How does rhizobium fix nitrogen?

Rhizobia generally coexist with leguminous plants. During their lives, rhizobia will secrete some organic matter into the soil, thus greatly improving the fertility of the soil. Rhizobia can make soybeans or peanuts grow normally. For bean crops, there are many benefits. Everyone knows that if the soil contains organic matter, it will make plants grow very vigorously.

Molybdenum iron hemoglobin in rhizobia catalyzes the reduction of nitrogen to ammonia. Next, the ammonia reacts with the acidic substances in the rhizobia to form ammonium salts. Then, the ammonium salt is converted into organic amines, such as amino acids, by a series of aminoacids in the rhizobia. Then, part of the organic amines and inorganic ammonium of the rhizobia that infect the root cortex of the plant are released and used by the plant.

Although the air contains about 80% nitrogen, most plants cannot use it directly. Leguminous plants such as peanuts, soybeans, and alfalfa can convert molecular nitrogen in the air into ammonia nitrogen that can be used by plants through symbiotic nitrogen fixation with rhizobia. After the seeds germinate and take root, rhizobia invade the roots through the root hairs and, under certain conditions, form nodules with the ability to fix nitrogen. Under the action of nitrogenase, the bacteroids in the nodules convert molecular nitrogen into ammonia nitrogen. At the same time, each nodule is a miniature nitrogen fertilizer plant, continuously delivering nitrogen to the plants for use. The number of indigenous rhizobia around leguminous crops is very small and cannot meet the needs of crop growth. All legume crops in the world need to be artificially inoculated with rhizobia. The nitrogen fertilizer produced by rhizobia for crops will not pollute the environment, does not require long-distance transportation, and there is no nitrogen loss during use. The loss rate of artificially applied chemical nitrogen fertilizers is often greater than 50%.

When leguminous crops germinate and grow root hairs, rhizobia are stimulated and attracted by the lectins secreted by the root hairs and gather in large numbers in the rhizosphere and root surface. After the root hairs come into contact with rhizobia, the cell wall first softens and the root hairs curl, then the cell wall invaginate, and the rhizobia then invade the root hairs until the root cortex, where the rhizobia multiply in large numbers and transform into bacteroids. At this time, the root cortex proliferates rapidly, forming tumor-like tissue, which finally protrudes from the root to form nodules. When the cross-section of the effective nodule appears pink, it indicates that the nodule has entered the mature stage, begins to fix nitrogen, and provides nitrogen to the plant. Artificial inoculation of rhizobia with excellent nodulation and nitrogen fixation properties can improve the efficiency of symbiotic nitrogen fixation.

Soil Biology and Soil Biochemistry

A type of rod-shaped bacteria that live in symbiosis with leguminous plants, forming root nodules and fixing nitrogen from the air for plant nutrition. A type of Gram-negative aerobic rods that can promote abnormal growth of plants.

Normal cells move by flagella and do not have spores. It can utilize a variety of carbohydrates and produce considerable amounts of extracellular mucus. For example, Rhizobium and Slow Rhizobium can invade the roots of leguminous plants through the root hairs to form nodules, and become branched polymorphic cells in the nodules, called bacteriovores.

There is also the genus Agrobacterium, which can invade a variety of dicotyledons and gymnosperms through external wounds, causing plant cells to transform into abnormally proliferating tumor cells, resulting in root cancer, hairy roots or stem galls, etc.