Functional characterization of CsSTK in floral organ development and its positive regulation by CsBPC5 in tea plants.

Background: As an important economic crop, the development of floral organs in tea plants plays an essential role in tea yield, quality, and varietal improvement. Previous studies have shown significant downregulation of the CsSTK gene in the pistil-deficient mutant 'Yuncha Qirui' (CRQS) during flower development, suggesting its involvement in floral organ and pistil development.

Results: This study investigated CsSTK function and regulatory mechanisms, concurrently conducting a systematic identification of the BPC transcription factor family members in tea plants. The results showed that CsSTK is specifically expressed in tea plant flowers (particularly in pistils) and fruit tissues. Notably, its expression level in CRQS flower buds was less than 1% of that in normal tea plants, whereas it was significantly upregulated in the sterile-flower mutant 'Yuncha Yirui' (ZDH), which exhibits abnormal fused carpels. Heterologous overexpression of CsSTK was found to promote early flowering in Arabidopsis, indicating its involvement in flowering time regulation. Additionally, genome-wide analysis identified 12 BPC family members in tea plants, which were divided into three subclasses (Class Ⅰ, Ⅱ, and Ⅲ). Except for CsBPC6.4, all other CsBPCs showed higher expression levels in normal flower buds compared to fully bloomed flowers, consistent with the CsSTK expression pattern. Further studies demonstrated that CsBPC5 positively regulates CsSTK expression.

Conclusions: This study elucidates the key regulatory role of CsSTK in tea plant floral organ development and explores the molecular mechanism underlying the CsBPC5-CsSTK regulatory module. These findings provide important theoretical insights into the molecular basis of floral organ development in tea plants.