Abstract
Dark-grown seedlings of dicots develop a hook-like structure at the apical part of the hypocotyl. It has generally been accepted that the hook opens upon exposure to light through phytochrome-mediated low-fluence response (LFR), because red light (R) promotes hook opening and far-red light (FR) reverses it in bean and pea seedlings. However, we have found that R as well as FR causes hook coiling (exaggerated hook curvature) in tomato, parsley, carrot and cucumber. In tomato and parsley, an FR pulse enhanced hook coiling as much as an R pulse. The tomato phyA-deficient mutant lacked this effect, indicating the involvement of very low fluence response (VLFR) of phytochrome. In Arabidopsis, cabbage and rape, in contrast, FR as well as R induced hook opening. The distinct characteristics of VLFR and LFR among species are kinetically analyzed.
