An embryo-derived peptide signal directs endosperm polarity in Arabidopsis.

Angiosperm seed formation requires the coordinated development of the products of double fertilization-the embryo and the endosperm. The endosperm mediates efficient nutrient transfer from surrounding maternal tissues to the developing embryo. This function requires a polarized tissue organization, which manifests as early polar gene expression and polar cellularization dynamics. We show that the receptor kinase HAIKU2 (IKU2) acts in coordination with the transcription factor WRKY10/MINISEED3 (MINI3) to ensure robust endosperm polarity establishment through the activity of the homeodomain transcription factors WUSCHEL-RELATED HOMEOBOX 8 (WOX8) and WOX9. This process depends on egg cell fertilization and is mediated through the peptide PATHOGEN-INDUCED PEPTIDE-LIKE 7 (PIPL7), which acts as a potential IKU2 ligand. Our results reveal how a molecular paracrine dialog between the embryo and endosperm ensures optimal seed developmental coordination.