Abstract
The lipid bilayer matrix of the thylakoid membrane of cyanobacteria and chloroplasts of plants and algae is mainly composed of uncharged galactolipids, but also contains anionic lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) as major constituents. The necessity of PG for photosynthesis is evident in all photosynthetic organisms examined to date, whereas the requirement of SQDG varies with species. In plants, although PG and SQDG are also found in non-photosynthetic plastids, their importance for the growth and functions of non-photosynthetic organs remains unclear. In addition, plants synthesize another anionic lipid glucuronosyldiacylglycerol (GlcADG) during phosphorus starvation, but its role in plant cells is not elucidated yet. To understand the functional relationships among PG, SQDG, and GlcADG, we characterized several Arabidopsis thaliana mutants defective in biosynthesis of these lipids. The mutants completely lacking both PG and SQDG biosynthesis in plastids showed developmental defects of roots, hypocotyls, and embryos in addition to leaves, which suggests that these lipids are pleiotropically required for the development of both photosynthetic and non-photosynthetic organs. Furthermore, our analysis revealed that SQDG, but not GlcADG, is essential for complementing the role of PG, particularly in photosynthesis under PG-deficient conditions such as phosphorus starvation.
Highlights
Thylakoid membranes of both cyanobacteria and plant chloroplasts contain four major lipids, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG)
Because mitochondria of the PGP1 knockout mutant accumulated cardiolipin, the mitochondrion-specific lipid synthesized from PG, to the wild-type level [11], ER-localized PGP2 would function in PG biosynthesis to support the activity of mitochondria
Complete lack of SQDG and loss of PG biosynthesis in plastids
Summary
Thylakoid membranes of both cyanobacteria and plant chloroplasts contain four major lipids, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG). Knockout mutations of the PGP synthase gene (pgsA) in cyanobacteria severely impaired formation and functions of photosystems, photosystem II (PSII) and cell growth [7,8,9]. A knockout mutation of the PGP1 gene by a T-DNA insertion (pgp1-2) decreased PG content in leaves by 88% of the wild-type level, which resulted in severe impairments of thylakoid membrane development, chlorophyll accumulation, photochemical reactions of PSII, and the photosynthetic electron transport [14,15,16]. The single pgp knockout mutant showed no obvious defective phenotype, the pgp pgp double mutant could not produce PG and cardiolipin and caused embryonic lethality [12] These data suggest a subsidiary role of the ER pathway by PGP2 in A. thaliana
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