Analysis of a chimeric class-I patatin-GUS gene in transgenic potato plants: High-level expression in tubers and sucrose-inducible expression in cultured leaf and stem explants.

Patatin is a family of lipid acyl hydrolases that accounts for 30 to 40% of the total soluble protein in potato (Solanum tuberosum L.) tubers. To examine the regulation of the patatin genes, we constructed a chimeric gene containing 2.5 kb of 5' flanking sequence from the class I patatin genomic clone PS20 transcriptionally fused to β-glucuronidase (GUS) and introduced it into potato plants using an Agrobacterium tumefaciens Tiplasmid vector. While the chimeric gene was expressed at high levels in tubers and in stolons attached to developing tubers, it was not normally expressed in leaves, stems, roots, or in stolons before tuberizatization. However, the expression of the class I patatin-GUS construct was not "tuber-specific" since leaf and stem explants cultured on medium containing 300 to 400 mM sucrose showed GUS activity equal or greater than that of tubers. The sucrose induction of GUS activity in leaf and stem explants was accompanied by the accumulation of patatin protein and large amounts of starch, but not by the morphological changes that normally are associated with tuberization. In contrast, the GUS reporter gene under the control of the 35S promoter of cauliflower mosaic virus showed an essentially uniform pattern of expression in transgenic potato plants and was not induced by sucrose.