JOURNAL OF BIOLOGICAL RHYTHMS / December 2001Yanovsky et al. / PHYTOCHROMES AND CRYPTOCHROMES IN ARABIDOPSIS Resetting of the Circadian Clock by Phytochromes and Cryptochromes in

Abstract The authors sought to investigate the role of phytochromes A and B (phyA and phyB) and cryptochromes 1 and 2 (cry1 and cry2) in the synchroniza-tion of the leaf position rhythm in Arabidopsis thaliana. The seedlings were trans-ferred from white light–dark cycles to free-running conditions with or without exposure to a light treatment during the final hours of the last dark period. The phase advance caused by a far-red light treatment was absent in the phyAmutant, deficient in the fhy1 and fhy3 mutants involved in phyA signaling, and normal in the cry1 and cry1 cry2 mutants. The phase shift caused by blue light was normal in the cry2 mutant; reduced in the phyA, cry1, phyA cry1, and cry1 cry2 mutants; and abolished in the phyA cry1 cry2 triple mutant. The phase shift caused by red light was partially retained by the phyA phyB double mutant. The authors conclude that cry1 and cry2 participate as photoreceptors in the blue light input to the clock but are not required for the phyA-mediated effects on the phase of the circa-dian rhythm of leaf position. The signaling proteins FHY1 and FHY3 are shared by phyA-mediated photomorphogenesis and phyA input to the clock. Key words Arabidopsis, circadian rhythms, cryptochrome, leaf movement, light input, phytochrome Light is acknowledged as the most important envi-ronmental cue involved in resetting the circadian clocks. Our understanding of the light input to the clock has improved very significantly in recent years. The discovery of cryptochromes first in plants (cry1