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.