Supervirulent pseudorecombination and asymmetric synergism between genomic components of two distinct species of begomovirus associated with severe tomato leaf curl disease in India.

Isolates of two distinct begomovirus species, the severe strain of the species Tomato leaf curl New Delhi virus (tomato leaf curl New Delhi virus-[India:New Delhi:Severe:1992]; ToLCNDV-[IN:ND:Svr:92], bipartite) and the Varanasi strain of the species Tomato leaf curl Gujarat virus (tomato leaf curl Gujarat virus-[India:Varanasi:2001]; ToLCGV-[IN:Var:01], mono/bipartite) infect tomato (Lycopersicon esculentum) and cause severe yield losses in northern India. This study investigated the infectivity properties of genomic components of these two species. Both pseudorecombinants were infectious in Nicotiana benthamiana, Nicotiana tabacum and L. esculentum. Enhanced pathogenicity was observed when DNA-A of ToLCNDV-[IN:ND:Svr:92] was trans-complemented with ToLCGV-[IN:Var:01] DNA-B, and was consistently associated with an increase in accumulation of ToLCGV-[IN:Var:01] DNA-B. Mixed infection of ToLCNDV-[IN:ND:Svr:92] and ToLCGV-[IN:Var:01] always showed extremely severe symptoms, suggesting a synergistic interaction between these two viruses. Southern blot analysis of viral DNAs from infected plants showed a significantly higher level of accumulation of both ToLCNDV components and DNA-B of ToLCGV-[IN:Var:01] with no alteration to levels of DNA-A of ToLCGV-[IN:Var:01]. Symptom development and/or higher infectivity of the supervirulent pseudorecombinants correlated with the increased levels of DNA-B accumulation. Protoplast replication assays revealed that enhanced infectivity by the pseudorecombinant occurred at the level of replication, as DNA-A of ToLCNDV-[IN:ND:Svr:92] enhanced ToLCGV-[IN:Var:01] DNA-B replication, whose accumulation was in turn increased by ToLCGV-[IN:Var:01] DNA-A. This is the first report demonstrating a virulent pseudorecombinant between two distinct species of begomoviruses that infect tomato, and is the second report on synergism between begomoviruses. The results revealed that ToLCGV-[IN:Var:01] DNA-B is capable of associating with different DNA-A components, despite having different iteron sequences.