Unique organization of photosystem II supercomplexes and megacomplexes in Norway spruce

Photosystem II (PSII) complexes are organized into large supercomplexes with variable amounts of light‐harvesting proteins (Lhcb). A typical PSII supercomplex in plants is formed by four trimers of Lhcb proteins (LHCII trimers), which are bound to the PSII core dimer via monomeric antenna proteins. However, the architecture of PSII supercomplexes in Norway spruce[Picea abies (L.) Karst.] is different, most likely due to a lack of two Lhcb proteins, Lhcb6 and Lhcb3. Interestingly, the spruce PSII supercomplex shares similar structural features with its counterpart in the green alga Chlamydomonas reinhardtii [Kou?il et al. (2016) New Phytol. 210 , 808–814]. Here we present a single‐particle electron microscopy study of isolated PSII supercomplexes from Norway spruce that revealed binding of a variable amount of LHCII trimers to the PSII core dimer at positions that have never been observed in any other plant species so far. The largest spruce PSII supercomplex, which was found to bind eight LHCII trimers, is even larger than the current largest known PSII supercomplex from C. reinhardtii. We have also shown that the spruce PSII supercomplexes can form various types of PSII megacomplexes, which were also identified in intact grana membranes. Some of these large PSII supercomplexes and megacomplexes were identified also in Pinus sylvestris, another representative of the Pinaceae family. The structural variability and complexity of LHCII organization in Pinaceae seems to be related to the absence of Lhcb6 and Lhcb3 in this family, and may be beneficial for the optimization of light‐harvesting under varying environmental conditions.     

Proteomic characterization of hierarchical megacomplex formation in Arabidopsis thylakoid membrane

Conversion of solar energy into chemical energy in plant chloroplasts concomitantly modifies the thylakoid architecture and hierarchical interactions between pigment–protein complexes. Here, the thylakoids were isolated from light‐acclimated Arabidopsis leaves and investigated with respect to the composition of the thylakoid protein complexes and their association into higher molecular mass complexes, the largest one comprising both photosystems (PSII and PSI) and light‐harvesting chlorophyll a/b‐binding complexes (LHCII). Because the majority of plant light‐harvesting capacity is accommodated in LHCII complexes, their structural interaction with photosystem core complexes is extremely important for efficient light harvesting. Specific differences in the strength of LHCII binding to PSII core complexes and the formation of PSII supercomplexes are well characterized. Yet, the role of loosely bound L‐LHCII that disconnects to a large extent during the isolation of thylakoid protein complexes remains elusive. Because L‐LHCII apparently has a flexible role in light harvesting and energy dissipation, depending on environmental conditions, its close interaction with photosystems is a prerequisite for successful light harvesting in vivo. Here, to reveal the labile and fragile light‐dependent protein interactions in the thylakoid network, isolated membranes were subjected to sequential solubilization using detergents with differential solubilization capacity and applying strict quality control. Optimized 3D‐lpBN‐lpBN‐sodium dodecyl sulfate–polyacrylamide gel electrophoresis system demonstrated that PSII–LHCII supercomplexes, together with PSI complexes, hierarchically form larger megacomplexes via interactions with L‐LHCII trimers. The polypeptide composition of LHCII trimers and the phosphorylation of Lhcb1 and Lhcb2 were examined to determine the light‐dependent supramolecular organization of the photosystems into megacomplexes.     

Different response of photosystem II to short and long‐term drought stress in Arabidopsis thaliana

Short‐ and long‐term drought stress on photosystem II (PSII) and oxidative stress were studied in Arabidopsis thaliana. Under drought stress, chlorophyll (Chl) content, Chl fluorescence, relative water content and oxygen evolution capacity gradually decreased, and the thylakoid structure was gradually damaged. Short‐term drought stress caused a rapid disassembly of the light‐harvesting complex II (LHCII). However, PSII dimers kept stable under the short‐term drought stress and significantly decreased only after 15 days of drought stress. Immunoblotting analysis of the thylakoid membrane proteins showed that most of the photosystem proteins decreased after the stress, especially for Lhcb5, Lhcb6 and PsbQ proteins. However, surprisingly, PsbS significantly increased after the long‐term drought stress, which is consistent with the substantially increased non‐photochemical quenching (NPQ) after the stress. Our results suggest that the PSII–LHCII supercomplexes and LHCII assemblies play an important role in preventing photo‐damages to PSII under drought stress.     

