Genetic Determinants of Host-Specificity in Bipartite Geminivirus DNA A Components

Geminiviruses are small, ssDNA-containing plant viruses. Bean golden mosaic virus (BGMV) and tomato golden mosaic virus (TGMV) have bipartite genomes, the components of which are designated A and B. Although they are closely related, BGMV and TGMV nevertheless exhibit distinct host-specific phenotypes, with BGMV being well adapted to beans and TGMV being well adapted to Nicotiana benthamiana.A previous study showed that the two open reading frames (ORFs) of DNA B only partially determine the host-adapted phenotypes of BGMV and TGMV. We have now investigated the contributions of A component ORFs to host adaptation. Co-inoculated TGMV DNA A enhances the accumulation of BGMV in N. benthamiana.Using mutant and hybrid TGMV A components, the determinant of this phenotype was mapped to a region encompassing the overlapping AL2and AL3ORFs ( AL23). BGMV- and TGMV-based hybrid A components containing the heterologous AL23region each displayed host-specific gain-of-function phenotypes, which indicates that these sequences contribute to host adaptation in both viruses. In N. benthamiana, al2and al3mutants of either virus can be complemented in transby the heterologous A component, so adaptation of the AL23region to this host is likely mediated through a virus nonspecific, trans-acting factor. In beans, however, co-inoculated BGMV A does not affect the accumulation of TGMV, and TGMV did not complement BGMV al2or al3mutants. Thus host-adaptation of the AL23region may have a different mechanistic basis in beans than it does in N. benthamiana.Although our experiments did not reveal significant host adaptation of the coat protein, which is encoded by the AR1ORF, a virus-specific effect on viral ssDNA accumulation was observed.