ABI5 (ABA insensitive 5), a bZIP (Basic leucine zipper) transcription factor, has been shown to be a major mediator of plant ABA responses during seed germination. Although the molecular basis of ABI5-modulated processes has been well demonstrated in Arabidopsis thaliana, its identity and function in cabbage (Brassica oleracea var. capitata L.) remain elusive. Here, we describe our identification of BoIABI5 (an ABI5 orthologue in B. oleracea) as a functional bZIP transcription factor in the modulation of plant ABA responses. Expression of BoIABI5 was dramatically induced by drought stress and exogenous ABA. Heterogeneous expression of BoIABI5 rescued the insensitive phenotype of Arabidopsis abi5-1 to ABA during seed germination. Subcellular localization and trans-activation assays revealed that BoIABI5 was localized in the nucleus and possessed DNA binding and trans-activation activities. Deletion of the bZIP domain generated BoIABI5 Delta bZIP, which no longer localized exclusively in the nucleus and had almost no detectable DNA-binding or trans-activation activities. Overall, these results suggest that BoIABI5 may function as ABI5 in the positive regulation of plant ABA responses. (C) 2012 Elsevier Inc. All rights reserved.