The Fie1-PRC2 complex regulates H3K27me3 deposition to balance endosperm filling and development in cereals

Polycomb repressive complex 2 (PRC2)-mediated H3K27me3 deposition is vital for cell fate determination during Arabidopsis thaliana endosperm development. Unlike the transient endosperm in Arabidopsis, cereals follow a different developmental program after the cellularization phase, producing a persistent endosperm. In contrast to the single, constitutively expressed Fertilization Independent Endosperm (FIE) gene in Arabidopsis, cereals have evolved a duplicated, grain-specific counterpart, such as maize ZmFie1 and rice OsFie1. However, their functions remain unclear. We applied Cleavage Under Targets and Tagmentation (CUT&Tag) to profile the dynamics of the H3K27me3 mark during maize endosperm development. We observed a genome-wide elevation of H3K27me3 levels at early stages, followed by a rapid reduction after seed filling. We identified common regions and designated them as Filling Specific Peaks (FSPs), which are largely regulated by ZmFie1. Indeed, knockout of ZmFie1 results in earlier cellularization and slightly enhanced mitosis during endosperm filling, leading to smaller kernels that accumulate more zeins. Consistently, H3K27me3 levels on α-zein genes, located as tandem repeats on chromosome 4, are dramatically decreased in zmfie1. Moreover, it indirectly inhibits cell proliferation by mediating H3K27me3 deposition at ZmMADS loci, thereby balancing endosperm development and filling. Intriguingly, OsFie1 imposes H3K27me3 modification at the loci of 13-kDa prolamin genes and OsMADSs, leading to their repressed expression. Collectively, our findings reveal the conserved function of H3K27me3 deposition mediated by ZmFie1/OsFie1 in cereal endosperm development. The newly evolved, cereal grain-specific FIE1-PRC2 complex plays a key role in balancing storage substance synthesis and cell proliferation during persistent endosperm development.