Metabolism of starch synthesis in developing grains of the shx shrunken mutant of barley (Hordeum vulgare)

Abstract

The shx mutant of barley (Hordeum vulgare L. cv. Bomi) produces grains greatly decreased in starch content and containing smaller A‐starch granules together with normal B‐granules. Soluble starch synthase (SSS) and ADP‐glucose pyrophosphorylase (AGP) are reduced to about 20% normal activity. To help determine the position of the block to starch synthesis in shx, we measured enzyme activities and metabolite levels in developing grains of normal and mutant genotypes in a cv. Bomi background. We demonstrate that sucrose, free hexose, hexose phosphates and, critically, ADP‐Glc accumulate as a result of the mutation. In addition 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 together indicate that the metabolic block is at the end of the starch synthetic pathway, with a primary effect on a type I, primer‐independent soluble starch synthase.