Cellular differentiation-in the development of multicellular organisms invariably involves changes-in patterns of gene expression. In many developmental programs, certain key changes in patterns of gene expression are epigenetic in nature; that is, such changes in gene expression remain stable through subsequent cell divisions in the absence of the original signal that induced the changes. In animals, epigenetic switches often "lock in" developmental fates such as occurs in establishment of the body plan during embryogenesis. In many plants, a comparable epigenetic switch resulting from vernalization is used to govern reproductive competence. Vernalization is the process by which plants measure the duration of exposure to winter's cold and, after perceiving a sufficient duration of cold, acquire competenceto flower the following spring. Competence to flower results from the epigenetic silencing of a flowering represser, FLOWERING LOCUS C (FLC), via a series of modifications to histones of FLC chromatin. We will combine studies of natural variation, mutant screens, protein interaction screens, and genome-wide gene expression patterns to identify constitutive elements of the vernalization network. We will define the specific changes in FLC chromatin that accompany the acquisition of competence. We will then address the roles and interaction of the genes product involved in vernalization. A long-term goal of our work is to understand how genetic regulatory networks operate. The regulatory network that we propose to study has similarities to those involved in body plan and organ formation in animals. Moreover,the epigenetic switch that we are studying is reset during meiosis which is also a feature of genomic imprinting in mammals. Improper regulation of epigenetic processes can underlie abnormal developmental events and some disease states such as cancer. A general understanding of epigenetic regulation in a range of multi-cellular organisms should ultimately contribute to the development of strategies to treat disease states.