Disease resistance signature of the leucine-rich repeat receptor-like kinase genes in four plant species

Receptor-like kinase (RLK) genes may be a potential reservoir for plant disease resistance (R) genes as defense response has been proved to be a main category of their biological functions. Although genome-wide identification of RLKs has been accomplished in various plant species, little was done to distinguish the genes related to plant disease resistance. To discover more R gene candidates in plants, we explored all the leucine-rich repeat (LRR) RLKs in four fully sequenced genomes, Arabidopsis thaliana, rice, poplar and grapevine. Phylogenetic analysis showed that LRR-RLKs in these four species could be mainly divided into two opposite groups: lineage-specific Expanded vs. ortholog-unambiguous Nonexpanded. About 39.2% of the total LRR-RLKs residing in 16 major clades belonged to the Expanded group, exhibiting similar characteristics with respect to typical functional R genes. A high proportion (72%) of these genes was located in tandem duplications, indicating that lineage-independent expansion played an essential role in their evolution. Contrarily, genes in the Nonexpanded group tended to function in plant growth and development and basal defense responses. Positive selection driven by highly variable pathogen effectors and gene expansion should be two major factors making the differences between the two groups of LRR-RLKs.