First plant circadian phase mutant identified

Abstract

Circadian rhythms exhibited by plants include leaf movements, CO2 fixation, stomatal conductance and gene expression. They have evolved in response to, and are entrained by, environmental periodicities (zeitgeber), primarily daily changes in light intensity, such that the endogenous oscillation adopts the 24 h periodicity of the zeitgeber and a particular phase relationship to it. The phytochrome (PHY) and cryptochrome (CRY) photoreceptor families mediate light input to the clock and the importance of several of these proteins in establishing period length has recently become apparent. However, how the phase of plant rhythms is determined is unknown. Indeed, the Arabidopsis Genome Initiative revealed no plant orthologues to clock proteins involved in phase determination in animal and prokaryote circadian systems.‘…this study identifies PHYB as an important regulator of white light-mediated phase determination…’In a new study, Patrice A. Salome and colleagues [1xThe out of phase 1 mutant defines a role for PHYB in circadian phase control in Arabidopsis. Salome, P.A. et al. Plant Physiol. 2002; 129: 1674–1685Crossref | PubMed | Scopus (61)See all References][1] describe a new allele of PHYB, out of phase 1 (oop1), which confers an altered circadian phase phenotype. The Arabidopsis thaliana mutant was isolated by exploiting a rhythm of sensitivity to the air pollutant sulfur dioxide, selecting plants exhibiting damaged leaves at a time when wild-type plants were resistant. After 7 days entrainment in a 12-h:12-h (white) light–dark regime, the peaks of leaf movement and CO2 assimilation rhythms in oop1 plants occurred 3.6 and 3.4 h earlier, respectively, than in wild-type plants. Period lengths were unaltered. Sequencing revealed two mutations in PHYBoop1; one introducing an early stop codon such that a truncated protein lacking most of the C-terminal kinase domain is expressed. Because oop1 seedlings displayed light-induced inhibition of hypocotyl elongation under blue or far- red light but not red light, the authors propose that the altered phase phenotype results primarily from impaired red-light photoreception through PHYB. Indeed, when phyB-9, a previously identified complete loss-of-function allele, was crossed into plants harbouring a transcriptional fusion of the LIGHT-HARVESTING CHLOROPHYLL a/b BINDING PROTEIN promoter and luciferase (LHCB::LUC), LHCB::LUC rhythms were coincident with those exhibited by the corresponding oop1 cross. However, the data suggest that the oop1 mutation also interferes with other light-signalling pathways because the elongated hypocotyl phenotype was also observed when oop1 seedlings were illuminated by white light or combinations of red light with high fluence blue light. This implies a red light-dependent alteration of blue-light signalling, presumably through CRY1, the primary blue-light photoreceptor at high fluence rates.In characterizing oop1, the first circadian phase mutant of a plant, this study identifies PHYB as an important regulator of white light-mediated phase determination, and, as such, represents an important milestone in circadian biology. Observations that oop1 plants exhibit wild-type period length and do not show the altered phase phenotype when entrained to temperature cycles indicate that oscillator function itself is not affected. However, whether impaired PHYB signalling affects light input to the clock or components of the output pathway downstream of the oscillator remains to be determined. Future studies on PHY INTERACTING FACTOR 3, a target of PHYB signalling, which activates transcription of two known clock components in plants should help resolve this question and provide further insight into the molecular basis of phase determination in plants.