Science 324, 1447–1451 (2009); Science 324, 1451–1454 (2009)

The distribution of methyl groups attached to DNA is thought to be the main route by which genes are 'imprinted', or expressed differently depending on the parent from which they are inherited. Two teams reveal that, in the plant Arabidopsis thaliana, extensive DNA demethylation occurs in the seed endosperm — the tissue that provides nutrients for the developing embryo — and show how it underlies imprinting.

Steven Henikoff and his colleagues at the Fred Hutchinson Cancer Research Center in Seattle, Washington, used gene-expression and endosperm-demethylation patterns to predict five new imprinted genes. And Robert Fischer, Daniel Zilberman and their colleagues at the University of California, Berkeley, found that extensive hypermethylation in the embryo accompanies endosperm gene demethylation.

The authors suggest that demethylation in the endosperm and production of small RNA molecules helps to silence disruptive transposable elements — short DNA sequences that can copy and insert themselves throughout the genome — in the embryo.