Activation of the imprinted Polycomb Group Fie1 gene in maize endosperm requires demethylation of the maternal allele

Abstract

Imprinting refers to the epigenetic regulation of gene expression that is dependent upon gene inheritance from the maternal or paternal parent. Previously, we have identified two maize homologs of the single Arabidopsis Polycomb Group gene FIE. Here, we report on the expression pattern of these genes in individual gametes before and after fertilization, and on the role of DNA methylation in determining the maternal expression of the Fie1 gene. We found that Fie1 is neither expressed in the sperm, egg cell nor central cell before fertilization. Activation of the Fie1 maternal allele occurs around two days after pollination (DAP) in the primary endosperm and peaks at 10-11 DAP coinciding with endosperm transition from mitotic division to endoreduplication. In contrast, Fie2 is expressed in the egg cell and more intensively in the central cell similar to Arabidopsis FIE, which strongly supports the hypothesis that it functions as a repressor of endosperm development before fertilization. Using MSRE-PCR and bisulfite sequencing, we could show that the methylated inactive state is the default status of Fie1 in most tissues. In the endosperm the paternal Fie1 allele remains methylated and silent, but the maternal allele appears hypomethylated and active, explaining mono-allelic expression of Fie1 in the endosperm. Taking together, these data demonstrate that the regulation of Fie1 imprinting in maize is different from Arabidopsis and that Fie1 is likely to have acquired important novel functions for endosperm development.

Balance between maternal and paternal alleles sets the timing of resource accumulation in the maize endosperm

“…Ubiquitously expressed FIE2 and MEZ1 and endosperm-specific FIE1 are reported to be imprinted in maize, expressed only from the female alleles (Haun et al 2007;Hermon et al 2007), and it is striking that expression of FIE2 and MEZ1 is unaffected under conditions of MGE and PGE (as is almost certainly FIE1, although there is some intersample variation between 5 DAP 4n  2n endosperms following 25 and 30 PCR cycles). This similarity of expression levels is also unexpected in view of the presence of two extra sets of female alleles in MGE.…”

Disruption of Imprinting byMutatorTransposon Insertions in the 5′ Proximal Regions of theZea mays Mez1Locus

“…Regions in the maize imprinted genes Mez1, ZmFie1, and ZmFie2 also display differential DNA methylation between the parental alleles in the imprinted tissue. For the ZmFie1 and ZmFie2 genes, transposon and repeat sequences, respectively, are present in the 59-proximal region but are not targeted for differential DNA methylation (Gutierrez-Marcos et al 2006;Hermon et al 2007). Additionally, Gutierrez-Marcos et al (2006) demonstrated imprinted expression and differential DNA methylation of ZmFie1-GUS and ZmFie2-GFP transcriptional fusions containing $4 kb of promoter and downstream sequence elements.…”

Combinatorial Networks Regulating Seed Development and Seed Filling

“…During male gametogenesis, these genes were repressed in sperm cells by the action of MET1 (Jullien et al, 2006;Tiwari et al, 2008). During endosperm development, the inherited paternal allele remains silenced by MET1, whereas the inherited maternal alleles are transcriptionally active (Gutiérrez-Marcos et al, 2006;Hermon et al, 2007;Tiwari et al, 2008). It was therefore suggested that the differential expression between the two parental alleles is established by the status of DNA methylation that has been epigenetically inherited from the gametes (Huh et al, 2007).…”