Cis-elements important for the expression of the ADP-glucose pyrophosphorylase small-subunit are located both...

ADP-glucose pyrophosphorylase (AGP) is a key regulatory enzyme in the biosynthesis of starch in higher plants. Previous studies have suggested that, unlike other plants that display tissue-specific AGP genes, potato expresses the same AGP small-subunit gene (sAGP) in multiple tissues. This view was confirmed by the spatial patterns of expression of the sAGP gene in transgenic potato plants observed when a promoter-dependent-beta-glucuronidase (beta-GUS) system was used. sAGP-beta-GUS chimeric gene fusions were expressed at high levels in tubers and in many other starch-containing cells throughout the plant. Deletional analysis of the 5'-upstream region of sAGP revealed that the observed spatial patterns of expression were due to different regions of the promoter of sAGP functioning in combination to confer cell- and organ-specific patterns of expression. Depending on the tissue examined, the patterns of reporter-gene expression were enhanced, suppressed, or altered when the 3'-nopaline-synthase terminator was replaced by the 3'-flanking sequence of sAGP. The observed cellular expression patterns of sAGP only partially overlap with the reported expression patterns of the major large-subunit gene (lAGP) in leaves. Since AGP is a heterotetrameric enzyme, composed of two sAGP and two lAGP subunits, this difference in the cellular expression patterns as well as quantitative differences in expression of the two AGP genes may account for the observed post-transcriptional regulation, i.e., relatively high levels of transcript but low levels of sAGP subunit in leaves.