Supramolecular organization of thylakoid membrane proteins in green plants

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.     

Consequences of state transitions on the structural and functional organization of Photosystem I in the green alga Chlamydomonas reinhardtii

State transitions represent a photoacclimation process that regulates the light‐driven photosynthetic reactions in response to changes in light quality/quantity. It balances the excitation between photosystem I (PSI) and II (PSII) by shuttling LHCII, the main light‐harvesting complex of green algae and plants, between them. This process is particularly important in Chlamydomonas reinhardtii in which it is suggested to induce a large reorganization in the thylakoid membrane. Phosphorylation has been shown to be necessary for state transitions and the LHCII kinase has been identified. However, the consequences of state transitions on the structural organization and the functionality of the photosystems have not yet been elucidated. This situation is mainly because the purification of the supercomplexes has proved to be particularly difficult, thus preventing structural and functional studies. Here, we have purified and analysed PSI and PSII supercomplexes of C. reinhardtii in states 1 and 2, and have studied them using biochemical, spectroscopic and structural methods. It is shown that PSI in state 2 is able to bind two LHCII trimers that contain all four LHCII types, and one monomer, most likely CP29, in addition to its nine Lhcas. This structure is the largest PSI complex ever observed, having an antenna size of 340 Chls/P700. Moreover, all PSI‐bound Lhcs are efficient in transferring energy to PSI. A projection map at 20 Å resolution reveals the structural organization of the complex. Surprisingly, only LHCII type I, II and IV are phosphorylated when associated with PSI, while LHCII type III and CP29 are not, but CP29 is phosphorylated when associated with PSII in state2.     

Supramolecular organization of photosystem II in green plants

Green plant photosystem II (PSII) is involved in the light reactions of photosynthesis, which take place in the thylakoid membrane of the chloroplast. PSII is organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. These supercomplexes are dimeric and contain usually 2-4 copies of trimeric LHCII complexes and have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. This review focuses on the overall composition and structure of the PSII supercomplex of green plants and its organization and interactions within the photosynthetic membrane. Further, we present the current knowledge how the thylakoid membrane is three-dimensionally organized within the chloroplast. We also discuss how the supramolecular organization in the thylakoid membrane and the PSII flexibility may play roles in various short-term regulatory mechanisms of green plant photosynthesis. This article is part of a Special Issue entitled: Photosystem II.     

Light acclimation involves dynamic re‐organization of the pigment–protein megacomplexes in non‐appressed thylakoid domains

Thylakoid energy metabolism is crucial for plant growth, development and acclimation. Non‐appressed thylakoids harbor several high molecular mass pigment–protein megacomplexes that have flexible compositions depending upon the environmental cues. This composition is important for dynamic energy balancing in photosystems (PS) I and II. We analysed the megacomplexes of Arabidopsis wild type (WT) plants and of several thylakoid regulatory mutants. The stn7 mutant, which is defective in phosphorylation of the light‐harvesting complex (LHC) II, possessed a megacomplex composition that was strikingly different from that of the WT. Of the nine megacomplexes in total for the non‐appressed thylakoids, the largest megacomplex in particular was less abundant in the stn7 mutant under standard growth conditions. This megacomplex contains both PSI and PSII and was recently shown to allow energy spillover between PSII and PSI (Nat. Commun., 6, 2015, 6675). The dynamics of the megacomplex composition was addressed by exposing plants to different light conditions prior to thylakoid isolation. The megacomplex pattern in the WT was highly dynamic. Under darkness or far red light it showed low levels of LHCII phosphorylation and resembled the stn7 pattern; under low light, which triggers LHCII phosphorylation, it resembled that of the tap38/pph1 phosphatase mutant. In contrast, solubilization of the entire thylakoid network with dodecyl maltoside, which efficiently solubilizes pigment–protein complexes from all thylakoid compartments, revealed that the pigment–protein composition remained stable despite the changing light conditions or mutations that affected LHCII (de)phosphorylation. We conclude that the composition of pigment–protein megacomplexes specifically in non‐appressed thylakoids undergoes redox‐dependent changes, thus facilitating maintenance of the excitation balance between the two photosystems upon changes in light conditions.     

