The out of phase 1 Mutant Defines a Role for PHYB in Circadian Phase Control in Arabidopsis

Abstract

Arabidopsis displays circadian rhythms in stomatal aperture, stomatal conductance, and CO₂ assimilation, each of which peaks around the middle of the day. The rhythmic opening and closing of stomata confers a rhythm in sensitivity and resistance, respectively, to the toxic gas sulfur dioxide. Using this physiological assay as a basis for a mutant screen, we isolated mutants with defects in circadian timing. Here, we characterize one mutant, out of phase 1 (oop1), with the circadian phenotype of altered phase. That is, the timing of the peak (acrophase) of multiple circadian rhythms (leaf movement, CO₂ assimilation, andLIGHT-HARVESTING CHLOROPHYLL a/b-BINDING PROTEINtranscription) is early with respect to wild type, although all circadian rhythms retain normal period length. This is the first such mutant to be characterized in Arabidopsis. oop1 also displays a strong photoperception defect in red light characteristic ofphytochrome B (phyB) mutants. Theoop1 mutation is a nonsense mutation ofPHYB that results in a truncated protein of 904 amino acids. The defect in circadian phasing is seen in seedlings entrained by a light-dark cycle but not in seedlings entrained by a temperature cycle. Thus, PHYB contributes light information critical for proper determination of circadian phase.