Control of reproduction by Polycomb Group complexes in animals and plants

Abstract

In both mammals and plants, Polycomb Repressive Complexes 2 (PRC2) are conserved and appear to be involved in the transition between vegetative or somatic and reproductive state in plants and mammals. In plants at least three different PRC2 control temporal aspects of development, and mutations in PcG cause heterochronies. Such heterochronic mutations affect the transition to flowering. During gametogenesis the Fertilization-Independent Endosperm-MEDEA-PRC2 (FIE-MEA PRC2) complex controls gametogenesis in synergy with a Retinoblastoma-dependent pathway. Several genes of the FIE-MEA pathway are imprinted as shown by their uniparental allele expression in the endosperm, the interface controlling maternal nutrition of the embryo in the seed. Imprinting is also a major feature for genes expressed in the placenta in mammals. Recent data have shown that imprinting in both placenta and endosperm likely share similar mechanisms involving cooperation between the PRC2 complexes and DNA methylation. KEY WORDS: polycomb, endosperm, flowering, reproduction, placentaFlowering plants and mammals are characterised by full maternal control of their sexual reproduction. Fertilisation occurs inside the maternal reproductive tract and the mother nurtures the embryo through a specialised interface, the placenta in mammals and the endosperm in flowering plants. The placenta develops from a specific mass of cell, the trophoblast, set apart after the first divisions of the embryo. The endosperm is the product of fertilisation of a secondary female gamete, the central cell. The central cell is genetically identical to the egg cell. In most plant species the central cell contains two copies of the maternal genome, leading to triploid endosperm. The fertilised egg cell gives rise to the embryo. As the two male gametes of a given pollen tube originate from a single mitosis, they are genetically identical and their zygotic fusion products are also genetically identical. In spite of their unique genetic identity, the embryo and the specialised maternal interface structure (endosperm or placenta) follow very divergent developmental pathways. Such divergence can only be accounted for by different controls of the genome expression potential. Epigenetic regulation involving changes in DNA methylation and in the histone code are likely mechanisms responsible for large-scale expression changes. Histone 3 modifications are mediated in part by Polycomb Group (PcG) class complexes. In plants, PcG genes have a strong impact on the control of the transition to reproductive phase and in endosperm development. In mammals there has been recent evidence that Polycomb Repressive Complexes 2 (PRC2) class of chromatin remodelling Int. J. Dev. Biol. 49: 707-716 (2005) complexes control placenta development. These recent findings will be reviewed and put in perspective with recent advances obtained in plants. PRC2 type PcG complexes control H3 methylationDNA in eukaryotic nuclei is compacted into a structure called chromatin. The basic unit of chro...

Bypassing genomic imprinting allows seed development

“…These results have been interpreted in light of the parental conflict theory (kinship theory) according to which mothers and fathers have a different interest in allocation of resources to their offspring 8,9 . Hence, it has been proposed that paternal genes promote seed growth, whereas maternal genes rather reduce growth; or, conversely, that in the maternal genome growth promoting factors are inactivated.Indeed, flowering plants have been found to imprint certain genes during endosperm development, which is similar to the situation observed for the placenta of mammals [10][11][12][13][14] . Interestingly, the imprinting machinery seems to regulate itself [15][16][17] .…”

“…Indeed, flowering plants have been found to imprint certain genes during endosperm development, which is similar to the situation observed for the placenta of mammals [10][11][12][13][14] . Interestingly, the imprinting machinery seems to regulate itself [15][16][17] .…”

Plant SET domain-containing proteins: Structure, function and regulation

“…As reviewed in Gutton and Berger [49], homologs corresponding to these PcG proteins are also present in Arabidopsis PcG complexes that function in mediating various aspects of plant development. In Arabidopsis, CURLY LEAF (CLF), SWINGER (SWN) and MEDEA (MEA) constitute the E(Z) homologs of SET domain-containing PcG proteins (Table 1).…”

“…In Drosophila, an additional polycomb repressive complex, PRC1, was found to impede chromatin remodeling mediated by hSWI/SNF [57,58]. Although no complex corresponding to PRC1 has been identified in plants, the presence of a family of multiple PcG members in Arabidopsis suggested that they may constitute alternative functional PRC2 complexes [49,53]. For example, Makarevich et al [59] recently showed that at least two different MEA, CLF or SWN-containing PcG complexes are involved in histone H3K27 trimethylation at PHERES1 (PHE1) or FUSCA3 (FUS3) loci during seed development.…”

Different Polycomb group complexes regulate common target genes in Arabidopsis

“…In Arabidopsis, there are three E(Z) homologues (MEDEA (MEA), CURLY LEAF (CLF) and SWINGER (SWN); Guitton & Berger, 2005), three SUZ12 homologues (FERTILIZA-TION INDEPENDENT SEED2 (FIS2), VERNALIZATION2 and EMBRYONIC FLOWER2; Guitton & Berger, 2005) and five p55 homologues (MULTICOPY SUPPRESSOR OF IRA1 (MSI1-5); Hennig et al, 2005). Genetic evidence suggests that there are different PcG complexes in plants, regulating different developmental pathways.…”