In Arabidopsis, the abscisic acid (ABA)-Insensitive ABI1 and ABI3 genes have been proposed to act in separate ABA signalling cascades. Recessive mutations in ABI3 alter various physiological processes during seed development, whereas the dominant abi1 mutation inhibits ABA responses largely in vegetative tissues. The seed-specific ABI3 gene was ectopically expressed in the vegetative tissues of transgenic Arabidopsis plants carrying a transcriptional fusion between the CaMV 35S promoter and the ABI3 cDNA. Genetic interactions between the ectopically expressed ABI3 and endogenous ABI1 genes were investigated by monitoring diverse ABA responses in vegetative tissues. Ectopic expression of ABI3 conferred to plantlets the ability to accumulate the seed-specific At2S33 and AtEm1 mRNAs in response to ABA, and the abi1 mutation inhibited this ABI3-dependent induction of AtEm1 by ABA. Furthermore, ectopic expression of ABI3 also influenced ABI1-dependent responses that occur in wild-type vegetative tissues. Expression of ABI3 increased ABA induction of the Rab18 mRNA and ABA inhibition of root growth, and both responses were sensitive to the abi1 mutation in the presence as in the absence of ABI3. Finally, although ABI3 is thought to be a transcription activator and stomatal regulation is not known to involve transcriptional events, the ectopically expressed ABI3 gene suppressed the effect of the abi1 mutation on stomatal regulation. The present data demonstrate that ABI1 and ABI3 genetically interact in controlling diverse ABA responses in transgenic vegetative tissues. The possibility that the endogenous ABI1 and ABI3 genes may similarly act in a common ABA signalling pathway in seed is discussed with previous phenotypic studies of the abi1 and abi3 mutants.