INFLUENCE OF HIGH-CARBOHYDRATE CONTENT ON THE ACTIVITY OF PLASTIDIC AND CYTOSOLIC ISOENZYME PAIRS IN PHOTOSYNTHETIC TISSUES

The carbohydrate content of photosynthetic cells or tissues was increased by feeding glucose to autotrophic Chenopodium rubrum cell suspension cultures, by feeding glucose via the transpiration stream to detached spinach leaves, and by expressing yeast-derived invertase in the apoplast of tobacco leaves. Extracts were prepared, and plastidic and cytosolic isoenzymes were separated by electrophoresis and assayed by in situ activity staining. In all three systems, compared to control treatments, accumulation of carbohydrate led to decreased activity of plastid starch phosphorylase and phosphoglucose mutase, but not of the corresponding cytosolic isoenzymes. It led to increased activity of cytosolic 6-phosphogluconate dehydrogenase but not of the plastid isoenzyme. The activities of cytosolic and plastidic aldolase, triose-phosphate isomerase, and phosphoglucose isomerase were unaltered. The transcript and activity of nitrate reductase and pyrophosphate:fructose-6-phosphate phosphotransferase (both cytosolic enzymes) increased. The transcript levels for the S-gene of ADP-glucose pyrophosphorylase (a plastid enzyme) increased, but the overall enzyme activity decreased slightly. It is concluded that high carbohydrate leads to a selective change in the enzyme complement of the plastid, retaining enzymes which are required for glycolysis and losing enzymes which are needed for photosynthesis or starch mobilization.