Epigenetic and genetic control of imprinting at the Mez1 locus in maize.

Abstract

Genomic imprinting is the mono-allelic expression of gene based on its parent-of-origin and is important for normal progeny development in plants. The goal of this research was to better classify the epigenetic modifications at the Zea mays (maize) imprinted gene Mez1, while also investigating the phenotypic consequence of a loss-of-imprinting. The Mez1 gene in maize is imprinted in endosperm tissue, displaying expression solely from the maternal allele. A differentially methylated region (DMR) was identified in the 5' cis -proximal region of Mez1 in endosperm tissue. In this DMR, the paternal allele displays significantly higher levels of both CpG and CpNpG DNA methylation relative to the corresponding region of the maternal allele. The chromatin modifications of the maternal and paternal alleles of Mez1 and a second imprinted gene, ZmFie1, were studied using allele-specific chromatin immunoprecipitation (ChIP). HistoneH3 and HistoneH4 acetylation are maternally-enriched in endosperm tissue, while HistoneH3 Lysine27 tri-methylation (and to a lesser extent HistoneH3 Lysine27 di-methylation) show paternal allele enrichment. HistoneH3 Lysine9 di-methylation and HistoneH3 Lysine9 tri-methylation do not show parent-specific enrichment. These results suggest DNA methylation and histone modifications are involved in the epigenetic regulation of imprinting in plants.

Numerous studies have focused on understanding the mechanism of imprinting, however relatively little is known about the phenotypic consequence of expressing the normally silent allele of an imprinted gene. Several different alleles containing Mu transposon insertions into the 5' cis -proximal region of Mez1 were characterized. Both maternal and paternal inheritance of mez1-mu alleles can result in a loss-of-imprinting. This suggests that Mu transposon insertions at the Mez1 locus can act by disrupting the production of a trans -acting factor or interfering with the cis -acting elements involved in imprinting. Interestingly, the mez1-mu insertions do not effect plant vegetative growth or seed development. These results suggest allelic communication is important between the two parental alleles of imprinted loci.