Abiotic and biotic environmental context dependency of plant-microbial interactions

Background/Question/Methods

Most terrestrial legumes form associations with both rhizobia and arbuscular mycorrhizal fungi (AMF). Because each symbiont is nutritionally-based, with AMF providing phosphorus and rhizobia providing nitrogen, soil resource availability is important in evaluating the cost and benefits of each interaction. However, the biotic environment (i.e. additional species interactions) can also impose significant variation contributing to pairwise interactions. We conducted a fully-factorial greenhouse experiment manipulating the presence of two AMF species and two rhizobia strains on the native tallgrass legume, Amorpha canescens (leadplant). Simultaneously, we evaluated how variation in abiotic environmental resource levels of nitrogen and phosphorus influence the interactions between plants, AMF, rhizobia, and the microbial partners interactive effects. In this experiment we determined the treatment effects on the fitness of the host plant and each microbial symbiont.

Results/Conclusions

Both AMF (p<0.001; F_{1, 37}=630.76) and rhizobia (p<0.001; F_{1, 37}) infection promoted plant growth compared to control treatments. While we found no effect of AMF species on plant biomass, one rhizobia strain benefited plants more than the other strain (p =0.024; F_1,37=5.13). AMF and rhizobia interacted synergistically to promote plant growth (p<0.001; F_1,37=319.69). Overall, plants had higher biomass when grown in high nitrogen level environments (p=0.001; F_1,37=10.89). Plants also demonstrated higher growth under low phosphorus conditions (p=0.046; F_1,37=4.01). Interestingly, we found evidence for interactive effects between AMF species and rhizobia strains that depend on abiotic environmental conditions, with a significant three-way interaction between AMF, rhizobia, and phosphorus level (p=0.032; F_1,37=4.60). The presence of AMF influences the number of nodules produced, with more nodules formed on plants infected with both AMF and rhizobia, than rhizobia alone (p<0.001, F_{1, 37}=62.91). While AMF and nitrogen-fixing organisms have been shown to positively influence plant growth, few studies have found synergistic interactions between these symbionts, especially from the perspectives of both the plant and rhizobia. Furthermore, we demonstrate that the nature of these pairwise interactions is mediated by both biotic and abiotic environmental variation.