Functioning of mycorrhizal associations along the mutualism–parasitism continuum*

Abstract

3 SUMMARYA great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts. In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and differences in fungal effects on, and responses to, the plant. Developing and testing quantitative models of mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry, and restoration.

Specialized cheating of the ectomycorrhizal symbiosis by an epiparasitic liverwort

“…The mycorrhizal specificity of Cryptothallus reported here is the most evolutionarily distant example of the epiparasitic syndrome; so far there are no exceptions to the pattern of epiparasitic plants specializing on phylogenetically narrow clades of fungi. Specialized mycorrhizal cheating is remarkable for several reasons: (i) it exploits one of the most widespread mutualisms in terrestrial ecosystems; (ii) it is one of the few nonanimal examples supporting the view that parasites are more specialized than mutualists (Price 1980); (iii) it represents one extreme of the mycorrhizal continuum where plant-fungus interactions range from parasitism of either partner at the ends to mutualism at the centre (Johnson et al 1997); (iv) it demonstrates that for some plants the high biodiversity of mycorrhizal communities lacks redundancy (Bruns 1995;Bidartondo et al 2000);and (v) it shows that mycorrhizal networks provide conduits for net fluxes of carbon from plants to fungi to plants. The latter two points have direct implications for myco-heterotrophic plant conservation strategies: specific soil fungi and photosynthetic plants must be taken into account.…”

“…Parasites of mutualisms, or cheaters, extract benefits normally exchanged exclusively between mutualists (Yu 2001). In the case of the mycorrhizal mutualism the autotroph supplies carbon to its fungal partners in exchange for mineral nutrients (Smith & Read 1997), and cheating of this symbiosis arises when the myco-heterotroph induces these same fungal partners to colonize it, thereby gaining access to photosynthate ( Johnson et al 1997).…”

Diversity and classification of mycorrhizal associations

“…Mycorrhizal associations have been considered to be parasitic in cases where the costs outweigh the benefits, as occurs in several cropping systems (Hendrix, Jones & Nesmith, 1992;Johnson et al, 1997). However, crops that benefit from mycorrhizas may eventually replace those that do not, because of crop rotation or the implementation of sustainable agricultural systems.…”

Gene expression in mycorrhizal orchid protocorms suggests a friendly plant–fungus relationship

“…The plant-fungus interactions in mycorrhizal symbioses have been described as a continuum ranging from mutualism to parasitism (Johnson et al 1997). Starting from early observations (Bernard 1909;Burges 1939), orchid mycorrhiza has often been portrayed as an example of parasitism because of the observation that the fungi occasionally attack and destroy the protocorm.…”