Movement of arbuscular mycorrhizae along the mutualism-parasitism continuum

Most plants associate with arbuscular mycorrhizal (AM) fungi, which facilitate plant uptake of soil nutrients in exchange for plant carbon.  This interaction is often mutually beneficial to both associates, however, AM fungi can have no effect or be parasitic on their plant host.  The processes that move the interaction along this mutualism-parasitism continuum are poorly understood.  Theoretically the degradation of the plant-mycorrhizal mutualism is predicted as a consequence of the cost fungi endure in acquiring and delivering phosphorus to its host.  Consistent with this expected cost of mutualism, I have found that the benefit that plants receive from their mycorrhizal fungal community decreases over time, indicating that the best growth promoting fungi are the poorest competitors.  Partner choice has been suggested as a mechanism to prevent the degradation of mutualism, however the potential for partner choice within AM fungi seems unlikely given the low specificity of association between plants and fungi.  We used a carbon labeling experiment to demonstrate that plants can preferentially allocate carbon resources to the better AM fungal mutualist.  We then demonstrated that this preferential allocation can prevent the degradation of the mycorrhizal mutualism provided there is an adequate level of spatial structure within the plant root system.