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Gravity and Light: The Role of Membrane Components in Tropic Responses.

Heike Winter Sederoff¹, Mariya Khodakovskaya¹, Jeffrey M. Kimbrough¹, Raul Salinas-Mondragon¹, and Christopher S. Brown^1,2

¹Dept. Plant Biology, and ²Kenan Institute for Science, Technology& Engineering, North Carolina State University, Raleigh, NC 27695-7612.

   Plant roots use light and gravity as directional signals to grow down into the soil. The response to light and gravity requires an integration and prioritization of a growth response. We have shown that the transcriptional response in Arabidopsis root tips occurs within less than 2 min and is distinct from the transcriptional response to mechanical stimulation (Kimbrough et al. 2004). Inositol 1,4,5 triphosphate (InsP₃) is an early signal transduction elements in gravitropism (Perera et al. 2006). We show here that InsP₃ mediates specific activation and repression of transcript abundance changes in response to gravity and light stimulation in root apices of the transgenic Arabidopsis plants. These gene-specific effects can be mimicked by inhibition of Phospholipase C. We have identified one of the fast InsP₃-mediated light and gravity regulated genes to be a cholesterol and cardiolipin binding protein, possibly involved in cholesterol transport between organells. Using One-Hybrid analysis to identify possible cis-regulatory elements involved in transcriptional regulation of light and gravity responsive genes, we fished a C2-domain DNA-binding protein. In vitro binding studies showed that the C2 domain binds to galactosylceramide-3-O-sulfate. We will present a hypothetical model for the possible roles those membrane components in the signal transduction and response pathways to light and gravity in roots.

(Supported by NASA grant NAG2-1566)

Perera, I. Y., C. Y. Hung, S. Brady, G. K. Muday and W. F. Boss (2006) Plant Physiol 140(2): 746-60.