Impact of drought on flowering time in rice

Drought is responsible for severe yield losses in Southern developing countries every year. Plants have adopted different strategies to cope with reduced water availability and some of them involve modulating flowering time in order to avoid the drought period. For example, in Arabidopsis thaliana this phenomenon has already been deeply characterized and described as drought escape. Plants adopting this strategy tend to flower earlier when exposed to drought stress. Conversely, drought treatments imposed to rice plants (Oryza sativa) before or during panicle development determine a flowering delay. Among the different pathways that control flowering time, the photoperiod pathway plays a prominent role to activate transcription of florigenic genes, including HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1). The effects of drought treatments imposed during floral transition and their impact on subsequent panicle initiation have not been thoroughly studied. We first set up a protocol to synchronize drought and the floral transition. Plants were grown on soil and exposed to inductive photoperiods while reducing the water content. We reproducibly obtained a delay of flowering upon exposure of plants to a mild drought stress. Global profiling of the leaf transcriptome (RNA-seq) and quantification of marker genes expression during drought and floral induction showed that both treatments were effective, and that the delay of flowering was determined by suppression of Hd3a transcription in leaves. Quantification of gene expression and the use of available mutant lines allowed us to identify EARLY HEADING DATE 1 (Ehd1) as integrator of flowering and drought signals. The results of this work will open new perspectives for connecting two important biological processes.