Polycomb Repressive Complex 2 (PRC2), which mediates the deposition of H3K27me3 histone marks, is important for developmental decisions in animals and plants. In the shoot apical meristem, TALE (Three Amino acid Loop Extension) family KNOX/BELL transcription factors are key regulators of meristem cell pluripotency and differentiation. Here, we identified a PRC2-associated coiled-coil protein (PACP) that interacts with KNOX/BELL transcription factors in rice (Oryza sativa) shoot apex cells. A loss-of-function mutation of PACP resulted in differential gene expression similar to that observed in PRC2 gene knockdown plants, reduced H3K27me3 levels, and reduced genome-wide binding of the PRC2 core component EMF2b. The genomic binding of PACP displayed a similar distribution pattern to EMF2b, and genomic regions with high PACP and EMF2b binding signals were marked by high levels of H3K27me3. We show that PACP is required for the repression of cell differentiation-promoting genes targeted by a rice KNOX1 protein in the shoot apical meristem. PACP is involved in the recruitment or stabilization of PRC2 to genes targeted by KNOX/BELL transcription factors to maintain H3K27me3 and gene repression in dividing cells of the shoot apex. Our results provide insight into PRC2-mdiated maintenance of H3K27me3 and the mechanism by which KNOX/BELL proteins regulate shoot apical meristem development.

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