11 - Ericoid mycorrhizas

Abstract

This chapter discusses the current understanding of the structural, taxonomic, and some functional aspects of ericoid mycorrhizas (ERM). Roots of ericoid plants are delicate structures, the anatomy of which shows considerable uniformity across all tribes of the Ericaceae, which are characterized by this type of mycorrhizas. Their most distinctive feature is the absence of root hairs. Because of their narrow diameters, which range from 100 μm to <50 μm in distal regions, such structures are referred to as “hair roots.” Hair roots have a monarch stele containing only a single file of very small aracheids and sieve elements within a ring of pericycle cells. Surrounding the vascular elements is a two-layered cortex consisting of an endodermis and a suberized exodermis. The outermost cell layer, the epidermis, provides both the interface with the soil, and it is exclusively these cells that are colonized by ERM fungi. During the colonization process of ERM, the hyphae of ERM fungi form a loose network over the zone of the hair root, which contains a mature epidermis. As the root apex extends, new epidermal cells are differentiated and the hyphae at the leading edge of the network advance to colonize them. The application of molecular techniques has improved the understanding of the interrelationships both of the plants, which form ERM and of the fungi involved in the associations. Cultural characteristics had revealed that, while there were apparently unifying features among ERM fungal isolates, there were also identifiable differences between them. Experimental examination of the nutrition of ericoid and epacrid endophytes growing in axenic culture and of the enzymes involved in the mobilization of structural and nutritient containing polymers have revealed that these fungi possess a wide range of saprotrophic capabilities.