Arbuscular mycorrhizal fungi are all placed in the phylum (division) Glomeromycota. They form the widespread arbuscular mycorrhiza symbiosis with land plants (Embryophyta). The fungi are obligate symbionts that cannot be cultured without a plant as a 'host' (symbiotic partner). Arbuscular mycorrhizal fungi have co-evolved with land plants for >400 million years, and probably were essential for the emergence of land plants.