In flowering plants, the embryo is surrounded by the endosperm, which
facilitates transfer of nutrient between embryo and the mother. The
endosperm is distinct as it contains only one copy of the father's genome
and two copies of the mother's. This maternal to paternal ratio is
remarkable because of its importance to seed viability and development.
Seeds with extra genomes that alter this critical ratio undergo a process
known as interploidy seed abortion due to defective endosperm development.

The enzyme RNA Pol IV is specific to plant genomes that generate small RNA
molecules silencing gene expression from transposons and repetitive DNA,
playing a major role in defending the genome against viruses and
transposable elements. A study published in Plant Cell shows that RNA Pol IV
plays a key role in interploidy seed abortion.

The study shows that RNA Pol IV targets genes in the father through the
"canonical" RNA-directed DNA methylation pathway. This results in
interploidy seed abortion. The researchers then compared gene transcription
in the endosperm of aborted interploidy seeds with seeds that were viable
due to the loss of paternal RNA Pol IV. They found that transposons and
thousands of genes, even imprinted ones, were misregulated in both living
and dying seeds and learned that misregulation of a relatively small number
of genes sets living seeds apart from aborting ones.

[www.plantcell.org]