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A NOVEL RED-LIGHT-BASED PHOTOSENSORY SYSTEM THAT MEDIATES POSITIVE PHOTOTROPISM IN ARABIDOPSIS ROOTS.  N.J. Ruppel¹, R.P. Hangarter², J.Z. Kiss¹. ¹Botany Dept., Miami Univ., Oxford, OH 45056; ²Biology Dept., Indiana Univ., Bloomington IN 47405.

The interaction between light and gravity is critical in determining the final form of a plant. However, while phototropism has been well-characterized in stems and stem-like organs, there have been relatively few studies of root phototropism. Our experiments suggest that there are two photosensory systems that elicit phototropic responses in roots of Arabidopsis thaliana: a previously identified blue-light photoreceptor system mediated by phototropin (= NPH1 protein) and a novel red-light-based mechanism. Results from three independent types of experiments (i.e., time course, unilateral illumination, orientation studies with light from above and below) confirmed the novel red-light response. The phototropic responses in roots are much weaker than the graviresponse, which competes with and often masks the phototropic response. It was through the use of mutant plants with a weakened graviresponse that we were able to identify the activity of the red-light-dependent phototropic system. In addition, the red-light-based photoresponse in roots is even weaker compared to the blue-light response. While it appears that the positive phototropism is controlled by phytochrome, studies are in progress to determine which member(s) of the phytochrome family is involved in mediating this response. (Financial support was provided by NASA grant NAG 2-1017 and the Howard Hughes Summer Internship program at Miami University.)