Rhizobium is a genus of bacteria commonly found in soil, known for their ability to form a close relationship with the roots of certain plants. This interaction is a defining characteristic of rhizobia and a fundamental component of soil nutrient cycles.
The Symbiotic Partnership
The relationship between rhizobium bacteria and their host plants is a form of symbiosis known as mutualism, where both organisms benefit. The bacteria receive a protected and stable environment inside the plant’s roots, along with a steady supply of carbohydrates. These carbohydrates, produced by the plant through photosynthesis, provide the energy the bacteria need to survive and function.
In return for this shelter and food, the rhizobia provide the plant with a usable form of nitrogen. This exchange is particularly important for leguminous plants, a family that includes peas, beans, lentils, clover, and alfalfa. The interaction is highly specific, with certain species of rhizobia only forming relationships with particular legume species.
This partnership allows legumes to thrive in soils where nitrogen might be scarce, giving them a competitive advantage. The bacteria act as a natural fertilizer source within the plant’s root system. This close association ensures an efficient transfer of nutrients, supporting the health of both partners.
The Process of Nitrogen Fixation
Earth’s atmosphere is composed of nearly 80% nitrogen gas (N₂), but plants cannot absorb it directly in this gaseous state. Rhizobia perform a process called biological nitrogen fixation, which converts this atmospheric nitrogen into ammonia (NH₃), a form that plants can readily use.
This process is facilitated by a specialized enzyme complex within the bacteria called nitrogenase. The nitrogenase enzyme is responsible for breaking the strong triple bond of the N₂ molecule and combining it with hydrogen to form ammonia. This reaction is extremely energy-intensive, which is why the bacteria depend on the carbohydrates supplied by their host plant.
The ammonia produced by the bacteria is then transferred to the plant, where it is used to build proteins, chlorophyll, and other organic molecules necessary for growth. The nitrogenase enzyme is sensitive and can be irreversibly inactivated by oxygen, which is why the structure of the root nodule is so important for its function.
Formation of Root Nodules
The symbiotic relationship begins with a chemical conversation between the plant’s roots and nearby rhizobia in the soil. When a legume plant needs nitrogen, its roots release chemical signals called flavonoids into the soil. These flavonoids act as an attractant for compatible rhizobia, signaling them to move toward the root.
Once attracted, the rhizobia produce their own signaling molecules called Nod factors. These Nod factors cause the plant’s root hairs to curl and deform, entrapping the bacteria. The bacteria then penetrate the root hair cell and form a structure called an infection thread. This tubular thread grows inward, allowing the bacteria to travel safely through the outer layers of the root into the cortex.
Upon reaching the root cortex, the bacteria stimulate the plant’s cells to divide rapidly. This controlled cell division leads to the formation of a specialized organ on the root known as a nodule. Inside these nodules, the rhizobia are released and transform into a state where they can begin fixing nitrogen, creating a visible lump that houses the bacterial partners.
Agricultural and Ecological Significance
Nitrogen fixation by rhizobia has significant applications in agriculture and contributes to ecological stability. Farmers and gardeners can harness this symbiotic relationship by using commercial products called inoculants, which contain specific strains of rhizobia. These inoculants are often applied as a coating to legume seeds before planting to ensure that a high population of effective bacteria is present in the soil.
This practice can lead to increased crop yields and improved nutritional quality, such as higher protein content in seeds. It also enhances overall soil fertility. When the leguminous plant dies and decomposes, the nitrogen-rich organic matter is returned to the soil, benefiting subsequent crops planted in the same field.
Relying on this biological process reduces the need for synthetic nitrogen fertilizers. This provides economic and environmental benefits, lowering costs for farmers and mitigating issues from fertilizer runoff, such as water pollution and greenhouse gas emissions. For this reason, legumes are often included in crop rotation systems to naturally enrich the soil with nitrogen for other non-leguminous crops.