Plant Physiol, April 2001, Vol. 125, pp. 1710-1722

Two Loci Control Phytoglycogen Production in the Monocellular Green Alga Chlamydomonas reinhardtii¹

Laboratoire de Chimie Biologique, Unité Mixte de Recherche du Centre National de la Recherche Scientifique, No. 8576, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France (D.D., C.C., G.M., C.d.H., S.G.B.); Institut National de la Recherche Agronomique, Centre de Recherches Agroalimentaires, Rue de la Géraudière, Boîte Postale 71627, 44316 Nantes cedex 03, France (A.B., D.J.G., B.B.); Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, G.P.O. Box 1600, Canberra, Australian Capital Territory 2601, Australia (M.K.M.); and Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, Iowa 50011 (A.M.M.)

The STA8 locus of Chlamydomonas reinhardtii was identified in a genetic screen as a factor that controls starch biosynthesis. Mutations of STA8 cause a significant reduction in the amount of granular starch produced during nutrient limitation and accumulate phytoglycogen. The granules remaining in sta8 mutants are misshapen, and the abundance of amylose and long chains in amylopectin is altered. Mutations of the STA7 locus, which completely lack isoamylase activity, also cause accumulation of phytoglycogen, although sta8 and sta7 mutants differ in that there is a complete loss of granular starch in the latter. This is the first instance in which mutations of two different genetic elements in one plant species have been shown to cause phytoglycogen accumulation. An analytical procedure that allows assay of isoamylase in total extracts was developed and used to show that sta8 mutations cause a 65% reduction in the level of this activity. All other enzymes known to be involved in starch biosynthesis were shown to be unaffected in sta8 mutants. The same amount of total isoamylase activity (approximately) as that present in sta8 mutants was observed in heterozygous triploids containing two sta7 mutant alleles and one wild-type allele. This strain, however, accumulates normal levels of starch granules and lacks phytoglycogen. The total level of isoamylase activity, therefore, is not the major determinant of whether granule production is reduced and phytoglycogen accumulates. Instead, a qualitative property of the isoamylase that is affected by the sta8 mutation is likely to be the critical factor in phytoglycogen production.