Body of Abstract: Rice is an herbaceous annual plant of Asian origin, whose flowering is accelerated under short-day (SD) conditions. Photoperiod is measured in leaves and, when conditions are favorable, florigens are produced and induce flowering. Florigens in rice are encoded by Heading Date 3a (Hd3a) and Rice Flowering Locus T1 (RFT1), both homologs of Arabidopsis thaliana FT. These proteins are transmitted to the Shoot Apical Meristem (SAM), where they form the Florigen Activation Complex (FAC), a multiproteic complex whose targets include genes involved in the vegetative-to-reproductive transition.
Among the genes directly regulated by rice florigens at the SAM, one of the most strongly induced is FT-L1, a gene belonging to the FT-family, as well as the closest homolog of Hd3a. Its transcription at the SAM distinguishes FT-L1 from traditional florigens, whose transcription is restricted to leaves, indicating non-canonical regulation and function in rice flowering.
Expression analysis at both transcriptional and protein levels showed that FT-L1 is present in the SAM at different stages of early panicle development, persisting also in flowers. Genetic analysis using both CRISPR- and chemically-induced mutations indicate that loss of FT-L1 causes a delay in flowering time, as well as increasing the number of secondary or higher order branches. Both phenotypes are exacerbated when ft-l1 mutants are combined with hd3a or rft1. These data suggest FT-L1 has a dual role as positive regulator of the vegetative-to-reproductive transition and of the transition from indeterminate branch meristem to determinate spikelet meristem fate. Taken together, these data allow proposing the existence of a triple system based on florigens and a florigen-like protein determining reproductive commitment. Ongoing research is addressing the molecular antagonism between florigenic and antiflorigenic proteins, to understand how their activities are integrated into the complex regulation of inflorescence architecture.