Honors College Thesis
 

Geographic variation in the seed mycobiome of Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii)

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  • Seeds are an essential component of plant life histories, and seed endophytes have the potential to influence germination, seedling establishment and development. That said, seed endophytes are a relatively new area of study, both in the factors that influence which taxa are present and how these microbes alter plant function. The objectives of my thesis were to characterize the fungal endophytes present in native and introduced populations of Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) seeds, and to test whether some of these endophytes affect seedling survival and growth in response to drought. Using culture-based techniques, endophytes were isolated from eight native populations of Douglas-fir seeds in the United States and from three introduced populations in New Zealand. All seeds had zero or one fungal endophyte; total endophyte isolation frequency was 5.3% in the United States populations and 9.2% in the New Zealand populations. These results are consistent with previous work documenting a bottleneck in the plant microbiome at the seed stage. Based on DNA sequence data (ITS), the dominant taxa present in the sampled seed populations were Trichoderma spp. and Sydowia polyspora. Endophyte community composition differed among US seed provenances; future work could further investigate how these communities vary along environmental and plant genetic gradients. To test the hypothesis that endophytes confer drought tolerance, Douglas fir seedlings were inoculated with one of two isolates of seed-borne Trichoderma spp., or sterile water for controls, and grown under drought conditions. Drought reduced the survival of seedlings (i.e., increasing the time to death compared to un-inoculated controls); however, inoculation with endophytes did not significantly alter this result. Further study is needed to more fully characterize the functional roles of seed endophytes in Douglas-fir response to abiotic stresses, with applications in mitigating plant stress due to climate change.
  • KEY WORDS: Ascomycota, endophytes, Primary Symbiont Hypothesis, provenance, Pseudotsuga menziesii, Sydowia polyspora, Trichoderma, maternal transmission
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  • Ongoing Research
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  • 2019-06-04 to 2020-07-05
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