ABSCISIC-ACID-DEPENDENT AND ABSCISIC-ACID-INDEPENDENT REGULATION OF GENE-EXPRESSION BY PROGRESSIVE DROUGHT IN ARABIDOPSIS-THALIANA

Four clones corresponding to Arabidopsis thaliana transcripts regulate d by progressive drought stress were isolated. Abundance of the AtDi8, AtDi19 and AtDi21 and mRNAs increased in both roots and leaves during progressive drought. The AtDr4 mRNA was expressed in a root-specific manner in regularly watered plants, and became undetectable under drou ght conditions. In all cases, the drought-induced modifications of mRN A abundance could be reversed by subsequent rehydration. The predicted AtDr4 protein displays extensive similarity to various members of the Kunitz protein family, suggesting that AtDr4 might be a root-specific protease inhibitor. Of these four genes, only AtDi8 and AtDi21 respon ded to an exogenous supply of abscisic acid (ABA). Analysis of the ABA -deficient aba mutant demonstrated that endogenous ABA indeed particip ates in the drought regulation of these two transcripts. This ABA-depe ndent response was differentially affected in the various classes of A BA-insensitive Arabidopsis mutants. The AtDi19 and AtDr4 mRNAs both re sponded to drought in an ABA-independent manner, but at distinct thres holds of the progressive drought stress. Regulation of these four targ et genes by progressive drought stress thus appears to be mediated by at least three distinct signals, only one of which is ABA.