The oxygen scavenging in Frankia induced root nodules for nitrogen fixation, is carried out by:

Question

Accepted Answer

The correct option is 4: Hopanoids

This question delves into the biochemistry of nitrogen fixation, specifically in actinorhizal plants, which form symbiotic associations with the filamentous bacterium Frankia. Nitrogen fixation, the conversion of inert atmospheric nitrogen (N2) into biologically usable ammonia (NH3), is catalyzed by the enzyme *nitrogenase*. This enzyme is extremely sensitive to oxygen, as even trace amounts can permanently inactivate it. Therefore, a critical mechanism to ensure the continuation of nitrogen fixation in the root nodules is oxygen scavenging. Unlike the *Rhizobium*-legume symbiosis, which uses leghaemoglobin, *Frankia* employs a unique structural defense. The *Frankia* bacteria differentiate into specialized, thick-walled compartments called vesicles or sporangia within the nodule, where the nitrogenase is housed. The walls of these vesicles are significantly thickened due to the accumulation of lipids, including the pentacyclic triterpenoid molecules known as **Hopanoids**. These Hopanoids form a rigid, highly impermeable layer within the vesicle membrane, acting as a physical and chemical diffusion barrier that severely restricts the entry of oxygen, thus maintaining the strictly anaerobic or microaerobic conditions essential for nitrogenase activity. It is this unique membrane component that primarily performs the necessary oxygen scavenging (by blocking oxygen diffusion) to protect the enzyme and ensure efficient nitrogen fixation in the *Frankia* nodule.

The incorrect option is 1: Heterocysts
Heterocysts are specialized, thick-walled cells, but they are characteristic of filamentous cyanobacteria (blue-green algae), not the *Frankia* symbiosis in root nodules. These cells are developed by free-living or symbiotic cyanobacteria to protect their nitrogenase enzyme, similar to the need in *Frankia*. They exclude oxygen using a thickened cell envelope and high respiratory rates, allowing for nitrogen fixation to proceed. Although they are a dedicated structure for oxygen protection, they operate in a different phylum and context than the *Frankia* system described in the question, which is located inside a plant's root nodule. Therefore, Heterocysts represent an analogous structure in a different group of nitrogen-fixing organisms, not the scavenging agent in *Frankia* nodules.

The incorrect option is 2: Leghaemoglobin
Leghaemoglobin is often considered the classic oxygen-scavenging agent in plant symbiosis, but it is exclusively found in the root nodules of leguminous plants infected with *Rhizobium* or related soil bacteria. This protein is a red pigment, a hybrid molecule where the globin component is produced by the host plant and the heme component by the bacteria. Its function is to bind oxygen reversibly, delivering just enough oxygen for the bacteroids' aerobic respiration while maintaining a crucial low-oxygen environment (microaerobic) to keep the nitrogenase active. Actinorhizal nodules induced by *Frankia* are structurally and biochemically different and do not generally produce or utilize leghaemoglobin for this purpose.

The incorrect option is 3: Nitrogenase
Nitrogenase is the metalloenzyme complex that is the central player in the process, as it is the enzyme that catalyzes the actual chemical reaction of converting N2 to NH3. Crucially, the nitrogenase enzyme is the component that *requires* oxygen protection, rather than being the component that *provides* the protection or performs the scavenging. Its extreme sensitivity to oxygen is the biological problem that the cell and the nodule must solve via oxygen scavenging mechanisms. Thus, nitrogenase is the *target* of the oxygen scavenging, not the scavenger itself.