Monomeric light harvesting complexes enhance excitation energy transfer from LHCII to PSII and control their lateral spacing in thylakoids

Proper assembly of plant photosystem II, in the appressed region of thylakoids, allows for both efficient light harvesting and the dissipation of excitation energy absorbed in excess. The core moiety of wild type supercomplex is associated with monomeric antennae that, in turn, bind peripheral trimeric LHCII complexes. Acclimation to light environment dynamics involves structural plasticity within PSII-LHCs supercomplexes, including depletion in LHCII and CP24. Here, we report on the acclimation of NoM, an Arabidopsis mutant lacking monomeric LHCs but retaining LHCII trimer. Lack of monomeric LHCs impaired the operation of both photosynthetic electron transport and state transitions, despite the fact that NoM underwent a compensatory over-accumulation of the LHCII complement compared to the wild type. Mutant plants displayed stunted growth compared to the wild type when probed over a range of light conditions. When exposed to short-term excess light, NoM showed higher photosensitivity and enhanced singlet oxygen release than the wild type, whereas long-term acclimation under stress conditions was unaffected. Analysis of pigment-binding supercomplexes showed that the absence of monomeric LHCs did affect the macro-organisation of photosystems: large PSI-LHCII megacomplexes were more abundant in NoM, whereas the assembly of PSII-LHCs supercomplexes was impaired. Observation by electron microscopy (EM) and image analysis of thylakoids highlighted impaired granal stacking and membrane organisation, with a heterogeneous distribution of PSII and LHCII compared to the wild type. It is concluded that monomeric LHCs are critical for the structural and functional optimisation of the photosynthetic apparatus.     

Proteomic approach to characterize the supramolecular organization of photosystems in higher plants

A project to investigate the supramolecular structure of photosystems was initiated, which is based on protein solubilizations by digitonin, protein separations by Blue native (BN)-polyacrylamide gel electrophoresis (PAGE) and protein identifications by mass spectrometry (MS). Under the conditions applied, nine photosystem supercomplexes could be described for chloroplasts of Arabidopsis, which have apparent molecular masses between 600 and 3200 kDa on BN gels. Identities of the supercomplexes were determined on the basis of their subunit compositions as documented by 2D BN/SDS-PAGE and BN/BN-PAGE. Two supercomplexes of 1060 and approximately 1600 kDa represent dimeric and trimeric forms of photosystem I (PSI), which include tightly bound LHCI proteins. Compared to monomeric PSI, these protein complexes are of low abundance. In contrast, photosystem II mainly forms part of dominant supercomplexes of 850, 1000, 1050 and 1300 kDa. According to our interpretation, these supercomplexes contain dimeric PSII, 1-4 LHCII trimers and additionally monomeric LHCII proteins. The 1300-kDa PSII supercomplex (containing four LHCII trimers) is partially converted into the 1000-kDa PSII supercomplex (containing two LHCII trimers) in the presence of dodecylmaltoside on 2D BN/BN gels. Analyses of peptides of the trypsinated 1300-kDa PSII supercomplex by mass spectrometry allowed to identify known subunits of the PSII core complex and additionally LHCII proteins encoded by eight different genes in Arabidopsis. Further application of this experimental approach will allow new insights into the supermolecular organization of photosystems in plants.     

Light‐dependent reversible phosphorylation of the minor photosystem II antenna Lhcb6 (CP24) occurs in lycophytes

Evolution of vascular plants required compromise between photosynthesis and photodamage. We analyzed representative species from two divergent lineages of vascular plants, lycophytes and euphyllophytes, with respect to the response of their photosynthesis and light‐harvesting properties to increasing light intensity. In the two analyzed lycophytes, Selaginella martensii and Lycopodium squarrosum, the medium phase of non‐photochemical quenching relaxation increased under high light compared to euphyllophytes. This was thought to be associated with the occurrence of a further thylakoid phosphoprotein in both lycophytes, in addition to D2, CP43 and Lhcb1‐2. This protein, which showed light intensity‐dependent reversible phosphorylation, was identified in S. martensii as Lhcb6, a minor LHCII antenna subunit of PSII. Lhcb6 is known to have evolved in the context of land colonization. In S. martensii, Lhcb6 was detected as a component of the free LHCII assemblies, but also associated with PSI. Most of the light‐induced changes affected the amount and phosphorylation of the LHCII assemblies, which possibly mediate PSI–PSII connectivity. We propose that Lhcb6 is involved in light energy management in lycophytes, participating in energy balance between PSI and PSII through a unique reversible phosphorylation, not yet observed in other land plants.     

