ABI5 and ABI3 mediate FIS-PRC2 recruitment to IKU2 and modulate endosperm cellularization during seed development.

Seed size is tightly associated with endosperm proliferation and the onset of cellularization. The mitotic proliferation rate of the early endosperm and the initiation timing of cellularization act as key developmental nodes that determine final seed volume and weight. In Arabidopsis thaliana, the FERTILIZATION-INDEPENDENT SEED Polycomb Repressive Complex 2 (FIS-PRC2) performs crucial functions throughout gametogenesis and seed development. Here, we report that the transcription factors ABSCISIC ACID-INSENSITIVE 5 (ABI5) and ABSCISIC ACID-INSENSITIVE 3 (ABI3) directly bind to the promoter of the core endosperm regulatory gene IKU2 and repress its transcription. Moreover, FIS-PRC2 is recruited to the IKU2 promoter via physical interaction with promoter-bound ABI5. FIS-PRC2 further facilitates the deposition of the repressive H3K27me3 epigenetic mark at the IKU2 locus. Collectively, our results reveal that ABI5 and ABI3 modulate the expression of endosperm developmental genes by recruiting the FIS-PRC2 complex.