Enhanced vascular activity of a new chimeric promoter containing the full CaMV 35S promoter and the plant XYLOGEN PROTEIN 1 promoter.

To develop a new strategy that controls vascular pathogen infections in economic crops, we examined a possible enhancer of the vascular activity of XYLOGEN PROTEIN 1 promoter (Px). This protein is specifically expressed in the vascular tissues of Arabidopsis thaliana and plays an important role in xylem development. Although Px is predicted as vascular-specific, its activity is hard to detect and highly susceptible to plant and environmental conditions. The cauliflower mosaic virus 35S promoter (35S) is highly active in directing transgene expression. To test if 35S could enhance Px activity, while vascular specificity of the promoter is retained, we examined the expression of the uidA reporter gene, which encodes β-glucuronidase (GUS), under the control of a chimeric promoter (35S-Px) or Px by generating 35S-Px-GUS and Px-GUS constructs, which were transformed into tobacco seedlings. Both 35S-Px and Px regulated gene expression in vascular tissues. However, GUS expression driven by 35S-Px was not detected in 30- and 60-day-old plants. Quantitative real-time PCR analysis showed that GUS gene expression regulated by 35S-Px was 6.2-14.9-fold higher in vascular tissues than in leaves. Histochemical GUS staining demonstrated that 35S-Px was strongly active in the xylem and phloem. Thus, fusion of 35S and Px might considerably enhance the strength of Px and increase its vascular specificity. In addition to confirming that 35S enhances the activity of a low-level tissue-specific promoter, these findings provide information for further improving the activity of such promoters, which might be useful for engineering new types of resistant genes against vascular infections.