| Plant receptor-like kinases (RLKs) are transmembrane proteins with an N-terminal ligand-binding region and a C-terminal kinase domain. RLKs are involved in a myriad of responses and perceive extracellular stimuli on the cell surface. To understand the functions of RLKs, the first RLK cloned from Arabidopsis, TMK1, was used for detailed analysis. TMK1 is a glycosylated protein that is localized to the plasma membrane. The expression of TMK1 protein is nearly ubiquitous within Arabidopsis. A transgenic line showing TMK1 cosuppression was isolated. However, there is no discernible phenotype when the TMK1 cosupressor was compared to wildtype. Subsequently, an additional three genes with high sequence similarity, identical gene structure, and nearly identical domain organization were found in the Arabidopsis genome, suggesting that functional redundancy may be responsible for the lack of phenotypes in the TMK1 cosuppresor. To understand the diversity and evolution of RLKs in Arabidopsis, a genome-wide analysis was conducted. In Arabidopsis alone, more than 1033 kinases are found. Among these 1033 genes, phylogenetic analysis indicates that 610 genes belong to a single gene family, the RLK family, including more than 400 RLKs and kinases without transmembrane regions. Interestingly, the RLK homologs in animal genomes are the Pelle kinases responsible for both animal development and innate immune responses. Moreover, a total of merely 5 Pelle kinases are found in the completed genomes of worm, fly, and human, suggesting that the RLK gene family may have undergone large-scale expansion in the plant lineage. This notion is supported by an analysis of available ESTs for RLK homologs. Two of the major mechanisms for the expansion of the RLK family are tandem duplication and polyploidization of Arabidopsis genome. The creation of RLKs likely involved exon shuffling since a strong phase bias for the presence of a phase 1 intron is found at the junction of signal sequence, N-terminal domain, transmembrane regions, and the C-termin kinase in more than 200 RLKs. |
