Biology of Nodulation
Nodulation is a very specific symbiotic event between a host plant and a bacterium [e.g. soybean (Glycine max – Bradyrhizobium japonicum)]. The rhizobium is a normal soil bacterium that survives in the soil. However, when a compatible plant host germinates nearby the rhizobium is attracted to the plant roots where it initiates the infection process (see timelapse video). In soybean and many other legumes, infection takes place through the root hair cell. The specificity of the symbiotic interaction is largely determined by the recognition of specific signal molecules produced both by the symbiotic bacteria and the plant host. For example, the rhizobia recognize their compatible host by specific flavonoid molecules released either from the seed or root. This recognition event triggers gene transcription in the bacterium leading to the expression of the nodulation genes (e.g., nodABC). In turn, the nodulation genes encode enzymes that synthesize a very special signal molecule, a lipo-chitooligosaccharide (Figure 1). This lipo-chitin Nodulation signal is excreted and recognized by the plant through specific receptors. The Nod signal alone at concentrations as low a 1 pMol will elicit specific responses in the plant host associated with the nodulation process. Among the first events that can be easily seen is the curling of the root hair to form a pocket in which the bacteria multiply. In a process that is not well understood, the rhizobium gain entry into the plant by invagination of the root hair membrane and the formation of an infection thread (Figure 2, also shown by arrows in time lapse film).
Time-lapse film of the infection of clover root hairs by rhizobia.
The infection thread grows down through the root hair and eventually enters and ramifies in the plant root cortex (Figure 3). Cortical cells are triggered to divide in advance of the infection thread, eventually forming the nodule primordium. The infection thread terminates in a cell that will become infected. The rhizobia are released via endocytosis into the infected cell. The plant cell forms a membrane that surrounds the intracellular symbiont forming what we call a “symbiosome” (Figure 4). It is within the symbiosome that the rhizobia fix nitrogen that is provided to the plant. The nodule is a true organ with tissue differentiation. For example, there are infected cells where nitrogen fixation takes place and associated uninfected cells that play a role in nitrogen assimilation (Figure 5).
The symbiosis is an example of mutualism. The plant gains a steady supply of nitrogen, which is often a limiting nutrient in many environments. In return, the bacterium gains a steady supply of carbon. Over several seasons, the population of rhizobia in the soil significantly increases when grown in the presence of a compatible host. Figure 6 shows the finished product, nodules on the roots of soybean.