Evidence that the rug3 locus of pea (Pisum sativum L.) encodes plastidial phosphoglucomutase confirms that the imported substrate for starch synthesis in pea amyloplasts is glucose-6-phosphate

Mutants at the rug3 locus give rise to seeds that, unlike those of the wild-type pea, are severely wrinkled at maturity. In the present analysis, the starch contents of the mature rug3rug3 seeds were found to be greatly reduced relative to near-isogenic wild-type peas, with the most severe phenotypes having only 1% of the seed dry weight as starch, compared with 50% in the wild type. At the microscopic level, the severely reduced starch content of mutant embryos was reflected by the presence of starchless plastids. The leaves of the rug3rug3 plants also had dramatically reduced starch contents that could be distinguished readily from the wild type by iodine staining. Biochemical investigations showed that the activity of the enzyme plastidial phosphoglucomutase (PGM) was not detectable in mutant tissues. In wild-type pea embryos, this enzyme catalyses the conversion of glucose-6-phosphate into glucose-l-phosphate in the plastids and therefore provides the substrate for the committed pathway of starch biosynthesis, The requirement for the plastidial PGM activity in order that starch synthesis can occur in pea embryos and leaves indicates that glucose-l-phosphate cannot be imported into the plastids and provides further evidence that the imported substrate in pea embryos is glucose-6-phosphate. Genetic analysis showed that the rug3 locus is tightly linked to the flower morphology locus k. This finding is consistent with the known genetic map location of the Pgm-p isozyme locus and supports the hypothesis that the rug3 mutations affect the plastidial PGM structural gene.