Abstract
Earlier studies of phototropic fluence-response curves with maize coleoptiles identified first and second pulse-induced phototropisms and time-dependent phototropism. In this study, we established a phototropism assay system that allowed quantitative determination of hypocotyl phototropism and investigated the molecular basis of multiphasic fluence-response relationships by using phototropic mutants of Arabidopsis. With this assay system in which red light-adapted seedlings were used and phototropic stimulation was directed parallel to the symmetrical plane of the seedling shoot, the first pulse-induced and time-dependent phototropisms were resolved and, in addition, a weak second pulse-induced phototropism was identified in high fluences. It was uncovered that all these responses are mediated by phot1 with no apparent contribution from phot2. It was also uncovered that NPH3 limits all the responses whereas RPT2 is involved specifically in time-dependent phototropism. Further analysis using continuous blue light stimulation indicated that phot2 can cause a weak phototropism at high fluence rates when phot1 is absent, but it does not mediate phototropism in wild-type hypocotyls.
