Drought severity gradient affects root exudate composition in blue grama (Bouteloua gracilis)

Authors: ULRICH, DANIELLE - Montana State University; CLENDINEN, CHAEVIEN - Pacific Northwest National Laboratory; ALONGI, FRANKLIN - Montana State University; Mueller, Rebecca; CHU, ROSALIE - Los Alamos National Research Laboratory; TOYODA, JASON - Pacific Northwest National Laboratory; GALLEGOS-GRAVES, LA VERNE - Los Alamos National Research Laboratory; GOEMANN, HANNAH - Montana State University; PEYTON, BRENT - Montana State University; SEVANTO, SANNA - Los Alamos National Research Laboratory; DUNBAR, JOHN - Los Alamos National Research Laboratory

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2022
Publication Date: 7/10/2022
Citation: Ulrich, D.E., Clendinen, C.S., Alongi, F., Mueller, R.C., Chu, R.K., Toyoda, J., Gallegos-Graves, L., Goemann, H.M., Peyton, B., Sevanto, S., Dunbar, J. 2022. Drought severity gradient affects root exudate composition in blue grama (Bouteloua gracilis). New Phytologist. https://doi.org/10.1038/s41598-022-16408-8.
DOI: https://doi.org/10.1038/s41598-022-16408-8

Interpretive Summary: Plant survival during times of environmental stress, such as drought, is linked to their associations with microbes in the soil. To recruit their soil microbes, termed the root microbiome, plants secrete exudates that contain key nutrients, including carbohydrates and proteins, but whether the composition of these exudates changes when plants experience drought has not been examined. Focusing on a widespread grass, blue grama (Bouteloua gracilis) we used a gradient of drought (none, intermediate and severe) to document plant responses to drought, focusing on plant physiological responses and root exudate abundance and composition. As expected, markers of plant physiological stress increased with increasing drought severity. The amount of carbon secreted belowground via root exudates increased in severe drought treatments, indicating that plants alter their carbon allocation patterns during drought, potentially in an effort to increase the abundance or diversity of the root microbiome.

Technical Abstract: Plant survival during environmental stress is a major component of ecosystem carbon (C) cycling, and plant-microbe interactions are central to plant stress survival. The release of C-rich root exudates is a key mechanism to manage the plant microbiome, attracting beneficial microbes and/or suppressing harmful microbes to help plants withstand environmental stress. However, a critical knowledge gap is how plants alter root exudate concentration and composition under varying stress levels. In a greenhouse study of blue grama (Bouteloua gracilis Kunth Lag. Ex Griffiths), we imposed three drought treatments (control, mild, severe) and measured plant physiology and root exudate concentration and composition using GC-MS, NMR, and FTICR. With increasing drought severity, predawn leaf water potential and photosynthesis declined concurrently with increases in root exudate total C and organic C. Root exudate composition mirrored the physiological gradient of drought severity treatments (control to mild to severe). Specific compounds that are known to alter plant drought responses and the rhizosphere microbiome caused the drought-severity treatment separation. Despite reducing C uptake, these plants actively invested C to root exudates under severe drought due to specific compounds. Patterns of plant physiology and root exudate concentration and composition co-varied along a gradient of drought severity.