Glomus (Chapter 27)

Abstract

Glomus is the most common and largest genus within the phylum Glomeromycota that form symbiotic relationships with plant roots. The genus includes species of arbuscular mycorrhizal fungi that frequently form abundant spores in soil and roots. It is morphologically characterized by spores that develop at the tip of sporogenous hyphae or as intercalary subglobose swellings on the inside of sporogenous hyphae. Spores are usually attached to a single subtending hypha which is simple, straight, or recurved, and rarely swollen. Spores are formed singly or in loose/compact sporocarps which are usually without or with peridium. Spores are mostly globose to subglobose; rarely ellipsoidal; hyaline, yellow, brown, or black colored with at least two or three (rarely up to five) distinct wall layers (usually inner-wall layers are laminated); and the laminated layer is generally associated with the evanescent layer. Spore germination mostly occurs with the emergence of a germ tube through the subtending hypha or directly through the spore wall. At maturity, contents of the spore are separated from the subtending hypha by a septum or occluded by spore wall thickenings. The genus forms long infection units with "H" connections between parallel strands, intraradical vesicles, and arbuscules that stain dark with Trypan blue. Phylogenetically, the genus is several times polyphyletic.