Plant Polycomb‐Group Proteins: Epigenetic Regulators of Development

Abstract

Although all cells in a multicellular organism contain the same set of genes, the spatiotemporal expression of these genes needs to be dynamically regulated for proper development (i.e. morphogenesis and life cycle transitions) to take place. Epigenetic mechanisms (i.e. heritable but reversible changes to chromatin) are important to maintain such gene expression patterns. Polycomb‐group proteins are evolutionarily conserved epigenetic regulators that function – via epigenetic marks such as histone modifications and alterations to chromatin structure – to maintain the repression of developmentally important genes so that these genes are expressed appropriately. Although they maintain specific patterns of gene repression, Polycomb‐group proteins are ubiquitously expressed. How their activity is regulated is a largely unexplored area but recent research in both metazoans and plants suggests myriad methods of regulation, including control of mRNA (messenger ribonucleic acid) and protein abundance and stability, post‐translational modifications and interactions with proteins and noncoding RNAs.

Key Concepts

  • Chromatin comprises of DNA folded up to form higher order structures, the basic unit of which is a nucleosome, which consists of DNA wrapped around histone proteins.
  • Epigenetic changes can regulate transcription and include alterations to DNA packaging and covalent modifications of histones or DNA.
  • Epigenetic changes are important for development because they can cause gene expression patterns that are heritable through cell divisions but more easily reversible than changes to DNA sequence.
  • Polycomb‐group proteins are conserved epigenetic regulators that maintain the repression of developmentally important genes in plants and animals.
  • Polycomb‐group proteins form various complexes that can modify chromatin (by adding epigenetic marks such as H3K27me3 and H2Aub to histones and altering chromatin structure and accessibility).
  • Polycomb‐group proteins are expressed ubiquitously and therefore need to be finely regulated to allow for specific and dynamic repression of their target genes.
  • Polycomb‐group protein levels or activity may be regulated by post‐translational modifications and controlled degradation.
  • Polycomb‐group proteins associate with other proteins or long noncoding RNAs that may regulate their activity or recruitment to specific targets.
  • Polycomb‐group repression can be reversed by antagonistic transcriptional activators and removers of Polycomb‐group epigenetic marks, some of which are part of the evolutionarily conserved trithorax‐group of proteins.

Keywords: Polycomb‐group; H3K27me3; PRC2; PRC1; repression; epigenetic regulators

Figure 1. Polycomb‐group members are involved in regulating many plant life cycle transitions. After gametogenesis, a PRC2 complex most likely containing FIS2, MEA, FIE and MSI1 prevents precocious development of the embryo and endosperm (a nutritive tissue derived from the double fertilisation event that occurs in flowering plants). Additionally, PRC2 complexes containing EMF2/VRN2 ensure proper seed coat development. The post‐germinative morphogenesis of organs such as leaves and flowers is also regulated by multiple Polycomb‐group proteins (both PRC2 and PRC1‐like), which help maintain cells in a differentiated state and repress regulators of seed development. Severe loss of Polycomb‐group function at this stage leads to dedifferentiation and embryonic callus‐like tissue formation. Reductions in Polycomb‐group function also lead to morphologically abnormal flowers. Polycomb‐group proteins also regulate the floral transition both by preventing precocious flowering and mediating the vernalisation response. PRC2 members are shown in blue and PRC1‐like components in red.
Figure 2. Polycomb‐group proteins associate in complexes and use several different enzymatic activities to modify chromatin. A summary of published interactions and associations between different Polycomb‐group components is shown. There is stronger support for the formation of PRC2 complexes and some PRC1‐like components (LHP1, EMF1) are likely to be present in some PRC2 complexes, pointing to connection between PRC2 and PRC1‐like Polycomb‐group proteins. PRC2 members are shown in blue and PRC1‐like components in red. (Interactions reviewed in Mozgova and Hennig, .)
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Perera, Pumi, and Goodrich, Justin(Sep 2016) Plant Polycomb‐Group Proteins: Epigenetic Regulators of Development. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023751]