(R.M., J.P.)GIGANTEA (GI) is a nuclear protein involved in the promotion of flowering by long days, in light input to the circadian clock, and in seedling photomorphogenesis under continuous red light but not far-red light (FR). Here, we report that in Arabidopsis (Arabidopsis thaliana) different alleles of gi have defects in the hypocotyl-growth and cotyledon-unfolding responses to hourly pulses of FR, a treatment perceived by phytochrome A (phyA). This phenotype is rescued by overexpression of GI. The verylow-fluence response of seed germination was also reduced in gi. Since the circadian clock modulates many light responses, we investigated whether these gi phenotypes were due to alterations in the circadian system or light signaling per se. In experiments where FR pulses were given to dark-incubated seeds or seedlings at different times of the day, gi showed reduced seed germination, cotyledon unfolding, and activity of a luciferase reporter fused to the promoter of a chlorophyll a/b-binding protein gene; however, rhythmic sensitivity was normal in these plants. We conclude that while GI does not affect the highirradiance responses of phyA, it does affect phyA-mediated very-low-fluence responses via mechanisms that do not obviously involve its circadian functions.
Phytochromes (phy) A and B provide higher plants the ability to perceive divergent light signals. phyB mediates red/far-red light reversible, low fluence responses (LFR). phyA mediates both very-low-fluence responses (VLFR), which saturate with single or infrequent light pulses of very low fluence, and high irradiance responses (HIR), which require sustained activation with far-red light. We investigated whether VLFR, LFR, and HIR are genetically coregulated. The Arabidopsis enhanced very-low-fluence response1 mutant, obtained in a novel screening under hourly far-red light pulses, showed enhanced VLFR of hypocotyl growth inhibition, cotyledon unfolding, blocking of greening, and anthocyanin synthesis. However, eve1 showed reduced LFR and HIR. eve1 was found allelic to the brassinosteroid biosynthesis mutant dim/dwarf1. The analysis of both the brassinosteroid mutant det2 in the Columbia background (where VLFR are repressed) and the phyA eve1 double mutant indicates that the negative effect of brassinosteroid mutations on LFR requires phyA signaling in the VLFR mode but not the expression of the VLFR. Under sunlight, hypocotyl growth of eve1 showed little difference with the wild type but failed to respond to canopy shadelight. We propose that the opposite regulation of VLFR versus LFR and HIR could be part of a context-dependent mechanism of adjustment of sensitivity to light signals.Light perceived by phytochromes strongly affects growth and development throughout the life cycle of plants. The relevant light signals are widely divergent in different developmental contexts as illustrated by the following examples. First, whereas a brief exposure to light is often enough to promote the germination of weed seeds during soil tillage (Scopel et al., 1991), prolonged exposure to light is required to achieve full seedling de-etiolation. Second, stem growth inhibition is initiated by seedling emergence under high as well as under low red light (R) to far-red light (FR) ratios (Yanovsky et al., 1995; Smith et al., 1997). However, this R/FR ratio-compensated light control of axis growth (i.e. regulation buffered against changes in R/FR) is lost during the deetiolation process itself and plants become competent to respond to reductions in R/FR ratio caused by vegetation canopies (Holmes et al., 1982). Third, deetiolation is partially buffered against the different photoperiods that the seedling can face according to the date and place (latitude) of emergence from the soil (Mazzella and Casal, 2001). However, photoperiod is a key signal controlling the timing of flowering once the plant has surpassed the juvenile phase of development.The wide array of light signals that phytochromes can perceive has been conceptualized as three modes of action (for review, see Casal et al., 1998). The very-low fluence response (VLFR) mediated by phytochrome A (phyA) is induced by radiation between 300 and 780 nm (Botto et al., l996; Shinomura et al., 1996). Brief light exposures are enough (although in some cases these exposures have t...
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