The relationship between the rate of starch synthesis, the adenosine 5'-diphosphoglucose concentration and the amylose content of starch in developing pea embryos
Mutations that reduced the rate of starch synthesis in pea (Pisum sativum L.) embryos through effects on enzymes on the pathway from sucrose to adenosine 5'-diphosphoglucose (ADPglucose) also led to a reduction in the amylose content of the starch of developing embryos. Evidence is presented that this relationship between rate of synthesis and the composition of starch is due to the fact that amylopectin-synthesising isoforms of starch synthase have higher affinities for ADPglucose than the amylose-synthesising isoform. First, developing mutant embryos (rb, rug3 and rug4 mutants) displayed both reduced amylose contents in their starches and reduced ADPglucose contents relative to wild-type embryos. Second, incubation of detached, wild-type embryos for 6 h at high and low glucose concentrations resulted in differences in both ADPglucose content and the relative rates of amylose and amylopectin synthesis. At 0.25 M glucose both ADPglucose content and the proportion of synthesised starch that was amylose were about twice as great as at 25 &mgr;M glucose. Third, S(0.5) values for soluble (amylopectin-synthesising) starch synthases in developing embryos were several-fold lower than that for granule-bound (amylose synthesising) starch synthase. Estimates of the expected amylose contents of the starch of the mutant embryos, based on the reduction in their ADPglucose contents and on the S(0.5) values of the starch synthases, were very similar to the measured amylose contents. The implications of these results for the determination of starch composition are discussed.