Legume nodulation: successful symbiosis through short- and long-distance signalling

Nodulation in legumes provides a major conduit of available nitrogen into the biosphere. The development of nitrogen-fixing nodules results from a symbiotic interaction between soil bacteria, commonly called rhizobia, and legume plants. Molecular genetic analysis in both model and agriculturally important legume species has resulted in the identification of a variety of genes that are essential for the establishment, maintenance and regulation of this symbiosis. Autoregulation of nodulation (AON) is a major internal process by which nodule numbers are controlled through prior nodulation events. Characterisation of AON-deficient mutants has revealed a novel systemic signal transduction pathway controlled by a receptor-like kinase. This review reports our present level of understanding on the short- and long-distance signalling networks controlling early nodulation events and AON.

Short root mutant of Lotus japonicus with a dramatically altered symbiotic phenotype

Legume plants carefully control the extent of nodulation in response to rhizobial infection. To examine the mechanism underlying this process we conducted a detailed analysis of the Lotus japonicus hypernodulating mutants, har1-1, 2 and 3 that define a new locus, HYPERNODULATION ABERRANT ROOT FORMATION (Har1), involved in root and symbiotic development. Mutations in the Har1 locus alter root architecture by inhibiting root elongation, diminishing root diameter and stimulating lateral root initiation. At the cellular level these developmental alterations are associated with changes in the position and duration of root cell growth and result in a premature differentiation of har1-1 mutant root. No significant differences between har1-1 mutant and wild-type plants were detected with respect to root growth responses to 1-aminocyclopropane1-carboxylic acid, the immediate precursor of ethylene, and auxin; however, cytokinin in the presence of AVG (aminoetoxyvinylglycine) was found to stimulate root elongation of the har1-1 mutant but not the wild-type. After inoculation with Mesorhizobium loti, the har1 mutant lines display an unusual hypernodulation (HNR) response, characterized by unrestricted nodulation (hypernodulation), and a concomitant drastic inhibition of root and shoot growth. These observations implicate a role for the Har1 locus in both symbiotic and non-symbiotic development of L. japonicus, and suggest that regulatory processes controlling nodule organogenesis and nodule number are integrated in an overall mechanism governing root growth and development.