Soybean receptor-like protein kinase genes: paralogous divergence of a gene family

Receptor-like protein kinases (RLKs) in plants play major roles in cellular processes and stress responses. Three soybean (Glycine max) orthologs of Arabidopsis thaliana RLK were isolated and designated GmRLK1, GmRLK2, and GmRLK3. GmRLK1, GmRLK2, and GmRLK3 are similar in sequence, with GmRLK2 and GmRLK3 being nearly identical. The deduced amino acid sequences of GmRLK1, GmRLK2, and GmRLK3 possess characteristics of a transmembrane leucine-rich repeat RLK, AtCLV1. DNA fingerprinting and PCR analyses of a bacterial artificial chromosome library identified five GmRLK contigs (I-V): three for GmRLK1 (I, II, and V), one for GmRLK2 (III), and one for both GmRLK2 and GmRLK3 (IV). Phylogenetic analysis of the soybean RLKs together with other plant RLKs indicates that soybean and A. thaliana CLV1s generate a CLV1 branch, while soybean, A. thaliana, and rice RLKs generate an RLK branch. Thus, the AtCLV1 orthologs may have evolved later than the other pathogen-, environmental stress-, plant hormone-, and development-associated RLKs. A common ancestral GmRLK gene may have duplicated to give rise to GmRLK1, GmRLK2, and GmRLK3, or GmRLK2 and GmRLK3 may have resulted from a recent duplication event(s). Several amino acid replacements in the kinase domain of GmRLK1 compared with those of GmRLK2 and GmRLK3 may reflect evolutionary divergence of individual family members.