Roles of Flavonoids in Symbiotic Root–Rhizosphere Interactions

Flavonoids serve multiple functions as signals in roots and the rhizosphere. Owing to their diverse structures, flavonoids can act as antioxidants, auxin transport regulators, and defense compounds during signaling in symbiosis. This review highlights some of the function of flavonoids in the rhizosphere, in particular for the interaction of roots with symbiotic microorganisms. Flavonoids can activate or inhibit rhizobial nod gene expression; act as chemoattractants for rhizobia toward the root, affect quorum sensing; stimulate mycorrhizal spore germination and hyphal branching; and also act to alter the soil, for example, by chelating soil nutrients. Our knowledge of the role of flavonoids in the rhizosphere is hampered by a lack of studies involving real rhizosphere conditions. In addition, we have much to learn about the mechanisms of root exudation of flavonoids and about the mobility, turnover, and concentration of flavonoids in the rhizosphere.