Bipartite Geminivirus Host Adaptation Determined Cooperatively by Coding and Noncoding Sequences of the Genome

Bipartite geminiviruses are small, plant-infecting viruses with genomes composed of circular, single-stranded DNA molecules, designated A and B. Although they are closely related genetically, individual bipartite geminiviruses frequently exhibit host-specific adaptation. Two such viruses are bean golden mosaic virus (BGMV) and tomato golden mosaic virus (TGMV), which are well adapted to common bean (Phaseolus vulgaris) and Nicotiana benthamiana, respectively. In previous studies, partial host adaptation was conferred on BGMV-based or TGMV-based hybrid viruses by separately exchanging open reading frames (ORFs) on DNA A or DNA B. Here we analyzed hybrid viruses in which all of the ORFs on both DNAs were exchanged except for AL1, which encodes a protein with strictly virus-specific activity. These hybrid viruses exhibited partial transfer of host-adapted phenotypes. In contrast, exchange of noncoding regions (NCRs) upstream from the AR1 and BR1 ORFs did not confer any host-specific gain of function on hybrid viruses. However, when the exchangeable ORFs and NCRs from TGMV were combined in a single BGMV-based hybrid virus, complete transfer of TGMV-like adaptation to N. benthamiana was achieved. Interestingly, the reciprocal TGMV-based hybrid virus displayed only partial gain of function in bean. This may be, in part, the result of defective virus-specific interactions between TGMV and BGMV sequences present in the hybrid, although a potential role in adaptation to bean for additional regions of the BGMV genome cannot be ruled out.