ELUCIDATION OF A HORMONAL RESPONSE PATHWAY

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The mechanism of action of phytohormones is not completely understood. A part of the transduction pathway involves mechanisms that alter the levels of gene products that are required for the physiological response. The regulation of this hormone-induced gene expression is in part controlled at the level of transcription. This proposal will focus on the characterization of the genes coding for nuclear proteins that bind to regions of plant promoters that are functionally important in the regulation of gene expression by the phytohormone abscisic acid (ABA). We have isolated cDNA clones of two genes whose proteins bind to different functional regions of an ABA-regulated promoter (Em) from wheat. An AT-rich region of the Em promoter appears to stimulate the level of expression in a hormone-independent manner. The binding protein that recognizes this region has the properties of a plant high mobility group (HMG) nuclear protein. A different region of the Em promoter contains an ABA response element and binds a different protein which has the sequence of a transcriptional regulatory protein possessing a leucine zipper motif. Experiments are proposed to: characterize each gene and isolate other members of their families; follow the expression of each gene in different tissues with respect to the levels of ABA and expression of Em; define the specific binding regions of the protein/DNA interaction; and, test the functionality of the proteins with respect to their possible roles in the ABA response pathway by using plant and yeast in vivo assay systems. All of these approaches should result in the identification of intermediates in the ABA response chain and help elucidate whether this ABA response pathway is analogous to hormonal regulation of gene expression in other eukaryotes.