Lotus japonicus Cytokinin Receptors Work Partially Redundantly to Mediate Nodule Formation

Abstract

Previous analysis of the Lotus histidine kinase1 (Lhk1) cytokinin receptor gene has shown that it is required and also sufficient for nodule formation in Lotus japonicus. The L. japonicus mutant carrying the loss-of-function lhk1-1 allele is hyperinfected by its symbiotic partner, Mesorhizobium loti, in the initial absence of nodule organogenesis. At a later time point following bacterial infection, lhk1-1 develops a limited number of nodules, suggesting the presence of an Lhk1-independent mechanism. We have tested a hypothesis that other cytokinin receptors function in at least a partially redundant manner with LHK1 to mediate nodule organogenesis in L. japonicus. We show here that L. japonicus contains a small family of four cytokinin receptor genes, which all respond to M. loti infection. We show that within the root cortex, LHK1 performs an essential role but also works partially redundantly with LHK1A and LHK3 to mediate cell divisions for nodule primordium formation. The LHK1 receptor is also presumed to partake in mediating a feedback mechanism that negatively regulates bacterial infections at the root epidermis. Interestingly, the Arabidopsis thaliana AHK4 receptor gene can functionally replace Lhk1 in mediating nodule organogenesis, indicating that the ability to perform this developmental process is not determined by unique, legume-specific properties of LHK1.

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

“…This strain formed a significantly higher number of nodules on wild-type roots compared with the reference strain S. meliloti 1021 (Supplemental Figure 1). Under our growth conditions in agar plates, both strains usually did not lead to nodule formation in the cre1 mutant (Supplemental Figure 1), and when nodules formed, this was at a significantly lower level than in the wild type, as reported previously for the cre1 mutant (Plet et al, 2011) and other cytokinin receptor mutants (Murray et al, 2007;Held et al, 2014).…”

ROLE OF PHYTOHORMONES IN THE CONTROL OF SYMBIOTIC NODULE DEVELOPMENT IN LEGUME PLANTS. I. CYTOKININS (review)

“…Rare nodulation in cre1 and lhk1 mutants has been time-shifted and controlled by additional cytokinin receptors (LjLHK1A and LjLHK3 in L. japonicus) [70]. In M. truncatula mutant cre1 plants and in those with RNAi knockdown of cytokinin receptor MtCRE1 the nodulation was disrupted, so that the number of forming nodules reduced sharply [63,71].…”

“…According to the alleged involvement of cytokinins in the regulation of nodule organogenesis and rhizobial infection, LjLHK1 receptor and the cytokinin response regulators are located in the dividing cells of the outer root cortex and in the root hair cells of L. japonicus [62,70]. These genes are expressed in substantially wider zone in emerging nodules, while in mature nodules the expression is limited to the meristem and vascular bundles [62,70].…”

“…These genes are expressed in substantially wider zone in emerging nodules, while in mature nodules the expression is limited to the meristem and vascular bundles [62,70].…”

“…Thus, an additional symrk14 mutation in the double mutant L. japonicus symrk14 lhk1 (recall that SYMRK is a component of signaling pathway activated by Nod factors, and LjLHK1 is a cytokinin receptor) prevented the development of infectious threads and bacterial release from these threads into the root hairs [70]. When lhk1 single mutants were inoculated, some bacteria reached the cells of root cortex and induced sporadic nodules due to additional cytokinin receptors (LjLHK1A, LjLHK3), whereas in double mutants the nodule development was completely blocked [70]. This suggests that the symbiotic events in the epidermis and the activation of cytokinin receptors to in the root cortex are related.…”