Imprinting and seed development.

Imprinted genes are expressed predominantly from one allele in a parent-of-origin–specific manner. The endosperm, a seed tissue that mediates the transfer of nutrients from the maternal parent to the embryo, is an important site of imprinting in flowering plants. Imprinted genes have been identified in maize and Arabidopsis thaliana, but crosses in a variety of species suggest that the effect of imprinting on seed development is widespread throughout the angiosperms. The mechanism of imprinting in plants, as yet only partially understood, likely involves differences in DNA methylation and chromatin structure between differentially expressed maternal and paternal alleles.