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theses

Duckweeds are small aquatic plants with applications in wastewater treatment, biofuel production and animal feed additive. Although duckweeds grow rapidly all over the world, man-made farming remains a challenge for large-scale production. Our growing world population requires sustainable agricultural practices that involve introduction of novel crops such as duckweeds and improved farming methods for existing crops. Beneficial microbes are of interest to improve plant health and yield. Thus, understanding interactions between plants and bacteria would be necessary for proper selection of bacterial strains and application procedures. Many bacteria are known to improve plant growth by production of phytohormones such as indole-3-acetic acid (IAA), an auxin commonly found in nature. In this thesis, we aim to 1) Characterize the duckweed microbiome by auxin production to identify potential growth promoting strains, 2) Determine the mechanisms of plant-bacteria interactions via the auxin pathway, and 3) Determine the mechanisms of auxin signaling within tripartite interactions.
Isolation of forty-seven strains of duckweed associated bacteria (DABs) from various duckweed clones provided an excellent resource for studying the diversity of interactions within the duckweed microbiome. Characterizing the duckweed microbiome by auxin production led to evidence for an association between the type of indole related compound produced by the DAB and the duckweed genus that it was isolated from. Binary association assays with DABs capable of producing indole related compounds revealed two genetically similar Microbacterium strains DAB 1A and DAB 33B. DAB 1A, but not DAB 33B, caused an auxin associated short root phenotype on the model plant A. thaliana. Yet both strains can cause similar, but not identical, transcriptional responses that included known auxin-responsive genes in the plant, indicating various roles of auxins in signaling between bacteria and the plant host. Co-inoculation of DAB 1A and Herbaspirillum strain DAB 5E onto A. thaliana resulted in suppression of the short root phenotype. Based on results from auxin-response reporter plant lines and auxin stability studies with bacteria culture, we propose that DAB 5E degrades IAA at high levels, which can enhance its growth and competition among other strains, while maintaining IAA response in the plant to escape detection. Altogether, these results suggest various roles of indole related compounds, such as IAA, in assembly of the plant microbiome.

ETD_10219

Ph.D.

Includes bibliographical references

doi:10.7282/t3-hrpb-9s78

ETD doctoral

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