EMF2b-1 regulates endosperm cell cycling through PRC2 recruitment in maize

Polycomb Repressive Complex 2 (PRC2) establishes H3K27me3 to regulate gene expression, yet how its genomic targeting is achieved in large crop genomes remains unclear. Cereal endosperm development relies on tightly coordinated transitions between mitosis and endoreduplication, which is essential for determining final kernel size. Here we identify the maize SUZ12 homolog EMF2b-1 as a key regulator of endosperm cell cycling and kernel growth. Loss-of-function mutations in EMF2b-1 lead to reduced kernel size and enhanced endoreduplication. We also find emf2b-1 mutants exhibit decreased cell size and delayed cell differentiation, accompanied by a substantial decrease in H3K27me3 levels. Transcriptome and H3K27me3 ChIP-seq analysis identify 353 downstream target genes with reduced H3K27me3 and increased expression, among which 98 are direct target genes enriched for functions in cell cycling and endoreduplication. Notably, the seed size regulator ZmDA1, a homolog of DA1 that suppresses cell division while promoting endoreduplication, is directly targeted by the EMF2b 1-PRC2 complex and is upregulated in emf2b-1 mutants, consistent with its known role in restricting kernel size. Further motif enrichment and EMF2b-1 immunoprecipitation-mass spectrometry demonstrate that the endosperm specific transcription factor, BZR1-9, recruits EMF2b-1-PRC2 complex to the ZmDA1, thereby repressing its expression to regulate cell cycling and endoreduplication. These findings demonstrate that EMF2b-1 is essential for cell cycling during maize endosperm development, and uncover a transcription factor-guided mechanism of PRC2 recruitment in maize, an evolutionarily conserved strategy paralleling Polycomb targeting in Arabidopsis and Drosophila.