Hd3a, RFT1 and Ehd1 integrate photoperiodic and drought stress signals to delay the floral transition in rice

Plants show a high degree of developmental plasticity in response to external cues, including day length and environmental stress. Water scarcity in particular can interfere with photoperiodic flowering, resulting in the acceleration of the switch to reproductive growth in several species, a process called drought escape. However, other strategies are possible and drought stress can also delay flowering, albeit the underlying mechanisms have never been addressed at the molecular level. We investigated these interactions in rice, a short day species in which drought stress delays flowering. A protocol that allows the synchronization of drought with the floral transition was set up to profile the transcriptome of leaves subjected to stress under distinct photoperiods. We identified clusters of genes that responded to drought differently depending on day length. Exposure to drought stress under floral-inductive photoperiods strongly reduced transcription of EARLY HEADING DATE 1 (Ehd1), HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1), primary integrators of day length signals, providing a molecular connection between stress and the photoperiodic pathway. However, phenotypic and transcriptional analyses suggested that OsGIGANTEA (OsGI) does not integrate drought and photoperiodic signals as in Arabidopsis, highlighting molecular differences between long and short day model species. Rice plants grown in rainfed areas can experience periods of water scarcity, culminating in drought stress. How drought signals impact on developmental switches, including the transition from vegetative to reproductive growth, has never been thoroughly assessed. We set up a protocol that allows monitoring the effects of drought stress while plants are undergoing reproductive phase change, and using a genome-wide transcriptomic approach we identified clusters of genes differentially responding to drought under conditions that either promote or repress the floral transition. The emerging picture suggests that rice plants experiencing drought adopt a distinct strategy, compared to Arabidopsis, to regulate plant reproduction, and that specific genes in the flowering network act as hubs where abiotic and developmental signals converge.