STARCH SYNTHESIS IN DEVELOPING PEA EMBRYOS

The pea embryo stores about half of its carbon as starch and has proved to be an excellent system on which to study the nature and regulation of the pathway of starch synthesis. The developing embryo receives its carbon as sucrose, which is metabolized via glycolysis in the cytosol of cotyledonary cells. Glucose 6-phosphate enters the amyloplast - probably via a phosphate-exchange translocator - where it is converted to ADPglucose via phosphoglucomutase and ADPglucose pyrophosphorylase. ADPglucose pyrophosphorylase is the site of action of a mutation at the rb locus, which reduces activity by more than 90 % and the rate of starch synthesis by about 50 %. Study of mutant and wildtype embryos reveals that one of four putative subunits of the enzyme is eliminated by the mutation. Three distinct isoforms of starch synthase catalyze the incorporation of the glucosyl moiety of ADPglucose into starch. Two of these are probably active in the soluble phase of the amyloplast and become incorporated into the granule as it grows, while the third is almost exclusively granule-bound. Analysis of cDNA clones for starch synthases shows that the exclusively granule-bound form is very similar to the `waxy' gene product believed to be responsible for amylose synthesis in cereal endosperms. The soluble starch synthases show some similarities to the 'waxy' proteins, but clearly belong to a different and previously undescribed class of starch synthases. The pea embryo contains two forms of starch branching enzyme, which are encoded by different genes, are maximally expressed at different times in development, and have different kinetic properties. It is likely that they play different roles in the synthesis of the granule. A mutation at the r locus, which reduces the rate of starch synthesis by about 50 % and increases the amylose content of the starch from 30 % to 70 %, consists of a transposon-like insertion in the gene encoding starch-branching enzyme I. Activity of this isoform is abolished by the mutation.