Potato (Solanum tuberosum) plants were transformed with a cDNA encoding the 59-kD subunit of the potato tuber NAD-dependent malic enzyme (NADME) in the antisense orientation. Measurements of the maximum catalytic activity of NADME in tubers revealed a range of reductions in the activity of this enzyme down to 40% of wild-type activity. There were no detrimental effects on plant growth or tuber yield. Biochemical analyses of developing tubers indicated that a reduction in NADME activity had no detectable effects on flux through the tricarboxylic acid cycle. However, there was an effect on glycolytic metabolism with significant increases in the concentration of 3-phosphoglycerate and phosphoenolpyruvate. These results suggest that alterations in the levels of intermediates toward the end of the glycolytic pathway may allow respiratory flux to continue at wild-type rates despite the reduction in NADME. There was also a statistically significant negative correlation between NADME activity and tuber starch content, with tubers containing reduced NADME having an increased starch content. The effect on plastid metabolism may result from the observed glycolytic perturbations.

The mtPDC occupies a central point in cellular energy metabolism, linking glycolytic carbon metabolism with the citric acid cycle. Since the reaction catalyzed by mtPDC is irreversible, this represents the committed step of carbon entry into the citric acid cycle. Plants are unique in that, in addition to mtPDC, they contain a second isoform of PDC localized in plastids. This isoform is believed to be involved in the provision of acetyl-COA for fatty acid, isoprenoid, and amino acid biosynthesis (Camp and Randall, 1985).Our knowledge of the structural organization of mtPDC is based largely on studies of the mammalian enzyme. The complex has a molecular weight of approximately 7 x 106 and is composed of enzymic, regulatory, and structural components. It contains the enzymes E1 (EC 1.2.4.1), E2 '

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