A TRUNCATED VERSION OF AN ADP-GLUCOSE PYROPHOSPHORYLASE PROMOTER FROMPOTATO SPECIFIES GUARD CELL-SELECTIVE EXPRESSION IN TRANSGENIC PLANTS

ADP-glucose pyrophosphorylase (AGPase) is a key regulatory enzyme in s tarch biosynthesis in higher plants. A 3.2-kb promoter of the large su bunit gene of the AGPase from potato has been isolated and its activit y analyzed in transgenic potato and tobacco plants using a promoter-be ta-glucuronidase fusion system. The promoter was active in various sta rch-containing cells, including guard cells, tuber parenchyma cells, a nd the starch sheath layer of stems and petioles. No expression was ob served in mesophyll cells. Analysis of various promoter derivatives sh owed that with respect to expression in petioles and stems, essential elements must be located in the 5' distal region of the promoter, wher eas elements important for expression in tuber parenchyma cells are lo cated in an internal fragment comprising nucleotides from positions -5 00 to -1200. Finally, a 0.3-kb 5' proximal promoter fragment was ident ified that was sufficient to obtain exclusive expression in guard cell s of transgenic potato and tobacco plants. The implications of our obs ervations are discussed with respect to starch synthesis in various ti ssues and the use of the newly identified promoter as a tool for stoma tal biology.