Early seed development includes an early stage of endosperm proliferation and a late stage of embryo growth at the expense of the endosperm tissue in Arabidopsis.Abscisic Acid (ABA) is often suggested to act in the late seed development stage, however, whether and how ABA mediates the early seed development remains elusive.In this study, we found that mutation in ABAx, a unique short-chain dehydrogenase/reductase gene in the biosynthesis of ABA, resulted in decreased ABA content but increased seed size and mass.