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-'%. 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 c 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.

The function of phytochrome A

“…the low-fluence response (LFR), is observed largely above Pfr/P = 1% and shows red light (R)/far-red light (FR) reversibility (Botto, Sanchez & Casal 1995). Recent experiments using phyA and phyB mutants of Arabidopsis thaliana have demonstrated that the VLFR is mediated by phyA whilst the LFR is mediated by phytochrome B (phyB) and by phytochrome(s) other than phyA (Botto et al 1995(Botto et al , 1996Shinomura et al 1996) (Fig. 1).…”

The germination of seeds of Epiphyllum phyllanthus (L.) Haw. (Cactaceae) is controlled by phytochrome and by nonphytochrome related process

“…The data were submitted to factorial analysis and One Way ANOVA followed by Tukey test at α ≤ 0.05 (Sokal & Rohlf 1981 Simão et al 2007) and those two factors have more effect on the germination rates than on germination percentage, as determined in Epiphyllum phyllanthus seed germination. Here, the temperature had no significant effect on final germination percentage except at 45 °C where no germination was observed and at 10 °C in darkness but the absence of white light decreased significantly (p < 0.05) the germination rates (Figures 1a and 1b) (Botto et al 1996).…”

“…Several species in plant communities present seed germination responses, at least partially mediated by phytochrome (Botto et al 1996). However, among species there is a large variation in the light-sensitive seeds, mainly in the function of the morphological and physiological characteristics (Amaral-Barolli & Takaki 2001, Rojas-Aréchiga et al 2001 besides of the temperature effect (Válio & Scarpa 2001, Sugahara & Takaki 2004) which can or not change the sensitivity of seeds to light.…”

“…For instance, in highly light sensitive seeds of Arabidopsis thaliana the far-red light causes only partial reversal of the red light induction and the far-red alone can promotes germination in darkness (Botto et al 1996). Thus, for some species, at specific conditions short period of light at very low fluence such as from dim green safe light used in photomorphogenic studies, is sufficient to trigger the germination process (Baskin, J.M.…”

Genetic approaches in plant physiology

“…From the moment these well defined mutants were available they have been used to specify the modes of action of these different phytochromes, e.g. in seed germination (Botto et al, 1995), anthocyanin formation (Kerckhoffs et al, 1997) and fiowering (Bagnall et al, 1995).…”