Sponsor
Portland State University. Department of Biology
First Advisor
Daniel J. Ballhorn
Term of Graduation
Winter 2025
Date of Publication
3-4-2025
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Biology
Department
Biology
Language
English
Subjects
Frankia, Plant-microbe interactions, Red alder, Rhizobia, Root nodule endophytes, Scotch broom
Physical Description
1 online resource (x, 92 pages)
Abstract
Endophytes are a hyper diverse group of bacterial or fungal organisms residing within plant tissues without causing visible symptoms of disease. While their overall diversity and ecological functions are not well-understood, some endophytes can provide a range of important benefits to their host such as enhanced growth and increased tolerance to environmental stress. Nitrogen-fixing bacteria residing within nodules--specialized structures on the roots of legumes and actinorhizal plants--play an integral role in the plant microbiome by facilitating nitrogen availability in nutrient-poor environments. Remarkably, the patterns of diversity including both bacterial and fungal endophytes in root nodules are virtually known, except in a few agricultural crop legumes. This study represents the first comparison of the culturable microbial communities associated with the root nodules of red alder (Alnus rubra Bong.), an actinorhizal plant native to the Pacific Northwest of the United States, and Scotch broom (Cytisus scoparius L.), an invasive legume, which often co-occur in similar environments. Red alder and Scotch broom root nodules collected at a sympatric location near Sandy, OR, revealed high numbers of cultivable endophytes. Culturable bacterial endophytes were detected in 98% (n = 44) of red alder nodules and 93% (n = 42) of Scotch broom nodules. Out of the 306 bacterial sequences obtained from red alder and Scotch broom root nodules, a total of 88 operational taxonomic units (OTUs) were identified, with 58% (n = 51) unique to red alder, 25% (n = 22) unique to Scotch broom, and 17% (n = 15) shared between the two host species. Red alder hosted 30 bacterial genera, while Scotch broom hosted 20 genera, with red alder exhibiting significantly greater bacterial diversity than Scotch broom (PERMANOVA: p = 0.001, R² = 0.1158). Although red alder hosted a more diverse root nodule microbial community, both hosts displayed a high frequency of Bradyrhizobium and Mesorhizobium within their root nodules. While typically associated with legumes, these genera have previously been isolated from alder and shown to enhance its growth. However, this is the first time rhizobia have been reported as the most frequently observed bacterial taxa in red alder root nodules. Fungal endophytes were less prevalent, found in 53% (n = 24) of red alder nodules and 56% (n = 25) of Scotch broom nodules. Out of the 147 fungal sequences obtained from red alder and Scotch broom root nodules, a total of 51 operational taxonomic units (OTUs) were identified, with 55% (n = 28) unique to red alder, 29% (n = 15) unique to Scotch broom, and 16% (n = 8) shared between the two host species. Red alder hosted six fungal orders, while Scotch broom hosted seven, with Eurotiales--driven by the prevalence of Penicillium--being the dominant order in both hosts. Despite red alder exhibiting slightly higher fungal diversity, the fungal communities were not significantly different between the two hosts (PERMANOVA: p = 0.224, R² = 0.0802). This contrasts with previous studies that reported significant variation in fungal root nodule endophytes between legumes and non-legumes and even among sympatric alder species. Red alder exhibited a higher frequency of microbial co-occurrence within its root nodules, with 96% (n = 43) of nodules containing multiple microbes compared to 73% (n = 33) in Scotch broom. This may suggest that red alder nodules are not only more diverse but also provide a more hospitable environment for microbial endophytes to coexist, potentially fostering synergistic interactions. Overall, the results revealed distinct patterns of bacterial diversity and composition within root nodules, significantly influenced by host type, with red alder hosting a more diverse bacterial community than Scotch broom. Although red alder and Scotch broom are fundamentally different plants, they often co-occur in similar environments and can alter soil nitrogen levels through associations with nitrogen-fixing bacteria. This study serves as a foundational step towards understanding host-specific plant-microbe interactions happening belowground, with significant implications for ecosystem recovery, land management, and the role of plant-microbe interactions in plant invasions.
Rights
© 2025 Felisha (Fay) Rene Pooler
In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).
Persistent Identifier
https://archives.pdx.edu/ds/psu/43192
Recommended Citation
Pooler, Felisha (Fay) Rene, "Culturable Microbial Communities in Root Nodules of Red Alder (Alnus rubra Bong.) and Scotch Broom (Cytisus scoparius L.): Patterns of Diversity and Composition" (2025). Dissertations and Theses. Paper 6798.
