Plant diversity and soil legacy can affect the plant metabolome
Recently, research on the response of plants to plant-plant interactions and soil feedbacks has been expanded beyond the common morphological and physiological traits. The consideration of the plant metabolome,i.e., the entirety of metabolites synthesized by a plant (Oliver et al., 1998) gave rise to a new discipline, eco-metabolomics, which uses metabolome analyses, or metabolomics, to illuminate the chemical mechanisms underpinning ecological and environmental processes (Peñuelas & Sardans, 2009; Peters et al., 2018). Eco-metabolomics has been employed to investigate if plants respond on a molecular level to plant community composition and soil biota diversity (Huberty et al., 2020; Ristok et al., 2019; Scherling et al., 2010).
Plant-plant interactions, for instance, can induce shifts in foliar metabolic profiles of multiple grassland plant species, with more than 100 metabolites changing in their concentration (Scherling et al., 2010). In addition, differential selection due to growing in monocultures or plant species mixtures can select for plants with distinct metabolomes (Zuppinger-Dingley et al., 2015). Similarly, in the presence of soil biota, plants produce species-specific shoot and root metabolomes that differ from those of plants grown in sterile conditions (Ristok et al., 2019). Furthermore, these plant-soil interactions often affect the diversity of a plant’s metabolome and can exert stronger metabolomic shifts than foliar herbivory (Huberty et al., 2020). Root parasites, pathogens, and herbivores as well as mutualistic symbionts can change the concentration of primary and secondary metabolites in leaves and roots in multiple ways, e.g. , up- or down-regulation of specific metabolites (van Dam & Heil, 2011; van der Putten et al., 2013). These responses are generally species-specific, context-dependent, and can affect subsequent biotic interactions (Bezemer & van Dam, 2005; Ristok et al., 2019).
Herbivory-induced defenses can be altered by biotic interactions
One important interaction type is that between plants and herbivores. Plants have evolved a plethora of indirect and direct chemical defenses to deal with attackers (Karban & Baldwin, 1997). Of special interest are induced defenses, i.e. , changes in the concentration of metabolites following an attack by parasites, pathogens or herbivores, or after interactions with beneficial microbes (Ferlian et al., 2018). Such induced responses can affect the plant metabolome locally or systemically (Bezemer & van Dam, 2005). Both plant-plant interactions and plant-soil interactions can modulate the induction of defensive metabolites. Plant-plant interactions can affect induced defenses through plant competition, which forces the plant to either invest resources into growth or defense (Broz et al., 2010; Fernandez et al., 2016; i.e., growth-defense trade off; van Dam & Baldwin, 2001). In addition, volatile organic compounds can induce defensive responses immediately or prime for future attacks (Baldwin et al., 2006). Plant-soil interactions with microbes, nematodes, and mycorrhizal fungi cannot just induce defenses locally in roots, but also systemically in foliar tissues (van Dam & Heil, 2011). Either of these groups of soil biota can up- or down-regulate specific primary metabolites, such as amino acids and sugars, or secondary metabolites, such as glucosinolates and iridoid glycosides, in aboveground plant tissues (Hol et al., 2010; Rivero et al., 2015; Wurst et al., 2010).
Taken together, both plant-plant interactions and plant-soil interactions play significant roles in modulating the plant’s metabolome, thereby affecting resistance to aboveground herbivores (Ristok et al., 2019; van Dam & Heil, 2011). Thus far, however, not much is known about the individual impact of plant-plant interactions or plant-soil interactions within plant communities. This is likely due to the fact that plant-plant and plant-soil interactions are tightly linked in natural communities. In addition, most microcosm studies only focus on plant-soil interaction effects (Huberty et al., 2020; see e.g. Ristok et al., 2019). Here, we explicitly investigate to which extent plant-plant-interactions (PPI) or plant-soil interactions (PSI) affect the metabolomes of three forb species in a similar microcosm set-up. Both the PPI and PSI experiment covered the same range of diversity levels and plant community compositions; either as assembled plant communities grown in sterile soil (PPI) or via the inoculation of sterile substrate with conditioned field soil of communities with similar plant diversity levels (PSI). In addition, a subset of all plants was infested with larvae of the generalist herbivoreSpodoptera exigua to induce defense responses. We analyzed all samples using an untargeted metabolomics approach focusing on profiling plant secondary metabolites in leaves and roots. We hypothesized that (1) both plant diversity and soil legacy can alter the overall plant metabolome, as well as affect the regulation of specific metabolites. In addition, we hypothesized that (2) the induced defense to herbivory is differently affected by plant diversity and soil legacy.