Molecular analysis of plant gene expression by phytohormone abscisic acid.

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Environmental stresses, such as drought, low temperature and salt stresses, trigger the production of plant hormone abscisic acid (ABA), which in turn induces various genes. ABA functions in stomata closure, seed maturation and dormancy. Many ABA inducible genes are also expressed at the late stage of embryogenesis during seed development.To understand the molecular process of signal transduction from initial water-stress signal to gene expression, we cloned 25 cDNAs (named RD or ERD) for genes that are induced by water stress by differential screening. The GUS reporter gene driven by these rd promoters was induced at significant level by dehydration and ABA in transgenic plants. We precisely analyzed the rd29A promoter and identified a novel cis-acting element containing 9 bp, TACCGACAT (DRE,Dehydration Responsive Element), that is involved in dehydration-responsive expression. ABRE (YACGTGGC) functions as one of the ABA responsive elements. Nuclear protein(s) that bind to the cis-acting element was detected with gel retardation assay.The induction of rd22 is mediated by ABA,but requires protein biosynthesis for its ABA-dependent expression. A 67-bp region of the rd22 promoter is essential for its ABA-responsive expression, and contains several conserved motifs of DNA binding proteins, such as MYC and MYB,but no ABREs. Recently, a cDNA for a transcription factor MYC homologue was cloned by southwestern method using the 67-bp DNA as a probe, which suggests that drought-inducible MYC homologue may function in the ABA-inducible expression of rd22. These results indicates that drought- or ABA-inducible transcription factor(s) function in the ABA-responsive gene expression under drought stress, and that there are two independent transcription system for ABA-responsive gene expression, one is ABRE/bZIP system and the other may require de novo synthesis of ABA-inducible transcription factor(s).