title: Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis.
abstract: Seeds of the wild type (WT) and of the phyA and phyB mutants of Arabidopsis thaliana were exposed to single red light (R)/far-red light (FR) pulses predicted to establish a series of calculated phytochrome photoequilibria (Pfr/P). WT and phyB seeds showed biphasic responses to Pfr/P. The first phase, i.e. the very-low-fluence response (VLFR), occurred below Pfr/P = 10-1%. The second phase, i.e. the low-fluence response, occurred above Pfr/P = 3%. The VLFR was similarly induced by either a FR pulse saturating photoconversion or a subsaturating R pulse predicted to establish the same Pfr/P. The VLFR was absent in phyA seeds, which showed a strong low-fluence response. In the field, even brief exposures to the very low fluences of canopy shade light (R/FR ratio < 0.05) promoted germination above dark controls in WT and phyB seeds but not in the phyA mutant. Seeds of the phyA mutant germinated normally under canopies providing higher R/FR ratios or under deep canopy shade light supplemented with R from light-emitting diodes. We propose that phytochrome A mediates VLFR of A. thaliana seeds.
authors.affiliation: Ifeva, Departamento de Ecologia, Facultad de Agronomia Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires, Argentina (J.F.B., R.A.S., J.J.C.).
authors.forename: J. F.
authors.initials: JF
authors.lastname: Botto
authors.forename: R. A.
authors.initials: RA
authors.lastname: Sanchez
authors.forename: G. C.
authors.initials: GC
authors.lastname: Whitelam
authors.forename: J. J.
authors.initials: JJ
authors.lastname: Casal