The FIS-PRC2 establishes H3K27me3 in the central cell, which is maintained on the maternal alleles after fertilization. ![]() In contrast, paternally biased expression is predominantly connected to the presence of H3K27me3, H3K9me2, and CHGm on maternal alleles ( Klosinska et al., 2016 Moreno-Romero et al., 2019 Moreno-Romero et al., 2016 Moreno-Romero et al., 2017). Genomic imprinting leading to maternally biased expression is predominantly connected to DNA demethylation of maternal alleles mediated by the DNA glycosylase DEMETER ( Gehring et al., 2009 Hsieh et al., 2011 Park et al., 2016). ![]() Imprinted genes are epigenetically modified during gametogenesis, establishing parental-specific expression patterns that are maintained after fertilization. Both processes have been linked to genomic imprinting, an epigenetic phenomenon causing parent-of-origin specific gene expression ( Batista and Kohler, 2020 Gehring and Satyaki, 2017 Piskurewicz et al., 2016). The endosperm furthermore regulates seed dormancy, a process preventing germination of seeds under favourable conditions ( Nonogaki, 2019). The endosperm regulates the transfer of nutrients to the developing embryo, similar as the placenta in mammals. Seed development in flowering plants is initiated by a double fertilization event, where one of the sperm cells fertilizes the haploid egg, giving rise to the diploid embryo and the other sperm cell fertilizes the diploid central cell, initiating development of the triploid endosperm ( Zhou and Dresselhaus, 2019). Binding of REF6 to this motif is negatively affected by non-CG methylation ( Qiu et al., 2019), revealing cross-talk between different epigenetic modifications. The zinc-finger (ZnF) domains of REF6 recognize the sequence motif CTCTGYTY (Y = T or C) ( Cui et al., 2016 Li et al., 2016). RELATIVE OF EARLY FLOWERING 6 (REF6) and EARLY FLOWERING 6 (ELF6) function redundantly as major erasers of H3K27me3 during Arabidopsis development ( Lu et al., 2011 Yan et al., 2018). Furthermore, redistribution of H3K27me3 to heterochromatic regions occurs when constitutive heterochromatin is disrupted, indicating that specific features of heterochromatin prevent PRC2 recruitment ( Deleris et al., 2012 Jamieson et al., 2016 Peters et al., 2003 Wiles and Selker, 2017).Įrasure of H3K27me3 and H3K9me2 is mediated by different types of JUMONJI lysine demethylases (KDMs) that belong to the KDM4 and KDM3 type, respectively ( Cheng et al., 2020). In animals and plants, H3K27me3 and H3K9me2 are generally exclusive repressive marks ( Wiles and Selker, 2017) however, they co-occur in certain tissue types like the plant endosperm ( Klosinska et al., 2016 Moreno-Romero et al., 2016 Weinhofer et al., 2010), as well as in specific organisms as filamentous fungi and bryophytes ( Carlier et al., 2020 Montgomery et al., 2020). The establishment of H3K9me2 in Arabidopsis depends on the SU(VAR)3–9 homologous proteins (SUVHs), KRYPTONITE (or SUVH4), SUVH5 and SUVH6 that act in a feedback loop with CHG methylation (CHGm) established by the CHROMOMETHYLTRANSFERASE 3 (CMT3) ( Kenchanmane Raju et al., 2019 Zhang et al., 2018). In Arabidopsis, the EMBRYONIC FLOWER (EMF) and VERNALIZATION (VRN) PRC2 regulate sporophyte development, while the FERTILIZATION-INDEPENDENT SEED (FIS) PRC2 is active in the central cell of the female gametophyte and the descendent endosperm ( Mozgova and Hennig, 2015). In plants, there are distinct PRC2 complexes that are active during different stages of plant development ( Mozgova and Hennig, 2015). The establishment of H3K27me3 is mediated by the evolutionary conserved Polycomb repressive complex 2 (PRC2). Repression mediated by the Polycomb system is transient and dynamically regulated, while repression mediated by H3K9me2/3 is generally stable during development ( Mozgova and Hennig, 2015 Saksouk et al., 2015 Schuettengruber et al., 2017 Wendte and Schmitz, 2018). In contrast, the repressive di- or trimethylation on lysine 9 of histone H3 (H3K9me2/3) is mainly associated with constitutively silenced heterochromatin, present in repeat-rich regions and TEs ( Saksouk et al., 2015 Wendte and Schmitz, 2018). The Polycomb repressive system applies the repressive trimethylation on lysine 27 of histone H3 (H3K27me3), which mainly serves to silence genes during defined stages of development ( Mozgova and Hennig, 2015 Schuettengruber et al., 2017). Animals, fungi, and plants make use of two major epigenetic repressive modifications on histones that are destined to silence genes and transposable elements (TEs).
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