Studies on genome structure and function of fruit tree viruses

Abstract

In this study, we investigated the complete nucleotide sequence and genome organization of Apple stem grooving virus (ASGV), Apple chlorotic leaf spot virus (ACLSV), Apple stem pitting virus (ASPV), which causes apple topworking disease in Japan, citrus tatter leaf virus, which causes citrus bud-union disorder, Grapevine berry inner necrosis virus (GINV) and Apple latent spherical virus (ALSV) to clarify their taxonomic positions. Sequence analysis of virus isolates also showed that ASGV, ASPV, and ACLSV isolates from fruit trees comprise at least two to four ‘‘sequence variants’’ that differ considerably from each other in nucleotide sequence. We also showed that P50 encoded by the ORF2 of ACLSV genome has multifunctional roles, i.e., it functions as a virus movement protein (MP), it acts as a suppressor of systemic silencing, and it interferes specifically with the functions of the MP encoded by GINV. We constructed infectious cDNA clones of ALSV-RNAs, which were modified into viral vectors for the expression of foreign genes in plants. ALSV vectors effectively induce reliable VIGS of endogenous genes among a broad range of plants including Rosaceae fruit trees. Using the ALSV vector expressing Flowering locus T (AtFT) gene from Arabidopsis thaliana (AtFTALSV), we established a system that promoted flowering time of apple seedlings within 2 months after germination and shortened the generation time from seed germination to next-generation seed maturation to within 7 months when pollen from precocious flowers was used for pollination.