In plants, the H3K27me3 modification, established by the subunit of the Polycomb Repressive Complex 2 showing a methyltransferase activity, is present at more than 4400 genes, which are mostly transcription factors involved in major plants developmental processes. The first aim of this thesis is to test whether nascent RNA could be present at PRC2 targeted genes and whether these nascents RNA could act as anchor point for cis-elements to recruit PRC2 to a specific locus. The presence of nascent RNA at Polycomb repressed genes was demonstrated in Arabidopsis before adapting a new innovative chromatin tool, called Chromatin Isolation by RNA Precipitation (ChIRP), to isolate and identify proteins interacting with nascent RNA. The fully optimized method revealed nineteen candidate proteins to interact specifically with the nascent RNA at the FLC locus (Chapter 3) but the link to PRC2 recruitment is yet to be shown and should be investigated in the future. The second aim of this thesis is to determine the effects of the H3K27me3 erasers on plant development. The flowering time, the fertility and the embryo to seedling transition were studied in different Arabidopsis plants lacking the different demethylases, REF6, ELF6 and JMJ13. It showed antagonist phenotype suggesting very distinct roles. Flowering time studies identified four potential targets of JMJ13, FLC, SNZ, AGL24 and SOC1 that require further validation (Chapter 4) while the effect of ELF6 and REF6 on the flowering time have been described several years ago. Fertility studies showed JMJ13 promotes stamen elongation while ELF6 suppresses stamen and promotes pistil elongations. Seed analysis revealed that the seed size and weight are negatively affected by the demethylases and that REF6 and ELF6 share redundancy (Chapter 5). Germination rate analysis showed JMJ13 suppresses germination while ELF6 and REF6 promote it and the embryo-to-seedling transition study revealed ELF6 is partially required to generate true leaves (Chapter 6). The characterisation of the effects of the demethylases on Arabidopsis development has started recently and should be investigated to better understand the regulation of plant life cycle by the histone demethylases.