Mechanisms of Symbiotic Nitrogen Fixation in Legumes
Introduction
Symbiotic nitrogen fixation is a biological process that occurs in legumes, where nitrogen gas (N2) from the atmosphere is converted into ammonia (NH3). This process is facilitated by symbiotic bacteria known as rhizobia, which colonize the root nodules of legumes. The symbiotic relationship between legumes and rhizobia is crucial for the nitrogen cycle, as it contributes to the replenishment of nitrogen in the soil.
Mechanism of Symbiotic Nitrogen Fixation
The mechanism of symbiotic nitrogen fixation involves several steps: recognition and attachment of the bacteria to the root hairs of the legume, infection and invasion of the root hairs, formation of infection threads, bacterial release into the plant cells, and the formation of nitrogen-fixing nodules.
Recognition and Attachment
The process begins when the rhizobia recognize specific flavonoids secreted by the legume roots. These flavonoids act as chemical signals that trigger the bacteria to produce nodulation (Nod) factors. The Nod factors induce changes in the root hairs, causing them to curl and trap the bacteria. This is followed by the attachment of the bacteria to the root hairs.
Infection and Invasion
Once attached, the rhizobia invade the root hairs by creating an infection thread, a tube-like structure that allows the bacteria to penetrate the root cells. The infection thread grows towards the base of the root hair, into the root cortex.
Formation of Infection Threads
The infection threads are formed by the invagination of the plant cell membrane, which engulfs the bacteria. The bacteria multiply within the infection threads, moving deeper into the root cortex.
Bacterial Release and Nodule Formation
When the infection threads reach the root cortex, the bacteria are released into the plant cells. Here, they differentiate into bacteroids, the nitrogen-fixing form of rhizobia. The plant cells housing the bacteroids develop into specialized structures called nodules.
Nitrogen Fixation
Within the nodules, the bacteroids convert atmospheric nitrogen into ammonia, a form of nitrogen that can be used by the plant. This process is catalyzed by the enzyme nitrogenase, which is produced by the bacteroids. The ammonia is then assimilated into amino acids, which are used by the plant for protein synthesis.
Role of Leghemoglobin
A key component of the nitrogen fixation process is leghemoglobin, a plant protein found in the nodules. Leghemoglobin binds to oxygen, creating a low-oxygen environment within the nodules. This is crucial because the enzyme nitrogenase, which catalyzes the conversion of nitrogen to ammonia, is sensitive to oxygen and functions optimally in low-oxygen conditions.
Significance of Symbiotic Nitrogen Fixation
Symbiotic nitrogen fixation plays a crucial role in the nitrogen cycle, contributing to the replenishment of nitrogen in the soil. This process is particularly important in agricultural systems, where nitrogen is often a limiting nutrient. By forming symbiotic relationships with rhizobia, legumes can access a source of nitrogen that is otherwise unavailable to most plants.