GIGANTEA is a Negative Regulator of Abscisic Acid Transcriptional Responses and Sensitivity in Arabidopsis

Transcriptional reprogramming plays a key role in drought stress responses, preceding the onset of morphological and physiological acclimation. The best-characterised signal regulating gene expression in response to drought is the phytohormone abscisic acid (ABA). ABA-regulated gene expression, biosynthesis and signalling are highly organised in a diurnal cycle, so that ABA-regulated physiological traits occur at the appropriate time of the day. The mechanisms that underpin such diel oscillations in ABA signals are poorly characterised. Here we uncover GIGANTEA (GI) as a key gatekeeper of ABA-regulated transcriptional and physiological responses. Time-resolved gene expression profiling by RNA sequencing under different irrigation scenarios indicates that gi mutants produce an exaggerated ABA response, despite accumulating wild-type levels of ABA. Comparisons with ABA-deficient mutants confirm the role of GI in controlling ABA-regulated genes and the analysis of leaf temperature, a read-out for transpiration, supports a role for GI in the control of ABA-regulated physiological processes. Promoter regions of GI/ABA-regulated transcripts are directly targeted by different classes of transcription factors, especially PHYTOCHROME-INTERACTING FACTORs, and (ABRE)-BINDING FACTOR, together with GI itself. We propose a model whereby diel changes in GI control oscillations in ABA responses. Peak GI accumulation at midday contributes to establishing a phase of reduced ABA sensitivity and related physiological responses, by gating DNA binding or function of different classes of transcription factors that cooperate or compete with GI at target regions.