Plant Physiol.
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Plant Physiology 89:1270-1274 (1989)
© 1989 American Society of Plant Biologists

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Metabolism and Enzymology

Studies on the Entry of Fructose-2,6-Bisphosphate into Chloroplasts 1

Sjef Smeekens2, Fraser D. Macdonald and Bob B. Buchanan

Division of Molecular Plant Biology, University of California, Hilgard Hall, Berkeley California 94720

The regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P2) has an important function in controlling the intermediary carbon metabolism of leaves. Fru-2,6-P2 controls two cytosolic enzymes involved in the interconversion of fructose-6-phosphate and fructose-1,6-bisphosphate (fructose-1,6-bisphosphatase and pyrophosphate, fructose-6-phosphate 1-phosphotransferase) and thereby controls the partitioning of photosynthate between sucrose and starch. It has been demonstrated that Fru-2,6-P2 is present mainly in the cytosol. Here we present evidence that Fru-2,6-P2 can be taken up by isolated intact chloroplasts but at a very slow rate (about 0.01 micromoles per milligram of chlorophyll per hour). This uptake is time and concentration dependent and is inhibited by PPi. When provided a physiological concentration of Fru-2,6-P2 (10 micromolar), chloroplasts accumulated up to 0.6 micromolar Fru-2,6-P2 in the stroma. Elevated plastid Fru-2,6-P2 levels had no effect on overall photosynthetic rates of isolated chloroplasts. The results indicate that, while Fru-2,6-P2 enters isolated chloroplasts at a sluggish rate, caution should be exercised in ascribing physiological importance to effects of Fru-2,6-P2 on chloroplast enzymes.

2 Recipient of an EMBO Long Term Fellowship. Present address: Department of Molecular Cell Biology, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands.

1 Supported by a grant from the National Science Foundation.






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ASPB Publications PLANT PHYSIOLOGY THE PLANT CELL
Copyright © 1989 by the American Society of Plant Biologists