4.2 Plant diversity and soil legacy effects on herbivore-induced responses
We also observed alterations in the herbivore-induced metabolomic response due to plant diversity and soil legacy. Together, plant-plant and plant-soil interactions regulated 82 metabolites in control plants and 141 metabolites in herbivore-induced plants.
As shown above, plant-plant interactions can modulate growth-defense trade-offs that likely vary in strength with changes in plant diversity. In mixed communities, a combination of niche complementarity but increased competition for light, as well as a reduction of herbivory by specialized herbivores via dilution effects, may lead to a higher investment of resources into growth than defense compared to monocultures (Castagneyrol et al., 2014; Eisenhauer et al., 2019; Finch & Collier, 2000; van Moorsel et al., 2018). In fact, earlier work revealed that plants growing in mixed communities invested more resources into growth than defense-related metabolites compared to plants growing in monoculture (Broz et al., 2010), potentially reducing herbivore resistance. While we did not find differences in the overall metabolome composition of herbivore-induced plants in response to increasing plant diversity, we observed induced metabolite regulation in mixed communities. Our results suggest that the identity of the neighboring plant species determines the extent and direction of the plant-plant interaction. This has potential consequences for our understanding of plant-herbivore interactions in mixed communities, but further research is needed to confirm this hypothesis.
Plant-soil interactions, on the other hand, can prepare a plant for future attack, also called priming (Conrath et al., 2006). Systemic priming in plants can occur following interactions with soil microbes, nematodes, and mycorrhizal fungi, allowing the plant to better respond to subsequent herbivory (Kaplan et al., 2008; Martinez-Medina et al., 2016). While we have not explicitly tested for priming, it may explain why the absolute number of up-regulated metabolites in herbivore-induced plants (in comparison to control plants) was highest when plants had grown in different soil legacies. However, other possible mechanisms, such as systemic acquired resistance to microbial pathogens, exist that could also explain the patterns of metabolite regulation in our study (Ryals et al., 1996).
Finally, we observed differences in the regulation of herbivore-induced metabolites among our plant species. In R. acris plants, plant-plant interactions resulted in a strong down-regulation of induced metabolites, while plant-soil interactions resulted in a strong up-regulation of induced metabolites. The response to either type of biotic interaction was much more attenuated in G. pratense andL. vulgare , suggesting differences in the plant species-specific adaptability which requires future research before general assumptions can be made on the effects of plant diversity versus soil legacy on herbivore resistance.
While the present experiment provides novel insights into how metabolomic profiles, and thereby herbivore resistance, respond to changes in plant and soil biodiversity, it also calls for future studies. To allow for the comparison of plant-plant and plant-soil interactions in our study, we inoculated sterile substrate with liquid field soil inoculum from the Trait-Based Experiment (Ebeling et al., 2014) in the PSI experiment. This, however, meant that the soil biota communities were adapted and “linked” to the plot-specific plant communities and that the sand-peat mixture that was used may have created a different environment than the one the microbes were accustomed to. To fully disentangle plant from soil biodiversity effects on the plant metabolome, one would need to expose plants to artificially constructed soil communities (see e.g. de Souza et al., 2020), also including larger soil organisms (see e.g. Lohmann et al., 2009). While this was not feasible in the scope of this study, it would also be important to explore the specific effects of pre-selected functional soil biota groups, such as nematodes (e.g. Bezemer et al., 2005). Moreover, future studies should explore potential shifts in growth-defense trade-offs in more detail by exploring the performance of plants and herbivores. To our knowledge, this kind of comparable experimental design to disentangle plant-plant and plant-soil interaction effects has rarely been employed (but see Kos et al., 2015) and results and conclusions can vary between studies. Hence, we advocate for additional experiments of that kind to generate the necessary data for more reliable conclusions.