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 AL2 and AL3 ORFs (AL23). BGMV- and TGMV-based hybrid A components containing the heterologous AL23 region each displayed host-specific gain-of-function phenotypes, which indicates that these sequences contribute to host adaptation in both viruses. In N. benthamiana, al2 and al3 mutants of either virus can be complemented in trans by the heterologous A component, so adaptation of the AL23 region 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 al2 or al3 mutants. Thus host-adaptation of the AL23 region 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 AR1 ORF, a virus-specific effect on viral ssDNA accumulation was observed.