Auxin-regulated Organ-and Tissue-Specific Gene Expression

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The plant hormone, indoleacetic acid, appears to play key roles in regulating or inducing growth and developmental processes such as cell division, elongation, and differentiation. Exogenous application of this hormone or its synthetic analogs results in rapid changes in gene expression in seedlings or organs that are induced to undergo cell division or elongation. Dr. Guilfoyle has described a variety of auxin-induced mRNAs, and some of these, the SAUR of Small Auxin Up-regulated RNAs, are most abundant and most inducible in elongating regions of soybean hypocotyls. "Tissue printing" and in situ hybridization have been used to show that the SAUR genes are expressed primarily in epidermal and cortical cells of the elongating hypocotyl region, and furthermore, the mRNAs rapidly redistribute within the upper and lower portions of a gravistimulated soybean hypocotyl. The abundance of these small RNAs increases within 2.5 minutes after auxin application to elongating soybean hypocotyl sections, and half maximal steady state levels of these mRNAs are reached within 10 minutes after auxin administration. Dr. Guilfoyle has shown that the increase in RNA abundance after auxin addition results, at least in part, from increased transcription rates on the SAUR genes. On the other hand, an increase in RNA abundance after cycloheximide administration does not involve transcriptional activation, suggesting that post transcriptional events such as RNA stabilization may be involved. Recent results indicate that the SAUR genes are clustered within the soybean nuclear genome, and that the soybean genes are induced by auxin addition when the entire gene cluster is transferred via Agrobacterium T-DNA to petunia plats. Dr. Guilfoyle now proposes to 1) identify the cis-acting elements that confer auxin inducibility to the SAUR genes, 2) isolate, purify, and characterize trans acting factors that interact with the cis acting elements, 3) identify the cis acting elements that regulate the rapid turnover of the SAUR RNAs, 4) determine the patterns of auxin and gravity induced gene expression at the tissue, cellular, and subcellular levels using hybridization and immunological approaches, and 5) examine dwarf and gravitropic mutants for possible altered expression patterns of the SAUR genes and the phenotypes of transgenic plants which overproduce or underproduce the auxin induced RNAs. %%% The long range objectives of the proposed research are aimed at providing insight into how auxin activates specific genes in specific tissues, what proteins mediate the auxin induced gene activation, and what roles the auxin-induced gene products play in plant growth and development.