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Ethylene Regulation of Gravitropic Curvature in Arabidopsis Stems.  M. A. Harrison and M. L. Brown. Dept. of Biological Sciences, Marshall University, Huntington, WV. 

   Horizontal placement of a plant stem causes the redistribution of the soluble hormone auxin and stimulates biosynthesis of the gaseous hormone ethylene. While auxin is the primary plant hormone engaged in gravitropic responses through stimulation of growth, ethylene plays a modulating role in regulating the kinetics of this process. Ethylene often acts as an inhibitor of shoot and root growth, slowing gravitropic curvature, but has been reported to stimulate growth under certain conditions. Ethylene is produced by the oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC), which is formed from S-adenosyl methionine. The regulation of ACC synthesis by ACC synthase (ACS) serves as the rate-controlling step in ethylene biosynthesis. ACS enzymes are encoded by a gene family whose expression is differentially regulated in various tissues. Our major research objective is to evaluate individual ACS forms in the regulation of gravitropism in dark-grown Arabidopsis seedlings. Potential changes in expression of the various ACS forms were evaluated in transgenic plants carrying ACS promoter-GUS fusions and by RT-PCR. Preliminary results do not reveal distinct changes in ACS expression in curving hypocotyls in wild type plants. The role of each ACS member in hypocotyl growth and gravitropic curvature was determined by comparing wild-type responses with those of mutants that do not express specific ACS forms. Gravitropic curvature and growth rate were measured from digital images taken at 0, 3, 5, and 7 hours after horizontal placement. Compared to wild-type seedlings, mutants with increased ethylene production showed significantly increased curvature by 7 hours after horizontal placement. Other acs mutants that did not exhibit increased ethylene production did not show changes in curvature kinetics. Overall, these results indicate a stimulatory role for ethylene in gravitropic curvature for Arabidopsis hypocotyls. (Supported by grants from the USDA National Research Initiative Competitive Grants Program, the American Society of Gravitational and Space Biology, and the WV Space Grant Consortium.)