Binds preferentially double-stranded supercoiled DNA (PubMed:14756567). Required for karyogamy during female gametophyte development, when the two polar nuclei fuse to form the diploid central cell nucleus (PubMed:16698901). A number of isoforms are produced. According to EST sequences. Expressed at low levels in lateral roots, root tips, cotyledons, leaves and flowers (including pedicels, but excluding styles). Phosphorylated. Failure of fusion of the polar nuclei during megagametogenesis. Belongs to the HMGB family.