A mycorrhiza (Greek: μυκός, mykós, "fungus" and ριζα, riza, "roots", pl. mycorrhizae or mycorrhizas) is a symbiotic association composed of a fungus and roots of a vascular plant. In a mycorrhizal association, the fungus colonizes the host plant's roots, either intracellularly as in arbuscular mycorrhizal fungi (AMF or AM), or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry. The association is generally mutualistic, but occasionally weakly pathogenic.