Collaborative Research: Building and testing predictions of context dependence of synergistic benefits to plants from multiple symbionts

Microbial symbionts help plants acquire resources and thereby improve plant productivity. Many plant species simultaneously interact with multiple symbionts. Notably, legumes, including economically important crops such as beans, associate with both soil fungi called arbuscular mycorrhizal fungi (AMF) and rhizobia bacteria, which improve uptake of soil phosphorus and nitrogen, respectively. Legumes can realize greater benefits from associating with both symbionts than expected from the effect each symbiont has on its own. That is, plants can derive synergistic benefits from multiple symbionts. However, when legumes will receive synergistic benefits is not understood. Understanding the contexts under which legumes are likely to receive synergistic benefits will help land managers and restoration ecologists manipulate environmental conditions to help establish desirable legumes in managed and degraded systems. In agricultural systems, understanding these context dependencies could help improve food production while potentially lessening the need for fertilizer inputs on legume crops. The goal of this project is to develop and test a predictive framework for synergistic benefits within the legume-AMF-rhizobia system. The investigators will also develop educational modules using legumes, rhizobia, and AMF for use in middle and high school classrooms, and train students within a collaborative environment. Although individual studies of legume-AMF-rhizobia interactions have shown synergism, synergism is not the general result according to meta-analyses. This suggests that the impact of simultaneous interaction with AMF and rhizobia on legume performance may depend on environmental context or plant and microbial characteristics. The investigators predict that synergism can occur when symbionts provide plants with complementary, limiting resources required for growth (phosphorus and nitrogen). This stoichiometric complementarity hypothesis suggests that synergism will occur when nitrogen and phosphorus are co-limiting and when the symbionts are effective at providing complementary resources. Moreover, synergism may require sustained reinvestment in symbionts over time and consequently be more likely in perennial than in short-lived annual legumes. The investigators will test these predictions by evaluating synergism in legume-AMF-rhizobia interactions while manipulating soil nitrogen and phosphorus availability, symbiont quality, and legume life history. The investigators will couple the experimental work with the development of theoretical models to extend mechanistic understanding of these symbioses to predictions of synergism. This work will improve understanding of the role that symbionts play in plant ecology and evolution and guide development of regenerative agricultural systems that maximize the value of plant microbiomes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.