Intracellular Compartmentalisation of Rhizobium

Intracellular Compartmentalisation of Rhizobium

	Intracellular Compartmentalisation of Rhizobium Following dissolution of the IT wall, Rhizobium may be considered released into the host cell cytoplasm though they remain bound in membrane envelope. Thus physiologically they are extracellular. In an effective root nodule, no bacteria have been observed to be free inside the host cell. Release of bacteria from the IT is followed by an increase in plasma membrane biosyntheses which keeps pace with the bacterial proliferation. As a result of an increased population of bacteria and a high demand for oxygen, the intracellular environment of the host cell becomes hypoxic.
In this environment, bacteria differentiate into bacteroids. Once the rhizobia are released, they lose their motility and reproducing abilities. They round off and may exist in various shapes like 'Y', 'L' etc. Now they are called bacteroids. Depending on the legume, bacteroids are surrounded by membrane envelopes whose origin is hypothetical. They are formed de novo after the release of the bacteria from the IT. They are extensions of the endoplasmic reticulum of host cells. They are derived from plasma membrane by a process of phagocytosis.
The number of bacteroid in any envelope is determined by the host. Large number of bacteroid per membrane envelope may be due to the fact that the membrane does not grow fast enough than the bacteria. The root nodule can be subdivided into specific zones. The outer periderm consists of loosely pack d cells and lenticels through which gases diffuse into the outer cortical layer. These gases include the N₂ required for N₂ fixation, as well as O₂ required for plant and bacterial respiration. The cells of the outer cortex are also loosely packed, and contain large air spaces which offer little resistance to the diffusion of gases.
However, in the inner cortex, the cells are smaller and much closer together, and here the gases may have to diffuse through the contents of the cortical cells of ammonia to reach the central zone of the nodule, since open intercellular spaces are infrequent. This part of the nodule is thought to act as a barrier to gas diffusion. The central zone of the nodule contains plant cells that are infected with thousands of bacteria. In their symbiotic form, these bacteria are called bacteroids, and a typical soybean root may provide a home for 2,000,000,000 of these bacteroids.
	The bacteroids are responsible for reducing the N₂ gas that diffuses into the central zone to ammonia, which can then be assimilated by the plant cells. More important is the exchange of materials that must occur between the plant and the bacteroids to allow for the fixation of N₂ and the assimilation. A carbon source from the plant is supplied via the phloem to root nodules housing the N₂ fixing bacteria. The carbon source is partially metabolised by the plant and the resulting carbon compounds are imported into the bacteria.

The bacteria contain the enzyme nitrogenase, which is responsible for catalyzing the reduction of N₂ gas to ammonia.

The carbon compounds entering the bacteria are metabolized to produce the ATP and reductant required in the nitrogenase reaction. The ammonia produced in the reaction is transported to the plant where it is assimilated into organic nitrogenous compounds. These are exported in the xylem to the rest of the plant where they are used in the synthesis of amino acids, nucleic acids and other nitrogen-containing compounds.

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