A novel shrunken endosperm mutant of barley

Although mutations affecting several enzymes of the starch synthetic pathway in developing cereal endosperm have been isolated, none has a major effect on soluble starch synthase We report a new recessive shrunken endosperm mutant in barley (Hordeum vulgare L. cv. Bomi‐like), shx, which has 25% of normal starch content. We have assayed the activity of sucrose synthase (EC 2.4.1.13), ADP and UDP‐glucose pyrophosphorylases (EC 2.7.7.27 and 2.7.7.9), branching enzyme (EC.2.4.1.18), and granule‐bound and soluble starch synthase (EC 2.4.1.21) in shx. Sucrose synthase activity is reduced by 49% and UDP‐glucose pyrrphosphorylase is 80% of the normal level. Branching enzyme and starch‐bound starch synthase activities are normal, but ADP‐glucose pyrophosphorylase activity is reduced by 72%. The soluble starch synthase that is primer‐independent in the presence of sodium citrate shows 14% of normal activity in shx. whereas the primer‐dependent form is unaffected. This lower starch synthase activity in shx cannot be explained by inhibition, substrate destruction or lack of primer. Although several starch‐synthetic enzymes are affected, it is suggested that the primer independent from of soluble starch synthase may be the primary‐site of the mutation in shx.