Plants need nitrogen because it is a major component of chlorophyll, which is the pigment used in photosynthesis. In addition, nitrogen is necessary to form amino acids, which are used to build proteins. Nitrogen is the most abundant element in Earth's atmosphere; 78% of the atmosphere is nitrogen. However, this atmospheric nitrogen is not available for plants to use.
However, legumes (peas and beans) can obtain nitrogen from their associations with rhizobia, many kinds of bacteria that live within the roots of the plants. The bacteria are brought to the roots when the plants release chemicals into the soil. These chemicals also encourage the activation of genes within the bacteria that form a nodule, a structure found on the roots of nitrogen-fixing plants that contain symbiotic bacteria that fix nitrogen. The bacteria enter the roots by way of an infection thread, which is a tube the bacteria use to move into the plant root. The bacteria enter into the cytoplasm of the cells of the nodule, where they can fix nitrogen from the atmosphere.
The main function of rhizobia is to pull nitrogen from the air and convert it into a usable form for the plant. Plants cannot use atmospheric nitrogen because the triple bond holding the nitrogen molecules together is very stable and strong. It requires a lot of energy to break it apart. The rhizobia engage the conversion of atmospheric nitrogen into ammonia, a form that is more usable by plants through the process of nitrogen fixation. This happens because of the actions of an enzyme called nitrogenase. It adds three pairs of hydrogen ions (H+) to the nitrogen gas to form two ammonia molecules. This effort requires a lot of energy from ATP, and a reducing agent (a molecule that donates an electron) must be present.Only legumes form these symbiotic associations, but the legumes store a great deal of fixed nitrogen in their root nodules, which break down. The nitrogen in their tissues is released back into the environment to be taken in again by new plants. This process, along with the mycorrhizal associations, provides sufficient nitrogen for all plants.
The symbiotic relationship between plant roots and fungus found within the soil is called mycorrhiza (plural, mycorrhizae). This mutualistic relationship happens in more than 90% of all plants. The fungus wraps itself around plant roots and penetrates into the root cells. The plants send out signals that help the fungus find and wrap around them. Plant roots produce a special type of molecule called a strigolactone that signals to attract mycorrhizae hyphae. Strigolactones encourage the growth of hyphae, which are small filaments branching off the fungus. When a fungus comes in contact with strigolactones, the fungus produces signals activating special "root-symbiosis" genes. These genes cause the growth of the hyphae into the interior parts of the root cortex through special areas called prepenetration apparati (PPA). Here plants trade the sugars produced by photosynthesis (which the fungus can use as energy source) for soil nutrients such as phosphate (which can be supplied by the fungus), but the organisms never share cytoplasm. The cell membranes of each species stay intact. Without these associations, plant roots often could not obtain these nutrients. For example, fungi can extend hyphae into clay or acidic soils, whereas plants cannot. Thus fungi can access nutrients locked in these soils. In addition fungi help break down organic matter that is slow to decay, such as wood. This releases the nutrients in them for use by plants.Fungi can expand the root structures of their host plants, sometimes by more than 1,000 times. This is highly beneficial because it gives that particular plant a much greater chance of finding the nutrients it needs, even if they are far away. The hyphae are much thinner than normal roots, so they can penetrate even the smallest spaces between soil particles in search for phosphorus, the primary nutrient obtained in this manner.