| Summary: | 碩士 === 臺灣大學 === 植物病理與微生物學研究所 === 95 === Banana is one of the most important fruit crops around the world. Banana bunchy top disease caused by Banana bunchy top virus (BBTV) is a destructive disease for banana. BBTV is isometric and belongs to the genus Babuvirus, in the family Nanoviridae. BBTV consists of six circular single-stranded integral DNA components, DNA 1-6. In some isolates, however, in additional to the integral components other replication competent components (additional Reps) are also associated with BBTV infection. Currently, 8 different additional Reps, Y, W1, W2, W3, W4, S1, S2, and S3, have been reported. To understand the roles of BBTV additional Reps in BBTV infection, we collected samples from field and using primer pairs specific to each BBTV additional Reps to detect the additional Reps. The samples can be grouped into five groups by the presence of additional Reps in a single infected plant. One isolate were selected from each group and use as source to inoculate healthy bananas by using of banana aphids (Pentalonia nigronervosa). Briefly, BBTV associated with S1, or Y plus S2 induced the most severe symptoms, and BBTV associated with Y induced comparatively slight severe symptoms. In addition, BBTV with no additional Reps association induced less severe symptoms. Phylogenetic analysis of additional Reps ORFs (open reading frames) encoded by Nanoviridae reveled that BBTV additional Reps evolved from a common ancestor and later evolved to two directions and form 6 monoclade. And we find that SRCR (stem-loop right site common region; a specific region conserved among BBTV additional Reps) within ORF evolved with a strong selection pressure; which suggest this region contains an unidentified protein motif. To further study the interaction between integral components and additional Reps, we construct BBTV integral components DNA 1-6 and additional Reps Y and S1. Each infectious clone consists of greater-than-unit (range 1.2 to 1.6 mers). We have demonstrated the replication of BBTV DNA 1 infectious clones in banana suspension cells.
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