Phytophthora species and arbuscular mycorrhizal (AM) fungi follow analogous steps to establish a root interaction. Following chemical cross-talk, the microbe germinates and forms attachment and penetration structures, termed appressoria and hyphopodia, respectively. Penetration occurs through or between cells, and in the case of AM fungi intracellular hyphae are supported by a plant-derived pre-penetration apparatus . Specialized intracellular interfaces, termed haustoria and arbuscules, form within plant root cells. Phytophthora infections eventually result in cell death of the infected tissue, while obligate biotrophic AM fungi continuously reside in living plant roots.