Identification and characterization of plant resistance genes (R genes) in sorghum and their involvement in plant defense against aphids

Plants are constantly exposed to numerous biotic stresses; thus, they have evolved to defend themselves from their enemies such as pathogen or insect pests. Plant resistance (R) genes encoded by nucleotide binding site-leucine rich repeat (NBS-LRR) or LRR-containing transmembrane-receptor proteins play important roles in plant defense against various biotic stresses. In this paper, we report the identification of 308 R genes, including NBS-LRR, 175 genes encoding receptor-like kinases (RLKs) and 65 genes coding for receptor-like proteins (RLPs) in sorghum [Sorghum bicolor (L.) Moench] through genome-wide sequence analysis. Those genes were dispersed across all ten chromosomes of sorghum. Phylogenetic analysis of the newly identified NBS-LRR, RLP and RLK genes classified them into five, eleven, and six clades, respectively, of which region-specific subclades were established. Investigation of exon/intron organization demonstrates the variations in number and location of introns within the family, and the genes that evolved close to their originals share identical gene structures. Also, gene duplication was noted, including 142 pairs of duplicated genes in the NBS family, 12 RLK genes and one RLP gene, respectively. Chromosomal locations analysis of these newly-identified R genes and 17 published QTLs indicated two hot-spot regions referring resistance to at least two biotic stresses. Among those R genes, some of them showed significantly differential expression during the infestation by greenbug biotype I based on the expression profile. Particularly, three R genes were validated by quantitative real time PCR for their up-regulation in response to aphids. In summary, the results of this study contribute not only a valuable genetic resource for further functional characterization of those newly-identified R genes, but also provide insight into genetic improvement of sorghum for resistance to greenbug aphids.