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The SHL1 and SHL5 Genes Influence both Red- and Blue-light-based Phototropism in Arabidopsis thaliana.  P. Kumar and J.Z. Kiss. 

   Plants receive sensory input from a large number of environmental stimuli including light, gravity, and touch.  Of these, light is one of the most important factors in the development throughout the life cycle of a plant.  As a consequence, plants have evolved several families of photoreceptor molecules.  The major groups of photoreceptors in flowering plants include the phytochromes (for red, far-red), cryptochromes (blue), and phototropins (blue).  While considerable progress has been made in understanding the photobiology of light sensing, relatively little is known about the events downstream to perception.  The shl (seedlings hypersenstive to light) mutants exhibit an inhibition in hypocotyl length even under low fluence rates, and the SHL gene family is hypothesized to be a negative regulator of photomorphogenesis.  In this report, we studied tropistic responses of seedlings of shl1 and shl5 and compared these responses to those of wild-type seedlings.  Roots of both mutants have an enhanced positive phototropic response to red light, and hypocotyls of shl5 have an enhanced positive phototropism relative to blue light.  However, roots of shl1 and shl5 seedlings exhibit a diminished negative blue-light phototropism.  In contrast, both shl mutants have little to no alteration in gravitropism and in growth rates of roots and hypocotyls.  Thus, SHL1 and SHL5 appear to be part of the signaling pathway downstream to the phototropins as well as to the cryptochromes and the phytochromes.  Our results support the hypothesis that the SHL genes act at the juncture of red and blue light signaling networks. 

(Supported by NASA Grant NCC2-1200).