2 - Colonization of roots and anatomy of arbuscular mycorrhizas

Abstract

This chapter provides an account of the main characteristics of arbuscular mycorrhizal (AM) roots of different types and shows how they develop from sources of in the soil. Colonization of roots by AM fungi can arise from three main sources of inoculum in soil: spores, infected root fragments, and hyphae—collectively termed propagules. Based on the variations in morphology of colonization AM have been classified as Arum-and Paris-typemycorrhizas. In Paris-typemycorrhizas, cortical colonization of roots is characterized by extensive development of intracellular coiled hyphae, which spread directly from cell to cell. While in Arum-type associations the fungus spreads relatively rapidly in the root cortex by intercellular hyphae, which extend along well-developed intercellular air spaces. Regardless of morphology, AM colonization of roots can be initiated from hyphae growing from any of the three sources of inoculum. Primary colonization of roots from discrete propagules can be initiated from as far away as 13 mm. Hyphal contact with the root is usually followed by adhesion of the hypha to the root surface and, after about 2–3 days, the formation of swollen appressoria. Following the formation of an appressorium and penetration of the epidermis and exodermal cells, infection units develop in both Arum-type and Paris-typemycorrhizas. The outcome of the colonization process is that the fungus comes to occupy two different apoplastic compartments in the root, the intercellular spaces between cortical cells, and a more specialized intracellular apoplast, surrounding arbuscules or coils. These interfaces play critical roles in bidirectional nutrient transfers between the symbionts. The morphological structures formed by the fungi, particularly in the root cortex, are highly varied and include both “classic” arbuscules as well as less well-known and understood intracellular coils and arbusculate coils.