The promoter of the sunflower HD-Zip protein gene Hahb4 directs tissue-specific expression and is inducible by water stress, high salt concentrations and ABA

Abstract

In the present work, we have analysed the promoter region of the sunflower nuclear gene Hahb4, encoding an homeodomain-leucine zipper protein involved in water stress responses. This region is represented in two different but highly conserved alleles of 1015 and 1221 bp, respectively, in the sunflower hybrid studied. To gain insight into the structure and function of these promoter forms, we have obtained plants stably transformed with different fragments fused to the β-glucuronidase ( gus) reporter gene. Histochemical staining indicated that both Hahb4 promoter forms direct expression in roots, cotyledons and leaves during the entire plant life cycle. No expression was observed in reproductive organs. The analysis of progressive upstream deletions of the promoters suggested that a minimal 417/421 bp fragment is required for basal expression. The presence of positive regulatory elements between nucleotides −601/608 and −818/−1024 from the transcription initiation site (depending on the promoter) and a sequence required for specific expression in the root central cylinder between −818/−1024 and −1015/−1221 has been detected. Water stress, ABA or NaCl treatments induced Hahb4 promoter-dependent β-glucuronidase expression as observed by Northern blot hybridization experiments. Putative regulatory elements involved in the regulation of other genes were detected in the promoter fragment required for expression. These elements, together with experimental evidence, were analysed with the aim of elucidating the molecular mechanisms that participate in the expression of this gene.