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 d evelopment in the absence of pollination, were fused to beta-glucuronidase (GUS) to study their activity pattern. The FIS2::GUS product is found in th e embryo sac, in each of the polar cell nuclei, and in the central cell nuc leus. After pollination, the maternally derived FIS2::GUS protein occurs in the nuclei of the cenocytic endosperm. Before cellularization of the endos perm, activity is terminated in the micropylar and central nuclei of the en dosperm and subsequently in the nuclei of the chalazal cyst. MEA::BUS has a pattern of activity similar to that of FIS2::GUS, but FIE::GUS protein is found in many tissues, including the prepollination embryo sac, and in embr yo 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 sugges t that these proteins operate in the same system of control of seed develop ment. Maternal and not paternal FIS2::GUS. MEA::GUS, and FIE::GUS show acti vity in early endosperm. so these genes may be imprinted. When fis2, mea, a nd fie mutants are pollinated. seed development is arrested at the heart em bryo stage. The seed arrest of mea and fis2 is avoided when they are fertil ized 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 co ntrol some gene(s) that have key roles in seed development.