Differential response of diverse solanaceous hosts to tomato leaf curl New Delhi virus infection indicates coordinated action of NBS-LRR and RNAi-mediated host defense

Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite begomovirus (family Geminiviridae) that infects a wide range of plants. ToLCNDV has emerged as an important pathogen and a serious threat to tomato production in India. A comparative and molecular analysis of ToLCNDV pathogenesis was performed on diverse solanaceous hosts (Capsicum annuum, Nicotiana benthamiana, N. tabacum, and Solanum lycopersicum). N. benthamiana was found to be the most susceptible host, whereas C. annuum showed resistance against an isolate of ToLCNDV collected in New Delhi from tomato (GenBank accession no. U15015 and U15017). S. lycopersicum and N. tabacum developed conspicuous symptoms and allowed virus to accumulate to significantly high titers. The viral DNA level was concurrent with symptom severity. ToLCNDV-specific siRNA levels were directly proportional to the amount of viral DNA. To investigate the basis for the differences in response of these hosts to ToLCNDV, a comparative expression analysis of selected defense-related genes was carried out. The results indicated differences in expression levels of genes involved in the posttranscriptional gene silencing machinery (RDR6, AGO1 and SGS3) as well as basal host defense responses (nucleotide-binding site and leucine-rich repeat [NBS-LRR] proteins and lipid transfer protein [LTP]). Among these, expression of NBS-LRR genes was found to be significantly higher in C. annuum following ToLCNDV infection. Our analyses suggest that the expression of host defense responses determines the level of ToLCNDV accumulation and degree of symptom development.