GENETIC AND MOLECULAR DISSECTION OF THE DROUGHT ESCAPE RESPONSE IN A. THALIANA

Modulation of the transition to flowering plays an important role in the adaptation to drought. The drought escape (DE) response allows plants to adaptively shorten their life cycle to make seeds before severe stress leads to death. However, the molecular basis of DE response is unknown. The screen of different Arabidopsis thaliana flowering time mutants under DE– triggering conditions revealed the central role of the flower–promoting gene GIGANTEA (GI) and the florigen genes FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF) in the DE response. Further screens showed that the phytohormone abscisic acid is required for DE response, positively regulating flowering under long day conditions (LDs). Drought stress promotes the transcriptional upregulation of the florigens in an ABA– and photoperiod– dependent manner, so that early flowering only occurs under LDs. Along with the florigens, the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) is also up-regulated in a similar fashion and contributes to the activation of TSF. The DE response was recovered under short days in the absence of the floral repressor SHORT VEGETATIVE PHASE (SVP) or in GI overexpressing plants. Our data reveal a key role for GI in connecting photoperiodic cues and environmental stress independently from the central FT/TSF activator CONSTANS. This mechanism explains how environmental cues may act upon the florigen genes in a photoperiodically–controlled manner, thus enabling plastic flowering responses.