FLOWERING LOCUS T genes MtFTb1 and MtFTb2 act redundantly to promote flowering under long days in Medicago truncatula

Flowering time is a crucial agronomic trait affecting yield and plant performance in different climates and photoperiods. While the molecular pathways regulating photoperiodic flowering are well-characterised in Arabidopsis and cereals, they remain poorly understood in many other groups of plants. FLOWERING LOCUS T (FT) genes are key components regulating the transition from vegetative to reproductive stages. In the legume Medicago truncatula, a forage crop, six FT orthologs are present. To date, only the role of MtFTa1 has been described, promoting flowering after prolonged exposure to cold (vernalization), while the other five remains unclear. Here we show that MtFTb1 and MtFTb2 together are essential for flowering under long-day (LD) photoperiods. Using CRISPR/Cas9, we generated Mtftb1 single mutants and Mtftb1/2 double mutants. MtFTb1 and MtFTb2 genes act redundantly and are required to up-regulate MtFTa1 under vernalised LD conditions. While Mtftb1 mutants flower normally under short-day (SD) and LD conditions, Mtftb1/2 mutants are delayed specifically under LD but retain a vernalization response. Transcriptomic analysis of wild-type and Mtftb1/2 mutants shows MtFTb1/2 are essential for MADS-box gene upregulation and modulate known and candidate flowering regulators under LD. These findings advance our understanding of flowering time control in legumes and may inform strategies to improve forage crop productivity. HighlightMtFTb1 and MtFTb2 genes control flowering under long days in Medicago truncatula, revealing clade-specific functions of MtFT genes and providing novel insights into the environmental regulation of flowering time in legumes.