Pathogen Species-Specific Differences in Induction of the Maize <i>Polyubiquitin</i> Gene Promoter in Transgenic Wheat

The maize polyubiquitin promoter (ZmUbi) is a mainstay in molecular biology for transgene expression and is used for constitutive expression of defense related gene products. Transgenic wheat lines were produced expressing a ZmUbi-RUBY reporter gene that produces the red pigment betalain. Some lines showed transgene silencing with reduced RUBY transcript accumulation and chimeric sectors of betalain. Infection of these plants with Blumeria graminis, Puccinia graminis f. sp. tritici (Pgt) or P. triticina (Pt) each resulted in localised betalain accumulation at infection sites and increased RUBY transcript accumulation. In contrast, two isolates of P. striiformis f. sp. tritici (Pst) caused no detectable RUBY transcript accumulation and no visible betalain accumulation at infection sites, although a modest betalain increase was detected in infected tissue extracts. Compared with Pst, Pgt more strongly induced host genes involved in transcriptional and post-transcriptional regulatory processes, although no obvious pathogen-induced changes in ZmUbi promoter methylation were observed. ZmUbi-GUS transgenic wheat plants were also pathogen challenged and, unlike Pst, both Pgt and Pt induced localised GUS staining at infection sites. Database mining showed the endogenous maize polyubiquitin gene from which ZmUbi is derived was pathogen inducible, albeit in a species-specific fashion. These pathogen differences in ZmUbi induction have implications when using this regulatory element to express defense related transgenes in wheat. Comparing the resistance efficacy of transgenes against different pathogens using this promoter is potentially influenced by significant, localised expression differences occurring at infection sites of different pathogen species.