ABA Signaling De-Repression Coordinates With Cytokinin to Drive Large Petal Formation in Brassica napus

Petal size is a critical agronomic trait influencing pollination efficiency and yield in field crops. However, the cellular basis and upstream hormonal interplay governing petal size in Brassica napus remain unclear. In this study, utilizing a natural variation population of "Zhongshuang 11", we investigated biological mechanisms underlying petal development through integrated phenotypic screening, paraffin section-based cytological analysis, targeted hormone metabolomics, and transcriptome. The results revealed that the single-flower area of large petals (BEP; 13.0 mm²) was significantly larger than that of wild type (8.5 mm²) and small petals (SEP; 6.8 mm²), a difference primarily attributable to increased petal width. The formation of BEP was due to a more than two-fold increase in cell number per unit area, rather than cell expansion. Furthermore, salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and cytokinins (CK) were significantly enriched in BEP petals, whereas brassinosteroid (BR) content was lowest. Correlation analysis indicated a significant positive correlation between ABA and JA (r = 0.95). Negative regulators of ABA signaling, BnaC09.ABI1 and BnaA01.PP2CA, were significantly downregulated, while core cell cycle genes were highly expressed in BEP. An "ABA de-repression" model was proposed; a hormonal milieu characterized by high concentrations of SA, JA, and CK, coupled with low BR levels, is associated with the downregulation of negative regulators of ABA, potentially alleviating constraints on cell cycle progression to facilitate cell proliferation. This study elucidates the molecular and physiological basis of petal size in rapeseed and provides a theoretical foundation for enhancing outcrossing and yield through floral trait modification.