THE PROMOTER OF THE GENE ENCODING THE C-4 FORM OF PHOSPHOENOLPYRUVATECARBOXYLASE DIRECTS MESOPHYLL-SPECIFIC EXPRESSION IN TRANSGENIC C-4 FLAVERIA SPP

The function of the C-4 mechanism of photosynthesis depends on the str ict compartmentation of the enzymes involved, Here, we investigate the regulatory mechanisms that ensure the mesophyll-specific expression o f the C-4 isoform of phosphoenolpyruvate carboxylase. We show that 2 k b of the 5' flanking region of the Flaveria trinervia C-4 PpcA1 gene i s sufficient to direct mesophyll-specific expression of the beta-glucu ronidase reporter gene in transgenic F. bidentis (C-4) plants. In youn g leaves of seedlings, the activity of this promoter is dependent on t he developmental stage of the mesophyll cells. It is induced in a basi petal fashion (leaf tip to base) during leaf development. The promoter region of the orthologous nonphotosynthetic Ppc gene of F. pringlei ( C-3) induces reporter gene expression mainly in the vascular tissue of leaves and stems as well as in mesophyll cells of transgenic F. biden tis plants. Our experiments demonstrate that during the evolution of t he C-4 Flaveria species, cis-acting elements of the C-4 Ppc gene must have been altered to achieve mesophyll-specific expression.