Working Principle of Rhizobium japonicum

The interaction between Rhizobium japonicum and soybean is a highly specific and coordinated symbiotic process, which can be divided into the following stages:

1.  Signal Recognition and Chemotaxis

    When soybeans are in the seedling stage, their roots secrete specific flavonoids. These substances can be recognized by Rhizobium japonicum in the soil, inducing the bacteria to produce nodulation factors (lipochitooligosaccharides). At the same time, flavonoids act as chemoattractants, guiding Rhizobium japonicum to move toward soybean roots.

2.  Root Hair Infection and Invasion Thread Formation

    Nodulation factors trigger deformation and curling of soybean root hairs. Rhizobium japonicum attach to the curled parts of root hairs and secrete enzymes to degrade the cell wall of root hairs, then invade the root hair cells and form invasion threads—tubular structures formed by the invagination of the plant cell membrane. The bacteria multiply in the invasion threads and spread to the cortical cells of the roots.

3.  Root Nodule Induction and Differentiation

    The invasion of Rhizobium japonicum and the secretion of related signals stimulate the division and proliferation of soybean root cortical cells, gradually forming visible root nodules on the roots. After entering the cortical cells, the bacteria differentiate into bacteroids with nitrogen fixation function. Meanwhile, the plant cells synthesize leghemoglobin (a pink protein unique to root nodules), which can bind oxygen in the root nodules to maintain a low-oxygen environment—this environment is essential for the activity of nitrogenase (oxygen will inactivate nitrogenase).

4.  Biological Nitrogen Fixation

    In the low-oxygen environment of root nodules, bacteroids use nitrogenase to catalyze the reduction of inert atmospheric nitrogen (N₂) into ammonia (NH₃). Ammonia is then converted into amino acids (such as glutamine) through a series of enzymatic reactions in plant cells, which can be directly transported to various parts of the soybean for growth and metabolism.