Molecular mechanism of imprinting of the maize endosperm gene fie1
The main purpose of this study was to characterize the expression patterns of the maize fie genes and to identify possible modes of their regulation. Our study revealed that although the fie1 and fie2 genes in maize contain a 78% homology within their exonic regions, their promoter regions are complex and contain no similarity. The maize fie1 and fie2 genes also have different pattern of expression. fie1 has a highly regulated expression. fie1 transcription is detectable only in the endosperm at early stages of development. fie2 has a more ubiquitous expression, detectable in vegetative and reproductive tissues at all times of plant development. We showed that the fie1 gene is imprinted and that transcriptional silencing of the paternal allele takes place. We looked at DNA methylation as part of the imprinting mechanism that regulates fie1 expression. Our study showed that the fie1 gene and not the fie2 gene is methylated along the promoter and exonic regions of the gene. More importantly we showed that in the endosperm, where fie1 is expressed, only the silent paternal allele is methylated. In contrast, in tissues where the fie1 gene is not expressed, like leaves and embryo, both maternal and paternal alleles have high levels of methylation. We propose the following model of fie1 imprinting. Before fertilization, the fie1 maternal alleles are demethylated in the central cell of the female gametophyte, but remain methylated in the egg cell and in the male gametophyte. After fertilization, demethylation allows for expression of the maternal fie1 alleles in the endosperm. However, the paternal allele remains methylated in the endosperm and therefore silent and both paternal and maternal alleles also remain methylated and silent in the embryo.