Effects of multiple mutualistic partners in a tripartite mutualism between plants, fungal endophytes, and mycorrhizal fungi

Background/Question/Methods

Mutualism (i.e. positive species interactions in which all partners gain a net fitness benefit) are ubiquitous in nature and play a critical role in community and ecosystem functioning (e.g. coral-zoozanthelle foundation species). The study of mutualism has historically focused predominantly on pairwise interactions between co-evolved species, yet organisms often interact with more than one mutualist in nature.  When organisms associate with partners that confer unique benefits, non-additive effects on both their fitness and that of the partner species may result. Here, we use the tripartite mutualism between a common, native grass (Festuca subverticillata), an arbuscular mycorrhizal fungus (Glomus intraradices), and a systemic fungal endophyte (Neotyphodium sp.) to address:  1) How does having multiple partner mutualists affect the fitness of the shared host and the partner mutualists? 2) Do these fitness outcomes depend on the nutrient environment in which the interactions take place?  We conducted a factorial design greenhouse experiment independently manipulating the presence of each microbial mutualist and the levels of two key nutrients (nitrogen and phosphorus) involved in mutualist resource exchange. We assessed fitness for the plants via growth and biomass metrics, for the mycorrhizal fungi via percent root colonization, and for the fungal endophyte via leaf hyphal density.     

Results/Conclusions

While across all nutrient environments fungal endophytes increased plant fitness, mycorrhizal fungi did not have a significant main effect on plant fitness. However, there was a significant four-way interaction between the nitrogen, phosphorus, endophyte, and mycorrhizal treatments, indicating that the outcome of having multiple partners depended on the nutrient environment. Presence of microbial mutualists had greater impacts in high nitrogen, low phosphorus environments where endophyte increased plant fitness slightly in the absence of mycorrhizal fungi. The hyphal density of fungal endophytes was significantly affected by the interaction between mycorrhizal fungi, nitrogen, and phosphorus treatments, suggesting that both the presence of a second mutualistic partner species and the nutrient environment in which the interactions occurred may be important for endophyte fitness. In particular, hyphal density was lower in the presence of mycorrhizal fungi in a high nutrient environment. Preliminary data indicates that mycorrhizal colonization was present, but generally very low. This result may explain the lack of a main effect of mycorrhizal treatment on plant fitness and demonstrates an asymmetry in degree of interaction with these two types of mutualists. The context-dependency of our results emphasizes the importance of studying mutualistic interactions in both their biotic and abiotic environments.