Metabolomic Profiling of Root Exudates for Wheat-Microbial Interactions.

Under abiotic stress (e.g., drought and salinity), plants produce the phytohormone ethylene, which induces defense responses, including reduced root and shoot growth and reduced productivity. Certain plant growth-promoting rhizobacteria (ACC+ bacteria) degrade the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), which results in root elongation and greater resistance to water stress.  Previous studies have shown that the abundance, diversity, and composition of these bacteria vary among different winter wheat genotypes. We hypothesized that winter wheat genotypes differ in their root exudate profiles and their ability to produce metabolites that may select for ACC+ bacteria. To test this hypothesis, four genotypes of winter wheat (Byrd, Ripper, RonL, and WB-Cedar) were grown from seed in the greenhouse, in 1-gallon pots filled with fritted clay. Each genotype was grown in 12 pots, with one plant per pot. Pots were irrigated throughout the study. Seven weeks after germination, a section of roots was carefully excavated from the clay medium, rinsed, and placed into 50-ml syringes that were taped onto the side of the pot. Syringes were filled with inert sand, plugged by glass wool, and closed off with a stopcock. Sand was kept moist with daily irrigating. After two weeks, exudates from the roots were collected by attaching a vacuum to each syringe and pulling the solution into a sterile 50-ml centrifuge tube. Tubes were centrifuged, and the supernatants were analyzed by ultra high pressure liquid chromatography-coupled with mass spectrometry (UPLC-MS-MS). Our results confirm that root exudates vary quantitatively and qualitatively among the four genotypes, and specific exudate compounds were identified for future studies on ACC+ bacterial recruitment.