Distinct Roles of <i>GIGANTEA</i> in Promoting Flowering and Regulating Circadian Rhythms in Arabidopsis

TLDR

The circadian clock governs photoperiodic flowering in Arabidopsis through the GI–CO–FT pathway, with GI acting upstream of CO and FT and influencing both flowering and circadian rhythms. The study seeks to determine whether GI directly promotes flowering‑time gene expression in the nucleus, independent of its circadian clock role. The authors examined GI function by analyzing GI overexpression lines, gi loss‑of‑function mutants, and circadian‑clock‑defective hypocotyl mutants to assess its impact on flowering and circadian regulation. GI promotes flowering by up‑regulating CO and FT transcription, while also broadly controlling circadian gene expression, and its nuclear localization supports a direct transcriptional role.

Abstract

The circadian clock acts as the timekeeping mechanism in photoperiodism. In Arabidopsis thaliana, a circadian clock-controlled flowering pathway comprising the genes GIGANTEA (GI), CONSTANS (CO), and FLOWERING LOCUS T (FT) promotes flowering specifically under long days. Within this pathway, GI regulates circadian rhythms and flowering and acts earlier in the hierarchy than CO and FT, suggesting that GI might regulate flowering indirectly by affecting the control of circadian rhythms. We studied the relationship between the roles of GI in flowering and the circadian clock using late elongated hypocotyl circadian clock associated1 double mutants, which are impaired in circadian clock function, plants overexpressing GI (35S:GI), and gi mutants. These experiments demonstrated that GI acts between the circadian oscillator and CO to promote flowering by increasing CO and FT mRNA abundance. In addition, circadian rhythms in expression of genes that do not control flowering are altered in 35S:GI and gi mutant plants under continuous light and continuous darkness, and the phase of expression of these genes is changed under diurnal cycles. Therefore, GI plays a general role in controlling circadian rhythms, and this is different from its effect on the amplitude of expression of CO and FT. Functional GI:green fluorescent protein is localized to the nucleus in transgenic Arabidopsis plants, supporting the idea that GI regulates flowering in the nucleus. We propose that the effect of GI on flowering is not an indirect effect of its role in circadian clock regulation, but rather that GI also acts in the nucleus to more directly promote the expression of flowering-time genes.