COMPLEMENTABLE AND NONCOMPLEMENTABLE HOST ADAPTATION DEFECTS IN BIPARTITE GEMINIVIRUSES

Members of the geminivirus group of plant viruses collectively infect a broad spectrum of species. Individual viruses which are genetically very similar may nevertheless have distinct host ranges. Two such gemi niviruses are tomato golden mosaic virus (TGMV) and bean golden mosaic Virus (BGMV), for which common hosts have not previously been reporte d. Each virus has two genome components, designated A and B. The A com ponent is capable of autonomous replication and encapsidation, whereas the B component provides viral functions required for the spread of i nfection in plants. To investigate the basis for the distinctive host ranges of BGMV and TGMV, we have introduced plasmids containing cloned viral genome components into Nicotiana bemthamiana, a good host for T GMV, and bean, Phaseolus vulgaris, a good host for BGMV. We found that TGMV has a low specific infectivity for bean and is virulent, whereas BGMV has a high specific infectivity for N. benthamiana, but infectio ns are asymptomatic and viral DNA accumulation is low. To investigate which Viral functions were defective in the poor host in each case, we attempted to complement them by co-inoculation with the well-adapted virus. After inoculation of beans with both viruses, only BGMV was det ected. Thus, TGMV exhibits a noncomplementable host adaptation defect in beans. This suggests that the defect has a cis-acting or virus-spec ific trans-acting genetic basis. In contrast, the BGMV phenotype of lo w DNA accumulation in N. benthamiana was partially complemented by TGM V A alone and complemented further by the complete TGMV genome. This s uggests that a virus nonspecific, trans-acting factor encoded by the B GMV A component is poorly adapted to N. benthamiana. The results of th is study indicate that bipartite geminivirus host range may be limited by defective virus-host interactions of more than one kind. (C) 1995 Academic Press, Inc.