Identification of genes that confer a semi-determinate growth habit in faba bean

In faba bean (Vicia faba L.), growth habit determines plant architecture and significantly influences flowering, podding patterns, and maturity. A recently identified semi-determinate germplasm exhibits favorable traits for mechanized harvesting and high yield potential, making it useful for developing a semi-dwarf faba bean ideotype with a high harvest index. To uncover the genetic basis of the semideterminate growth habit, we investigated developmental differences in the shoot apical meristems (SAMs) of four faba bean germplasms with distinct growth habits using scanning electron microscopy. The phenotypic differences among these germplasms were attributed to variations in the duration of the overlap periods between vegetative and reproductive growth. Through bulked-segregant analysis (BSA) combined with linkage mapping in an F₂ population and a recombinant inbred line (RIL) population derived from Qingcan 17 (indeterminate) × RF25 (semi-determinate), we identified two quantitative trait loci (QTL): one located within a 3.73-Mb region on chromosome 5 (VfSdt1) and the other within an 8.68-Mb region on chromosome 1L (VfSdt2). We selected Terminal flower 1 (VfTFL1) as the candidate gene for VfSdt1, as a functional single nucleotide polymorphism (T-to-G) in this gene was found to be responsible for terminal inflorescence formation. VfTFL1 expression is downregulated before the SAM transitions into an inflorescence meristem and the subsequent termination of differentiation. VfFD was identified as the candidate gene for VfSdt2, as it harbors two nonsynonymous polymorphisms within its coding sequence that are associated with variation in growth habit. VfFD is specifically expressed in the shoot apex. These polymorphisms may promote vegetative growth of the SAM in the sdt1/sdt1 genetic background, resulting in a semi-determinate phenotype. These findings offer valuable insights for molecular design breeding aimed at developing semi-dwarf faba bean varieties.