Plant and Algal PSII-LHCII Supercomplexes: Structure, Evolution and Energy Transfer.

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy in chemical forms. The light-harvesting, excitation transfer, charge separation and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledge about the architectures and assemblies of plant and green algal PSII-light harvesting complex II (LHCII) supercomplexes, with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII-LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution and about the energy transfer pathways that define their light-harvesting and photoprotective properties.

Polyclonal antibodies against the TLA1 protein also recognize with high specificity the D2 reaction center protein of PSII in the green alga Chlamydomonas reinhardtii.

The Chlamydomonas reinhardtii DNA-insertional transformant truncated light-harvesting antenna 1 (tla1) mutant, helped identify the novel TLA1 gene (GenBank Accession # AF534570-71) as an important genetic determinant in the chlorophyll antenna size of photosynthesis. Down-regulation in the amount of the TLA1 23 kDa protein in the cell resulted in smaller chlorophyll antenna size for both photosystems (in Tetali et al. Planta 225:813-829, 2007). Specific polyclonal antibodies, raised against the recombinant TLA1 protein, showed a cross-reaction with the predicted 23 kDa TLA1 protein in C. reinhardtii protein extracts, but also showed a strong cross-reaction with a protein band migrating to 28.5 kDa. Questions of polymorphism, or posttranslational modification of the TLA1 protein were raised as a result of the unexpected 28.5 kDa cross-reaction. Work in this paper aimed to elucidate the nature of the unexpected 28.5 kDa cross-reaction, as this was deemed to be important in terms of the functional role of the TLA1 protein in the regulation of the chlorophyll antenna size of photosynthesis. Immuno-precipitation of the 28.5 kDa protein, followed by LC-mass spectrometry, showed amino acid sequences ascribed to the psbD/D2 reaction center protein of PSII. The common antigenic determinant between TLA1 and D2 was shown to be a stretch of nine conserved amino acids V-F-L(V)LP-GNAL in the C-terminus of the two proteins, constituting a high antigenicity "GNAL" domain. Antibodies raised against the TLA1 protein containing this domain recognized both the TLA1 and the D2 protein. Conversely, antibodies raised against the TLA1 protein minus the GNAL domain specifically recognized the 23 kDa TLA1 protein and failed to recognize the 28.5 kDa D2 protein. D2 antibodies raised against an oligopeptide containing this domain also cross-reacted with the TLA1 protein. It is concluded that the 28.5 kDa cross-reaction of C. reinhardtii protein extracts with antiTLA1 antibodies is due to antibody affinity for the GNAL domain of the D2 protein and has no bearing on the identity or function of the TLA1 protein.

Sustained H2 production in a Chlamydomonas reinhardtii D1 protein mutant.

In the present investigation, a detailed biochemical analysis of the high H2 producer D1 protein mutant strain L159I-N230Y of Chlamydomonas reinhardtii, carrying a double amino acid substitution, was made. The leucine residue L159 was replaced by isoleucine, and the N230 asparagine was replaced by tyrosine. The performance of this strain was compared to that of the cc124 strain. The mutant showed a sustained capacity to donate electrons by means of direct biophotolysis for H2 production, as demonstrated by the higher efficiency of utilization of the hydrogenase enzyme when carried out under anaerobic conditions. The latter property was maintained also under sulfur deprivation. Furthermore, when compared to the cc124, the mutant showed a higher amount of D1 protein content, a higher carbohydrate storage capacity and a sustained PSII direct contribution to the H2 production during sulfur deprivation. The addition of DCMU to the cells showed that as much as 7.0 mL H2 liter of culture h⁻¹ were produced by means of direct biophotolysis. The maximum apparent light-to-hydrogen conversion efficiency expressed on PAR (photosynthetically active radiation) reached 3.22%, while PSII efficiency to perform direct biophotolysis was calculated to be 2.03%. These values are significantly higher than what has been reported in the literature.

The occurrence of the psbS gene product in Chlamydomonas reinhardtii and in other photosynthetic organisms and its correlation with energy quenching.

