Multiple Responses of Rhizobia to Flavonoids During Legume Root Infection

In the formation of legume-rhizobia symbioses flavonoids released from roots and seeds are best known as inducers of the bacterial nodulation genes that control the synthesis of reciprocal chitolipooligosaccharide signals (Nod factors) to the prospective host plant. However, successful symbiotic development requires the transmission of a variety of other signals from rhizobia to plant roots and flavonoids initiate or mediate the production of most of them. This review considers the wide range of rhizobial responses to these secondary plant metabolites in the early phases of symbiotic interaction, including chemotaxis, growth stimulation, degradation, Nod factor synthesis, protein secretion by type I and III systems, surface polysaccharide production and expression of many new genes and proteins whose functions are only beginning to be analysed. Attention is also drawn to aspects of flavonoid-rhizobia interaction, such as release of compounds from inoculated roots and the reception of nodulation gene inducers by regulatory NodD proteins, that will need to be revisited by researchers if a complete understanding of the molecular dialogue between the partners is to be achieved.