A W-box is required for full expression of the SA-responsive gene SFR2

Abstract

Transcripts of SFR2, a member of the S family of receptor kinase genes, accumulate rapidly in Brassica oleracea leaves in response to wounding, bacterial infection and following treatment with salicylic acid (SA). Expression of a chimeric gene consisting of the SFR2 5' flanking sequence fused to the gusA reporter gene is also induced in wounded and SA-treated Arabidopsis plants indicating that the observed response is conferred by the SFR2 promoter. We show here that, in Arabidopsis plants carrying the salicylate hydroxylase (NahG) transgene, wound induction of the SFR2 promoter-gusA reporter fusion was abolished, indicating that, as has previously been shown for the response to bacterial infection, SA is required for the response to wounding. Deletion analysis of the SFR2 promoter identified a region necessary for full expression following SA treatment. This region, which includes two putative W-boxes, is conserved in the promoter of the Arabidopsis SFR2 homologue, ARK3. Deletion of a 12 bp region containing the two W-box motifs reduced the response to SA treatment. Tandem repeats of the W-box-containing element fused upstream of a CaMV 35S minimal promoter enhanced reporter gene expression in transgenic Arabidopsis both in the absence and presence of SA. Gel-mobility shift assays showed that Arabidopsis leaf extracts contained factors that bound to a fragment of the promoter spanning the putative W-boxes and that a fragment in which these motifs were mutated was unable to compete for binding. In summary, induction of the SFR2 promoter in response to bacterial infection and wounding requires SA, and full expression of the induced gene requires the presence of a functional element containing W-box motifs in the SFR2 promoter. The involvement of two W-boxes indicates that transcription factors of the WRKY family may play a key role in mediating these responses.