THE ARABIDOPSIS HOMEOBOX GENE ATHB-7 IS INDUCED BY WATER-DEFICIT AND BY ABSCISIC-ACID

Homeodomain-leucine zipper (HD-Zip) proteins are putative transcriptio n factors encoded by a class of recently discovered homeobox genes as yet found only in plants. This paper reports on the characterization o f one of these genes, ATHB-7, in Arabidopsis thaliana. ATHB-7 transcri pts were present in all organs of the plant at low levels, but express ion was induced several-fold by water deficit, osmotic stress as well as by exogenous treatment with abscisic acid (ABA), a response being d etectable at 10(-8) M and reaching a maximum at 10(-6) M ABA. The ATHB -7 transcript was detected within 30 min after treatment with ABA and the transcript level was rapidly reduced after removal of the hormone. The induction of ATHB-7 was shown to be mediated strictly via ABA, si nce no induction of ATHB-7 was detectable in the ABA-deficient mutant aba-3 subjected to drought treatment. Induction levels in two ABA-inse nsitive mutants abi2 and abi3 were similar to the wild-type response. In the abi1 mutant, however, induction was impaired as 100-fold higher concentrations of ABA were required for a maximum induction as compar ed with wild-type. In this mutant the ATHB-7 response was reduced also after drought and osmotic stress treatments. These results indicate t hat ATHB-7 is transcriptionally regulated in an ABA-dependent manner a nd may act in a signal transduction pathway which mediates a drought r esponse and also includes ABl1.