Membrane lipid remodeling is required for photosystem II function under low CO₂

Area of Science:

• Plant and Microalgae Physiology
• Biochemistry
• Molecular Biology

Background:

• Membrane lipid remodeling is critical for plant and microalgal survival under nutrient stress.
• Aquatic microalgae rely heavily on sugar-derived lipids (glycolipids) like monogalactosyldiacylglycerol (MGDG) due to limited CO2 availability.
• Cyanobacteria, like Synechocystis sp. PCC 6803, face similar CO2 limitations impacting photosynthesis.

Purpose of the Study:

• To investigate a novel membrane lipid remodeling system in response to CO2 levels in Synechocystis sp. PCC 6803.
• To explore the biological significance of altered lipid content, specifically phosphatidylglycerol (PG) and MGDG, under low CO2 conditions.
• To determine the role of the sll0545 gene, encoding a putative phosphatidic acid phosphate (oxPAP), in membrane lipid adaptation and phototrophic growth.

Main Methods:

• Comparative analysis of membrane lipid composition between high CO2 (HC) and low CO2 (LC) conditions in Synechocystis sp. PCC 6803.
• Generation and characterization of a Synechocystis sp. PCC 6803 transformant overexpressing the sll0545 gene (oxPAP).
• Assessment of Photosystem II (PSII) activity, growth rates, and lipid profiles (PG, MGDG, sulfoquinovosyldiacylglycerol) in wild-type and oxPAP strains under varying CO2 conditions.

Main Results:

• Under LC conditions, wild-type Synechocystis increased PG at the expense of MGDG.
• Overexpression of sll0545 (oxPAP) in Synechocystis led to reduced PG and increased MGDG and sulfoquinovosyldiacylglycerol under LC.
• The oxPAP strain exhibited inhibited PSII activity and growth rates under LC, which were restored by HC conditions or PG supplementation.

Conclusions:

• A novel CO2-responsive membrane lipid remodeling system involving PG and MGDG was identified in Synechocystis.
• Increased PG content appears essential for optimal Photosystem II (PSII) function and phototrophic growth under low CO2 conditions.
• The sll0545 gene plays a role in regulating membrane lipid composition and adaptation to CO2 availability.