Into the Root: How Cytokinin Controls Rhizobial Infection

Leguminous plants selectively initiate primary responses to rhizobial nodulation factors (NF) that ultimately lead to symbiotic root nodule formation. Functioning downstream, cytokinin has emerged as the key endogenous plant signal for nodule differentiation, but its role in mediating rhizobial entry into the root remains obscure. Nonetheless, such a role is suggested by aberrant infection phenotypes of plant mutants with defects in cytokinin signaling. We postulate that cytokinin participates in orchestrating signaling events that promote rhizobial colonization of the root cortex and limit the extent of subsequent infection at the root epidermis, thus maintaining homeostasis of the symbiotic interaction. We further argue that cytokinin signaling must have been crucial during the evolution of plant cell predisposition for rhizobial colonization.

TrendsCytokinin regulates many aspects of plant development, including symbiotic root nodule formation in leguminous plants.The role of cytokinin as the endogenous plant inducer of nodule primordia formation has now been firmly established.Current data implicate cytokinin in the regulation of root colonization by nitrogen-fixing bacteria, but the underlying mechanism remains unclear.Plant mutants with defects in cytokinin signaling (receptors) show aberrant rhizobial infection phenotypes.