Flavones and isoflavones as inducing substances of legume nodulation.

Rhizobia are soil bacteria that can form symbiotic associations with leguminous plants leading to the fixation of atmospheric nitrogen to ammonia which the plant can use. This is an interaction which involves the exchange of many signals between the plant and the bacterium. To start this interaction, rhizobia have adapted to use flavonoid compounds, released by the plant root, as part of a regulatory system to initiate the transcription of their infection (nodulation, nod) genes. The development of an assay system for the detection of plant-derived stimulatory biofactors has now led to the isolation and identification of the compounds which are responsible for the activation of the nod genes. Stimulatory compounds now have been isolated from plants: from clovers, 7,4'-dihydroxyflavone; from alfalfa, luteolin; from peas, apigenin; and from soybeans, the isoflavones daidzein and genistein. These hydroxylated flavonoid compounds are derived from the phenylpropanoid pathways which are responsible for the synthesis of many important plant phenolic compounds, including the phytoalexin molecules which are thought to be involved in plant defence systems. The current hypothesis on the regulation of the nodulation genes in Rhizobium strains is that the gene product of the regulatory nod gene, nodD, requires the presence of the plant signals to convert it to an active form. This altered NodD protein then induces the expression of the other nodulation genes. This bacterium, induced by plant biofactors, now is able to infect legume root hairs.