Date of Award

2016

Document Type

Thesis

Degree Name

Bachelors

Department

Natural Sciences

First Advisor

Clore, Amy

Area of Concentration

Biology

Abstract

Gravitropism is the response that allows plants to reorient themselves and for the stems to grow against the gravity vector. Grass stems, such as that of Zea mays L., are excellent models for studying gravitropism because the response to gravistimulation is isolated to pulvini (pulvinus sing.). The pulvinus is a disk of tissue apical to each node that is responsible for cell elongation and causes bending when the stem is displaced from the vertical position. The signaling pathway required for plant gravitropism is not yet fully understood. Previous work in our lab suggested a role for reactive oxygen species (ROS) release in the pulvinus gravitropic signaling pathway, likely associated with starch-filled plastids called amyloplasts. ROS levels were found to rapidly increase in or around these organelles, primarily in cells surrounding vascular bundles throughout the response period, eventually concentrating in the lower half of the pulvinus within 72 hrs of gravistimulation. In the present study, to further test the hypothesis that ROS release is associated with amyloplast sedimentation, B73 maize plants were placed in complete darkness for 7 days to ―burn-off‖ starch present in the plastids. These plants, along with positive controls grown in normal light cycles, were then gravistimulated for 1 min, 30 min, 5 hrs, or 72 hrs and examined for ROS using the cytochemical stain 3,3‘-diaminobenzidine (DAB) and the fluorescent dye 2`,7`-dichlorodihydrofluorescein diacetate (H2DCFDA). Maize plants kept in darkness had measurably less starch than those in light-grown plants present in their pulvini. These results were consistent with the amount of bending in response to gravistimulation, as less starch was correlated with an attenuated bending response. The results of DAB staining and H2DCFDA fluorescence further suggest that starch-filled amyloplasts are necessary for the full extent of ROS release.

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