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Abstract

Nodule symbiosis with diazotrophic Frankia or rhizobium occurs in plant species belonging to ten taxonomic lineages within the related orders Fabales, Fagales, Cucurbitales, and Rosales. Phylogenomic studies indicate that this nitrogen-fixing nodulation trait has a single evolutionary origin. In legume model plants, the molecular interaction between plant and rhizobium microsymbiont is mapped to a significant degree. A specific LysM-type receptor kinase, LjEPR3 in Lotus japonicus and MtLYK10 in Medicago truncatula, was found to act in a secondary identity-based mechanism, controlling intracellular rhizobium infection. Furthermore, LjEPR3 showed to bind surface exopolysaccharides of Mesorhizobium loti, the diazotrophic microsymbiont of L. japonicus. EPR3 orthologous genes are not unique to legumes. Surprisingly, however, its ortholog EXOPOLYSACCHARIDE RECEPTOR (EPR) is pseudogenized in Parasponia, the only lineage of non-legume plants that nodulate also with rhizobium.