Impacts of the Interaction between Viral Pathogens and Mutualistic Fungi on Plant Performance Under Elevated CO2

Predicting the effect of increasing CO2 on plants may require that we understand how such increases alter the strength and nature of multiple, co-occurring species interactions. Here we investigate the interactive effects of fungal mutualists, a viral pathogen, and phosphorus availability on plant response to increased CO2 supply. We predicted that the interaction of arbuscular mycorrhizal fungi (AMF) infection and virus infection would result in plants better able to handle virus infection such that virus-infected plants associating with AMF should be larger than those infected plants not associating with AMF. This effect should be amplified under elevated CO2 during which carbon supply is greatest. We also predicted increased carbon supply would increase mycorrhizal hyphal colonization. Finally, we predicted that increases in plant carbon supply would counterbalance the disruption of the carbon supply by the virus, resulting in increased biomass for those plants infected with virus and grown under elevated CO2 compared to those plants also infected with virus but grown under ambient CO2. To test these hypotheses we grew two exotic invasive annual grasses, Avena fatua and Bromus hordeaceus, and factorially manipulated atmospheric CO2 concentration, soil phosphorus, mycorrhizal association and Barley yellow dwarf virus–PAV infection. The interaction of mycorrhizal and viral infections did not result in increased biomass for virus infected plants. However, elevated CO2 significantly altered AMF colonization for plants of both species grown with additional phosphorus or infected with virus. Both mycorrhizal association and phosphorus addition independently altered viral titer in plants of both species. Thus, increases in plant phosphorus either through AMF or via soil amendments contribute to increases in viral titer. Most predictions of plant responses to elevated CO2 fail to include the impacts of both microbial mutualists and pathogens. Our research indicates that the inclusion of these microbes is important in order to fully understanding plant responses to increased CO2 availability