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.