We all know that rhizobia, a diazotrophic bacterium, helps fix nitrogen, specifically with the help of leguminous plants (Fabaceae/legume family). But here, in this article, we will explore some step-by-step processes that help improve nitrogen. Nitrogen is one of the essential macronutrients for plants; it is crucial for plant growth. If the plants starve from nitrogen, that would lead to poor yield and less protein synthesis and will also affect chlorophyll. The plant root region is called the rhizosphere, where we can witness numerous microbiomes that are the habitat for enormous microorganisms.
What is the relationship between bacteria and legume plants?
The answer to this question is there is a symbiotic relationship between these bacteria and plants where the bacteria can get carbon, which is the energy source for their daily living, and, in turn, fix the nitrogen.
What are the steps of nitrogen fixation?
- Nitrogen Fixation.
- Assimilation.
- Ammonification.
- Nitrification.
- Denitrification
Ιmage source: https://extension.umn.edu/yard-and-garden-news/inoculating-garden-legumes
Plant roots generally release exudates containing sugars, amino acids, etc. They also tend to release specific compounds like flavonoids, which will be host-specific, and the bacteria detect those flavonoids in the rhizosphere region using nod D genes (nodulation gene – present in the rhizobia); this interaction activates the nod D genes, which will activate the nod A, B, and C (common genes for all the rhizobia) and some host-specific gene like P, Q, H, F, E, and L. This wholly forms the NOD factor, a compound of lipo-chitooligosaccharides derivative of chitin, which will be specific for the hosts if they match the host crop. That NOD factor will be sent back to the plants' roots again. If they fit, the plant will produce nodulin genes, which the bacteria attach to the root by rhicadhesion; they affect the calcium in the cell wall of the plant roots. So, root curling occurs, sending the mitogenic signals to the cortex for cell division.
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This causes the formation of infection thread into the cortex of the plant root, where the rhizobia travels and differentiates inside the cortical cells to form bacteroids; inside the Bacteroids, they contain an enzyme called nitrogenase, which is crucial in fixing the nitrogen. This bacteroid formation inside the root cells causes hormonal change, so they swell from the root nodules, which can be prominently seen in our naked eye. They are surrounded by a peribacteriod membrane, which keeps them away from oxygen.
They are pink in color because of the presence of leghemoglobin, which is called an oxygen scavenger. After all, the oxygen would poison the enzyme. So this is present to carry the oxygen and helps the nitrogenase enzyme to thrive well and fix the nitrogen.
Which plants fix the most nitrogen?
Other grain legumes, such as peanuts, cowpeas, soybeans, and fava beans, are excellent nitrogen fixers and will meet all their nitrogen requirements except those absorbed from the soil. These legumes can fix up to 250 pounds of nitrogen per acre and typically are not fertilized (Walley et al., 1996; Cash et al., 1981).
Why do we need nitrogen fixation?
Plants can’t take di-nitrogen (N2), which is dominant in the atmosphere. The acceptable form of nitrogen is nitrate (NO3-) and Ammonia (NH3), so these rhizobia bacteria help fix the dinitrogen by converting N2 to NH3, so plants can easily use it for their growth. If the nitrogen fixation doesn’t happen in real life. Plants will starve from nitrogen deficiency, or else we will need to add plenty of synthetic fertilizers to compensate for the needs of the plants, which will lead to economic and environmental loss.
References:
Book: Soil Microbiology, Ecology and Biochemistry, Fourth Edition, 2015, Author: Eldor A. Paul.