VvWUS increases carpel number by upregulating VvAG2 expression in grapevine.

Key message: The positive effect of VvWUS on carpel number variation in Vitis vinifera 'Xiangfei' is accomplished via forming the heterodimer with VvSTMa and VvSTMb proteins, which enhances the downstream VvAG2 gene expression during grapevine carpel formation. The WUSCHEL-CLAVATA3 signalling pathway plays an essential role in the shoot apical meristem (SAM) maintenance and floral organ development, influencing carpel number and fruit morphology in horticultural plants. This study investigates the role of VvWUS in regulating carpel number in the grapevine cultivar 'Xiangfei', which frequently produces flowers with multiple carpels. We found a positive correlation between VvWUS expression levels and carpel number variation among grapevine cultivars. Transgenic tomato plants overexpressing VvWUS exhibited an increased carpel number and a concomitant expression of SAM-related genes. Further experiments demonstrated that VvWUS interacts with VvSTMa and VvSTMb, forming heterodimers that are crucial for grapevine carpel development. VvWUS was also confirmed to activate VvAG2 expression. Our findings indicate that VvWUS positively regulates carpel number in grapevine by interacting with VvSTMa and VvSTMb and upregulating VvAG2 expression. These results provide a basis for molecular breeding aimed at increasing grape yield and fruit size.