Flavonoids as Signaling Molecules and Regulators of Root Nodule Development

Flavonoids are a diverse class of secondary plant metabolites, synthesized from phenylpropanoid precursors, which play an array of important functions in plants, ranging from floral pigments for the attraction of insect pollinators to antioxidants and auxin transport inhibitors. Many plant species also use flavonoids as signal molecules for beneficial microorganisms in the root rhizosphere, and as antimicrobial defense compounds in their interactions with pathogenic microbes. In legumes, flavonoids also play a critical role in promoting nitrogen-fixing symbiosis with rhizobia. Legume root-exuded flavonoids act both as chemo-attractants for symbiotic rhizobia, and signaling molecules for the activation of the expression of rhizobial nod genes, which are responsible for the synthesis of Nod factors, the bacterial signals that are necessary for the initiation of a new plant organ, the nodule. In addition, flavonoids also play a key role in enabling the initiation of differentiation of nodule primordia by inhibiting auxin transport in roots in response to rhizobial Nod factors. This review reports our present level of understanding on the role of flavonoids in the establishment of legume-Rhizobium symbiosis. Also described are the limits of our knowledge in this area of research, and how functional genomic strategies will help in clarifying the regulatory roles of individual flavonoids in mediating nod gene induction in rhizobial cells and auxin transport inhibition in the legume roots during the course of the development of legume-Rhizobium symbiosis. _____________________________________________________________________________________________________________