Minisymposium 10: Tropisms

Abs # 23001: Attenuation of high-light phototropism

Presenter:	Whippo, Craig W, cwhippo@indiana.edu
Authors	Whippo, Craig W (A)   Hangarter, Roger P (A)
Affiliations:	(A): Indiana University

Etiolated Arabidopsis hypocotyls are highly phototropic towards low-intensity unilateral blue light (<10 μmol m^-2 s^-1 ). Our research shows that high-intensity unilateral blue light (>10 μmol m^-2 s^-1 ), on the other hand, attenuates the phototropic growth of etiolated hypocotyls. To identify the photoreceptors controlling the high-light attenuation response, we used time-lapse imaging to measure the kinetics of the high-light phototropic responses of phototropin, cryptochrome, and phytochrome mutants. Our results indicate that the early stage of the attenuation response is mediated by phot1 and phyA. Following this early phot1- and phyA-mediated phase, our research suggests that the cryptochromes maintain the attenuation response. It is possible that the attenuation of phototropism by high light is reflective of desensitization of the photoreceptors or effectors promoting phototropism. However, based on changes in hypocotyl length following phototropic growth and similarities with blue-light mediated growth inhibition (Parks et al. 2001), it appears that the attenuation of high-light phototropism might also be at least partially due to growth inhibition stimulated by high intensity blue-light. To better understand the attenuation of high-light phototropism, we have screened for and identified mutants with an enhanced high-light phototropic response . These mutants retain a normal low-light phototropic response but have longer hypocotyls than wild-type when grown under continuous light. Although mediated by the same photoreceptors, the phenotypes of these mutants indicate that the attenuation of high-light phototropism and light–mediated growth inhibition are independent from the promotion of phototropism.