title: Effects of granule-bound starch synthase I-defective mutation on the morphology and structure of pyrenoidal starch in Chlamydomonas.
abstract: Lowering of the CO₂ concentration in the environment induces development of a pyrenoidal starch sheath, as well as that of pyrenoid and CO₂-concentrating mechanisms, in many microalgae. In the green algae Chlamydomonas and Chlorella, activity of granule-bound starch synthase (GBSS) concomitantly increases under these conditions. In this study, effects of the GBSS-defective mutation (sta2) on the development of pyrenoidal starch were investigated in Chlamydomonas. Stroma starch- and pyrenoid starch-enriched samples were obtained from log-phase cells grown with air containing 5% CO₂ (high-CO₂ conditions favouring stromal starch synthesis) and from those transferred to low-CO₂ conditions (air level, 0.04% CO₂, favouring pyrenoidal starch synthesis) for 6h, respectively. In the wild type, total starch content per culture volume did not increase during the low-CO₂ conditions, in spite of the development of pyrenoidal starch, suggesting that degradation of some part of stroma starch and synthesis of pyrenoid starch simultaneously occur under these conditions. Even in the GBSS-deficient mutants, pyrenoid and pyrenoid starch enlarged after lowering of the CO₂ concentration. However, the morphology of the pyrenoid starch was thinner and more fragile than the wild type, suggesting that GBSS does affect the morphology of pyrenoidal starch. Surprisingly normal GBSS activity is shown to be required to obtain the high A-type crystallinity levels that we now report for pyrenoidal starch. A model is presented explaining how GBSS-induced starch granule fusion may facilitate the formation of the pyrenoidal starch sheath.
authors.affiliation: School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Horinouchi, Hachioji, Tokyo 192-0392, Japan.
authors.forename: Asako
authors.initials: A
authors.lastname: Izumo
authors.forename: Shoko
authors.initials: S
authors.lastname: Fujiwara
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Sakurai
authors.forename: Steven G
authors.initials: SG
authors.lastname: Ball
authors.forename: Yoshimi
authors.initials: Y
authors.lastname: Ishii
authors.forename: Hikaru
authors.initials: H
authors.lastname: Ono
authors.forename: Mayumi
authors.initials: M
authors.lastname: Yoshida
authors.forename: Naoko
authors.initials: N
authors.lastname: Fujita
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Nakamura
authors.forename: Alain
authors.initials: A
authors.lastname: Buléon
authors.forename: Mikio
authors.initials: M
authors.lastname: Tsuzuki
mesh_terms: D000687:Amylopectin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D002236:Carbohydrate Conformation; D002245:Carbon Dioxide / Q000378:metabolism; D002696:Chlamydomonas / Q000201:enzymology* / Q000235:genetics* / Q000648:ultrastructure; D003594:Cytoplasmic Granules / Q000201:enzymology / Q000648:ultrastructure*; D017343:Genes, Plant; D008855:Microscopy, Electron, Scanning; D009154:Mutation; D010788:Photosynthesis; D013213:Starch / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013214:Starch Synthase / Q000235:genetics / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D002245:Carbon Dioxide; D013213:Starch; D000687:Amylopectin; C434607:granule-bound starch synthase I; D013214:Starch Synthase