Interactions between plants and phytophagous insects play an important part in shaping the biochemical composition of plants. Reciprocally plant metabolites can influence major life history traits in these insects and largely contribute to their fitness. Plant rhizospheric microorganisms are an important biotic factor modulating plant metabolites and adaptation to stress. While plant-insects or plant-microorganisms interactions and their consequences on the plant metabolite signature are well-documented, the impact of soil microbial communities on plant defenses against phytophagous insects remains poorly known. In this study, we used oilseed rape (Brassica napus) and the cabbage root fly (Delia radicum) as biological models to tackle this question. Even though D. radicum is a belowground herbivore as a larva, its adult life history traits depend on aboveground signals. We therefore tested whether soil microbial diversity influenced emergence rate and fitness but also fly oviposition behavior, and tried to link possible effects to modifications in leaf and root metabolites. Through a removal-recolonization experiment, 3 soil microbial modalities ("high," "medium," "low") were established and assessed through amplicon sequencing of 16S and 18S ribosomal RNA genes. The "medium" modality in the rhizosphere significantly improved insect development traits. Plant-microorganism interactions were marginally associated to modulations of root metabolites profiles, which could partly explain these results. We highlighted the potential role of plant-microbial interaction in plant defenses against Delia radicum. Rhizospheric microbial communities must be taken into account when analyzing plant defenses against herbivores, being either below or aboveground.

Plants vary widely in how common or rare they are, but whether commonness of species is associated with functional traits is still debated. This might partly be because commonness can be measured at different spatial scales, and because most studies focus solely on aboveground functional traits. We measured five root traits and seed mass on 241 central European grassland species, and extracted their specific leaf area, height, mycorrhizal status and bud-bank size from databases. Then we tested if trait values are associated with commonness at seven spatial scales, ranging from abundance in 16-m 2 grassland plots, via regional and European-wide occurrence frequencies, to worldwide naturalization success. At every spatial scale, commonness was associated with at least three traits. The traits explained the greatest proportions of variance for abundance in grassland plots (42%) and naturalization success (41%) and the least for occurrence frequencies in Europe and the Mediterranean (2%). Low root tissue density characterized common species at every scale, whereas other traits showed directional changes depending on the scale. We also found that many of the effects had significant non-linear effects, in most cases with the highest commonness-metric value at intermediate trait values. Across scales, belowground traits explained overall more variance in species commonness (19.4%) than aboveground traits (12.6%). The changes we found in the relationships between traits and commonness, when going from one spatial scale to another, could at least partly explain the maintenance of trait variation in nature. Most importantly, our study shows that within grasslands, belowground traits are at least as important as aboveground traits for species commonness. Therefore, belowground traits should be more frequently considered in studies on plant functional ecology.

Plant below‐ground organs perform essential functions, including water and nutrient uptake, anchorage, vegetative reproduction and recruitment of mutualistic soil microbiota. Recently, multivariate analyses showed that root traits of species can largely be linked to a ‘conservation’ and a ‘collaboration’ gradient. Here, we tested whether this species‐level bidimensional below‐ground trait space also exists at the community level in grasslands. Furthermore, we tested whether the position of grassland communities in below‐ground trait space relates to environmental variables. For a total of 313 species, we collected data on eight below‐ground traits in greenhouse and common garden experiments and supplemented it with data on bud‐bank size and specific leaf area from databases. We calculated community weighted means (CWMs) of these 10 traits for 150 temperate grassland plots to investigate below‐ground plant‐trait dimensionality and its variation along 10 soil and land‐use parameters. Using PCA, we found that about 55% of variance in CWMs was explained by two main dimensions, corresponding to a mycorrhizal ‘collaboration’ and a resource ‘conservation’ gradient. Frequently overlooked traits such as rooting depth, bud‐bank size and root‐branching intensity were largely integrated in this trait space. The two plant‐strategy gradients were partially dependent on each other, with communities that do ‘outsourcing’ of resource uptake to mycorrhizal fungi along the collaboration gradient also being more ‘slow’ along the conservation gradient. (i.e. high root tissue density and high root weight ratio). ‘Outsourcing’ communities were also more often deep rooting and associated with soil parameters, such as low moisture and sand content, high topsoil pH, high C:N and low δ15N. ‘Slow’ communities had large bud banks and were associated with low land‐use intensity, high topsoil pH and low nitrate but high ammonium concentration in the soil. Surprisingly, we did not find an association of phosphorus availability with the mycorrhizal ‘collaboration’ gradient. Synthesis. The ‘collaboration’ and ‘conservation’ gradients previously identified among species scale up to the community level in grasslands, encompass more traits than previously described, and vary with the environment.