The Polycomb group protein MEDEA controls cell proliferation and embryonic patterning in Arabidopsis

Establishing the body plan of a multicellular organism relies on precisely orchestrated cell divisions coupled with pattern formation. In animals, cell proliferation and embryonic patterning are regulated by Polycomb group (PcG) proteins that form various multisubunit complexes (Grossniklaus and Paro, 2014). The evolutionary conserved Polycomb Repressive Complex 2 (PRC2) trimethylates histone H3 at lysine 27 (H3K27me3) and comes in different flavors in the model plant Arabidopsis thaliana (Forderer et al., 2016; Grossniklaus and Paro, 2014). The histone methyltransferase MEDEA (MEA) is part of the FERTILIZATION INDEPENDENT SEED (FIS)-PRC2 required for seed development4. Although embryos derived from mea mutant egg cells show morphological abnormalities (Grossniklaus et al., 1998), defects in the development of the placenta-like endosperm are considered the main cause of seed abortion (Kinoshita et al., 1999; Scott et al., 1998), and a role of FIS-PRC2 in embryonic patterning was dismissed (Bouyer et al., 2011; Leroy et al., 2007). Here, we demonstrate that endosperm lacking MEA activity sustains normal embryo development and that embryos derived from mea mutant eggs abort even in presence of a wild-type endosperm because MEA is required for embryonic patterning and cell lineage determination. We show that, similar to PcG proteins in mammals, MEA regulates embryonic growth by repressing the transcription of core cell cycle components. Our work demonstrates that Arabidopsis embryogenesis is under epigenetic control of maternally expressed PcG proteins, revealing that PRC2 was independently recruited to control embryonic cell proliferation and patterning in animals and plants.