Phytochrome A is an irradiance-dependent red light sensor

• Published in: The Plant journal : for cell and molecular biology Vol. 50; no. 1; pp. 108 - 117
• Main Authors: Franklin, Keara A, Allen, Trudie, Whitelam, Garry C
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.04.2007; Blackwell Publishing Ltd; Blackwell Science
• Subjects: Arabidopsis; Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis - radiation effects; Arabidopsis Proteins; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Arabidopsis Proteins - physiology; Arabidopsis thaliana; Biological and medical sciences; Botany; etiolation; Experiments; flowering; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation, Plant; genetically modified organisms; genetics; growth & development; Hypocotyl; Hypocotyl - genetics; Hypocotyl - growth & development; Hypocotyl - radiation effects; Immunoblotting; irradiance; Light; light intensity; metabolism; Microscopy, Fluorescence; mutants; Mutation; photomorphogenesis; photoprotection; photoreceptors; photostability; Physical agents; physiology; phytochrome; Phytochrome - genetics; Phytochrome - metabolism; Phytochrome - physiology; Phytochrome A; Phytochrome A - genetics; Phytochrome A - metabolism; Phytochrome A - physiology; plant architecture; Plant physiology and development; Plants, Genetically Modified; Proteins; proteolysis; radiation effects; red light; Seedlings; Vegetative apparatus, growth and morphogenesis. Senescence; Growth; Arabidopsis; Photoreceptor; phytochrome A; Arabidopsis thaliana; Hypocotyl; Cruciferae; Dicotyledones; Angiospermae; Phytochrome; Development; Flowering; irradiance; Spermatophyta; red light; Mutation; Photoprotection
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313x.2007.03036.x

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 μmol m⁻² sec⁻¹ of R when characterizing phytochrome functions. Here we reveal considerable phyA activity in R at higher (>160 μmol 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.