Figure 1.

Signal exchange in the legume-rhizobia interaction. Flavonoids and isoflavonoids secreted by legume roots activate the Nodulation protein D (NodD) transcription factor on compatible rhizobia inducing the transcription of nod genes, which are required for the synthesis of Nod factors. Nod factors are perceived by receptors present in the plasma membrane of root cells, triggering the signaling pathway required for the development and infection of the nodule, where bacteria are allocated, and nitrogen fixation occurs.