What happens during nodulation?

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. While in contact with their host, the rhizobia produce complex chitin-like substances termed "nod factors"- more technically lipo-oligosaccharides.

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. When the appropriate purified nod factor is applied to its host at concentrations as low as 10^-9 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.

Rhizobia are never really permitted open access to the host. As they penetrate the root hair, plant-derived mucilaginous material is deposited about them, and even as they move down the root hair in the direction of the root, they remain surrounded by a plant-derived infection thread.

Many infections do not develop into functional nodules. For this to occur rhizobia must contact cells in the host cortex that have been stimulated to division by the presence of the nod-factor. Rhizobia -- still enclosed within a plant derived barrier that mutes possible host defence responses -- are released into these cells, where their multiplication leads to the development of the typical nodule structure.

More detailed accounts of the infection process include articles by Boogerd and van Rossum (1997), Hirsch (1992), and John, et al. (1997).