The SHL1 and SHL5 genes influence both red- and blue-light-based phototropism in Arabidopsis thaliana

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 hyper-responsive to light) mutants exhibit an inhibition in hypocotyl length even under low fluence rates, and several of the genes implicated by the shl mutants are hypothesized to be negative regulators of photomorphogenesis. In this report, we performed detailed phenotypic analyses of the 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 addition, both shl mutants show a greater alteration in phototropism compared to changes in gravitropism and in growth rates of roots and hypocotyls. Our results support the hypothesis that the SHL genes may act at the juncture of red and blue-light signaling networks.