<I>Glomus custos</I> sp. nov., isolated from a naturally heavy metal-polluted environment in southern Spain

An undescribed species of the arbuscular mycorrhizal (AM) fungi (Glomeraceae, Glomeromycetes) was isolated from the bank side of the Rio Tinto River (Huelva, Spain), a naturally heavy-metal polluted environment. The species description is based on spore morphological parameters from in vitro root organ cultures, in vivo pot cultures, in vitro colony morphogenesis, and molecular analyses. Mature in vitro grown and pot cultured spores are pale to brownish yellow, globose to subglobose, 110-172 µm diameter and characterized by 4-layered walls. Phylogenetic analyses of the entire rDNA ITS region place the species into the Glomeraceae (group A) without closely related homology with known species. Glomus custos forms vesicular-arbuscular mycorrhizae with leek and clover plants under in vivo growing conditions and with excised carrot roots under in vitro propagation. The name custos (guardian) refers to the protective properties this fungus confers to host plants in terms of resistance to extreme pH and heavy metal concentrations in soil.