Mechanism of PHERES1 imprinting in Arabidopsis.

Genomic imprinting is a phenomenon where only one of the two alleles of a gene is expressed - either the maternally or the paternally inherited allele. Imprinting of the plant gene PHERES1 requires the function of the FERTILIZATION INDEPENDENT SEED (FIS) Polycomb group (PcG) complex for repression of the maternal PHERES1 allele. In this study we investigated the mechanism of PHERES1 imprinting and found that PcG silencing is necessary but not sufficient for imprinting establishment of PHERES1. We provide evidence that silencing of the maternal PHERES1 allele depends on a distantly located region downstream of the PHERES1 locus. This region needs to be methylated to ensure PHERES1 expression but must not be methylated for PHERES1 repression. This mechanism is analogous to the regulation of several imprinted genes in mammals, suggesting the employment of similar evolutionary mechanisms for the regulation of imprinted genes in mammals and flowering plants.