Flavonoids in plant rhizospheres: secretion, fate and their effects on biological communication

Flavonoids, one of the most-described group of plant specialized metabolites, consist of more than 10,000 structurally diverse compounds. Most flavonoids accumulate in plant vacuoles as glycosides, with some released by the roots into rhizospheres. These flavonoids are involved in biological communications with rhizobia, arbuscular mycorrhizal fungi, plant growth promoting rhizobacteria, pathogens, nematodes, and other plant species. Both aglycones and glycosides of flavonoids are found in root exudates and in soils. This review describes researches on the mechanisms of flavonoid secretion and the fate of flavonoids released into rhizospheres. This review also discusses the direction of future research that may elucidate the specific roles of flavonoids in biological communications in rhizospheres, enabling the utilization of flavonoid activities and functions in agricultural practice.