Storage, Photosynthesis, and Growth: The Conditional Nature of Mutations Affecting Starch Synthesis and Structure in Chlamydomonas.

Growth-arrested Chlamydomonas cells accumulate a storage polysaccharide that bears strong structural and functional resemblance to higher plant storage starch. It is synthesized by similar enzymes and responds in an identical fashion to the presence of mutations affecting these activities. We found that log-phase photosynthetically active algae accumulate granular [alpha](1->4)-linked, [alpha](1->6)-branched glucans whose shape, cellular location, and structure differ markedly from those of storage starch. That synthesis of these two types of polysaccharides is controlled by both a common and a specific set of genes was evidenced by the identification of a new Chlamydomonas (STA4) locus specifically involved in the biosynthesis of storage starch. Mutants defective in STA4 accumulated a new type of high-amylose storage starch displaying an altered amylopectin chain size distribution. It is expected that the dual nature and functions of starch synthesis in unicellular green algae will yield new insights into the biological reasons for the emergence of starch in the eukaryotic plant cell.