Integrated Transcriptomic and Developmental Analyses Provide Insights into the Intrafloral Stamen Differentiation in Cassia fistula L.

Selective pressure targeting male functions plays a crucial role in the evolution of floral morphological traits. In some angiosperm groups, flowers contain two or more sets of stamens that vary in size, color, and morphology, a phenomenon known as heteranthery. This reflects an evolutionary adaptation of stamens. However, the developmental basis and molecular mechanisms remain poorly understood. This study integrates transcriptomic and developmental approaches to elucidate the molecular and morphological mechanisms underlying intra-floral stamen differentiation in Cassia fistula L., an economic leguminous tree exhibiting heteranthery with three distinct stamen types: long stamens (LS), short stamens (SS), and degenerated stamens (St). We documented asynchronous stamen primordia initiation and development trajectories across stamen types. Transcriptomic profiling and protein-protein interaction analysis identified differentially expressed genes (DEGs) between filaments of the three stamen sets, with significant enrichment in brassinosteroid (BR) related pathways. CYP90D1 (Cf_f49903) and CYP90C1 (Cf_f56973) emerged as candidate genes related to stamen length differentiation in C. fistula. This study not only helped elucidate the developmental and genetic framework of heteranthery in C. fistula but also provided new insights for exploring floral organ evolution in leguminous plants.