The microbiome of sorghum phyllosphere mucilage and wax: A role for host resilience?

Marco Mechan-Llontop and Ashley Shade, Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, John Mullet, Great Lakes Bioenergy Research Center, Texas A&M University, College Station, TX, Ashley Shade, Department of Microbial and Molecular Genetics, Michigan State University, East Lansing, MI, Ashley Shade, Plant Soil and Microbial Sciences, Michigan State University, East Lansing, MI

Background/Question/Methods
During development, plants accumulate several exudates on their external aerial surfaces. Sorghum is a crop targeted for biofuel production, and with the Great Lakes Bioenergy Research Center, our goal is understand how to leverage the phyllosphere microbiome to optimize sorghum cultivation for biofuel production and crop adaptation to stressors like drought. Sorghum accumulates elevated levels of epicuticular wax on its stems and leaves to prevent water loss. Also, sorghum produces sugar-rich mucilage on its aerial roots to likely facilitate nutrient acquisition, including nitrogen fixation as has been reported in maize. Here, we hypothesize that the epicuticular wax structure selectively filters for microbiome members that confer drought tolerance to the host and that the aerial root mucilage harbors a diazotrophic microbiota that supports nitrogen acquisition. Thus, we assessed the microbiome associated with the aerial root mucilage from N-fertilized and non-fertilized sorghum plants at two points in the growing season, and the epicuticular wax from plants under non-limiting and limiting water conditions. We applied both cultivation-independent and dependent methods to capture microbiome members hypothesized to be important for host resilience. For the latter, we built a large bacterial collection by targeting a wide range of traits expected to be beneficial for the host plant during stress, including N-fixation, P-solubilization, and osmotic tolerance.
Results/Conclusions
Our initial results showed that Proteobacteria, Bacteroidetes, and Firmicutes were the dominant phyla in the mucilage compartment regardless of the fertilization treatment. We also identified specific bacterial taxa that were differentially more abundant on the mucilage of N-fertilized plants as compared to non-fertilized, and vice versa. We detected 151 ASVs with 100% occupancy in all mucilage samples (N+/N- subplots, n=10), and this set was preliminarily designated as “core” mucilage taxa. The dominant epicuticular wax microbial phyla were Proteobacteria and Firmicutes. This work broadens our understanding of the microbiome members that are enriched on (or selected by) the specialized phyllosphere exudates that support sorghum resilience and productivity.