The plant hormone abscisic acid mediates the drought-induced expression but not the seed-specific expression of rd22, a gene responsive to dehydration stress in Arabidopsis thaliana

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Abstract
Nine cDNA clones, corresponding to genes that are responsive to dehydration (named RD), have been isolated from Arabidopsis thaliana. The sequence of a putative protein encoded by one of the RD cDNA clones, RD22, exhibits considerable homology to an unidentified seed protein (USP) of Vicia faba. Northern analysis showed that RD22 mRNA is induced by salt stress as well as by water deficit but not by cold or heat stress. RD22 mRNA appeared after the application of abscisic acid (ABA), an indication that transcription of RD22 mRNA is induced by endogenous ABA, the production of which is triggered by drought and salt stress. The induction of RD22 mRNA by ABA was inhibited by cycloheximide. Thus, it appears that protein synthesis is required for the induction of this mRNA by ABA. By contrast, protein synthesis was not required for the ABA-responsive induction RD29 mRNA, which corresponds to another dehydration-responsive gene of A. thaliana. These results suggest that there are at least two mechansisms for the induction of dehydration-responsive genes by ABA. RD22 mRNA was also expressed during the early and middle stages of seed development, showing a pattern of expression similar to that of USP. The seed-specific expression of RD22 seems not to be regulated by ABA. Structural analysis of the RD22 genomic clone revealed that the structural gene (designated rd22) contains three introns, and only a single copy of the gene is present in the A. thaliana genome, while the gene for USP from V. faba is actually a family of genes with 10 to 20 members. The site of initiation of transcription was determined by primer extension. Possible cis-acting elements involved in the expression of rd22 are discussed.