Promoter analysis of iron-deficiency-inducible barley IDS3 gene in Arabidopsis and tobacco plants

Under conditions of iron deficiency, graminaceous plants induce the expression of genes involved in the biosynthesis of mugineic acid family phytosiderophores. We previously identified the novel cis-acting elements IDE1 and IDE2 (iron-deficiency-responsive element 1 and 2) through promoter analysis of the barley (Hordeum vulgare L.) iron-deficiency-inducible IDS2 gene in tobacco (Nicotiana tabacum L.). To gain further insight into plant gene regulation under iron deficiency, we analyzed the barley iron-deficiency-inducible IDS3 gene, which encodes mugineic acid synthase. IDS3 promoter fragments were fused to the β-glucuronidase (GUS) gene, and this construct was introduced into Arabidopsis thaliana L. and tobacco plants. In both Arabidopsis and tobacco, GUS activity driven by the IDS3 promoter showed strongly iron-deficiency-inducible and root-specific expression. Expression occurred mainly in the epidermis of Arabidopsis roots, whereas expression was dominant in the pericycle, endodermis, and cortex of tobacco roots, resembling the expression pattern conferred by IDE1 and IDE2. Deletion analysis revealed that a sequence within −305 nucleotides from the translation start site was sufficient for specific expression in both Arabidopsis and tobacco roots. Gain-of-function analysis revealed functional regions at −305/−169 and −168/−93, whose coexistence was required for the induction activity in Arabidopsis roots. Multiple IDE-like sequences were distributed in the IDS3 promoter and were especially abundant within the functional region at −305/−169. A sequence moderately homologous to that of IDE1 was also present within the −168/−93 region. These IDE-like sequences would be the first candidates for the functional iron-deficiency-responsive elements in the IDS3 promoter.