Arbuscular mycorrhizal fungi inside corn root cells. Arbuscules are the finely branched structures of fungi found in the roots of plants, where they form a symbiotic (mycorrhizal) relationship. Arbuscular mycorrhizal (AM) fungi help plants to capture nutrients, such as phosphorus, and micronutrients from the soil, exchanging them for carbon. This is one mechanism by which carbon dioxide from the air is sequestered in the soil. The carbon is used to form more and longer hyphal strands and to produce glomalin.