Abscisic acid (ABA) and gibberellins (GAs) are plant hormones which antagonistically mediate numerous physiological processes, and their optimal balance is essential for normal plant development. However, the molecular mechanism underlying ABA and GA antagonism still needs to be determined. Here, we report that ABA-INSENSITIVE 4 (ABI4) is a central factor in GA/ABA homeostasis and antagonism in post-germination stages.ABI4overexpression in Arabidopsis (OE-ABI4) leads to developmental defects including a decrease in plant height and poor seed production. The transcription of a key ABA biosynthetic gene,NCED6, and of a key GA catabolic gene,GA2ox7, is significantly enhanced byABI4overexpression. ABI4 activatesNCED6andGA2ox7transcription by directly binding to the promoters, and genetic analysis revealed that mutation in these two genes partially rescues the dwarf phenotype ofABI4overexpressing plants. Consistently,ABI4overexpressing seedlings have a lower GA/ABA ratio than the wild type. We further show that ABA inducesGA2ox7transcription while GA repressesNCED6expression in an ABI4-dependent manner; and that ABA stabilizes the ABI4 protein whereas GA promotes its degradation. Taken together, these results suggest that ABA and GA antagonize each other by oppositely acting onABI4transcript and protein levels.