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

ABSTRACT Single, double, triple and quadruple mutants of phyA , phyB , cry1 and cry2 were exposed to different sunlight irra-diances and photoperiods to investigate the roll played byphytochrome A, phytochrome B, cryptochrome 1 and cryp-tochrome 2 during de-etiolation of Arabidopsis thaliana seedlings under natural radiation. Even the quadruplemutant retained some hypocotyl-growth inhibition by sun-light. Hypocotyl length was strongly affected by interac-tions among photoreceptors. Double phyA phyB , phyAcry1 , and cry1 cry2 mutants were taller than expected fromthe additive action of single mutations. Some of theseredundant interactions required the presence of phy-tochromes A and/or B. Interactions among photoreceptorsresulted in a 44% reduction of the response to irradianceand a 70% reduction of the response to photoperiod. Thecomplex network of interactions among photoreceptors isproposed to buffer de-etiolation against changes in irradi-ance and photoperiod, i.e light fluctuations not related tothe positions of the shoot above or below soil level