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Biochemical and structural analyses of a higher plant photosystem II supercomplex of a photosystem I-less mutant of barley: Consequences of a chronic over-reduction of the plastoquinone pool

Université d'Aix-Marseille II, Faculté des Sciences de Luminy, Laboratoire de Génétique et de Biophysique des Plantes, LGBP, CNRS-CEA-Université de la Méditerranée, Marseille, France.
FEBS Journal (Impact Factor: 3.99). 11/2006; 273(20):4616-30. DOI: 10.1111/j.1742-4658.2006.05465.x
Source: PubMed

ABSTRACT Photosystem II of higher plants is a multisubunit transmembrane complex composed of a core moiety and an extensive peripheral antenna system. The number of antenna polypeptides per core complex is modulated following environmental conditions in order to optimize photosynthetic performance. In this study, we used a barley (Hordeum vulgare) mutant, viridis zb63, which lacks photosystem I, to mimic extreme and chronic overexcitation of photosystem II. The mutation was shown to reduce the photosystem II antenna to a minimal size of about 100 chlorophylls per photosystem II reaction centre, which was not further reducible. The minimal photosystem II unit was analysed by biochemical methods and by electron microscopy, and found to consist of a dimeric photosystem II reaction centre core surrounded by monomeric Lhcb4 (chlorophyll protein 29), Lhcb5 (chlorophyll protein 26) and trimeric light-harvesting complex II antenna proteins. This minimal photosystem II unit forms arrays in vivo, possibly to increase the efficiency of energy distribution and provide photoprotection. In wild-type plants, an additional antenna protein, chlorophyll protein 24 (Lhcb6), which is not expressed in viridis zb63, is proposed to associate to this minimal unit and stabilize larger antenna systems when needed. The analysis of the mutant also revealed the presence of two distinct signalling pathways activated by excess light absorbed by photosystem II: one, dependent on the redox state of the electron transport chain, is involved in the regulation of antenna size, and the second, more directly linked to the level of photoinhibitory stress perceived by the cell, participates in regulating carotenoid biosynthesis.

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    • "In our native gels using a much softer and partial solubilization of thylakoid with digitonin (see Supplemental Figure 1A online), we did not detect a difference in PSII supercomplex distribution between State I and II membranes. Thus, by considering our results (Figures 4A and 5), the possibility of differently interpreting previous reports (Dietzel et al., 2011; García-Cerdán et al., 2011), the fact that trimer S has a fundamental role in PSII structure (Dekker and Boekema, 2005; Caffarri et al., 2009), and that the minimum PSII antenna size includes S-LHCII (Morosinotto et al., 2006), we consider that S trimer detachment from PSII during state transition is rather unlikely and could have, at best, a limited impact on the whole process under physiological conditions. Even if we cannot exclude that a functional PSI-LHCII-PSII interaction exists in vivo, from our results, such a megacomplex would be maintained by very weak interactions (those between PSII and L trimers) that do not resist our very mild solubilization (see Supplemental Figure 1A online). "
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    • "n a 1 : 1 ratio ( B4 ; Caffarri et al . , 2009 ) was analyzed in the same gel system . A 1 . 1 times stronger binding to CP29 than to LHCII was found . CP29 is assumed to be present at stoichiometric amounts with the PSII core , based on its presence in the smallest C 2 S 2 ( dimeric core with two strongly bound LHCII trimers ) PSII supercomplex ( Morosinotto et al . , 2006 ; Caffarri et al . , 2009 ) . To our knowledge , no substantial amount of PSII core without associated Lhcb antenna can be present in wild - type plants . Eight repetitions from three different leaves per growth light treatment were used . The PSI : PSII ratio was calculated from the measured chlorophyll a : b ratio as described by Croc"
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    • "It was revealed that the redox state of the PQ-pool regulates the antenna size of PSII under different light conditions (Escoubas et al., 1995; Fey et al., 2005; Lindahl et al., 1995; Pfannschmidt et al., 1999; Yang et al., 2001). In the works (Frigerio et al., 2007; Morosinotto et al., 2006) it was found that the regulation of the antenna size is carried out by the changing of the antenna proteins quantity at the post-transcriptional level. However the signal from the PQ-pool for the light acclimation has still remained largely unsolved. "
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