The Arabidopsis PRC1-like ring-finger proteins are necessary for repression of embryonic traits during vegetative growth

Polycomb group genes play crucial roles in the maintenance of the transcriptionally silenced state of genes for proper cell differentiation in animals and plants. While components of the polycomb repressive complex2 (PRC2) are evolutionarily conserved and their functions are extensively studied in plants, PRC1 differs considerably between animals and plants, and its functions in plants are as yet not well described. Previous studies have identified the Arabidopsis AtRING1a and AtRING1b as homologues of the animal PRC1 subunit RING1. Here, we show that the Atring1a Atring1b double mutant exhibits derepression of embryonic traits during vegetative growth. Accordingly, several key regulatory genes involved in embryogenesis and stem cell activity are ectopically expressed in the mutant. Furthermore, we show that the mutant phenotypes and increased expression of regulatory genes are enhanced by the PRC2 mutant clf. Finally, we show that three homologues of the animal PRC1-subunit ring-finger protein BMI1, AtBMI1a, AtBMI1b and AtBMI1c, can bind with AtRING1a or AtRING1b, and in addition, AtBMI1c can bind with LHP1. The Atbmi1a Atbmi1b double mutant shows derepression of embryonic traits similar to that of the Atring1a Atring1b double mutant. Interestingly, expression levels of AtBMI1a, AtBMI1b and AtBMI1c are elevated in the Atring1a Atring1b mutant and those of AtBMI1c, AtRING1a and AtRING1b are elevated in the Atbmi1a Atbmi1b mutant, suggesting a self-regulatory feedback mechanism. Taken together, our results illuminate crucial functions of the PRC1-like ring-finger components in stable repression of embryonic traits and regulatory genes for proper somatic growth.