Hahb-4, a homeobox-leucine zipper gene potentially involved in abscisic acid-dependent responses to water stress in sunflower

Homeodomain-leucine zipper proteins constitute a family of transcription factors found only in plants. We have characterized a full-length cDNA encoding the sunflower homeobox-leucine zipper protein Hahb-4 (Helianthus annuus homeobox-4). The complete cDNA is 674 base pairs long and contains an open reading frame of 177 amino acids that belongs to the Hd-Zip I subfamily. Northern blot and RNAse A protection analysis show that the expression of Hahb-4 is rapidly, strongly and reversibly induced by water deficit in whole seedlings, roots, stems and leaves. A similar fast induction of Hahb-4 expression is observed when seedlings are subjected to a treatment with the hormone abscisic acid (ABA). Nuclei prepared from seedlings treated with ABA or subjected to water stress show a significant increase of protein(s) that specifically bind the sequence 5'-CAAT(A/T)ATTG-3', recognized in vitro by Hahb-4, suggesting that an active protein is synthesized in response to these treatments. The promoter region of the Hahb-4 gene contains sequences that fit the consensus for a G-box present in some ABA responsive elements (ABREs). We propose that Hahb-4 may function in signalling cascade(s) that control(s) a subset of the ABA-mediated responses of sunflower to water stress.