Abstract
Phosphatidylglycerol (PG) is the only major phospholipid in the thylakoid membrane in cyanobacteria and plant chloroplasts. Although PG accounts only for ~10% of total thylakoid lipids, it plays indispensable roles in oxygenic photosynthesis. In contrast to the comprehensive analyses of PG-deprived mutants in cyanobacteria, in vivo roles of PG in photosynthesis during plant growth remain elusive. In this study, we characterized the photosynthesis of an Arabidopsis thaliana T-DNA insertional mutant (pgp1-2), which lacks plastidic PG biosynthesis. In the pgp1-2 mutant, energy transfer from antenna pigments to the photosystem II (PSII) reaction center was severely impaired, which resulted in low photochemical efficiency of PSII. Unlike in the wild type, in pgp1-2, the PSII complexes were susceptible to photodamage by red light irradiation. Manganese ions were mostly dissociated from protein systems in pgp1-2, with oxygen-evolving activity of PSII absent in the mutant thylakoids. The oxygen-evolving complex may be disrupted in pgp1-2, which may accelerate the photodamage to PSII by red light. On the acceptor side of the mutant PSII, decreased electron-accepting capacity was observed along with impaired electron transfer. Although the reaction center of PSI was relatively active in pgp1-2 compared to the severe impairment in PSII, the cyclic electron transport was dysfunctional. Chlorophyll fluorescence analysis at 77K revealed that PG may not be needed for the self-organization of the macromolecular protein network in grana thylakoids but is essential for the assembly of antenna-reaction center complexes. Our data clearly show that thylakoid glycolipids cannot substitute for the role of PG in photosynthesis during plant growth.
Highlights
The thylakoid lipid bilayer formed by amphipathic glycerolipids serves as a matrix embedded with photosynthesis protein–cofactor complexes forming the electron transport chain
In the photosystem II (PSII) complex, 3 PGs were located around the primary electron acceptor (QA) binding site, one was present at the interface between D1 and CP43 and the other was located near the secondary electron acceptor (QB) binding site
To assess whether the photosystem I (PSI) activity is affected by the pgp1-2 mutation, we examined the redox state of the primary electron donor chlorophyll (P700) of the PSI reaction center under increased actinic light intensity (Figure 7A)
Summary
The thylakoid lipid bilayer formed by amphipathic glycerolipids serves as a matrix embedded with photosynthesis protein–cofactor complexes forming the electron transport chain. Many lipid molecules are present in photosystem I (PSI) and PSII complexes. Crystallography analysis of the PSII dimer complex from Thermosynechococcus vulcanus at 1.9-Å resolution identified 20 lipid molecules per monomer in the structure; 5 molecules were PG, which were buried near the reaction center with their head groups facing the cytoplasmic side (Umena et al, 2011). In the crystal structure of the PSI complex from Thermosynechococcus elongatus at 2.5-Å resolution, 4 lipid molecules were assigned per monomer, and 3 of the 4 molecules were PG (Jordan et al, 2001). One of the 3 PG molecules was located near the reaction center, and the other 2 located between the PsaB and PsaX subunits and near the monomer–monomer interface of the trimeric PSI complex, respectively. PG is structurally involved in LHCII; in LHCII crystal structures from spinach at 2.7 Å (Liu et al, 2004) and pea at 2.5 Å (Standfuss et al, 2005), one PG molecule was buried at the monomermonomer interface with the trans- 3-hexadecenoic acid at the sn-2 position penetrating the deep binding pocket of the trimer
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