Mycorrhiza

Abstract

Plants come into contact with the soil environment through the rhizosphere, a complex interface between the root surface and soil. Specialized rhizosphere fungi colonize plant roots and form symbiotic structures termed ‘mycorrhiza’. In the mycorrhizal symbiosis, the root and fungus together constitute a mutualism that controls the metabolism of both plant and fungus. There are different types of mycorrhiza, distinguished primarily by the morphology of the contact zone between the partners. The roots of many important forest trees such as spruce, pine, fir, and larch naturally form obligate fungal associations termed ectomycorrhiza (ECM). In this type of interaction, hyphae extending from a mycelial layer cover the surface of fine roots (called the mantle or sheath) and penetrate between root cells and form characteristic structures (called the Hartig net) in the cortex. When not in contact with the mycelium, the root systems of many ectomycorrhizal tree species, but particularly those of pine and spruce, while functional, perform poorly in terms of total length growth, branching pattern, and ability to exploit the soil in which they develop. It is likely that these tree species co-evolved with their fungal partners and in so doing developed the strategy whereby carbon is allocated to fungal rather than root structures to facilitate nutrient absorption.