The answer that best fits the blank is GLOMEROMYCETE. The glomeromycete clade belongs to the division glomeromycota of the kingdom Fungi. These species are commonly found in the roots of land plants or trees. In addition, arbuscular mycorrhizae is also a kind of fungus that are also found in the roots of the plants and this fungus is believed to improve the phosphorus uptake and water absorption of land plants.
