Nitrogen fixation is a crucial process in which certain bacteria convert atmospheric nitrogen into a form that plants can use. This process is primarily carried out by rhizobia, a type of gram-negative soil bacteria that form a symbiotic relationship with legumes, plants belonging to the Fabaceae family. These bacteria reside in specialized structures called nodules, which are swollen areas on the roots of legumes. Although this relationship is mutually beneficial, it is technically a bacterial infection, as the swelling indicates the presence of the bacteria within the plant tissues.

Legumes attract rhizobia by releasing flavonoids into the soil, which signal the bacteria to respond with nod factors. These nod factors facilitate the entry of rhizobia into the plant's root hairs, leading to a morphological change that allows the bacteria to penetrate the root cortex through an infection thread. Once inside, rhizobia establish a mutualistic relationship with the plant, where the plant provides carbohydrates and protection, while the bacteria supply usable nitrogen. This exchange is vital, as nitrogen is a key component of nucleic acids and proteins.

Additionally, legumes produce leghemoglobin, a specialized molecule that binds oxygen to protect the nitrogenase enzyme from oxygen toxicity. Nitrogenase is essential for the conversion of gaseous nitrogen into ammonia, making it available for plant use.

Besides rhizobia, plants can also obtain nitrogen through detritivores, particularly fungi. Fungi form mycorrhizal associations with plant roots, enhancing nutrient absorption. There are two main types of mycorrhizae: ectomycorrhizae and arbuscular mycorrhizae. Ectomycorrhizae wrap around plant cells without penetrating them, while arbuscular mycorrhizae penetrate the cortical cells of plant roots, increasing the surface area for nutrient absorption. Both types of mycorrhizae play a significant role in breaking down organic matter, releasing nitrogen and phosphorus, which are essential for plant growth.

In summary, the interactions between legumes and rhizobia, as well as the relationships between plants and mycorrhizal fungi, are vital for nutrient acquisition, particularly nitrogen, which is essential for plant health and development.

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