Post-translational histone modifications contribute to the maintenance of chromatin states associated with a defined gene expression programme. The Polycomb Group (PcG) machinery plays a key role in repressing genes not necessary for a particular cell identity. However, only some PcG proteins are known in plants. The PLANTPCGSILENCING project has unveiled the different PcG regulatory mechanisms in plants and their biological functions. PcG proteins are found in several families of multiproteic complexes, including the Polycomb Repressive Complexes, PRC1 and PRC2. So far, much is known about plant PRC2 complexes but plant PRC1 components have only just been discovered. Evidence shows that both complexes are required for stable repression of target genes but details again are missing. The researchers focused on the role of PRC1 in PcG repression throughout plant development. For this, they focused on the AtBMI1 proteins that have been proposed as possible PRC1 components. Results showed that activity of the AtBMI1 proteins was crucial to establish and maintain the repression of seed maturation genes after germination. As recruitment of PcG complexes to specific targets in animals has long been thought to be a sequential process, this was an unexpected result because a reverse mechanism was indicated for the regulation of seed maturation genes. Analysis of the transcriptome of atbmi1 mutants indicated that in addition to switching off the seed maturation programme after germination, AtBMI1s promote the transition from one developmental phase to the next. This continues throughout development and control of cell proliferation during organ growth and development. Integration of datasets with previous data points to different PRC1 functional networks, where genes are regulated by AtBMI1 and/or Embryonic Flower 1 (EMF1) together with Chromo domain-containing protein 1(LHP1), and PRC2. Additional proteins are required to regulate distinct subsets of genes. Revealing an important difference between plants and animals, PLANTPCGSILENCING PRC1 activity can also recruit PRC2 for seed maturation genes. Results therefore challenge the use of the classical recruitment animal model of PcG complexes as a classical hierarchical model for PcG mark deposition to represent the sequence of events in the plant Arabidopsis.
Gene silencing, Arabidopsis, PcG, PLANTPCGSILENCING, PRC