It has become increasingly evident that the health of plants, like that of humans, depends largely on their close interaction with microbes. This hidden world of microbes influences processes critical to the storage of carbon in the soil, which in turn stabilizes the soil to prevent erosion, and provides a source of nutrients and water to support plant growth. Many plant species form associations with certain types of fungi that grow around their roots. These fungi, known as ectomycorrhizal fungi (EMF), engage in a symbiosis, exchanging nutrients for sugars, thereby promoting belowground storage of soil carbon. Using carbon from the host plant as fuel, EMF branch out from the root surface to form networks and improve surface area for nutrient uptake. However, when the carbon supply from the plant is limited, EMF may compete with free-living (non-symbiotic) microbes for both nutrients and carbon. Alternatively, instead of direct competition, EMF could also "cheat" or capture the degraded products generated by the action of other microbes in the community. This project supports ongoing dissertation research to establish whether EMF might use plant degradation products released by another organism (i.e. "cheat"). Results of this research will contribute to a better understanding of the role of fungi in plant litter degradation. The results will also improve predictions about the effects of climate change on decomposition. The broader impacts of this study include the training a doctoral student as well as opportunities for training of several undergraduate students.
Microbial species vary in their ability to break down recalcitrant plant polymers, such as cellulose and lignin, into easily metabolized monomers. The diversity of gene combinations for extracellular enzymes (ECE) seen in microbial genomes points to a differential ability to degrade these polymers. The preliminary tests of this project leveraged model organisms grown in controlled microcosms, thereby profiting from a controlled experimental setting and sequence data available for well-studied species. These tests demonstrated that polymer-degrading enzyme activity in EMF is not directly reflective of the presence of enzyme-degrading genes. The current project will bolster the initial results via metatranscriptomic analyses, shed light on competitive interactions between EMF and free-living saprotrophs at the gene expression level, and show how EMF optimize their success in a particular environment. The researchers will be comparing mRNA sequences to full genome sequences, using standard methods to map each sequence read to a corresponding genome. This will allow for quantification of expressed ECE genes in EMF in competition with other microorganisms and without a carbon supply from a host plant. Shifts in expression of these genes, coupled with physiological data, will allow the researchers to verify whether those organisms that consume polymer degradation products are also expressing ECE genes (i.e., whether they are cheating). The knowledge gained from this research will enhance our understanding of the role of EMF in decomposition and to incorporate their activities in carbon cycling models.
