Abscisic acid (ABA) regulates seed maturation, germination, and adaptation of vegetative tissues to environmental stresses. The mechanisms of ABA action and the specificity conferred by signaling components in overlapping pathways are not completely understood. The ABI5 gene (ABA insensitive 5) of Arabidopsis encodes a basic leucine zipper factor required for ABA response in the seed and vegetative tissues. Using transient gene expression in rice protoplasts, we provide evidence for the functional interactions of ABI5 with ABA signaling effectors VP1 (viviparous 1) and ABI1 (ABA insensitive 1). Co-transformation experiments with ABI5 cDNA constructs resulted in specific transactivation of the ABA-inducible wheat Em, Arabidopsis AtEm6, bean β-Phaseolin, and barley HVA1 and HVA22 promoters. Furthermore, ABI5 interacted synergistically with ABA and co-expressed VP1, indicating that ABI5 is involved in ABA-regulated transcription mediated by VP1. ABI5-mediated transactivation was inhibited by overexpression of abil-1, the dominant-negative allele of the protein phosphatase ABI1, and by 1-butanol, a competitive inhibitor of phospholipase D involved in ABA signaling. Lanthanum, a trivalent ion that acts as an agonist of ABA signaling, potentiated ABI5 transactivation. These results demonstrate that ABI5 is a key target of a conserved ABA signaling pathway in plants.