Abstract
Earlier studies of phototropic fluence-response curves with maize coleoptiles identified first and second pulse-induced phototropisms and time-dependent phototropism (Iino, 1990). In the present study, we analyzed the phototropic fluence-response curve of rice coleoptiles and investigated causal relationships between the phototropin signaling pathway and the components resolved in fluence-response curves. Owing to weak phototropic responsiveness, it has been difficult to analyze pulse-induced phototropism in rice coleoptiles (Neumann and Iino, 1997). We were able to obtain a full phototropic fluence-response curve using lazy1 mutants (Yoshihara and Iino, 2007), which show reduced gravitropism. In this mutant, first and second pulse-induced phototropisms and time-dependent phototropism were found and an additional pulse-induced response was identified in low fluences. By using double mutants generated by crossing lazy1 and cpt1 (Haga and Iino, 2005), it was uncovered that all the multiphasic fluence-response components are dependent on phototropin signaling.