PS 39-68 - Root nodules of red alder (Alnus rubra) and Sitka alder (Alnus viridis ssp. sinuata) are inhabited by taxonomically diverse cultivable microbial endophytes

Abstract: Similar to animals, plants harbor microbial communities in, and on, the different compartments of their bodies. The bacteria and fungi that comprise the phytomicrobiome can influence their host-plants in many beneficial ways including growth promotion, stress tolerance, and pathogen suppression. Systems with nitrogen-fixing symbionts are of particular ecological and economic importance and thus can be excellent models for understanding the dynamics of phytomicrobiomes and their impacts on host performance and ecosystem processes. A particularly critical symbiosis is the mutualistic interaction of vascular plants and root-nodule-forming, nitrogen-fixing bacteria. These symbioses are most studied in legumes due to their agricultural importance. In contrast, actinorhizal plants have received significantly less attention despite the fact that they are equally productive in terms of nitrogen-fixation rates and global, plant-available nitrogen provision.

Actinorhizal plants nodulate with Frankia, a genus of gram-positive, nitrogen-fixing bacteria. In temperate forests of the northern hemisphere, the most widely studied actinorhizal plant system is the genus Alnus. Red alder (A. rubra) is the most common hardwood tree species in the Pacific Northwest of the United States (and Canada). Despite its outstanding economic, ecological, and cultural value, there has been relatively little effort to establish systematic improvement programs of red alder, and microbial symbionts in this system remain a virtually unused resource. In this study, we used a culture-based approach to survey the taxonomic diversity and host-specificity of culturable non-Frankia root nodule endophytes in two sympatric alder species (A. rubra and A. viridis ssp. sinuata) on Mount St. Helens, Washington. In contrast to A. rubra, A. viridis has little or no commercial value as a timber species. It is, however, an important colonizer of disturbed areas at higher elevations with nitrogen deficient soils.

We found surprisingly high taxonomic diversity that was significantly different among individual trees, regardless of host species. We are reporting numerous fungal endophytes of alder root nodules (Sporothrix guttuliformis, Fontanospora sp., Cadophora spp., Ilyonectria destructans, Gibberella sp., Nectria ramulariae, Trichoderma sp., Mycosphaerella tassiana, Talaromyces sp., Coniochaeta sp., and Sistotrema brinkmanii) as well as bacteria (Collimonas, Psychrobacillus, Phyllobacterium, and Serratia) for the first time. While extensive work still needs to be done, these culturable isolates have the potential to be developed into inocula for silvicultural applications which, ultimately, will enable us to leverage the root nodule microbiome to maximize plant productivity while reducing synthetic fertilizers and pesticides and building more resilient silvicultural systems.