4.1 Plant diversity and soil legacy effects on plant metabolomes
While we did not find any overall changes in the foliar or root
metabolome composition in response to plant diversity and soil legacy,
we observed the unique regulation of 139 metabolites. This is in line
with previous work showing that plant diversity or soil legacy can
affect the regulation of foliar metabolites (Huberty et al., 2020;
Scherling et al., 2010). Our study not only adds to this body of
literature but also expands our knowledge by revealing that plant-plant
and plant-soil interactions also affect the regulation of root
metabolites.
Plant-plant and plant-soil interactions can range from positive, over
neutral, to negative (Barry et al., 2019; Cortois et al., 2016). In
particular negative plant-plant interactions, such as competition, can
affect the regulation of metabolites. In our study, we detected 45
metabolites that were significantly up-regulated and 36 metabolites that
were significantly down-regulated as a response to plant-plant
interactions. This shift in regulation is potentially a consequence of
competition for resources, such as light, nutrients, and water, that can
force the plant to either invest resources into growth or defense, as
well as affect the production of allelopathic metabolites (Fernandez et
al., 2016; Treutter, 2006). Positive plant-soil interactions with
mutualists, such as arbuscular mycorrhizal fungi and plant
growth-promoting bacteria, that can improve nutrient uptake and protect
against antagonists (Bardgett & van der Putten, 2014; Latz et al.,
2012; Wardle et al., 2004), can also affect the regulation of
metabolites. In our study, we detected 24 metabolites that were
significantly up-regulated and 34 metabolites that were significantly
down-regulated as a response to plant-soil interactions. This shift in
regulation may be a response to mycorrhization that, for instance, can
affect phenyl alcohol and vitamin associated pathways (Rivero et al.,
2015), and/or a response to negative plant-soil interactions with root
parasites, pathogens, and herbivores that can reduce root uptake
capabilities of resources (Bardgett & van der Putten, 2014; van der
Putten et al., 2013). The infection with nematodes, for instance, can
affect the regulation of iridoid glycosides (Wurst et al., 2010), while
the interaction among different types of soil organisms can further
influence the plant metabolome and defense (Lohmann et al., 2009). In
addition to these interaction-specific effects on foliar and root
metabolomes, leaves and roots have different functions and are in
different abiotic and biotic environments (van Dam, 2009). These
differences are the likely reason that certain metabolite classes in our
study, such as alkaloids and phenolics, show different levels of
concentration among leaves and roots (Kaplan et al., 2008). Our study
confirms that plant-plant and plant-soil interactions affect the
regulation of metabolites in leaves and roots. Among the regulated
metabolites, we tentatively identified some as flavonoids, iridoids, and
alkaloid glycosides. Flavonoids are known as physiologically active
compounds, playing important roles as signals in plant-soil biota
interactions, as allelochemicals in plant-plant interactions, or as
deterrents in plant-herbivore interactions (Treutter, 2006). Iridoids
and alkaloid glycosides are known for their significant roles in
plant-herbivore interactions (Bowers & Puttick, 1988; Mithöfer &
Boland, 2008). Moreover, we also show for the first time that the nature
of the regulated metabolites is unique to the tissue and type of biotic
interaction. This strongly suggests that plants can adjust their
constitutive metabolome and specifically react to their biological
environment. In light of the recent support of the interaction diversity
hypothesis (Whitehead et al., 2021) for the maintenance of chemical
diversity, our study presents two potentially additional avenues of
biotic interactions (plant-plant and plant soil interaction) aside from
plant-herbivore interactions that may explain the maintenance of
chemical diversity in the plant kingdom.