COS 204-1 - Capturing the chemical diversity of metabolites and lipids in the rhizosphere of a perennial energy grass grown on marginal soil

Abstract: The plant root- soil interface, known as the rhizosphere, is the center of dynamic biogeochemical activity and interkingdom interactions. Small molecule metabolites and lipids exuded by plant roots and their associated microbes play a central role in shaping plant-microbe interactions. Studies to date have mainly focused on a limited subset of root exudate metabolites (sugars, amino acids, organic acids) and the chemical diversity of the rhizosphere environment remains largely unexplored. A thorough characterization of the composition of small molecules in root exudates and rhizosphere soil is the first step towards elucidating the functional roles of small molecules that drive belowground interkingdom interactions.

The goal of this study is to comprehensively profile rhizosphere metabolites and lipids in mature tall wheatgrass (Thinopyrum ponticum) plants grown in our field site in Prosser, WA. Our field site features marginal, low carbon aridosols, ideal for investigating bioenergy and carbon sequestration potential. We collected a series of soil fractions with varying proximity to plant roots and adjacent unplanted plots. We also collected root exudates and root tissue to help determine the origin of metabolites (plant vs microbial). Untargeted metabolomics and lipidomics analysis using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) was performed. Over 500 unique metabolites and lipids from 11 chemical super classes were detected. Of these, 26% belonged to organic acids and derivatives, 19% to lipid and lipid-like molecules, 17% to benzenoids, 16% to organoheterocyclic compounds and 10% to phenylpropanoids and polyketides. Numerous metabolites displayed abundance trends based on the soil proximity to the plant root. Organic acids such as glutaric acid, succinic acid and coumaric acid were significantly more abundant in rhizosphere soil and decreased with distance from the roots. Saturated fatty acids, palmitic acid and stearic acid, were least abundant in rhizosphere soil suggesting microbial uptake. Total carbon and total nitrogen were significantly higher in rhizosphere soil. DNA yields (ng DNA/g soil) however, were highest in the soil fraction collected a few mm away from the rhizosphere soil. Ongoing efforts include characterization of bacterial and fungal community composition using 16S rRNA and ITS region amplicon sequencing and gene expression using metatranscriptomics.

Overall, our study reveals the immense chemical diversity of root exudates and rhizosphere soil and lays the foundation for follow-on experiments to probe origin and roles of bioactive small molecules that drive interactions between plants, soil, and microbial communities in the rhizosphere.