Lotus japonicus Cytokinin Receptors Work Partially Redundantly to Mediate Nodule Formation

Previous analysis of the Lotus histidine kinase1(Lhk1) cytokinin receptor gene has shown that it is required andalso sufficient for nodule formation in Lotus japonicus. TheL. japonicus mutant carrying the loss-of-functionlhk1-1 allele is hyperinfected by its symbiotic partner,Mesorhizobium loti, in the initial absence of noduleorganogenesis. At a later time point following bacterial infection,lhk1-1 develops a limited number of nodules, suggesting thepresence of an Lhk1-independent mechanism. We have tested ahypothesis that other cytokinin receptors function in at least a partially redundantmanner with LHK1 to mediate nodule organogenesis in L. japonicus. Weshow here that L. japonicus contains a small family of fourcytokinin receptor genes, which all respond to M. loti infection. Weshow that within the root cortex, LHK1 performs an essential role but also workspartially redundantly with LHK1A and LHK3 to mediate cell divisions for noduleprimordium formation. The LHK1 receptor is also presumed to partake in mediating afeedback mechanism that negatively regulates bacterial infections at the rootepidermis. Interestingly, the Arabidopsis thaliana AHK4 receptorgene can functionally replace Lhk1 in mediating noduleorganogenesis, indicating that the ability to perform this developmental process isnot determined by unique, legume-specific properties of LHK1.