Isolation and Characterization of NBS-Encoding Disease Resistance Gene Analogs in Watermelon against Fusarium Wilt

Fusarium wilt (Fusarium oxysporum f. sp. niveum) in watermelon is one of the deadliest diseases around the globe, and availability of disease-resistant varieties is moderate. Disease management utilizing resistance genes (R-genes)/resistance gene analogs (RGAs) has proven to be a promising and successful approach. In the present study, six watermelon RGAs were isolated from wild, fusarium wilt resistant genotype IIHR-82 (Citrullus lanatus var. citroides) using degenerate primers that identify nucleotide binding site-leucine-rich repeat (NBS–LRR) regions. Multiple sequence alignment of these RGAs identified the characteristic NBS– LRR motif, and BLASTp search revealed similarity of these RGAs with other pathogenesis-related proteins. Phylogeny and motif analysis revealed genetic diversity of RGAs within those isolated from watermelon and with other plant R-genes. The watermelon RGAs isolated in this study contained both TIR–NBS–LRR (TNL) and non-TIR–NBS–LRR (CNL) classes of Rgenes. Protein secondary structure prediction of these watermelon RGAs revealed the composition of proteins, including α -helix, β -strand, disordered region and other template-related information. Watermelon RGAs identified in the present study will help in the development of RGA-based markers for resistance to fusarium wilt of watermelon.