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Degree

Doctor of Philosophy, Miami University, Botany, 2006.

Abstract

Gravitropism, the directed growth of a plant in response to gravity, can be divided into the following phases: gravity perception, signal transduction, and the growth response. In flowering plants, gravity sensing occurs in specialized cells termed statocytes, which include the columella cells of the root cap and the endodermis (= starch sheath) of stem-like organs. These statocytes contain organelles (amyloplasts) that are packed with dense starch grains and move in response to changes in the orientation of the plant organ relative to gravity. Signal transduction occurs when changes in the potential and/or kinetic energy of gravistimulated amyloplasts is converted into a biochemical signal. Numerous reports indicate that the process is mediated by actin microfilaments (F-actin), although the exact nature of F-actin involvement is unknown. This study was undertaken to assess the relationship between amyloplasts and F-actin in statocytes of Arabidopsishypocotyls before and during gravitropic stimulation by reorientation. A pharmacological approach was employed throughout this project. Disruption of F-actin with latrunculin B (Lat-B) caused a promotion of gravitropic curvature despite a reduction in growth. Cryofixation, histology and microscopy were used to determine the effects of F-actin disruption on amyloplast positions before and after gravistimulation. Amyloplast mobility in hypocotyls was virtually eliminated after Lat-B treatment. Reduced amyloplast mobility after cytoskeletal disruption is consistent with an active mechanism of intracellular statolith transport and suggests that amyloplast movement in endodermal cells is dependent upon F-actin. To determine whether amyloplasts require myosin motor proteins to move along actin filaments, Arabidopsisseedlings were exposed to myosin ATPase inhibitor 2,3-butanedione monoxime (BDM), and the effects of gravistimulation and myosin ATPase inhibition on growth rate, gravitropic curvature and amyloplast kinetics were studied. BDM reduced growth and gravitropic curvature in a concentration dependent manner, enhanced amyloplast displacement in vertically-oriented hypocotyls, and reduced displacement after reorientation, suggesting that BDM reduces the ability of the actomyosin system to actively participate in amyloplast positioning in endodermal statocytes. These results support the hypothesis that amyloplasts interact with F-actin and myosin during gravitropism in shoots.

Keywords

Gravitropism; Arabidopsis; plant physiology; curvature; latrunculin B; BDM; growth; curvature; spaceflight; microgravity; signal transduction; gravity perception; amyloplast; statolith; tensegrity; actin tether; vacuole; endodermis; statocyte

Advisor

John Z. Kiss

Pages

143p.

Document number: miami1155230444
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=miami1155230444

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