Abstract
Many plant species simultaneously interact with multiple symbionts, which can, but do not always, generate synergistic benefits for their host. We ask if plant life history (i.e. annual vs perennial) can play an important role in the outcomes of the tripartite symbiosis of legumes, arbuscular mycorrhizal fungi (AMF), and rhizobia. We performed a meta-analysis of 88 studies examining outcomes of legume-AMF-rhizobia interactions on plant and microbial growth. Perennial legumes associating with AMF and rhizobia grew larger than expected based on their response to either symbiont alone (i.e. their response to co-inoculation was synergistic). By contrast, annual legume growth with co-inoculation did not differ from additive expectations. AMF and rhizobia differentially increased phosphorus (P) and nitrogen (N) tissue concentration. Rhizobium nodulation increased with mycorrhizal fungi inoculation, but mycorrhizal fungi colonization did not increase with rhizobium inoculation. Microbial responses to co-infection were significantly correlated with synergisms in plant growth. Our work supports a balanced plant stoichiometry mechanism for synergistic benefits. We find that synergisms are in part driven by reinvestment in complementary symbionts, and that time-lags in realizing benefits of reinvestment may limit synergisms in annuals. Optimization of microbiome composition to maximize synergisms may be critical to productivity, particularly for perennial legumes.