MUTATIONS IN THE GENE ENCODING STARCH-SYNTHASE-II PROFOUNDLY ALTER AMYLOPECTIN STRUCTURE IN PEA EMBRYOS

Mutations at the rug5 (rugosus5) locus have been used to elucidate the role of the major soluble isoform of starch synthase II (SSII) in amy lopectin synthesis in the developing pea embryo. The SSII gene maps to the rug5 locus, and the gene in one of three rug5 mutant lines has be en shown to carry a base pair substitution that introduces a stop codo n into the open reading frame. All three mutant alleles cause a dramat ic reduction or loss of the SSII protein. The mutations have pleiotrop ic effects on the activities of other isoforms of starch synthase but apparently not on those of other enzymes of starch synthesis. These mu tations result in abnormal starch granule morphology and amylopectin s tructure. Amylopectin contains fewer chains of intermediate length (B- 2 and B-3 chains) and more very short and very long chains than does a mylopectin from wild-type embryos. The results suggest that SSII may p lay a specific role in the synthesis of B-2 and B-3 chains of amylopec tin, The extent to which these findings can be extrapolated to other s pecies is discussed.