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Background   HIGH-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.    Results  The  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.    Conclusions  These 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.

High-level expression of sugar inducible gene2 (HSI2) is a negative regulator of drought stress tolerance in Arabidopsis