Regulation of photosynthetic sucrose synthesis: a role for calcium?

We have investigated whether changes of the cytosolic free-calcium concentration could regulate photosynthetic sucrose synthesis. Partially purified enzymes from spinach (Spinacea oleracea L.) leaves were assayed using calcium-EGTA buffers to obtain defined free-calcium concentrations in the low micromolar and submicromolar ranges. These concentrations of calcium did not directly affect sucrose-phosphate synthase activity. They inhibited the cytosolic fructose-1,6-bisphosphatase, acting competitively to magnesium. The Ki for calcium (1.2 μM) was 400-fold lower than the Km for magnesium. To investigate the in-vivo significance of these observations, the Ca(2+) antagonists lanthanum (La(3+)), trifluoperazine, and ruthenium red were supplied to barley leaves via the transpiration stream. All three antagonists selectively inhibited sucrose synthesis: they inhibited (14)C incorporation into sucrose at concentrations which did not affect starch synthesis; in low light, partitioning was changed towards starch without altering the rate of photosynthesis; there was a characteristic change in the induction kinetics of photosynthesis; and there was an accumulation of phosphorylated metabolites in the leaf. The detailed changes in metabolite levels indicate that La(3+) was acting via inhibition of the fructose-1,6-bisphosphatase, trifluoperazine via blockage of the turnover of inorganic pyrophosphate, and ruthenium red via changes in the activation state (phosphorylation) of sucrose-phosphate synthase. The results are discussed in terms of a possible contribution of calcium to the regulation of carbon metabolism.