Keel Petal Fusion in Soybean: Anatomical Insights and Transcriptomic Identification of Candidate Regulators

The fusion of keel petals is a defining trait of Papilionoideae flowers, contributing to floral architecture and promoting self-pollination but hindering hybridization in crops like soybean. Here, we investigated the cellular and molecular basis of keel petal fusion in Glycine max (L.) Merr. cv. Jack using anatomical and transcriptomic approaches. Microscopy revealed that keel petal fusion involves marginal cell reshaping and postgenital adhesion with defective cuticle continuity, consistent with fusion modes in other Papilionoideae species. Comparative transcriptome analysis between fused and unfused petal stages identified 23,328 differentially expressed genes, with lipid and cuticle metabolism genes showing coordinated downregulation during fusion. A set of 384 keel-enriched genes was identified, among which a previously uncharacterized gene, KPEG1 (Keel Preferential Expression Gene 1), was preferentially expressed in fused keel petals. Protein interaction network analysis revealed that KPEG1 co-expresses with epigenetics-related genes, suggesting a regulatory role in fusion through chromatin-mediated mechanisms. These findings uncover the cellular dynamics and transcriptional reprogramming underlying keel petal fusion in soybean and provide a candidate regulator for further functional studies.