Genome‐Wide Identification of YABBY Genes in Medicago truncatula Reveals a Role for MtYAB3 in Petal Lateral Expansion

YABBY transcription factors are plant‐specific regulators of lateral organ development, yet their functions in legume flower morphogenesis remain poorly understood. Here, we identified seven YABBY genes in the Medicago truncatula genome and assigned them to five conserved subfamilies. Among the two FIL ‐clade members, MtFIL and MtYAB3 , both showed preferential expression in shoot apices and reproductive tissues, but only loss of MtYAB3 caused an obvious floral phenotype. In the mtyab3‐1 mutant, keel and wing petals were markedly narrower and petal fusion was impaired, whereas early petal primordium initiation showed no obvious abnormality. Complementation restored the mutant phenotype, supporting the conclusion that disruption of MtYAB3 was responsible for the floral defects. GUS staining and RNA in situ hybridization further showed that MtYAB3 is expressed in floral tissues with clear abaxial polarity during organ development. In addition, several genes associated with cell proliferation and expansion were differentially expressed in mtyab3‐1 . These results identify MtYAB3 as an important regulator of petal lateral growth in M. truncatula and provide a useful framework for understanding petal morphogenesis in legumes.