Molecular characterization of two bipartite geminiviruses causing squash leaf curl disease: role of viral replication...

The genomes of two distinct, but highly homologous, bipartite geminiviruses have been identified in and cloned from extracts of squash leaf curl diseased field squash. These two squash leaf curl viruses (SqLCVs) have covalently closed, circular single-stranded DNA genomes with the same bipartite component organization characteristic of other whitefly-transmitted geminiviruses. Infectivity studies using virus preparations or cloned viral genomic components on different potential host plants demonstrated that these two SqLCVs have different host range phenotypes which can be explained by specific interactions among the different viral genomic components that act to influence viral replication and systemic movement in the plant. Analysis of Agrobacterium-inoculated leaf discs demonstrated that replication of the restricted virus was rescued in trans by the nonrestricted virus, providing an explanation for the mixtures of viral DNA components often found in particular hosts in the field. Sequence analysis of the common regions of these two SqLCVs identified a 13-base deletion in the restricted virus as compared to the nonrestricted virus, suggesting a potential sequence alteration likely to be involved in their host range phenotypic differences and strengthening the conclusion based on hybridization studies of their close evolutionary relationship. Also identified in the original field squash was a defective viral component which appeared to interfere with movement of the restricted SqLCV in its normally permissive hosts and accounted for another aspect of host range variation observed for this virus.