GIGANTEAActs in Blue Light Signaling and Has Biochemically Separable Roles in Circadian Clock and Flowering Time Regulation

Abstract

Circadian clocks are widespread in nature. In higher plants, they confer a selective advantage, providing information regarding not only time of day but also time of year. Forward genetic screens in Arabidopsis (Arabidopsis thaliana) have led to the identification of many clock components, but the functions of most of these genes remain obscure. To identify both new constituents of the circadian clock and new alleles of known clock-associated genes, we performed a mutant screen. Using a clock-regulated luciferase reporter, we isolated new alleles of ZEITLUPE, LATE ELONGATED HYPOCOTYL, and GIGANTEA (GI). GI has previously been reported to function in red light signaling, central clock function, and flowering time regulation. Characterization of this and other GI alleles has helped us to further define GI function in the circadian system. We found that GI acts in photomorphogenic and circadian blue light signaling pathways and is differentially required for clock function in constant red versus blue light. Gene expression and epistasis analyses show that TIMING OF CHLOROPHYLL A/B BINDING PROTEIN1 (TOC1) expression is not solely dependent upon GI and that GI expression is only indirectly affected by TOC1, suggesting that GI acts both in series with and in parallel to TOC1 within the central circadian oscillator. Finally, we found that the GI-dependent promotion of CONSTANS expression and flowering is intact in a gi mutant with altered circadian regulation. Thus GI function in the regulation of a clock output can be biochemically separated from its role within the circadian clock.

Rapid Assessment of Gene Function in the Circadian Clock Using Artificial MicroRNA in Arabidopsis Mesophyll Protoplasts

“…2A), generally consistent with the reported effects of loss-of-function mutants in these genes (Park et al, 1999;Somers et al, 2004;Kevei et al, 2006;Martin-Tryon et al, 2007). However, some reports show a long period for some gi alleles depending on the reporter and light conditions (Park et al, 1999;Martin-Tryon et al, 2007). When tested under blue light the results were similar (Table I; Fig.…”

REVEILLE1, a Myb-like transcription factor, integrates the circadian clock and auxin pathways

“…1 C and D). However, luciferase activity is low in these mutants, perhaps due to partial silencing of the reporter transgenes (26,27). Consistent with this suggestion, mRNA levels and clock regulation of two output genes, PHYTOCHROME INTERACTING FAC-TOR4 (PIF4) and PIF5 are similar to wild type in rve1 rve2 rve7 mutants ( Fig.…”

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

“…GI, a small nuclear protein whose biochemical function is still not understood, was first characterized for its role in promoting flowering under long-day conditions and circadian clock. 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).…”