The Arabidopsis Polycomb Repressive Complex 1 (PRC1) components AtBMI1A, B and C impact gene networks throughout all stages of plant development. Arabidopsis thaliana

PcG regulation in Arabidopsis is required to maintain cell differentiation and to allow developmental phase transitions. This is achieved by the activity of three PRC2s and the participation of a yet poorly defined PRC1. Previous results showed that apparent PRC1 components perform discrete roles during plant development, suggesting the existence of PRC1 variants; however, it is not clear in how many processes these components participate. We show that AtBMI1 proteins are required to promote all developmental phase transitions and to control cell proliferation during organ growth and development, expanding their proposed range of action. While AtBMI1 function during germination is closely linked to B3 domain transcription factors VAL1/2 possibly in combination with GT-box binding factors, other AtBMI1 regulatory networks require participation of different factor combinations. Conversely, EMF1 and LHP1 bind many H3K27me3 positive genes upregulated in atbmi1a/b/c mutants; however, loss of their function affects expression of a different subset, suggesting that even if EMF1, LHP1 and AtBMI1 exist in a common PRC1 variant, their role in repression depends on the functional context. Overall design: mRNA profiles of 10DAG Arabidopsis seedlings Col-0; single mutants atbmi1a and atbmi1b; strong double mutants atbmi1a/b and strong triple mutants atbmi1a/b/c