C4 Protein of Regulates Brassinosteroid Signaling Pathway through Interaction with AtBIN2 and Affects Male Fertility in .

Sweepoviruses have been identified globally and cause substantial yield losses and cultivar decline in sweet potato. This study aimed to investigate the interaction between sweepovirus and plant host by analyzing the function of the viral protein C4 of -Jiangsu (SPLCV-JS), a sweepovirus cloned from diseased sweet potato plants in East China. Ectopic expression of the C4 in altered plant development drastically with phenotypic changes including leaf curling, seedling twisting, deformation of floral tissues and reduction of pollen fertility, and seed number. Using bimolecular fluorescence complementation analysis, this study demonstrated that the SPLCV-JS C4 protein interacted with brassinosteroid-insensitive 2 (AtBIN2) in the plasma membrane of cells. The C4 AtBIN2 interaction was further confirmed by yeast two-hybrid assays. This interaction led to the re-localization of AtBIN2-interacting proteins AtBES1/AtBZR1 into the nucleus which altered the expression of brassinosteroid (BR)-response genes, resulting in the activation of BR-signaling pathway. The interaction of SPLCV-JS C4 and AtBIN2 also led to the down-regulated expression of key genes involved in anther and pollen development, including , and , which caused abnormal tapetal development, followed by defective exine pattern formation of microspores and pollen release. Consequently, male fertility in the transgenic was reduced. The present study illustrated how the sweepovirus C4 protein functioned in host cells and affected male fertility by interacting with the key components of BR-signaling pathway.