Honors Theses

Date of Award

Spring 5-8-2022

Document Type

Undergraduate Thesis



First Advisor

Yongjian Qiu

Relational Format



The plant stem grows against the gravitational force, a phenomenon called negative gravitropism. In the model dicotyledonous plant Arabidopsis thaliana, gravity is sensed by starch-filled amyloplasts, which trigger the shoot gravitropic response through the action of a growth hormone called auxin. In the darkness, a group of transcription factors named PHYTOCHROME-INTERACTING FACTORS (PIFs) promote stem negative gravitropism by inhibiting the starch metabolism in the endodermal amyloplasts. Red light promotes starch metabolism and converts amyloplasts into other types of plastids (e.g., chloroplasts) through the red/far-red light photoreceptors—phytochromes. Consequently, red-light-grown seedlings lose starch granules in amyloplasts and exhibit agravitropic growth. However, the detailed molecular mechanism by which starch metabolism is triggered in red light is still not well understood. Here, an Arabidopsis mutant that showed hypocotyl negative gravitropic growth in red light was identified. Interestingly, we observed that its negative gravitropism phenotype was also temperature dependent—specifically, stronger gravitropic response at warmer temperatures. Genetic analyses further demonstrated that PIFs are required for the mutant’s unique negative gravitropic phenotype in red light. The gene that was mutated encodes a key component involved in the microRNA pathway, suggesting a previously unknown role of small RNAs in regulating shoot negative gravitropism.

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