Pyrenoid Starch Sheath Is Required for LCIB Localization and the CO-Concentrating Mechanism in Green Algae.

Aquatic photosynthetic organisms induce a CO-concentrating mechanism (CCM) to overcome the difficulty of acquiring inorganic carbon under CO-limiting conditions. As part of the CCM, the CO-fixing enzyme Rubisco is enriched in the pyrenoid located in the chloroplast, and, in many green algae, several thick starch plates surround the pyrenoid to form a starch sheath. In , low-CO-inducible protein B (LCIB), which is an essential factor for the CCM, displays altered cellular localization in response to a decrease in environmental CO concentration, moving from dispersed throughout the chloroplast stroma to around the pyrenoid. However, the mechanism behind LCIB migration remains poorly understood. Here, we report the characteristics of an -less mutant (4-D1), which shows aberrant LCIB localization and starch sheath formation. Under very-low-CO conditions, 4-D1 showed retarded growth, lower photosynthetic affinities against inorganic carbon, and a decreased accumulation level of the HCO transporter HLA3. The aberrant localization of LCIB was also observed in another starch-sheathless mutant -, but not in -, which possesses a thinned starch sheath. These results suggest that the starch sheath around the pyrenoid is required for the correct localization of LCIB and for the operation of CCM.