Sink-cell-specific activity of a potato ADP-glucose pyrophosphorylase B-subunit promoter in transgenic potato and tomato plants

The 5'-flanking region of a B (st-nail) subunit gene of ADP-glucose pyrophosphorylase (EC 2.2.7.27! called the agpB1 promoter - was cloned from the potato cultivar Desiree and its activity studied in transgenic potato (Solanum tuberosum L.) and tomato (Lycopersicon esculentum Mill.) plants using the gusA reporter gene. In potato, high beta-glucuronidase (GUS) activity was found in the storage tissues of developing tubers, but gusA expression was also detected in phloem-associated parenchymas in stolons, stems and petioles, as well as in the starch sheath adjacent to the vascular tissues in leaf veins, stems and petioles. In leaves, promoter activity was limited to stomatal guard cells and to the starch sheath of the major veins, with no detectable activity in the mesophyll. No expression was observed in roots. beta-Glucuronidase activity was finally detected in pollen grains and in ovary placental tissues. The potato promoter::gusA construct was introduced in transgenic tomatoes and was shown to be highly regulated during fruit development, with a tight parallelism between GUS activity, the extractable ADP-glucose pyrophosphorylase activity and fruit starch content. In conclusion,;he agpB1 promoter appears to be specific to sink tissues, contrasting with the sAGP gene recently described by P.A. Nakata and T.W. Okita (1996 Mol Gen Genet 250: 581-592) which is transcribed in both sink and source tissues. Furthermore, and in contrast to sAGP, agpB1 transcription is stimulated by sucrose.