title: Phytochrome A is an irradiance-dependent red light sensor.
abstract: Plants perceive red (R) and far-red (FR) light signals using the phytochrome family of photoreceptors. In Arabidopsis thaliana, five phytochromes (phyA-phyE) have been identified and characterized. Unlike other family members, phyA is subject to rapid light-induced proteolytic degradation and so accumulates to relatively high levels in dark-grown seedlings. The insensitivity of phyA mutant seedlings to prolonged FR and wild-type appearance in R has led to suggestions that phyA functions predominantly as an FR sensor during the early stages of seedling establishment. The majority of published photomorphogenesis experiments have, however, used <50 micromol m(-2) sec(-1) of R when characterizing phytochrome functions. Here we reveal considerable phyA activity in R at higher (>160 micromol m(-2) sec(-1)) photon irradiances. Under these conditions, plant architecture was observed to be largely regulated by the redundant actions of phytochromes A, B and D. Moreover, quadruple phyBphyCphyDphyE mutants containing only functional phyA displayed R-mediated de-etiolation and survived to flowering. The enhanced activity of phyA in continuous R (Rc) of high photon irradiance correlates with retarded degradation of the endogenous protein in wild-type plants and prolonged epifluorescence of nuclear-localized phyA:YFP in transgenic lines. Such observations suggest irradiance-dependent 'photoprotection' of nuclear phyA in R, providing a possible explanation for the increased activity observed. The discovery that phyA can function as an effective irradiance sensor, even in light environments that establish a high Pfr concentration, raises the possibility that phyA may contribute significantly to the regulation of growth and development in daylight-grown plants.
authors.affiliation: Department of Biology, University of Leicester, Leicester LE1 7RH, UK. kaf5@le.ac.uk
authors.forename: Keara A
authors.initials: KA
authors.lastname: Franklin
authors.forename: Trudie
authors.initials: T
authors.lastname: Allen
authors.forename: Garry C
authors.initials: GC
authors.lastname: Whitelam
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D018506:Gene Expression Regulation, Plant; D018546:Hypocotyl / Q000235:genetics / Q000254:growth & development / Q000528:radiation effects; D015151:Immunoblotting; D008027:Light; D008856:Microscopy, Fluorescence; D009154:Mutation; D010834:Phytochrome / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D051942:Phytochrome A / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D030821:Plants, Genetically Modified
chemical_list: D029681:Arabidopsis Proteins; D051942:Phytochrome A; D010834:Phytochrome