METABOLISM OF STARCH SYNTHESIS IN DEVELOPING GRAINS OF THE SHX SHRUNKEN MUTANT OF BARLEY (HORDEUM-VULGARE)

The shx mutant of barley (Hordeum vulgare L. cv. Bomi) produces grains greatly decreased in starch content and containing smaller A-starch g ranules together with normal B-granules. Soluble starch synthase (SSS) and ADP-glucose pyrophosphorylase (AGP) are reduced to about 20% norm al activity. To help determine the position of the block to starch syn thesis in shx, we measured enzyme activities and metabolite levels in developing grains of normal and mutant genotypes in a cv. Bomi backgro und. We demonstrate that sucrose, free hexose, hexose phosphates and, critically, ADP-Glc accumulate as a result of the mutation. In additio n to AGP and SSS, several other enzyme activities are affected in the shx mutant, most of them showing activities 50-80% of normal. Northern blots showed that transcripts for the AGP small subunit are much less abundant, but of normal size, in shx. These and earlier results toget her indicate that the metabolic block is at the end of the starch synt hetic pathway, with a primary effect on a type I, primer-independent s oluble starch synthase.