Student Theses and Dissertations

Date of Award

1996

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

Understanding the mechanisms by which light signals are transmitted within a plant to elicit changes in physiology and gene expression is a primary goal of current research. Recently, significant insights have been gained regarding the signal transduction pathway that is regulated by the red/far-red light receptor phytochrome.

Moreover, it has become clear that the phytochrome signaling pathway functions within the context of a global regulatory network that exists in plants and permits fine control of a multitude of responses to various environmental conditions. In this study, an investigation of the basic mechanisms of phytochrome signal transduction, and the way this signaling pathway interacts with sugar signals was undertaken. Pharmacological studies of sucrose signal transduction in a soybean cell culture demonstrated that sucrose is a potent inducer of anthocyanin biosynthetic gene expression and that its inductive effects are mediated by the small signaling molecule cyclic GMP (cGMP). In addition, sucrose is an inhibitor of phytochrome-regulated gene expression, but the manner in which it achieves these effects remains undefined. It was also found that the signaling pathways of sucrose and phytochrome synergistically interact to induce the expression of chalcone synthase. A novel genetic screen was developed based on a phytochrome-dependent phenotype induced in constant red and far-red light by the cGMP-dependent phosphodiesterase inhibitor zaprinast in Arabidopsis thaliana. Mutations at a locus designated ZAP1 were found to confer resistance to zaprinast in both red and far-red light and resulted in plants with elongated hypocotyls relative to wild-type plants. Additional phytochrome-regulated responses were also found to be altered in zapl mutants. The late flowering phenotype of zapl mutants led to the discovery that the previously-identified ft mutant was resistant to zaprinast in red light. Studies of gene expression in this mutant led to the determination that mutations of the FT locus disrupt the synergistic integration of phytochrome and sucrose signals, causing reduced expression of anthocyanin biosynthetic genes. The results described here have strong implications for the nature of phytochrome signaling and the way it interacts with other pathways in a global regulatory network.

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