Abstract
Flavonoids are crucial signaling molecules in the symbiosis between legumes and their nitrogen-fixing symbionts, the rhizobia. The primary function of flavonoids in the interaction is to induce transcription of the genes for biosynthesis of the rhizobial signaling molecules called Nod factors, which are perceived by the plant to allow symbiotic infection of the root. Many legumes produce specific flavonoids that only induce Nod factor production in homologous rhizobia, and therefore act as important determinants of host range. Despite a wealth of evidence on legume flavonoids, relatively few have proven roles in rhizobial infection. Recent studies suggest that production of key "infection" flavonoids is highly localized at infection sites. Furthermore, some of the flavonoids being produced at infection sites are phytoalexins and may have a role in the selection of compatible symbionts during infection. The molecular details of how flavonoid production in plants is regulated during nodulation have not yet been clarified, but nitrogen availability has been shown to play a role.