Understanding the Effects of Populus—Mycorrhizal Associations on Plant Productivity and Resistance to Abiotic Stress
Melissa A. Cregger1* (firstname.lastname@example.org), María del Rosario Ramírez-Flores1, Alyssa A. Carrell1, Spencer Roth1, David Weston1, Dawn Klingeman1, Miranda Clark1, Sara Jawdy1, Dana L. Carper1, Gail Taylor2, Jamie McBrien1, Leah Burdick1, Ann Wymore1, and David McLennan1
1Oak Ridge National Laboratory; and 2University of California–Davis
The overarching goal of this project is to create sustainable, multipurpose bioeconomies whereby globally important feedstocks can be produced while simultaneously maximizing soil health and mitigating adverse impacts of climatic change. In this project, the unique ability of Populus species will be leveraged to associate with both ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi to examine how variation in these associations alters plant productivity, abiotic stress response, and belowground soil carbon cycling.
Within the myriad of possible plant-microbe interactions occurring belowground, plant-mycorrhizal associations are widespread with the two most common mycorrhizal types being ECM and AM. Belowground plant interactions with these fungi have been shown to increase water uptake and nutrient acquisition and alter soil carbon storage. It is unclear how these two dominant mycorrhizal fungal types differ in their abilities to offer benefits to the plant and change belowground carbon and nutrient cycling. Most plants associate with one type of mycorrhizal fungi, and individual plant species are less likely to associate with both AM and ECM fungal species. Populus species, however, uniquely associate with AM and ECM simultaneously in natural settings, thus providing an ideal experimental system for examining how mycorrhizal fungal types confer benefits to their host. The team will take advantage of high-throughput plant phenotyping, greenhouse, and field experiments to characterize how variation in Populus-mycorrhizal associations alters the plant response to drought, and researchers will manipulate plant-mycorrhizal interactions to influence plant productivity, increase plant drought tolerance, and enhance soil health.
In objective one of this project, researchers identified drought tolerant and drought susceptible genotypes of Populus trichocarpa and Populus deltoides x P. trichocarpa hybrids. In a series of replicated greenhouse experiments, researchers grew 37 genotypes of P. trichocarpa and 29 unique hybrid genotypes (P. trichocarpa x P. deltoides and P. deltoides x P. trichocarpa) in double autoclaved potting mix under well-watered and drought conditions. Before manipulating water availability, base line plant phenotypes (e.g., plant height, stomatal conductance, leaf chlorophyll, leaf protein) were measured. Next, the team initiated an acute drought on half of the plants and monitored soil volumetric water content over the course of 1 week. When plants began to wilt significantly, hyperspectral images were captured from leaf four using a Headwall camera from 900 to 2500 nm wavelength to identify early indications of drought tolerance in images. Further, researchers characterized water-use efficiency, leaf protein, leaf chlorophyll, and changes in plant height, leaf number, and above/below ground biomass. Overall, the team found significant variation in plant phenotype across genotypes and in response to acute drought. Populus genotypes that varied in drought tolerance will be used in upcoming manipulative experiments with mycorrhizal fungi. This work will be expanded to identify P. deltoides genotypes that vary in drought tolerance.
Within objective two, researchers will characterize variation in mycorrhizal community composition, colonization, and abundance across drought tolerant/susceptible Populus species/genotypes. In February and July of 2022, root and rhizosphere soil samples were collected from drought tolerant and susceptible P. trichocarpa in a genome-wide association study (GWAS) plantation in Davis, Calif. Across these genotypes, the team found that both AM and ECM fungi colonized the roots, and drought tolerant genotypes had a greater percentage of hyphae, greater number of arbuscules, and a larger hartig net compared to drought susceptible trees. Amplicon and metatranscriptomic sequencing are in progress to characterize the AM and ECM taxa across these trees. Further, culturing of these unique organisms is underway to be used in manipulative experiments.
Combined, these initial results highlight significant genetic variation in the response of Populus to drought when grown without microbial symbionts, and further demonstrates variation in belowground mycorrhizal communities across drought tolerant and susceptible genotypes. Plant and fungal resources resulting from these experiments will be used to evaluate how these differences drive changes in host abiotic stress tolerance and soil carbon cycling.
This work was supported by the U.S. Department of Energy Office of Science, through the Biological and Environmental Research (BER) Program’s Early Career Research Program. The P. trichocarpa GWAS plantation in Davis, California was developed and is maintained through the Center for Bioenergy Innovation, Bioenergy Research Center funded by BER.