Gamete-specific epigenetic mechanisms shape genomic imprinting.

Although most genes are expressed equally from both parental alleles, imprinted genes are differentially expressed depending on their parental origin. In flowering plants, imprinting depends on DNA methylation. Conversely, activation of the expressed allele requires DNA demethylation. This is achieved during female gametogenesis by the synergy between the DNA glycosylase DEMETER and the repression of DNA methylation by the Retinoblastoma pathway. DEMETER is only expressed in the central cell and the resulting DNA demethylation is propagated in the fertilized central cell developing into the endosperm, which nurtures embryo growth. In addition other imprinted genes are regulated by histone methylation by Polycomb Group activity. The identification of new imprinted genes in Arabidopsis and in maize supports a conservation of imprinting mechanisms. Evidence for a role of distant cis-elements in imprinting regulation and the discovery of new imprinted genes expand the scope of research in plant imprinting.