Effects of granule-bound starch synthase I-defective mutation on the morphology and structure of pyrenoidal starch in Chlamydomonas

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 6 h, 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.

Research highlights▶ GBSS affects morphology of pyrenoidal starch. ▶ Normal GBSS activity is required to obtain the high A-type crystallinity levels. ▶ GBSS-induced starch granule fusion may facilitate pyrenoidal starch sheath formation.