Expression and parent-of-origin effects for FIS2, MEA, and FIE in the endosperm and embryo of developing Arabidopsis seeds.

The promoters of MEA (FIS1), FIS2, and FIE (FIS3), genes that repress seed development in the absence of pollination, were fused to beta-glucuronidase (GUS) to study their activity pattern. The FIS2GUS product is found in the embryo sac, in each of the polar cell nuclei, and in the central cell nucleus. After pollination, the maternally derived FIS2GUS protein occurs in the nuclei of the cenocytic endosperm. Before cellularization of the endosperm, activity is terminated in the micropylar and central nuclei of the endosperm and subsequently in the nuclei of the chalazal cyst. MEAGUS has a pattern of activity similar to that of FIS2GUS, but FIEGUS protein is found in many tissues, including the prepollination embryo sac, and in embryo and endosperm postpollination. The similarity in mutant phenotypes; the activity of FIE, MEA, and FIS2 in the same cells in the embryo sac; and the fact that MEA and FIE proteins interact in a yeast two-hybrid system suggest that these proteins operate in the same system of control of seed development. Maternal and not paternal FIS2GUS, MEAGUS, and FIEGUS show activity in early endosperm, so these genes may be imprinted. When fis2, mea, and fie mutants are pollinated, seed development is arrested at the heart embryo stage. The seed arrest of mea and fis2 is avoided when they are fertilized by a low methylation parent. The wild-type alleles of MEA or FIS2 are not required. The parent-of-origin-determined differential activity of MEA, FIS2, and FIE is not dependent on DNA methylation, but methylation does control some gene(s) that have key roles in seed development.