Antisense inhibition of plastidial phosphoglucomutase provides compelling evidence that potato tuber amyloplasts import carbon from the cytosol in the form of glucose-6-phosphate

The aim of this work was to establish whether plastidial phosphoglucomutase is involved in the starch biosynthetic pathway of potato tubers and thereb y to determine the form in which carbon is imported into the potato amylopl ast. For this purpose, we cloned the plastidial isoform of potato PGM (StpP GM), and using an antisense approach generated transgenic potato plants tha t exhibited decreased expression of the StpPGM gene and contained significa ntly reduced total phosphoglucomutase activity. We confirmed that this loss in activity was due specifically to a reduction in plastidial PGM activity . Potato lines with decreased activities of plastidial PGM exhibited no maj or changes in either whole-plant or tuber morphology. However, tubers from these lines exhibited a dramatic (up to 40%) decrease in the accumulation o f starch, and significant increases in the levels of sucrose and hexose pho sphates. As tubers from these lines exhibited no changes in the maximal cat alytic activities of other key enzymes of carbohydrate metabolism, we concl ude that plastidial PGM forms part of the starch biosynthetic pathway of th e potato tuber, and that glucose-6-phosphate is the major precursor taken u p by amyloplasts in order to support starch synthesis.