Fruits and flowers house microbial communities that are unique from the rest of the plant. While a great deal is known about a handful of symbiotic microbes associated with roots and leaves, the microbial communities of fruits and flowers have received considerably less attention. Fruits are reproductive tissues that house, protect, and facilitate the dispersal of seeds, and thus they are directly tied to plant reproductive success. Fruit and flower microbial communities may, therefore, also impact plant fitness. This dissertation examines how fruit bacterial communities, as determined using the 16S rRNA gene marker, are shaped across spatial and environmental gradients and the role of pollinators in shaping floral bacterial communities among natural populations of Solanum carolinense. There have been limited studies on how spatial distance influences bacterial communities found in and on fruit tissue and the role of pollinators in shaping floral bacterial communities. The first study addresses how bacterial communities in fruit change across similar environmental conditions at fine spatial scales (2 to 450 m). Overall, no differences were found in observed richness or bacterial community composition. Next, the role that generalist pollinators might play in shaping these communities was tested using pollinator exclusion cages. Here we found that generalist pollinators do not play a large role in shaping floral bacterial communities in Solanum carolinense. Comparing bacterial community diversity between caged and uncaged flowers, via PCoA we found no significant clustering of samples. In contrast, significant clustering was detected between flowers and bee pollen baskets. Together these results suggested that environmental factors may be more important in shaping floral bacterial communities. To test this, we sampled 9 populations along a 337 km latitudinal transect and again used the 16S rRNA gene to characterize bacterial communities. We did not identify a significant correlation between distance and bacterial community composition in either the total nor endophytic community in the fruit. Results from these studies suggest that while there is some evidence for environmental effects shaping fruit and flower communities, other factors such as host selection (e.g., secondary compounds in fruit) also likely play an important role in shaping bacterial communities. / Doctor of Philosophy / Fruit and flowers are directly tied to plant reproduction, yet little is known about the bacterial communities associated with these important organs, especially compared to other plant tissues. This dissertation aims to address gaps in our knowledge regarding how spatial distance and pollinators influence fruit and floral bacterial communities. Specifically, how does bacterial community composition (what bacteria taxa are present or not and how abundant the bacterial taxa are) change based on spatial distance. Studies were conducted at both fine (under 0.5 km) and broad (337 km) scales to determine if a pattern was observed between increasing distance and how distinct the bacterial community composition is. There was no significant correlation between spatial distance and bacterial community composition at fine spatial distances, however there were high levels of dissimilarity in the bacterial communities sampled across fine spatial scales. This led to the investigation of pollinators, which directly interact with flowers and may act as a source of bacterial community transmission in the flower and fruit bacterial communities as they move around the landscape. To address the role that pollinators play in shaping bacterial communities in the fruits an exclusion cage study was used to prevent pollinators from interacting with a subset of flowers. Pollen baskets (pollen that was collected by the pollinator and is found on the legs of pollinator) were also collected from pollinators to determine what bacteria may be present on the pollinator. Pollen baskets may also represent what bacteria may have been picked up by the pollinator at the site. It was observed that caged and uncaged samples were similar to one another, which suggest that pollinators were not adding new bacteria nor changing the bacterial composition in the flowers. Yet, the pollen baskets (from the pollinator) were unique from the caged and uncaged floral samples. To further investigate what factors may be at play in shaping the fruit bacterial community the transect was expanded to 337 km to determine if there was an observable relationship between spatial distance and bacterial community composition. The relationship between spatial distance and bacterial community composition was not significant in either the endophytic (internal bacterial community) or the total fruit bacterial community (which represented external and internal bacterial communities). Similar to the fine spatial scale study, there were high levels of dissimilarity in the bacterial community that was observed across sites. Results from these studies may suggest that there are other factors that play a larger role in shaping bacterial communities in fruit and flower tissue. These could include the host plant and the production of secondary compounds, which in some cases can act as antimicrobial compounds, and enhance or inhibit the growth of specific bacterial taxa. Environmental factors such as wind and rainfall may have influenced the bacterial community composition. It is likely that environmental factors play a role in shaping floral and fruit bacterial communities. However, it is still unclear what factors shape fruit and floral bacterial communities are. This study provides the foundation for future studies to address additional factors that shape fruit and flower bacterial communities.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/115987 |
| Date | 03 August 2023 |
| Creators | Heminger, Ariel Renee |
| Contributors | Plant Pathology, Physiology and Weed Science, Haak, David C., Badgley, Brian Douglas, Barney, Jacob, Belden, Lisa Kay |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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