Tissue-specific expression conferred by the S-adenosyl-L-methionine synthetase promoter of Arabidopsis thaliana in transgenic poplar.

In Arabidopsis the promoter of the gene encoding S-adenosyl-L-methionine synthetase (SAM-S) Psam-1 confers expression preferentially in the vascular tissue. In search for promoters that drive expression in particular cells of the lignifying tissues in trees, we have analyzed the expression pattern conferred by the Psam-1 promoter in transgenic poplar. Histochemical analyses demonstrated beta-glucuronidase (GUS) activity mainly in phloem and cortex tissue throughout the plant, and in root tips. Fluorimetric assays showed high GUS activity in the tissues outside (phloem, cortex and cork) compared to those inside (xylem and pith) of the cambial layer. In contrast, the endogenous SAM-S activity was high in tissues inside and low in tissues outside of the cambial layer. RNA gel blot analysis demonstrated a high transcript level of the endogenous sam-s gene(s) in tissues both outside and inside the cambial layer. This indicates that the low SAM-S activity in the bark was at least partially due to translational and/or post-translational regulation of the endogenous sam-s gene(s). In dormant transgenics, the tissue specificity was conserved, but the activity levels were up to 10-fold reduced.