
To investigate the role of HSI2 in modulating drought responses, the levels and/or integrity of the gene were stably altered in transgenic Arabidopsis plants
Notable gene ontology (GO) terms enriched in drought-regulated genes differentially expressed in hsi2 vs. Col-0 at stage 0 or stage 1 include response to abiotic, temperature and chemical stimuli, and oxidoreductase activity (Table 2). These results indicate that the hsi2 mutant constitutively expresses a number of drought- and abscisic acid (ABA)-responsive genes but under wilting conditions, the induction or repression of drought- and ABA-regulated genes is attenuated relative to the wild-type
Summary
HIGH-LEVEL EXPRESSION OF SUGAR INDUCIBLE GENE2 (HSI2), known as VAL1, is a B3 domain transcriptional repressor that acts redundantly with its closest relative, HSI2-LIKE1 (HSL1), to suppress the seed maturation program following germination. Mutant hsi hsl seedlings are arrested early in development and differentially express a number of abiotic stress-related genes. To test the potential requirement for HSI2 during abiotic stress, hsi single mutants and plants overexpressing HSI2 were subjected to simulated drought stress by withholding watering, and characterized through physiological, metabolic and gene expression studies. With patterns of global climate change likely to increase the severity of drought stresses in the future [2], the development of crop plants better adapted to water-limited environments is a priority for sustainable agriculture. The accumulation of osmolytes and the synthesis of protective proteins are all correlated with drought-induced increases in endogenous ABA levels. Mutants defective in ABA synthesis or perception are more susceptible to drought, while treatment with exogenous ABA enhances drought tolerance and induces the expression of numerous dehydration-stress responsive genes and proteins. Several dehydration stress-induced genes do not respond to exogenous application of ABA in Arabidopsis thaliana (L.) [9,10], suggesting the existence of both ABA-dependent and -independent signal transduction pathways
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