Rhizobia are microbes that exploit host plants as a nutritional source but cause little or no host damage. They may provide, through biological nitrogen fixation, a valuable source of nitrogen for plant growth. Different rhizobia nodulate a limited range of plants. In this review we will show that host range specificity is determined by the success or otherwise of communication events between the interacting partners. To infect different plant species, a distinct cocktail of phenolic compounds (flavonoids) is recognised. Flavonoids of the correct structure induce the expression of several bacterial nodulation (nod) and other genes required for plant infection. Flavonoids of the incorrect, but related, structure can antagonise nod gene induction. Some nod genes are responsible for the synthesis of a small family of lipo-oligosaccharides necessary for the triggering of a defined but complex series of morphological responses in the host plant including root hair curling and cortical cell division. Lipo-oligosaccharides are active at concentrations of between 10-8 and 10-12 M. The appropriate lipo-oligosaccharide required for infection of one plant host can have antagonistic effects on other non-host plants and this effect appears to be determined by minor chemical changes to the basic lipo-oligosaccharide structure. Apart from host specificity operating at the genus level, other interdependent nod gene functions determine host specificity at the cultivar level. A complex interplay between positively and negatively acting nod genes and a single host gene affects cultivar specificity in a manner analogous to, but more complex than, the gene-for-gene interactions common amongst plant-pathogen interactions.

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