Plant homeodomain protein involved in transcriptional regulation of a pathogen defense-related gene

Transcription of the parsley pr2 gene, encoding pathogenesis-related protein 2 (PR2), is rapidly stimulated by fungal or bacterial elicitors. Previous work has revealed a 125-bp region within the pr2 promoter; this region encompasses all important cis-regulatory elements required for fungal elicitor-mediated expression. We now report the identification of a functionally relevant 11-bp DNA motif (CTAATTGTTTA) contained within this region; it specifically binds to factors present in both parsley and Arabidopsis nuclear protein extracts. From both plant species, full-length cDNA clones were isolated that encode proteins with high affinity fo this DNA motif. The proteins from both species contain stretches of 61 amino acids that are characteristic of homeodomain (HD) proteins. Binding studies and use of a polyclonal antiserum raised against a fusion polypeptide of glutathione S-transferase with the HD portion of the parsley protein indicated that the 11-bp DNA motif is a potential in vivo target site and that the HD protein is contained within the observed complex formed between the DNA motif and nuclear protein extracts. Transient expression studies using the authentic and a mutated target site suggested a functional role of the HD-DNA interaction in the regulation of the pr2 gene expression.