Honors Theses

Date of Award

Spring 8-25-2022

Document Type

Undergraduate Thesis



First Advisor

Yongjian Qiu

Second Advisor

Bryce Noonan

Third Advisor

Ryan Garrick

Relational Format



Gravitropism is a plant response to gravity in directing the growth of its organs, such as the root and the stem. Gravity is sensed by statocytes, unique cells that contain starch-filled amyloplasts, which trigger gravitropic growth response by modulating the distribution of growth hormone auxin in the elongation zone of the stem and root. Despite the fact that multiple factors have been identified as key regulators of shoot negative gravitropism, how these factors are regulated at the transcriptional level is largely unknown. In this study, we explore the role of transcriptional machinery in regulating shoot gravity sensing and responses by characterizing the functions of the Mediator of RNA polymerase II (Pol II) transcription. The Mediator complex is a key coactivator complex involved in regulating the transcription of almost all Pol II-dependent genes in plants. Through a reverse genetic screen, we identified a tail Mediator subunit as a crucial regulator of hypocotyl negative gravitropism. While the wild-type seedlings grow agravitropically and show random hypocotyl orientations in red light, the mediator mutants grow upright in the same condition. Interestingly, this phenotype is temperature-dependent as the hypocotyl negative gravitropism of the mediator mutants can be enhanced at warmer ambient temperatures (e.g., 27°C). Further genetic analysis by crossing the mediator mutants with the starch-deficient mutant pgm-1 suggests that the gravitropic responses of the mediator mutants may be caused by the accumulation of starch granules in the endodermal amyloplasts in their hypocotyls. This notion was further supported by the observation that knocking out four phytochrome-interacting factors (PIFs), which repress the conversion of amyloplasts to other plastids, abolishes the negative gravitropism of the mediator mutants. These data imply a critical role of transcriptional regulation in shoot gravitropic responses and lay the groundwork for future research on identifying transcription factors involved in gravitropism.

Accessibility Status

Searchable text

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Available for download on Tuesday, July 01, 2025