Barley sex6 mutants lack starch synthase lla activity and contain a starch with novel properties

Analysis of barley shrunken grain mutants has identified lines with a novel high amylose starch phenotype. The causal mutation is located at the sex6 locus on chromosome 7H, suggesting the starch synthase lla (sslla) gene as a candidate gene altered by the mutation. Consistent with this hypothesis, no evidence of SSlla protein expression in either the starch granule or soluble fractions of the endosperm was found. Sequences of the starch synthase lla gene, sslla, from three independent sex6 lines showed the presence of a stop codon preventing translation of the sslla transcript in each line. Perfect segregation of the starch phenotype with the presence of stop codons in the sslla gene was obtained, providing strong evidence for the lesion in the sslla gene being the causal mutation for the sex6 phenotype. The loss of SSlla activity in barley leads to novel and informative phenotypes. First, a decrease in amylopectin synthesis to less than 20% of the wild-type levels indicates that SSlla accounts for the majority of the amylopectin polymer elongation activity in barley. Secondly, in contrast to high amylose starches resulting from branching enzyme downregulation, the sex6 starches have a shortened amylopectin chain length distribution and a reduced gelatinisation temperature. Thirdly, the mutation leads to pleiotropic effects on other enzymes of the starch biosynthesis pathway, abolishing the binding of SSl, branching enzyme lla and branching enzyme llb to the starch granules of sex6 mutants, while not significantly altering their expression levels in the soluble fraction.