title: Phytochrome-imposed oscillations in PIF3 protein abundance regulate hypocotyl growth under diurnal light/dark conditions in Arabidopsis.
abstract: Arabidopsis seedlings display rhythmic growth when grown under diurnal conditions, with maximal elongation rates occurring at the end of the night under short-day photoperiods. Current evidence indicates that this behavior involves the action of the growth-promoting bHLH factors PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) at the end of the night, through a coincidence mechanism that combines their transcriptional regulation by the circadian clock with control of protein accumulation by light. To assess the possible role of PIF3 in this process, we have analyzed hypocotyl responses and marker gene expression in pif single- and higher-order mutants. The data show that PIF3 plays a prominent role as a promoter of seedling growth under diurnal light/dark conditions, in conjunction with PIF4 and PIF5. In addition, we provide evidence that PIF3 functions in this process through its intrinsic transcriptional regulatory activity, at least in part by directly targeting growth-related genes, and independently of its ability to regulate phytochrome B (phyB) levels. Furthermore, in sharp contrast to PIF4 and PIF5, our data show that the PIF3 gene is not subject to transcriptional regulation by the clock, but that PIF3 protein abundance oscillates under diurnal conditions as a result of a progressive decline in PIF3 protein degradation mediated by photoactivated phyB, and consequent accumulation of the bHLH factor during the dark period. Collectively, the data suggest that phyB-mediated, post-translational regulation allows PIF3 accumulation to peak just before dawn, at which time it accelerates hypocotyl growth, together with PIF4 and PIF5, by directly regulating the induction of growth-related genes.
authors.affiliation: Departament de Genètica Molecular, Center for Research in Agricultural Genomics, CSIC-IRTA-UAB-UB, Campus Universidad Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain.
authors.forename: Judit
authors.initials: J
authors.lastname: Soy
authors.forename: Pablo
authors.initials: P
authors.lastname: Leivar
authors.forename: Nahuel
authors.initials: N
authors.lastname: González-Schain
authors.forename: Maria
authors.initials: M
authors.lastname: Sentandreu
authors.forename: Salomé
authors.initials: S
authors.lastname: Prat
authors.forename: Peter H
authors.initials: PH
authors.lastname: Quail
authors.forename: Elena
authors.initials: E
authors.lastname: Monte
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000502:physiology* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000378:metabolism*; D057906:Circadian Clocks; D002940:Circadian Rhythm; D003624:Darkness; D018506:Gene Expression Regulation, Plant / Q000502:physiology*; D018546:Hypocotyl / Q000235:genetics / Q000254:growth & development / Q000502:physiology / Q000528:radiation effects; D008027:Light; D009154:Mutation; D017440:Photoperiod; D051939:Phytochrome B / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D059748:Proteolysis; D036226:Seedlings / Q000235:genetics / Q000254:growth & development / Q000502:physiology / Q000528:radiation effects; D015398:Signal Transduction / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D051939:Phytochrome B; C116162:PIF3 protein, Arabidopsis; C459466:PIF4 protein, Arabidopsis; C497729:PIF5 protein, Arabidopsis