Nod factor‐induced host responses and mechanisms of Nod factor perception

SUMMARY Soil bacteria belonging to the genera Rhizobium, Bradyrhizobium and Azorhizobium are able to invade the roots of their leguminous host plants, where they trigger the formation of a new organ, the root nodule. At least two steps of signal exchange between the bacterium and the host plant are involved in starting the interaction. In the first step, flavonoids excreted by the plant induce the transcription of bacterial nodulation genes (nod genes). The induction of these nod genes leads to the synthesis of specific lipooligosaccharides (called Nod factors) that can induce various root responses, e.g. root hair deformation, depolarization of the root hair membrane potential, induction of nodulin gene expression, and formation of nodule primordia. We will focus on the various nod factor‐induced plant responses. We will discuss the ways these responses can be used to improve our knowledge of the mechanism of Nod factor perception. Furthermore, plant genes, in particular the sym2 allele from Afghanistan pea, encoding proteins that probably are involved in Nod factor recognition, are discussed. Based on the Nod factor‐induced plant responses and the phenotype displayed by sym2 peas, a model will be presented of how Nod factors may be perceived.