Plant-Microbe Interactions
Host-specificity and functional diversity among arbuscular
mycorrhizal fungi
Klironomos, John
Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1 Canada
ABSTRACT
Arbuscular mycorrhizal (AM) fungi are mutualistic symbiotic associations between 150
species of fungi and the roots of approximately 300 000 species of plants. As a result of
this low fungus:host species ratio, it has been assumed that the fungi are not host-specific
and that there is a high functional redundancy among fungal species. In this study, fungi and
plants were isolated from an old field meadow and used in a series of experiments to test for
host-specificity and functional redundancy. AM fungi were not host specific - 96% of
combinations yielded successful colonization. However, there was strong evidence for
functional variability among fungal species. Differential responses in P uptake, protection
against non-mycorrhizal fungal infection, and productivity was detected for almost every
plant-fungus combination tested. The nature of the responses was highly fungus- and plant-
species specific. These results suggest that changes in the species composition of AM
fungal communities can strongly alter plant community structure.
Introduction
Few studies have explored the role of soil organisms, particularly mycorrhizas, in regulating
the diversity of plant species or guilds. This is despite the fact that most significant
ecosystem functions (ie. primary productivity, nutrient cycling, trophic patterns) are based
on feedback mechanisms between above and belowground organisms, and that mycorrhizal
fungi are known to act as critical mediators in many of these processes (Brundrett 1991;
Francis and Read 1994).
Many plant and fungal ecologists have assumed that AM fungi are functionally redundant
(Allen et al. 1995), at least with regard to plant fitness and community structure, and have
ignored the possible role of AM fungal diversity in regulating floral diversity and
productivity. Despite the proliferation of research on AM fungi, most of it has focused on
only a handfull of plant and fungal taxa (Klironomos and Kendrick 1993) and very few
researchers have examined feedbacks between the diversity of plants and AM fungi. Two
studies have examined the influence of plants on AM populations and both found that
different plant species stimulated different levels of AM propagation (Johnson et al. 1992;
Sanders and Fitter 1992), while a unique microcosm study (Grime et al. 1987) found that
plant diversity increased in response to a single AM species inoculation. A more recent
study (van der Heijden et al. 1998a) showed that plant species differed in their dependancy
on AM fungi and that specific fungal species had significantly different effects on plant
growth. Together, these studies suggest that different AM fungi play different roles within
ecosystems.
In this study we tested the hypothesis that AM fungi within a single habitat are not host-
specific in their ability to successfully colonize plant roots, but that AM fungal diversity is
