A TRIPLE FLORIGEN SYSTEM IS ESSENTIAL FOR FLOWERING AND PANICLE ARCHITECTURE IN RICE

The transition from a vegetative phase to a reproductive one has always been deeply studied in plants and in particular in the cereals due to the relevance of this trait for seed yield. The transition occurs upon the arrival of a mobile flowering signal, the florigen, at the shoot apical meristem (SAM). In rice (Oryza sativa), there are two different florigens: Heading Date 3a (Hd3a), that promotes flowering in short day (SD) conditions and Rice Flowering Locus T 1 (RFT1) that promotes flowering in both short and long- days (LD). They are transcribed and translated in leaves once environmental conditions are optimal, and then transported through the phloem to the SAM. After reaching the shoot they form a heterohexameric complex named Florigen Activation Complex (FAC) binding to two 14-3-3 proteins and to two bZIP transcription factors and activate conversion of the shoot apex into an inflorescence by promoting expression of inflorescence- identity genes. Here, we show that Hd3a and RFT1 arrival at the SAM activates FLOWERING LOCUS T-LIKE 1 (FT-L1), encoding a third florigen-like protein showing atypical characteristics. Expression data from qRT-PCR, in situ hybridization and transcriptional marker lines indicated that its mRNA is mainly expressed in the SAM and persists during all developmental stages of inflorescence development. Misexpression of FT-L1 from meristem- and tissue-specific promoters indicates that only when expressed in the SAM, FT-L1 has florigenic activity. Isolation and analysis of EMS and CRISPR mutants showed that FT-L1 strengthens the effects of Hd3a and RFT1 during conversion of the vegetative meristem into inflorescence, in an addictive manner. Furthermore, it controls panicle branching by inhibiting secondary branching and promoting spikelet meristem differentiation. Thus, we propose a triple florigenic module that promotes a balanced progression of inflorescence identity and determinacy.