Has a dual function during gametophyte development and early embryogenesis. Required for correct pollen maturation. Homodimer. In tricellular pollen, expressed in mature sperm cells. Not expressed in bicellular or unicellular pollen. Detected in ovules, roots and guard cells. Expressed in the embryo sac before cellularization, in the egg cell after cellularization, in the zygote/embryo immediately after fertilization and in the pollen vegetative cell. The extracellular domain (25-427) is sufficient for male and female gametophyte development before double fertilization, but not for early embryogenesis. The cytoplasmic domain (507-593) is necessary for correct early embryogenesis and mediates homodimer formation at the plasma membrane. Embryo lethal when homozygote.