Surface polysaccharides are important for bacterial interactions with multicellular organisms, and some are virulence factors in pathogens. In the legume–rhizobium symbiosis, bacterial exopolysaccharides (EPS) are essential for the development of infected root nodules. We have identified a gene in Lotus japonicus, Epr3, encoding a receptor-like kinase that controls this infection. We show that epr3 mutants are defective in perception of purified EPS, and that EPR3 binds EPS directly and distinguishes compatible and incompatible EPS in bacterial competition studies. Expression of Epr3 in epidermal cells within the susceptible root zone shows that the protein is involved in bacterial entry, while rhizobial and plant mutant studies suggest that Epr3 regulates bacterial passage through the plant’s epidermal cell layer. Finally, we show that Epr3 expression is inducible and dependent on host perception of bacterial nodulation (Nod) factors. Plant–bacterial compatibility and bacterial access to legume roots is thus regulated by a two-stage mechanism involving sequential receptor-mediated recognition of Nod factor and EPS signals. This paper describes the discovery of the exopolysaccharide receptor (Epr3) in plants, and shows that its expression is induced upon perception of the bacterial Nod factors; the EPR3 receptor recognizes exopolysaccharides on the surface of rhizobia, thus controlling the symbiotic infection of the roots of legumes. Rhizobium bacteria infect the roots of legumes, where they induce the formation of nitrogen-fixing root nodules. This symbiotic relationship is of agricultural importance as it reduces the need for nitrogen fertilizers. But how do legumes recognize these beneficial partners among thousands of incompatible soil bacteria they encounter? It is known that exopolysaccharides on the surface of bacteria are important for interactions of these microorganisms with multicellular organisms and here Jens Stougaard and coworkers identify an exopolysaccharide receptor (EPR3) that mediates recognition of rhizobia in the wild legume Lotus japonicus. EPR3 expression is induced upon perception of bacterial signalling molecules known as Nod factors. The receptor recognizes compatible exopolysaccharides, thus controlling the symbiotic infection.

Receptor-mediated exopolysaccharide perception controls bacterial infection