Isolation and characterization of NBS–LRR resistance gene analogues from mango

The nucleotide-binding site (NBS)-leucine-rich repeat (LRR) gene family is a class of R genes in plants. NBS genes play a very important role in disease defence. To further study the variation and homology of mango NBS-LRR genes, 16 resistance gene analogues (RGAs) (GenBank accession number HM446507-22) were isolated from the polymerase chain reaction fragments and sequenced by using two degenerate primer sets. The total nucleotide diversity index Pi was 0.362, and 236 variation sites were found among 16 RGAs. The degree of homology between the RGAs varied from 44.4% to 98.5%. Sixteen RGAs could be translated into amino sequences. The high level of this homology in the protein sequences of the P-loop and kinase-2 of the NBS domain between the RGAs isolated in this study and previously characterized R genes indicated that these cloned sequences belonged to the NBS-LRR gene family. Moreover, these 16 RGAs could be classified into the non-TIR-NBS-LRR gene family because only tryptophan (W) could be claimed as the final residual of the kinase-2 domain of all RGAs isolated here. From our results, we concluded that our mango NBS-LRR genes possessed a high level of variation from the mango genome, which may allow mango to recognize many different pathogenic virulence factors.