THE ROLE OF ABA IN THE FLORAL TRANSITION: SITE AND MECHANISM OF ACTION

The reproductive success of plants depends on their developmental plasticity that is the ability to modulate their growth in response to exogenous and endogenous stimuli. Plants efficiently integrate these signals to coordinate their life cycle according to the best conditions to increase their fitness. Light and water availability is a limiting factor for plants sustenance and growth. In Arabidopsis thaliana day length (photoperiod) and water status influence flowering time. In particular, water deficit accelerates flowering thus enabling the drought escape (DE) responses. Interestingly, such DE responses only occurs under inductive long day conditions (LDs, typical of spring and summer seasons) but not short day conditions (SDs) highlighting a link between photoperiod perception and drought responses. The phytohormone abscisic acid (ABA) mediates the DE response, by promoting the upregulation of the florigen genes FLOWERING LOCUS T (FT) and its paralogue TWIN SISTER OF FT (TSF), whose expression is activated mainly by LDs. The role of ABA in flowering regulation is controversial as the literature describes both positive and negative roles for ABA in flowering. My PhD work supports the idea that ABA acts as florigen-stimulating molecule under LDs and its activity depends on prior activation of the photoperiodic pathway. I demonstrated that the ABA-dependent activation of FT requires GIGANTEA (GI) and CONSTANS (CO) functions, two main components of the photoperiodic pathway that control florigen expression. The generation of transgenic plants over-expressing tagged versions of GI or CO proteins in different ABA genetic backgrounds allowed me to directly asses their activity and measure their accumulations under varying levels of ABA signalling. My results indicate that ABA promotes GI and CO function without affecting their protein stability. An intriguing perspective of my work is that ABA might regulate GI and/or CO accessibility to the FT promoter. Further studies are however necessary to test this hypothesis and to decipher the molecular mechanism by which ABA allow plants to coordinate flowering time according to the prevailing watering conditions.