The epigenetic mechanisms that control chromatin organization are known to regulate cell differentiation in both animals and plants. Nevertheless, why plants, unlike animals, so readily switch their cell fate in response to external signals is unclear. Furthermore, how plants regenerate a complete organism from a single somatic cell remains a mystery. Post-translational histone modifications contribute to the maintenance of chromatin states associated with a defined gene expression program. Among histone modifiers, the evolutionary conserved Polycomb Group (PcG) machinery has a key role in repressing the genes that are not required in a specific cell fate, thus, establishing cell identity. Unfortunately, our knowledge of this important regulatory mechanism is still incomplete, especially in plants in which only some PcG proteins are known. Thus far, plant PRC2-like complexes are relatively well studied, but the existence of plant PRC1 components has just been discovered. These components are homologs to animal PRC1 Ring finger proteins and are involved in H2A monoubiquitination. Plants impaired in PRC1 Ring finger components undergo dedifferentiation of somatic cells into embryonic cells, indicating that they play a central role in maintaining cell identity. Interestingly, the PRC1 Ring finger proteins apparently regulate only a subset of PRC2 targets; moreover, there are different plant-specific proteins that participate in the regulation of specific targets, suggesting that not all PcG targets are regulated by the same mechanism. Our main goal is to unveil the different PcG regulatory mechanisms in plants and their biological functions. Using a combination of genetic and biochemical approaches, we will purify the putative PRC1 complexes, determine the direct targets of the PRC1 Ring finger proteins and unveil the role of H2A monoubiquitination in the mechanism. These findings will shed light on important aspect of plant epigenetics and developmental biology.
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