Light-Stimulated Apical Hook Opening in Wild-Type Arabidopsis thaliana Seedlings.

Apical hook opening and cotyledon unfolding are characteristic responses that occur during deetiolation of dicotyledonous seedlings. Light-stimulated apical hook opening and cotyledon unfolding in etiolated Arabidopsis thaliana seedlings appears to involve the activities of multiple photosensory systems. Red, far-red, and blue light are all effective in stimulating these responses in Arabidopsis. Stimulation of hook opening by red light and low fluence blue light is inductive, far-red reversible, and exhibits reciprocity, as is characteristic of many low fluence-dependent phytochrome-mediated responses. Far-red and high-fluence blue light appear to stimulate hook opening and cotyledon unfolding through high-irradiance-response systems during long-term light treatments. Although a phytochrome high-irradiance-response system presumably mediates the responses in far-red light, the responses to high-fluence blue light may be mediated by a blue light-specific photosensory system.