Legumes form a symbiosis with noduleinducing bacteria

Initially it was thought that the nodules of legumes (Fig. 11.1) were caused by a plant disease, until their function in N2 fixation was recognized by Hermann Hellriegel (Germany) in 1888. He found that beans containing these nodules were able to grow without nitrogen fertilizer.

The nodule-inducing bacteria include, among other genera, Rhizobium, Bradyrhizobium, and Azorhizobium and are collectively called rhizobia. Species of Rhizobium form nodules with peas, species of Bradyrhizobium with soybean and species of Azorhizobium with the tropical legume Sesbania. The rhizobia are strictly aerobic gram-negative rods, which live in the soil and grow heterotrophically in the presence of organic compounds. Some species (Bradyrhizobium) are also able to grow autotrophically in the presence of H2, although at a low growth rate.

The uptake of rhizobia into the host plant is a controlled infection. The molecular basis of specificity and recognition is still only partially known. The rhizobia form species-specific nodulation factors (Nod factors). These are lipochito-oligosaccharides that acquire a high structural specificity (e.g., by acylation, acetylation, and sulfatation). They are like a security key with many notches and open the house of the specific host with which the rhizobia

Figure 11.1 Root system of Phaseolus vulgaris (bean) with a dense formation of nodules after infection with Rhizobium etli. (By P. Vinuesa-Fleischmann and D. Werner, Marburg.)

associate. The Nod factors bind to specific receptor kinases of the host, which are part of signal transduction chains (section 19.1). In this way the "key" induces the root hair of the host to curl and the root cortex cells to divide, forming the nodule primordium. After the root hair has been invaded by the rhizobia, an infection thread forms (Fig. 11.2), which extends into the cortex of the roots, branches there and infects the cells of the nodule primordium. A nodule thus develops from the infection thread. The morphogenesis of the nodule is of similarly high complexity as any other plant organ such as the root or shoot. The nodules are connected with the root via vascular tissues, which supply them with substrates produced by photosynthesis. The bacteria incorporated into the plant cell are enclosed by a peribacteroid membrane (also called a symbiosome membrane), which derives from the plasma membrane of the infected plant cell. The incorporated bacteria are thus separated from the cytoplasm of the host cell in a so-called symbiosome (Fig. 11.3). In the symbiosome, the rhizobia differentiate to bacteroids. The volume of these bacteroids can be 10 times the

Figure 11.2 Controlled infection of a host cell by rhizobia is induced by an interaction with the root hairs. The rhizobia induce the formation of an infection thread, which is formed by invagination of the root hair cell wall and protrudes into the cells of the root cortex. In this way the rhizobia invaginate the host cell where they are separated by a peribacteroid membrane from the cytosol of the host cells. The rhizobia grow and differentiate into large bacteroids.

Peribacteroid membrane

Peribacteroid membrane

Root hair cell

Infection thread (invagination of root hair membrane)

Infected cell

Root hair cell

Infection thread (invagination of root hair membrane)

Infected cell

Figure 11.3 Electron microscopic cross-section through a nodule of Glycine max cv. Caloria (soybean) infected with Bradyrhizobium japonicum. The upper large infected cell shows intact symbiosomes (S) with one or two bacteroids per symbiosome. In the lower section, three noninfected cells with nucleus (N), central vacuole (V), amyloplasts (A), and peroxisomes (P) are to be seen. (By E. Mörschel and D. Werner, Marburg.)

volume of an individual bacterium. Several of these bacteroids are surrounded by a peribacteroid membrane.

Rhizobia possess a respiratory chain which corresponds to the mitochondrial respiratory chain (see Fig. 5.15). In a Bradyrhizobium species, an additional electron transport path develops during differentiation of the rhizobia to bacteroids. This path branches at the cyt-bc complex of the respiratory chain and conducts electrons to another terminal oxidase, enabling an increased respiratory rate. It is encoded by symbiosis-specific genes.

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