Plants exude a large amount of photosynthesis-derived carbon (C) into the rhizosphere. These exudates shape rhizosphere community composition and potentially attract beneficial microbes that improve plant nutrition, reproduction, drought tolerance, and other stress responses. Additionally, plant-microbial interactions could define the future fate of root C, and specifically whether it is respired to the atmosphere or stabilized in soil. In this study, we combined mass spectrometry-based metabolomics with genomic analysis of rhizosphere isolates to uncover key chemical mechanisms of plant-microbe interactions in soil. These include release of exudates by two annual and perennial grasses, metabolite exchange between plant and rhizosphere microorganisms, substrate preferences and niche partitioning of rhizosphere microbiome linked to the dynamic root exudation and potential mechanisms how these interactions between microorganisms and plants could impact C stabilization in soil.
Results/Conclusions
Taking a multi-scale approach including field, greenhouse and highly controlled lab experiments our goal is to determine how dynamic plant exudation impacts plant-soil-microbial relationships. Here, we analyzed exudation patterns of the annual grass Avena and linked changes in the exudation to the substrate utilization preferences of the rhizosphere isolates. We identified 101 metabolite released by this grass during its development. Thirteen plant-exuded metabolites including nicotinic, shikimic, salicylic, cinnamic, were more preferentially consumed by the rhizosphere bacteria compared to bulk soil bacteria.
Additionally, we measured chemical succession of plant exudates during development and across different moisture and nutrient-limited conditions for perennial grass Panicum virgatum(switchgrass). We identified dynamics of more than 60 compounds released by switchgrass during 90 days of plant development. Aromatic acids (syringic, cinnamic) and quaternary amines (betaine, stahydrine) were two classes of compounds that switchgrass released the most at the later stages of the plant growth. To dissect specific mechanisms illustrating how changes in exudate chemistry shape microbial community and thus impact plant development and C flux, we are using a highly controlled system where we can measure metabolite exchanges between plants and a defined microbial community derived from bacteria isolated from the rhizosphere of switchgrass.
Our results indicate that in the rhizosphere of grasses, specific metabolites are exuded at different stages of plant development and under different nutrient limitations. These metabolites are selectively consumed by rhizosphere bacteria. We suggest that this substrate specialization of rhizosphere bacteria together with changing composition of root exudates, used by plant to attract beneficial bacteria from the complex soil community in order to eliminate stress, improve nutrient availability and suppress pathogenic bacteria.