Genomic Component Interactions Shape Host-Specific Systemic Infection in Tomato Leaf Curl New Delhi Virus

Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite begomovirus with a broad host range and marked geographic variation in host adaptation. Whereas Asian isolates readily infect solanaceous crops, Mediterranean isolates are mainly adapted to cucurbits and infect tomato poorly. The molecular basis of this divergence remains unclear. Here, we compared Indian (ToLCNDV-IN) and Spanish (ToLCNDV-ES) isolates using reciprocal pseudo-recombinants to infect Solanum lycopersicum and Nicotiana benthamiana. Both DNA-A components replicated in tomato leaves, indicating that the inability of ToLCNDV-ES to infect tomato is not due to defective replication. Pseudo-recombinant analyses revealed that the DNA-B-IN component efficiently supported systemic infection of the heterologous DNA-A-ES, whereas the reciprocal combination established systemic infection only at low frequency. Furthermore, DNA-A from the IN isolate was unable, or only rarely able, to establish systemic infection in tomato in the absence of DNA-B, demonstrating a strict requirement for DNA-B-mediated movement functions. In N. benthamiana, all viral combinations systemically infected plants. Although both DNA-A components were capable of systemic infection without DNA-B in this host, viral accumulation was significantly reduced compared with bipartite infections, indicating that DNA-B enhances infection efficiency but is not essential for long-distance movement. Unexpectedly, a coat protein (CP)-mutated version of ToLCNDV-ES failed to establish systemic infection in N. benthamiana and Cucumis melo despite normal replication, revealing an essential role for CP in systemic movement. Collectively, our findings identify DNA-B as a major determinant of tomato infectivity and reveal an essential role for CP in systemic movement of the Mediterranean ToLCNDV isolate.