Interactive signalling by phytochromes and cryptochromes generates de-etiolation homeostasis in Arabidopsis thaliana

Single, double, triple and quadruple mutants of phyA, phyB, cry1 and cry2 w ere exposed to different sunlight irradiances and photoperiods to investiga te the roll played by phytochrome A, phytochrome B, cryptochrome 1 and cryp tochrome 2 during de-etiolation of Arabidopsis thaliana seedlings under nat ural radiation. Even the quadruple mutant retained some hypocotyl-growth in hibition by sunlight. Hypocotyl length was strongly affected by interaction s among photoreceptors. Double phyA phyB, phyA cry1, and cry1 cry2 mutants were taller than expected from the additive action of single mutations. Som e of these redundant interactions required the presence of phytochromes A a nd/or B. Interactions among photoreceptors resulted in a 44% reduction of t he response to irradiance and a 70% reduction of the response to photoperio d. The complex network of interactions among photoreceptors is proposed to buffer de-etiolation against changes in irradiance and photoperiod, i.e lig ht fluctuations not related to the positions of the shoot above or below so il level.