Mass spectrometry imaging: Towards mapping the elemental and molecular composition of the rhizosphere

This short review provides perspective regarding the use of mass spectrometry imaging (MSI) to study the rhizosphere. It also serves to complement the multi-omic-focused review by White et al. (in press). MSI is capable of elucidating chemical distributions within samples of interest in situ, and thus can provide spatial context to MS omics data in complementary experimental endeavors. Most MSI-based studies of plant-microbe interactions have focused on the phyllosphere and on the "associated rhizosphere" (our term for material that is not removed during harvesting). Sample preparation for these in situ analyses tends to be a limiting factor. These studies, however, have provided valuable insights into the spatial arrangement of proteins, peptides, lipids, and other metabolites within these systems. We intend this short review to be a primer on the fundamentals of MSI and its role in plant-microbe analysis. Lastly, we offer a perspective on the future of MSI and its use in understanding the molecular transformations beyond what we call the associated rhizosphere, one which extends to the rest of rhizosphere and into the bulk soil.