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MOLECULAR CHARACTERIZATION OF THE THIGMIC STRESS RESPONSE IN SOYBEAN.   A. L. Santone, M. J. Vasconcelos,  K. G.  Raghothama, and C. A. Mitchell.   Dept. of Horticulture & Landscape Architecture, Purdue University, West Lafayette, IN.

     An experimental system based on dark-grown soybean (Glycine max (L.) Merr.) seedlings is being studied to better understand the genetic responses to mechanical stress.  Such a system allows for rapid experiments (complete in 1 week) and, since such seedlings elongate exclusively by axial cell enlargement in a specific region of the hypocotyl (the “hypocotyl elongation zone” or HEZ), results are not confounded by light-driven growth processes. Mild mechanical stress applied to the seedling hook just above the HEZ reduces seedling growth 33% compared to unstressed control seedlings when measured 24 hours post-stress. Supplied Ca²⁺ levels have been shown to negate this stress response.  An increase in Ca²⁺ concentration over two orders of magnitude reduced the effects of this stress by half.  This response to touch stimulation is thought to be related to expression of touch (TCH) genes in the HEZ.  A cDNA library prepared from HEZs of stressed and unstressed dark-grown soybean seedlings is being probed with clones of TCH and Expansin genes from Arabidopsis thaliana (L.) Heynh., as well as ESTs of soybean genes with high sequence similarity to the Arabidopsis TCH genes in order to find soy genes thought to play a role in thigmostress-induced growth reduction. 

     (Supported in part by NASA: NAG2-1389)