GIGANTEA Regulates Phytochrome A-Mediated Photomorphogenesis Independently of Its Role in the Circadian Clock

Abstract

(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.

TheGIGANTEA-Regulated MicroRNA172 Mediates Photoperiodic Flowering Independent ofCONSTANSinArabidopsis

“…It was constantly higher under LDs but lower under SDs. Together with the reduced miR172 abundance in the gi-2 mutant grown under LDs ( Figure 5B), these observations suggest that miR172 may be regulated by a clock-independent GI pathway (Mizoguchi et al, 2005;Oliverio et al, 2007).…”

Natural diversity in daily rhythms of gene expression contributes to phenotypic variation

“…Interestingly, we detected no significant relationship between GI expression and flowering in the NILs, despite an important function of GI being the promotion of flowering through the regulation of CONSTANS (CO) (45,46). GI regulates diverse traits through distinct molecular pathways (47)(48)(49), and it is possible that these pathways are not all equally sensitive to precise changes in GI expression. GI-mediated promotion of flowering might be more robust than growth to small perturbations of GI expression and function, an idea supported by a previous study in which an induced mutation of GI altered growth but not GI-dependent promotion of CO (48).…”

OWL1: An Arabidopsis J-Domain Protein Involved in Perception of Very Low Light Fluences

“…However, loss-of-function mutants also show an increased hypocotyl elongation under R and B light (Fowler et al, 1999;Park et al, 1999;Huq et al, 2000;Martin-Tryon et al, 2007). Recently, GI has been implicated in many VLFR, but not in HIR under continuous FR light (Oliverio et al, 2007). The involvement of brassinosteroids as positive regulators in the phytochrome signaling cascade was established by the analysis of deetiolated2 and the identification of the brassinosteroid biosynthesis mutant diminuto/dwarf1 as an enhanced very-low-fluence response1 mutant (Luccioni et al, 2002).…”

“…Pks1 and 2 mutants show enhanced phyA-mediated VLFR but are also involved in blue light-mediated phototropism (Lariguet et al, 2003(Lariguet et al, , 2006. Although GIGANTEA (GI) was first characterized as playing a role in the circadian clock and deetiolation under red light conditions, recent work showed that this protein is also important for hypocotyl growth, cotyledon opening, and germination under VLF conditions (Oliverio et al, 2007).…”