GLUCOSE-DEPENDENT AND ADPGLC-DEPENDENT STARCH SYNTHESIS IN ISOLATED CAULIFLOWER-BUD AMYLOPLASTS - ANALYSIS OF THE INTERACTION OF VARIOUS POTENTIAL PRECURSORS

Recently, we have demonstrated that isolated cauliflower-bud amyloplas ts incorporate glucose 6-phosphate at high rates into newly synthesize d starch (Neuhaus et al. (1993) Plant Physiol. 101, 573-578). Here we have analyzed the incorporation of radioactively labeled glucose and A DPglucose into newly synthesized starch. It could be shown that glucos e incorporation into starch exhibits a typical substrate saturation be haviour and is linear with time for at least 40 min. The incorporation of glucose is strongly dependent upon the intactness of the plastids and upon the presence of both, ATP and 3-phosphoglyceric acid. Using 4 ,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) (1) we showed that glucose is taken up into isolated cauliflower-bud amyloplasts as the f ree glucose molecule, rather than as glucose 6-phosphate. Glucose inco rporation into newly synthesized starch is strongly inhibited in the p resence of low concentrations of glucose 6-phosphate. The radioactivel y labeled glucose moiety of ADPglucose is also incorporated into starc h. This incorporation can be saturated at increased concentrations of ADPglucose. ATP significantly inhibits the incorporation of the glucos e moiety of ADPglucose into starch. This inhibition can be reinforced by the additional presence of glucose 6-phosphate. Glucose 6-phosphate -dependent starch synthesis is not strongly inhibited in the presence of glucose or ADPglucose indicating that glucose 6-phosphate is the pr ecursor for starch synthesis in isolated cauliflower-bud amyloplasts.