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NITRIC OXIDE AND cGMP DEPENDENT SIGNALING IN ARABIDOPSIS ROOT GROWTH AND GRAVITROPISM.   J.L. Jacobi1, J.J. Elmer1, K.L. Russell1, D.M. Porterfield2,3  1Dept. Biol. Sciences, University of Missouri – Rolla, Rolla, MO, 2Dept. of Ag. and Bio. Eng., and 3Dept. of Horticulture and Landscape Architecture, Purdue University, West Lafayette, IN.  

   Nitric oxide (NO) and cGMP are established components of many calcium dependent signaling pathways in animal systems. Roles for NO/cGMP in plants has been suggested in previous experiment to be involved with diverse functions in plants including Ca++ dependent polar cellular development in pollen (Prado et al., 2004, Development: 131: 2707-2714).  In these experiments we used a pharmacological approach to determine if a NO/cGMP dependent signaling pathway was active in regulating Arabidopsis thaliana root growth and gravitropism.  Our hypothesis is that a NO/cGMP dependent signaling pathway is activated by auxin to direct cell elongation in the root.  Arabidopsis thaliana plants were grown initially on agar/MS media and then transferred to media containing experimental drugs.  For gravitropism and vertical growth the plates were turned 90°, root length was marked and pictures were taken at regular time intervals. Root lengths and reorientation angles were measured from the digital images using Scion Image software.  Artificial NO donors (DEA and spermine NONOates) were shown to enhance root elongation at low concentrations, whereas high concentrations inhibited root elongation. Pharmacological accumulation of cGMP by adding dibutryl cGMP or by blocking cGMP specific phosphodiesterase activity, using the drug Viagra, also enhanced root elongation. The nitric oxide synthase inhibitor L-NAME also promoted root elongation in a dose dependent manner. All of the drugs that were used to alter the NO/cGMP pathway also modified root gravitropism. Supported by NSF: 0316876