Phytochrome is a plant photoreceptor important in the regulation of many developmental processes in plants. Our work is focused on understanding one particular aspect of phytochrome action -- how it can cause changes in the expression of specific genes. In the long term, a clear understanding of this signal transduction pathway should help elucidate other phytochrome responses and possibly other intersecting signal transduction pathways. We propose two kinds of strategies for understanding the mechanism(s) of phytochrome regulation of transcription. The first of these Is to use molecular and biochemical approaches to characterize the regulation of several specific genes whose transcription is either increased or decreased in response to phytochrome action. Promoter elements most directly involved In responsiveness to phytochrome action and transcription factors Interacting with these regions will be determined, characterized, and compared among two kinds of positively regulated (rbcS and cab) genes and two negatively regulated (NPR) genes in a single species, Lemna gibba. We will also compare the regulation of cab genes between Lemna and Arabidopsis thaliana. We should gain an understanding of the elements and factors involved In the phytochrome regulation of these genes and what, if anything they have in common. The way the factors themselves may be regulated will also be examined In the hope of taking our understanding one step further back in the signal transduction pathway. The second strategy, complementary to the first, is to use genetics to help define the components of the signal transduction pathway. Our objective is to make and Isolate Arabidopsis mutants defective in that pathway and to characterize the corresponding component by characterizing the phenotypes of the mutants and by cloning and characterizing the affected genes. We have devised a directed strategy for negative selection of such mutants. Ultimately, the results from the two different kinds of approaches should converge to give a clear picture of how phytochrome can regulate gene expression.