Geminivirus C4 proteins inhibit GA signaling via prevention of NbGAI degradation, to promote viral infection and symptom development in N. benthamiana

Phytohormone gibberellin (GA) is an important plant signaling molecule that regulates plant growth and defense against abiotic and biotic stresses. To date, the molecular mechanism of the plant responses to viral infection mediated by GA is still undetermined. DELLA is a repressor of GA signaling and is recognized by the F-box protein, a component of the SCFSLY1/GID2 complex. The recognized DELLA is degraded by the ubiquitin-26S proteasome, leading to the activation of the GA signaling. Here, we report that ageratum leaf curl Sichuan virus (ALCScV)-infected N. benthamiana plants showed dwarfing symptom and abnormal flower development. The infection of ALCScV alters the expressions of GA pathway-related genes and decreases the content of endogenous GA significantly in N. benthamiana. Further, ALCScV-encoded C4 protein interacts with the DELLA protein NbGAI, and interferes with the interaction between NbGAI and NbGID2 to prevent the degradation of NbGAI, leading to the inhibition of the GA signaling pathway. Silencing of NbGAI or exogenous GA3 treatment significantly reduces viral accumulation and disease symptoms in N. benthamiana plants. The same results were proved by the experiments with C4 protein encoded by tobacco curly shoot virus (TbCSV). Therefore, we propose a novel mechanism of geminivirus C4 proteins controling virus infection and disease symptom development through interfering GA signaling pathway. Author SummaryGibberellins (GAs) are plant hormones that are essential for many developmental processes in plants. It has indicated that plant virus infection can induce abnormal flower development and influence GA pathway resulting the plant dwarfing symptom, but the underlying mechanisms is still not well described. Here, we demonstrate that geminivirus - encoded C4 protein regulates the GA signaling pathway to promote viral accumulation and disease symptom development. Through directly interacting with NbGAI, the C4 protein interferes with the interaction between NbGAI and NbGID2, which inhibits the degradation of NbGAI. As a result, the GA signaling pathway is blocked, and the infected plants display symptoms of typical dwarfing and delayed flowering. Our results reveal a novel mechanism by which geminivirus C4 proteins influence viral pathogenicity via interfering the GA signaling pathway, and provide new insights into the interaction between virus and host.