Know About Nodule Formation

On the roots of plants, particularly legumes, root nodules are discovered that create symbiotic relationships with nitrogen-fixing bacteria. Under nitrogen-deficient situations, capable plants develop a symbiotic connection with a strain of bacteria called rhizobia that is particular to the host. This process has occurred numerous times in legumes and other species belonging to the Rosid group. Beans, peas, and soybeans are all legume crops.

Nitrogen gas (N2) from the atmosphere is converted within legume root nodules into ammonia (NH3), which is then assimilated into amino acids (protein building blocks), nucleotides (DNA and RNA building blocks, as well as the critical energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. Legumes are an attractive agricultural organism because of their capacity to fix gaseous nitrogen, which reduces their reliance on nitrogen fertiliser. Indeed, high nitrogen content inhibits nodule formation since the plant derives no advantage from the symbiosis. The energy required to split the nitrogen gas in the nodule is supplied by sugar translocated from the leaf (a product of photosynthesis). Malate, a breakdown product of sucrose, is the bacteroid’s direct carbon source. The nodule’s nitrogen fixation process is extremely oxygen-sensitive. To promote the diffusion of oxygen gas utilised in respiration, legume nodules contain an iron-containing protein termed leghaemoglobin, which is closely related to animal myoglobin.

Classification

In legumes, two distinct forms of nodules have been described: determinate and indeterminate.

Determinate nodules are found on several tropical legume tribes, including those belonging to the genera Glycine (soybean), Phaseolus (common bean), and Vigna, as well as on some temperate legumes, including Lotus. Due to the fact that these determinate nodules lose their meristematic activity shortly after start, growth occurs by cell expansion, resulting in mature nodules that are spherical in shape. A different type of determinate nodule is found on a variety of herbs, shrubs, and trees, including Arachis (peanut). These are always connected with the axils of lateral or adventitious roots and are created as a result of infection through the crevices where these roots emerge, rather than through root hairs. Their interior structure is extremely dissimilar to that of soybean nodules.

Indeterminate nodules are found in the majority of legumes belonging to all three subfamilies, whether they are found in temperate or tropical climates. They are found in Faboideae legumes such as Pisum (pea), Medicago (alfalfa), Trifolium (clover), and Vicia (vetch), as well as in all mimosoid legumes such as acacias, as well as in the few nodulated caesalpinioid legumes such as partridge pea. They are called “indeterminate” because they retain an active apical meristem that generates new cells for development throughout the nodule’s existence. As a result, the nodule is often cylindrical in shape and may be very branching. Due to their dynamic growth, indeterminate nodules exhibit zones that denote various stages of development/symbiosis:

Zone I —consists of the active meristem. This is the site of formation of fresh nodule tissue that will eventually differentiate into the other zones of the nodule.

Zone II—the zone of infection. This zone is infested with infection threads that are teeming with germs. The plant cells are now larger than they were in the preceding zone, and cell division has ceased.

Interzone II–III—This is the point at which bacteria enter plant cells with amyloplasts. They elongate and begin terminal differentiation into nitrogen-fixing symbiotic Bacteroides.

Zone III—the zone of nitrogen fixation. Each cell in this zone has a large central vacuole, and the cytoplasm is densely packed with completely differentiated bacteroids that are actively nitrogen fixating. Leghemoglobin is supplied by the plant to these cells, giving them their distinctive pink colour.

Zone IV—is referred to as the senescent zone. Plant cells and their bacteroid contents are destroyed in this location. Leghemoglobin’s heme component is degraded, resulting in a noticeable greening near the nodule’s base.

Although this is the most extensively studied type of nodule, the details are quite different in nodules of peanut and relatives, as well as some other important crops such as lupins, where the nodule is formed following direct infection by rhizobia through the epidermis and where infection threads never form. Nodules form a collar-like structure around the root. The centre infected tissue is homogenous in these nodules and in the peanut type, lacking the uninfected ells found in nodules of soybean and many indeterminate forms such as peas and clovers.

Actinorhizal nodules are structurally distinct from those found in non-legumes. Cells originating from the root cortex generate the infected tissue in this kind, and the prenodule develops into a mature nodule. Despite this apparent disparity, a single homeotic mutation can result in the formation of such nodules in legumes.

Symbiosis

Leguminous family

The legume family – Fabaceae – includes taxa such as kudzu, clovers, soybeans, alfalfa, lupines, peanuts, and rooibos. Within the nodules, they contain symbiotic bacteria called rhizobia, which produce nitrogen compounds that aid the plant in growing and competing with other plants. When the plant dies, the fixed nitrogen is freed, which helps feed the soil. Although the vast majority of legumes share this trait, a few taxa (for example, Styphnolobium) do not. To take advantage of this, many traditional farming practices rotate fields through multiple types of crops, which typically includes one composed primarily of a leguminous crop such as clover.

Non-leguminous

Although the legume family Fabaceae contains the vast majority of plants capable of forming nitrogen-fixing root nodules, there are a few notable exceptions:

Due to a symbiotic relationship with Frankia bacteria, actinorhizal plants such as adler and bayberry can develop (less complex) nitrogen-fixing nodules. These plants are classified into 25 genera and eight plant families. It comprises approximately 200 species and is responsible for almost the same amount of nitrogen fixation as rhizobial symbioses, according to a 1998 count. A critical structural distinction is that the bacteria in these symbioses are never liberated from the infection thread.

Parasponia, a Cannabaceae tropical genus, is also capable of interacting with rhizobia and forming nitrogen-fixing nodules. Due to the fact that related plants are actinorhizal, it is assumed that the plant evolved via a “partner flip.”

Conclusion

In general, nodules form as a result of soil bacteria infecting the roots. In the case of forage crops, the bacteria that infect the roots most frequently are those of the genus Rhizobium. Infection is the intricate process through which rhizobia infect plant roots.