Maternal gene products deposited in an animal egg determine the polarity of embryonic axes and regulate embryonic cell-cell communication important for morphogenesis. Here we report the first maternal-effect embryo-defective mutation discovered in a plant. Recessive mutations in the SHORT INTEGUMENT (SIN1) gene in Arabidopsis were previously shown to influence ovule development and flowering time. Here we show that a sin1 mutation has a pronounced maternal effect on zygotic embryo development. A homozygous sin1 mutant embryo is normal when nursed by a sin1/+ heterozygous maternal sporophyte. Strikingly, a sin1 or a sin1/+ embryo that is nursed by a sin1 homozygous maternal sporophyte develops morphogenetic defects in the apical-basal and radial axes. The defects resemble those seen in some zygotic-effect embryonic pattern formation mutants. These results imply that in maternal cells the SIN1 gene either codes for or controls the production of a diffusible morphogen necessary for proper zygotic embryogenesis.