The role of the flavonoid pathway in Medicago truncatula in root nodule formation. A review

Flavonoids are plant secondary metabolites with a wide array of structures and functions. In legumes, their most prominent role is in the determination of host specificity through their recognition by rhizobial NodD transcription factors. Specific flavonoid recognition leads to expression of nod genes and production of Nod factors in rhizobia. In addition, flavonoids play a role in chemoattraction of rhizobia toward the root. Silencing of different branches of the flavonoid pathway in Medicago truncatula demonstrated that root flavonoids are essential for nodulation, and that flavones function as nod gene inducers. However, other flavonoids, including isoflavonoids and chalcones, act as nod gene inducers and suppressors in M. truncatula. Flavonoids are also essential in M. truncatula nodule development, likely through their action as auxin transport inhibitors. The induction of flavonoids controlling auxin transport in the root following rhizobial infection is under the control of cytokinin signaling. Flavonols are the most likely candidates controlling auxin transport during nodule development. However, the actual bioactive flavonoids remain to be identified.