The mycorrhizal symbiosis

The rhizosphere and the surface of plant roots are inhabited by populations of several microorganisms. Among these populations, a group of filamentous fungi can be present quite consistently on the root surfaces or in the tissues or cells of the roots, so that dual organs of consistent morphological and histological patterns are formed (Harley, 1989). In these ‘mycorrhizas’ the fungus and the host co-exist actively for long periods in a state called a mutualistic symbiosis. Most plant species in the greater part of the world's ecosystems are infected with these mycorrhizal fungi (Harley & Smith, 1983; Harley & Harley, 1987, Janos, Chapter 10).

Under natural conditions the vast majority of the fungi involved in this association appear to be obligate symbionts, with little or no ability for independent growth. This seems to be less true for the autobionts. Their degree of mycorrhizal dependency is somewhat more variable. Some plant species are infected occasionally while most others (70% of the angiosperms according to Trappe, 1987) cannot complete their life cycles without mycorrhizas, at least in their natural environment. However, environmental factors as well as the presence of companion plants can determine whether a plant is mycotrophic or not in some settings (Miller, 1979; Molina, Massicotte & Trappe, 1992).