Fig. 2

Multifaceted roles of flavonoids mediating plant-microbe interactions. A A conceptual overview of the distinct roles of flavonoids mediating plant-microbe interactions. B Illustration of flavonoids mediating nodule formation in plant roots. PGPR, plant growth-promoting rhizobacteria; LPO, lipo-oligosaccharide. Flavonoids are secreted into the rhizosphere via distinct mechanisms (see “Transport of flavonoids in plants and exudation” section). Specific flavonoid molecules in the rhizosphere are perceived by Rhizobium, which induces the expression of the Nod gene. This gene expression results in the synthesis and secretion of Nod factors (LPO). Legume plants recognize these Nod factors in the receptor and via the signal transduction pathway, thus eliciting a series of physiological and biochemical responses. Besides, flavonoid, such as kaempferol inhibits auxin transport, which enhances auxin concentration in the cortical cells and accelerates cell division and growth. Together, these physiological and biochemical processes lead to the formation of the root nodule