Flavonoids Play Multiple Roles in Symbiotic Root-Rhizosphere Interactions

Flavonoids are a class of structurally highly diverse secondary compounds present in all plants. Flavonoids play multiple roles in plants as well as signals regulating the interactions of plants with many microorganisms. This chapter highlights the role of flavonoids in root symbioses including Rhizobium-legume symbiosis; actinorhizal-Frankia symbiosis, the association between plants and free-living nitrogen-fixing bacteria; and the vesicular-arbuscular mycorrhizal symbiosis. Flavonoids act as chemo-attractants for rhizobia and are specific signals that activate nod gene expression, regulation of exopolysaccharide synthesis, and possibly quorum sensing signaling. They also play a crucial role in the regulation of auxin transport required for indeterminate nodule formation. Their role in the actinorhizal symbioses is less defined but likely involves the regulation of infection and selectivity of the symbiont. In endophytic diazotrophs, flavonoids regulate the synthesis of exopolysaccharides, lipopolysaccharides and auxin, which could be involved in host colonization. While most of these studies have been carried out under controlled conditions, much remains to be discovered about flavonoid action under field settings.