There are actually several different mechanisms by which rhizobia induce nodule formation . The best studied, as found in plants such as clovers, alfalfa, bean and soybean, involves rhizobial infection of developing root hairs. However, in peanuts, rhizobia gain entry to their host via wounds made as lateral roots emerge, and in this species nodules are usually found in the angle made by the tap and lateral roots. Root hair infection involves complicated signalling between host and rhizobia. This is necessary because most of the genes needed for nodule formation by rhizobia are only activated in the presence of a suitable host plant.
Activation is achieved by the secretion of chemicals called flavonoids from the germinating root.
There are a number of these substances, and different legumes tend to produce a mix of flavonoids that is unique to that host. Rhizobia attach to still-growing root hairs in a region just behind the root tip, and fully mature root hairs are rarely infected. Irrespective of the Rhizobium involved, these substances all have a basically similar core structure, with small chemical differences in this structure determining which and how many legumes are nodulated. M, nodules are formed, even in the absence of rhizobia. Presence of the rhizobia causes modification in the structure of the root hair cell wall, and permits penetration by the rhizobia.