To avoid photodamage, photosynthetic organisms have developed mechanisms to evade or dissipate excess energy. Lumen overacidification caused by light-induced electron transport triggers quenching of excited chlorophylls and dissipation of excess energy into heat. In higher plants participation of the PsbS protein as the sensor of low lumenal pH was clearly demonstrated. Although light-dependent energy quenching is a property of all photosynthetic organisms, large differences in amplitude and kinetics can be observed thus raising the question whether a single common mechanism is in action. We performed a detailed study of PsbS expression/accumulation in Chlamydomonas reinhardtii and investigated its accumulation in other algae and plants. We showed that PsbS cannot be detected in Chlamydomonas under a wide range of growth conditions. Overexpression of the endogenous psbs gene showed that the corresponding protein could not be addressed to the thylakoid membranes. Survey of different unicellular green algae showed no accumulation of anti-PsbS reactive proteins differently from multicellular species. Nevertheless, some unicellular species exhibit high energy quenching activity, suggesting that a PsbS-independent mechanism is activated. By correlating growth habitat and PsbS accumulation in different species, we suggest that during the evolution the light environment has been a determinant factor for the conservation/loss of the PsbS function.

Studies of the Ndh complex and photosystem II from mesophyll and bundle sheath chloroplasts of the C4-type plant Zea mays.

In C(4) plants, granal mesophyll (MS) chloroplasts contain higher photosystem (PS) II and lower PS I activity than agranal bundle sheath (BS) chloroplasts. The maize NAD(P)H dehydrogenase or NAD(P)H-plastoquinone oxidoreductase (also named Ndh complex) from MS and BS chloroplasts, contains at least 11 subunits (NdhA-K) and is homologous to NADH dehydrogenase or Complex I from mitochondria and bacteria. The amount of Ndh complex is higher in BS compared with MS chloroplasts. However, there is little information about the interdependence of the PS II and Ndh complex in chlororespiration and linear and cyclic electron transport in C(4) plants. To characterize the expression of the PS II and Ndh complex in maize plastids, we used cytochrome b559 (cyt b559) antibodies and Ndh immunoglobulins (IgG) to analyze the Ndh complex and PS II in both MS and BS chloroplasts from maize leaves by Western blotting and immunolabeling. In Western blot experiments, it was found that the amount of cyt b559 (a marker for PS II) is 7-8 times higher in MS than BS chloroplasts. Conversely, the NdhH, -J, -K and -E content is 2.5-3 times higher in BS than MS chloroplasts. Similar results were obtained in immunolabeling experiments using Ndh IgGs and cyt b559 antibodies in MS and BS chloroplasts. These data suggest that in BS chloroplasts, ATP could be produced mainly by cyclic electron transport around PS I and Ndh complexes. Conversely, the linear electron transport in BS chloroplasts via PS II could have a lower production of ATP. These results also suggest that the contribution of the Ndh complex in the production of ATP in MS chloroplasts is minimal and that instead, this complex could have a chlororespiratory role.

Brassica rapa plants adapted to microgravity with reduced photosystem I and its photochemical activity.

The photosynthetic apparatus contains several protein complexes, many of which are regulated by environmental conditions. In this study, the influences of microgravity on PSI and PSII in Brassica rapa plants grown aboard the space shuttle were examined. We found that Brassica plants grown in space had a normal level of growth relative to controls under similar conditions on Earth. Upon return to Earth, cotyledons were harvested and thylakoid membranes were isolated. Analysis of chlorophyll contents showed that the Chl a/b ratio (3.5) in flight cotyledons was much higher than a ratio of 2.42 in the ground controls. The flight samples also had a reduction of PSI complexes and a corresponding 30% decrease of PSI photochemical activity. Immunoblotting showed that the reaction centre polypeptides of PSI were more apparently decreased (e.g. by 24-33% for PsaA and PsaB, and 57% for PsaC) than the light-harvesting complexes. In comparison, the accumulation of PSII complex was less affected in microgravity, thus only a slight reduction in D1, D2 and LHCII was observed in protein blots. However, there was a 32% decrease of OEC1 in the flight samples, indicating a defective OEC subcomplex. In addition, an average 54% increase of the 54 kDa CF1-beta isoform was found in the flight samples, suggesting that space-grown plants suffered from certain stresses, consistent with implications of the increased Chl a/b ratio. Taken together, the results demonstrated that Brassica plants can adapt to spaceflight microgravity, but with significant alterations in chloroplast structures and photosynthetic complexes, and especially reduction of PSI and its activity.