Resistance gene analogs in walnut (Juglans regia) conferring resistance to Colletotrichum gloeosporioides

Walnut (Juglans regia L.) resistance gene analogs (RGAs) of 35 nucleotide binding site RGAs (jrRGAPGs), 47 leucine-rich repeat RGAs (jrRGANLs), and 45 serine/threonine kinase RGAs (jrRGAPTs) conferring resistance to the pathogen Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. were isolated from the resistant cultivar ‘Qing Lin’, the susceptible cultivar ‘Yuan Lin’, and their F1 hybrids using a polymerase chain reaction-based strategy. The jrRGAPGs and jrRGAPTs occurred only in ‘Qing Lin’, whereas the jrRGANLs were found in both parent cultivars. In 85 F1 hybrid progeny, the jrRGAPGs were found only in resistant individuals, while the jrRGANLs and jrRGAPTs were present more frequently in resistant individuals than in susceptible ones. The jrRGAPGs were highly homologous to nucleotide binding site genes from other species. Multiple alignments revealed that jrRGAPGs had P-loop, kinase-2, kinase-3, and hydrophobic GLPL motifs. Phylogenetic analysis resolved the inferred jrRGAPG amino acid sequences into distinct TIR and non-TIR clades. The jrRGANLs had no matches in GenBank and were classified into multiple subfamilies in the phylogeny. The jrRGAPTs were similar to known kinase receptor/regulators and formed two major phylogenetic clades. These results will facilitate molecular breeding strategies for walnut anthracnose resistance.