Flavonoids and nod factors: Importance in legume-microbe interactions and legume improvement

Abstract

Biological nitrogen fixation is one of the most important physiological processes in which atmospheric nitrogen is reduced to ammonia by symbiotic bacteria called rhizobia belonging to α- and Β-Proteobacteria. Legume plants (Fabaceae) enter into mutualistic symbiosis with nitrogen-fixing rhizobia which enable them to grow in nitrogen-limited agricultural soils. Infection of legumes by rhizobia involves a series of sequential steps in which plant flavonoids and rhizobial Nod factors activate plant transmission signaling and initiate nodule development. Inside the nodule, rhizobia multiply and differentiate into nitrogen-fixing bacteroids. Here, besides an overview of symbiosis, the role of signal molecules, flavonoids, and Nod factors in legume growth and yield enhancement is highlighted. Recent progress in the understanding of the functions of the symbiotic signaling factors in initiation and development of symbiosis is likely to facilitate successful application thereof in sustainable agriculture to promote growth and nodulation of legume plants.