Regulatory Mechanisms of Symrk Kinase Activity

Abstract

Most land plants develop a symbiosis with arbuscular mycorrhizal fungi to benefit from phosphate uptake, whereas only members of four plant orders, e.g., legumes, interact with rhizobia to form a specialized nitrogen-fixing organ, the root nodule. Genetic analyses revealed that these two different types of symbiosis have common components and mutation of "common symbiosis (SYM) genes" cause defects in both symbiotic processes (Kistner and Parniske, 2002). Several common SYM genes have been identified in the model legumes, Lotus japonicus and Medicago truncatula. SYMRK (symbiosis receptor-like kinase) is a common SYM gene from Lotus japonicus, which is required for an early step of symbiosis. The SYMRK gene encodes an LRR-type receptor-like kinase, featuring a signal peptide, three LRR motifs, a transmembrane domain and a Ser/Thr kinase domain (Stracke et al., 2002). SYMRK orthologues were identified from Medicago truncatula (DMI2), Medicago sativa (NORK) and Pisum sativum (SYM19), indicating that SYMRK is evolutionarily conserved among legume species (Stracke et al., 2002; Endre et al., 2002). To understand how SYMRK regulates symbiosis signal transduction, we analysed the mutation points of plant SYMRK mutants. Several mutations were found in the conserved kinase domain, and assigned to conserved crucial residues for kinase activity. Therefore, we assumed that kinase activity is required for SYMRK function. To test this hypothesis, we introduced the same mutation into E. coli expression clones and analysed kinase activity in vitro. As expected, we observed a correlation between kinase activity and symbiosis phenotype, suggesting the kinase activity is important for SYMRK symbiosis function. SYMRK kinase domain autophosphorylates in an intermolecular manner. Intermolecular phosphorylation is important to activate animal receptor tyrosine kinases, therefore, the intermolecular autophosphorylation of SYMRK may be a first step for activation of Y-P. Wang et al, (eds.), Biological Nitrogen Fixation, Sustainable Agriculture and the Environment, 183-185.