Imbalanced carbon-for-phosphorus exchange between European arbuscular mycorrhizal fungi and non-native Panicum grasses-A case of dysfunctional symbiosis

- Arbuscular mycorrhiza often confers benefits to both plant and fungal partners.
- We assessed mycorrhizal response of C4 and C3Panicum under different P levels.
- Mycorrhiza reduced plant biomass and P uptake, while draining carbon belowground.
- Our experiment demonstrated functional incompatibility of mycorrhizal partners.
- Possible causes are discussed like provenance, ontogeny or other microbes.

Arbuscular mycorrhizal fungi (AMF) are globally widespread root symbionts of the majority of terrestrial plant species, they are present in almost all soils, and show particularly low levels of partner specificity. Usually, they benefit their plant hosts through increased nutrients (especially phosphorus, P) supply, improved growth, stress tolerance and fitness as compared to the non-mycorrhizal plants. In exchange for the symbiotic benefits, plant supplies the fungal partner with carbon (C), constituting symbiotic costs for the plant. Here we tested the effect of four soil treatments, combining removal of indigenous AMF communities and/or supplementation with mineral P to restore plant P nutrition, on plant growth and C fluxes from plant to soil as well as on mineral nutrition of a C3 and a congeneric C4 grass species. Contrary to all expectations, both plant species showed lower P and nitrogen contents, and grew smaller, though allocated more C belowground, when supplied with AMF-containing full soil inoculum as compared to AMF-free inoculum. Our results indicate possible incompatibility of symbiotic partners of different geographic origin (European AMF and tropical/subtropical grasses from Africa/Asia), leading to apparent parasitism of the plants by the AMF communities in terms of both growth and nutritional responses. Most likely, downregulation of the direct (root) P uptake pathway by the plants in response to mycorrhiza formation over-compensated the symbiotic (indirect) P acquisition via mycorrhizal hyphae. The observed effects could also have been caused (or contributed to) by the relatively young age of the experimental plants, and different composition of microbial communities in the two inoculant (containing or not the AMF).