Fig. 2.
Colonization of rhizobia and nodule formation. Flavonoids are produced by the host and act as a signal molecule for the production of nodulation factors by rhizobia. The perception of nodulation factors by the plant induces the root hair curling, facilitating the entry of bacteria in the cortical zone of plant tissue. Invaginations in the region of the hair bending form the infection cord that leads the bacteria to the region where the nodule will be formed. The mitogenic action of nodulation factors stimulates the proliferation of cortical cells in the plant and the nodular primordium formation or emerging nodule. Rhizobia colonize the emerging nodule, forming the symbiosome, and differ in to bacteroids at the N2 fixation stage