Peanuts are one of the most widely grown legume crops in the world. The rhizobia that can establish a symbiotic relationship with peanuts are mainly Bradyrhizobium.
Studies have shown that the nitrogen fixation of the peanut-bradiorhizobium symbiotic system can meet 30%-80% of the nitrogen required for peanut growth, and improve the yield of subsequent crops.
Under the premise of reducing chemical nitrogen fertilizer, bradiorhizobium can solve soil ecological problems and maintain sustainable development of soil environment, and have high value in peanut cultivation.
1. Infection Type of Peanut by Bradyrhizobium
As plant roots grow, organic secretions such as amino acids and flavonoids are released into the soil, and rhizobia respond to these secretions and reach the plant roots.
After rhizobia polysaccharides and lectins are recognized, plants will help rhizobia to colonize the root surface and root hairs in preparation for root infection.
The symbiosis between bradyrhizobium and peanut belongs to fissure infection, which refers to the process in which rhizobia enters the plant root through the crack of the epidermis of the plant root or the fissure between the lateral root and the main root and forms a root nodule.
2. Formation of peanut nodules
Bradyrhizobia moved to the nodule primordium located in the cortex in the intercellular space of peanut, and entered the nodule primordium by changing the structure of peanut cell wall.
The original cell wall of Bradyrhizobium is hydrolyzed by the cell wall-degrading enzymes secreted by peanut. With the gradual enlargement of the hydrolysis pores, the rhizobia protoplast is wrapped by the symbiont membrane of peanut, and finally swells and develops into a spherical mature symbiont. When all rhizobia have developed into symbionts, the nodules are fully mature and the nitrogen fixation capacity reaches its peak.
3. Mechanism of nitrogen fixation in peanut nodules
The symbionts of mature nodules contain nitrogenase, which converts N2 to NH3 in a microaerobic environment.
During nitrogen fixation, ferredoxin transfers electrons to the ferritin component of nitrogenase, and ferritin reduces the molybdenum ferritin of nitrogenase while hydrolyzing ATP. Molybdenum ferritin transfers electrons to molecular nitrogen and reduces it to NH3.
NH3 combines with H+ in the symbiont membrane to form NH4+, which in turn combines with glutamate in plant cells to form glutamine to provide a nitrogen source for plants, or glutamine transfers the amino group to aspartic acid in the form of asparagine supply nitrogen source.