Signalling and Control of Flavonoids in the Biological Nitrogen Fixation Process of Leguminous Plants: A Review

Biological nitrogen fixation in plants gives them a competitive edge by converting atmospheric nitrogen into ammonium. Rhizobia and leguminous plants have symbiotic relationships in which the latter produce nitrogen-fixing nodules in their roots. Rhizobial bacteria infiltrate leguminous plants through signal exchange. Under low nitrogen environments, host plant roots release flavonoids that cause rhizobia to produce Nod factors, which are lipo-chitooligosaccharide signalling molecules. Flavonoids play a crucial role in interactions between plants and microbes, facilitating symbiosis and defensive mechanisms. Flavonoids are a diverse class of phenolic compounds found in all higher plants and act as chemo-attractants, attracting compatible rhizobia, promoting or suppressing expression of rhizobial nod genes, suppressing root pathogens, promoting germination of mycorrhizal spores, quorum sensing and chelating soil nutrients. Flavonoids activate the bacterial regulatory protein NodD, which controls the transcription of nod genes required for the synthesis of the bacterial Nod factor. The legume-rhizobium interaction allows rhizobia to enter the host plants and stimulate flavonoids from legume roots.