Maintenance of genomic imprinting at the Arabidopsis medea locus requires zygotic DDM1 activity

In higher plants, seed development requires maternal gene activity in the h aploid (gametophytic) as well as diploid (sporophytic) tissues of the devel oping ovule. The Arabidopsis thaliana gene MEDEA (MEA) encodes a SET-domain protein of the Polycomb group that regulates cell proliferation by exertin g a gametophytic maternal control during seed development. Seeds derived fr om female gametocytes (embryo sacs) carrying a mutant mea allele abort and exhibit cell proliferation defects in both the embryo and the endosperm. In this study are show that the mea mutation affects an imprinted gene expres sed maternally in cells of the female gametophyte and after fertilization o nly from maternally inherited MEA alleles. Paternally inherited MEA alleles are transcriptionally silent in both the young embryo and endosperm. Mutat ions at the decrease in DNA methylation1 (ddm1) locus are able to rescue me a seeds by functionally reactivating paternally inherited MEA alleles durin g seed development. Rescued seeds are larger than the wild type and exhibit some of the abnormalities found in aborting mea seeds. Our results indicat e that the maintenance of the genomic imprint at the mea locus requires zyg otic DDM1 activity. Because DDM1 encodes a putative chromatin remodeling fa ctor, chromatin structure is likely to be interrelated with genomic imprint ing in Arabidopsis.