Symbiosome

Symbiosome

Root nodule containing symbiosomes

A symbiosome is a specialized structure found within the cells of certain host organisms, particularly in the context of symbiotic relationships between plants and nitrogen-fixing bacteria. These structures are crucial for the process of biological nitrogen fixation, which is essential for plant growth and soil fertility.

Structure

Symbiosomes are formed when nitrogen-fixing bacteria, such as those from the genus Rhizobium, infect the root cells of leguminous plants. The bacteria are engulfed by the plant cell membrane, creating a compartment known as the symbiosome. This compartmentalization allows the bacteria to fix nitrogen in a protected environment, while the plant provides the necessary nutrients and energy.

The symbiosome consists of the bacterial cell, the surrounding plant-derived membrane, and the space between them, known as the peribacteroid space. This space is crucial for the exchange of nutrients and signals between the plant and the bacteria.

Function

The primary function of the symbiosome is to facilitate the conversion of atmospheric nitrogen (N_) into ammonia (NH_), a form of nitrogen that plants can readily assimilate. This process is catalyzed by the enzyme nitrogenase, which is produced by the bacteria within the symbiosome.

The symbiosome environment is carefully regulated by the host plant to optimize conditions for nitrogen fixation. This includes maintaining a low oxygen concentration, as nitrogenase is sensitive to oxygen.

Formation

The formation of symbiosomes begins with the recognition and attachment of Rhizobium bacteria to the root hairs of the host plant. This interaction is mediated by specific signaling molecules, such as flavonoids released by the plant and nod factors produced by the bacteria.

Following attachment, the bacteria induce the formation of an infection thread, through which they enter the root cortex. Once inside the root cells, the bacteria are engulfed by the plant cell membrane, leading to the formation of the symbiosome.

Importance

Symbiosomes play a critical role in sustainable agriculture by reducing the need for synthetic nitrogen fertilizers. The ability of leguminous plants to fix atmospheric nitrogen through symbiosomes enhances soil fertility and supports plant growth, making them valuable in crop rotation and soil management practices.

Related pages

• Nitrogen fixation
• Rhizobium
• Legume
• Root nodule