title: Phytochrome A enhances the promotion of hypocotyl growth caused by reductions in levels of phytochrome B in its far-red-light-absorbing form in light-grown Arabidopsis thaliana.
abstract: We sought to determine if phytochrome B (phyB)-mediated responses to the red light (R)/far-red light (FR) ratio are affected by phytochrome A (phyA) activity in light-grown seedlings of Arabidopsis thaliana. Pulses of FR delayed into the dark period were less effective than end-of-day (EOD) FR in promoting hypocotyl growth over a given period in darkness. White light minus blue light interposed instead of darkness between the end of the white-light photoperiod and the FR pulse was sufficient to maintain responsivity to the decrease in phyB in FR-light-absorbing form in wild-type (WT) seedlings, but not in the phyA mutant. Compared with EOD R, hourly R+FR pulses provided throughout the night caused a stronger promotion of stem growth than a single EOD R+FR pulse in WT Arabidopsis, cucumber, mustard, sunflower, tobacco, and tomato, but not in phyA Arabidopsis or in the aurea mutant of tomato. WT seedlings of Arabidopsis responded to a range of high EOD R/FR ratios, whereas the phyA mutant required stronger reductions in the EOD R/FR ratio. In sunlight, phyA seedlings of Arabidopsis showed no response to the "early warning" signals of neighboring vegetation, and hypocotyl-growth promotion occurred at higher plant densities than in the WT. Thus, under a series of light conditions, the sensitivity or responsivity to reductions in the R/FR ratio were larger in WT than in phyA seedlings. A product of phyA is therefore proposed to enhance the hypocotyl-growth response to decreases in phyB in FR-light-absorbing form in light grown seedlings.
authors.affiliation: Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires, Argentina. jjcasal@criba.edu.ar
authors.forename: J J
authors.initials: JJ
authors.lastname: Casal
mesh_terms: D017360:Arabidopsis / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D029681:Arabidopsis Proteins; D018553:Cucumis sativus / Q000502:physiology; D003624:Darkness; D017343:Genes, Plant; D018546:Hypocotyl; D008027:Light; D008029:Lighting; D018551:Solanum lycopersicum / Q000502:physiology; D009149:Mustard Plant / Q000502:physiology; D010786:Photoreceptor Cells; D010834:Phytochrome / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D051942:Phytochrome A; D051939:Phytochrome B; D010946:Plants, Medicinal; D010947:Plants, Toxic; D014026:Nicotiana / Q000502:physiology; D014157:Transcription Factors
chemical_list: D029681:Arabidopsis Proteins; C497973:PHYA protein, Arabidopsis; C497952:PHYB protein, Arabidopsis; D051942:Phytochrome A; D014157:Transcription Factors; D010834:Phytochrome; D051939:Phytochrome B