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.