COS 204-4 - Increasing drought severity elicits a gradient response in the plant physiology, root exudation, and rhizosphere community composition of blue grama grass.

Abstract: Plants rely on a healthy root microbiome for resilience to environmental abiotic stress such as drought, similar to how humans rely on a healthy gut microbiome for resilience to illness. To maintain a healthy root microbiome, plants release carbon (C)-rich compounds as root exudates to stimulate microbial activity and increase nutrient availability. However, the enhanced performance comes at a cost: up to 44% of a plant’s C can be allocated to root exudates, diverting resources from growth and respiration. How this ‘C cost’ is managed and how root exudates alter the microbiome under different environmental conditions is a critical knowledge gap in plant-microbe ecology. Therefore, we conducted a greenhouse study using blue grama (Bouteloua gracilis), a drought-tolerant C4 grass native to shortgrass prairie across North American plains, as a model organism to study the effect of increasing drought severity on plant-microbe interactions. Blue grama seedlings were exposed to three drought conditions (no drought, mild, and severe) for 30 days. At the end of the treatment period, plant physiology measurements including photosynthesis and stomatal conductance were collected for each treatment, and plants were destructively harvested for root exudates and biomass. Root exudates were analyzed for chemical composition including total organic carbon, total nitrogen, and metabolite detection via GCMS. Rhizosphere soil was also collected and we performed high-throughput sequencing on the Illumina Miseq platform of 16S and LSU rRNA gene sequences to capture the bacterial/archaeal and fungal soil communities, respectively. We found that plant physiology, root exudate composition, and microbial community composition each followed a gradient of response to increasing drought stress. Under severe drought conditions, plant root exudate and rhizosphere microbiome composition shifted dramatically (PERMANOVA, p < 0.001). We further linked changes in relative abundance of specific taxa to the presence of released osmoprotectants and microbial stimulants. These results demonstrate the importance of root exudates in mediating plant-microbe interactions and warrant further investigation into the responses of plants and microbes to abiotic stresses through root exudation.