You have discovered a new species of fungus associated with plant roots. it invades the cells in the roots, forming arbuscular mycorhizzae. based just upon this character, you confidently determine that this fungus is member of the:

The fungus is member of the phylum Glomeromycota, and class Glomeromycetes.

Explanation:

The phylum Glomeromycota includes all the fungi species that develop arbuscular mycorrhizae in relation to the symbiotic interaction between the fungus hyphae and the plant's roots. The most important taxonomic character is the arbuscular structures inside the plant's roots.