Functional Scanning of Apple Geminivirus Proteins as Symptom Determinants and Suppressors of Posttranscriptional Gene Silencing

Apple geminivirus (AGV) is a recently identified geminivirus which is isolated from the apple tree in China. We carried out functional scanning of apple geminivirus proteins as symptom determinants and suppressors of posttranscriptional gene silencing (PTGS). Our results indicated that AGV V2 is an important virulence factor localized to the nucleus and cytoplasm that suppresses PTGS and induces severe symptoms of crinkling and necrosis. AGV C1 is also a virulence determinant which elicits systemic necrosis when expressed from a PVX-based vector. The AGV C4 is targeted to cytoplasm, plasma membrane, nucleus, and chloroplasts. The inoculation of PVX-C4 on N. benthamiana induced severe upward leaf curling, which implied that AGV C4 also functions as a symptom determinant, and mutation analyses suggested that the acylated residues on Gly2 and Cys8 play important roles in its subcellular localization and symptom development.