Metabolomics in the Rhizosphere: Tapping into Belowground Chemical Communication

The rhizosphere is densely populated with a variety of organisms. Interactions between roots and rhizosphere community members are mostly achieved via chemical communication. Root exudates contain an array of primary and secondary plant metabolites that can attract, deter, or kill belowground insect herbivores, nematodes, and microbes, and inhibit competing plants. Metabolomics of root exudates can potentially help us to better understand this chemical dialogue. The main limitations are the proper sampling of the exudate, the sensitivity of the metabolomics platforms, and the multivariate data analysis to identify causal relations. Novel technologies may help to generate a spatially explicit metabolome of the root and its exudates at a scale that is relevant for the rhizosphere community. Metabolomics is becoming accepted as a tool for the (untargeted) analysis of metabolites in root exudates. The importance of the rhizosphere microbiome for the functioning of plants is becoming increasingly clear. Statistical analysis and genetic mapping are used to establish relations between metabolites and (other) traits. The increased understanding of metabolic engineering in plants will allow the critical assessment of the role of individual compounds in plant–rhizosphere communication. Novel chemical-analytical platforms, such as laser-assisted electrospray ionisation (LAESI), will allow for spatial metabolomics on the scale of roots and interacting organisms.