library_construction_protocol: Frozen 16 DAP endosperms (0.2g) of nkd1, 2 and o2 single, double and triple mutants, as well as the wild type counterpart (two biological replicates each) were collected and ground in liquid nitrogen. Add 2mL of pre-chilled lysis buffer (15mM Tris-HCl pH7.5, 20 mM NaCl, 80 mM KCl, 0.5 mM spermine, 5mM 2-ME, 0.2% TritonX-100) to resuspend the ground powder. Then the resuspension was filtered three times through miracloth. The crude nuclei were stained with 4,6-Diamidino-2-phenylindole (DAPI, from Millipore Sigma, St. Louis, MO) (4uL DAPI per ~500uL filtered resuspension), and at least 50,000 triploid nuclei (3C) were sorted for ATAC-Seq library construction. The library was constructed using the Active Motif ATAC-Seq Kit following the manufacturer's manual. The library was sequenced using NovaSeq 6000 SP flow cell sequencing platform at 2X50 bp paired-end in 50 million bp depth.