DNA methylation is a crucial regulator of mammalian development. Mutations in DNA methyltransferases and methylcytosine binding protein genes cause genetic disease, and disrupted DNA methylation is a hallmark of cancer. Defects in DNA demethylation are associated with disorders of the central nervous system, tumor development, and failure to establish embryonic pluripotency. In addition, defective DNA demethylation prevents the in vitro derivation of induced pluripotent stem cells, a technology with the potentialto be used in the clinic for the regeneration of cells, tissues and organs. Whereas most common model organisms lack DNA methylation, flowering plants, including Arabidopsis thaliana, share highly conserved methylation and demethylation pathways with mammals. Both plants and mammals use DNA methylation to regulate genomic imprinting - epigenetic marking and resultant gene expression only from an allele inherited from either the male or the female parent. We discovered that DNA demethylation catalyzed by the Arabidopsis DME DNA glycosylase occurs exclusively in companion cells, the central cell and the vegetative cell, which are adjacent to the gametes, the egg and sperm, respectively. One sperm fertilizes the central cell to generate the endosperm, a nutritive extra-embryonic tissue. A second sperm fertilizes the egg to form the embryo. We showed that gene imprinting occurs in the endosperm, which is established by DNA demethylation in the central cell. Moreover, our recent results suggest that DNA demethylation of central cell and vegetative cell genomes generates small RNAs that move to the adjacent gametes and silence transposable elements by the RNA-directed DNA methylation pathway. In Aim 1, we will test our model that DME-mediated DNA demethylation in companion cells functions to reinforce TE silencing in gametes. In Aim 2, we will elucidate the critical relationship between chromatin structure and targeted DNA demethylation. In Aim 3, we will identify new genes required for DNA demethylation. Our studies will lead to a detailed understanding of the epigenetic reprogramming catalyzed by DNA demethylation during plant sexual reproduction.