Integrative transcriptomic and metabolomic analyses reveal the mechanisms underlying calcium-induced increase in female flower production in Cucumis sativus L.

Cucumber sexual differentiation is regulated by multiple factors, including genetic background, environmental conditions, and hormone levels. Previous studies have indicated that Ca2+ is involved in cucumber sexual differentiation, but the precise regulatory role of the Ca2+ signaling pathway remains unclear. In this study, we applied CaCl2 to the shoot apices of androecious line of cucumber (Banna 31-10), at different development stages. Transcriptomic and metabolomic analyses were conducted to identify key factors influencing sexual differentiation. A significant increase in the number of female flowers following CaCl2 treatment (p < 0.05) was observed. The identified differentially expressed genes and differentially accumulated metabolites were primarily enriched in metabolic pathways, biosynthesis of secondary metabolites, and plant hormone signal transduction. The response of cucumber to Ca2+ mainly involved the plant hormone signaling pathway, including abscisic acid, gibberellin, Indole-3-acetic acid, ethylene, cytokinin, jasmonic acid, and salicylic acid. Notably, the key sex-related gene CsACS2 (M) was significantly upregulated in the Ca-3W group. The regulation of sexual differentiation was associated with transcription factors such as AP2-ERF, bHLH, MYB, C2H2, and MADS. Additionally, we identified 11 calmodulin-like (CML) genes that interacted with the biosynthesis and signaling pathways. These genes exhibited a significant positive correlation with genes involved in female flower development (p < 0.05, cor > 0.9). This study provides new insights into the role of the Ca2+ signaling system in cucumber sexual differentiation and has important implications for improving cucumber yield and conserving high-quality germplasm resources.