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 alga Chlamydomonas, 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 pyrenoid starch were researched in Chlamydomonas. Stroma starch- and pyrenoid starch-enriched samples were obtained from log-phase cells grown with air containing 5% CO₂ (high-CO₂ conditions) and from those transferred to low-CO₂ conditions (air level, 0.04% CO₂) for 6 h, respectively. The morphology of the pyrenoid starch was thinner and more fragile than the wild type, suggesting that GBSS does affect the morphology of pyrenoid starch. Surprisingly normal GBSS activity is shown to be required to obtain the high A-type crystallinity levels that we now report for pyrenoid starch. A model is presented explaining how GBSS-induced starch granule fusion may facilitate the formation of the pyrenoidal starch sheath. (Izumo et al., Plant Science, in press)