Rhizobium

Rhizobium (“root living”) is a genus in the bacteria kingdom. It consists of gram-negative bacteria, which are able to participate in symbiotic associations with plants of the legume family.

In this symbiotic association the bacteria inside the nodules fix nitrogen for the plants and in return receive fuel in form of carbohydrates. Although close to 80% of the Earth’s atmosphere consists of N2, it cannot be assimilated by the plants and organic nitrogen, and unlike other minerals, is not replenished in the soil by the breakdown of rocks. Therefore, this relationship provides the plants with organic nitrogen in the form of ammonium (NH4+) or nitrate (NO3-). In fact, nitrogen fixation is an exclusively prokaryotic phenomenon. The importance of this symbiotic relationship can be observed by the fact both partners contribute to the biosynthesis of the molecule leghemoglobin, a hemoglobin-like iron containing protein that gives the nodules a reddish appearance. Leghemoglobin binds oxygen and thereby lowers it’s effective concentration in the nodules, an important factor, since the enzyme nitrogenase which is responsible for the fixing of nitrogen is strongly inhibited by oxygen.

Importance in Agriculture
It is therefore certainly obvious that nitrogen-fixing bacteria play a paramount role in the ecosystem and in the agricultural industry. Organic nitrogen, among all other minerals, is the one that most often limits productivity of the plants, since a meager nitrogen diet for the plants deteriorates their protein yield. At the same time, protein-deficiency is the most common form of malnutrition in humans, as the majority of the people of the world have a vegetarian diet.

Note: The neutrality of this section may be disputed.

Improving the protein yield of plants is therefore necessarily the best method of remedying protein-deficiency. Nitrogen-fixing bacteria have been playing an important role in agriculture for centuries. The agricultural practice of crop rotation takes advantage of the fact that root nodules often excrete excess ammonium into the soil, therefore increase its fertility for nonlegumes. This provides a more economical and ecologically-friendly alternative to the practice of commercially synthesizing fertilizer. It is also evident that by improving the efficiency of the symbiotic relationship of the bacteria with plants, the protein yield of the plants can be increased. By studying the molecular aspects of their symbiosis, one can hope to induce nodule formation in nonlegumes. In fact in Australia and China, chemicals have been produced that weaken the epidermis of root cells in wheat, and allow nitrogen-fixing bacteria to penetrate them and improve the nitrogen level.