Supermolecular organization of photosystem II and its associated light-harvesting antenna in Arabidopsis thaliana.

The organization of Arabidopsis thaliana photosystem II (PSII) and its associated light-harvesting antenna (LHCII) was studied in isolated PSII-LHCII supercomplexes and native membrane-bound crystals by transmission electron microscopy and image analysis. Over 4000 single-particle projections of PSII-LHCII supercomplexes were analyzed. In comparison to spinach supercomplexes [Boekema, E.J., van Roon, H., van Breemen, J.F.L. & Dekker, J.P. (1999) Eur. J. Biochem. 266, 444-452] some striking differences were revealed: a much larger number of supercomplexes from Arabidopsis contain copies of M-type LHCII trimers. M-type trimers can also bind in the absence of the more common S-type trimers. No binding of l-type trimers could be detected. Analysis of native membrane-bound PSII crystals revealed a novel type of crystal with a unit cell of 25.6 x 21.4 nm (angle 77 degrees ), which is larger than any of the PSII lattices observed before. The data show that the unit cell is built up from C2S2M2 supercomplexes, rather than from C2S2M supercomplexes observed in native membrane crystals from spinach [Boekema, E.J., Van Breemen, J.F.L., Van Roon, H. & Dekker, J.P. (2000) J. Mol. Biol. 301, 1123-1133]. It is concluded from both the single particle analysis and the crystal analysis that the M-type trimers bind more strongly to PSII core complexes in Arabidopsis than in spinach.     

Biogenesis of water splitting by photosystem II during de‐etiolation of barley (Hordeum vulgare L.)

Etioplasts lack thylakoid membranes and photosystem complexes. Light triggers differentiation of etioplasts into mature chloroplasts, and photosystem complexes assemble in parallel with thylakoid membrane development. Plastids isolated at various time points of de‐etiolation are ideal to study the kinetic biogenesis of photosystem complexes during chloroplast development. Here, we investigated the chronology of photosystem II (PSII) biogenesis by monitoring assembly status of chlorophyll‐binding protein complexes and development of water splitting via O₂ production in plastids (etiochloroplasts) isolated during de‐etiolation of barley (Hordeum vulgare L.). Assembly of PSII monomers, dimers and complexes binding outer light‐harvesting antenna [PSII‐light‐harvesting complex II (LHCII) supercomplexes] was identified after 1, 2 and 4 h of de‐etiolation, respectively. Water splitting was detected in parallel with assembly of PSII monomers, and its development correlated with an increase of bound Mn in the samples. After 4 h of de‐etiolation, etiochloroplasts revealed the same water‐splitting efficiency as mature chloroplasts. We conclude that the capability of PSII to split water during de‐etiolation precedes assembly of the PSII‐LHCII supercomplexes. Taken together, data show a rapid establishment of water‐splitting activity during etioplast‐to‐chloroplast transition and emphasize that assembly of the functional water‐splitting site of PSII is not the rate‐limiting step in the formation of photoactive thylakoid membranes.     

In pea stipules a functional photosynthetic electron flow occurs despite a reduced dynamicity of LHCII association with photosystems

The flexible association of the light harvesting complex II (LHCII) to photosystem (PS) I and PSII to balance their excitation is a major short-term acclimation process of the thylakoid membrane, together with the thermal dissipation of excess absorbed energy, reflected in non-photochemical quenching of chlorophyll fluorescence (NPQ). In Pisum sativum, the leaf includes two main photosynthetic parts, the basal stipules and the leaflets. Since the stipules are less efficient in carbon fixation than leaflets, the adjustments of the thylakoid system, which safeguard the photosynthetic membrane against photodamage, were analysed. As compared to leaflets, the stipules experienced a decay in PSII photochemical activity. The supramolecular organization of photosystems in stipules showed a more conspicuous accumulation of large PSII-LHCII supercomplexes in the grana, but also a tendency to retain the PSI-LHCI-LHCII state transition complex and the PSI-LHCI-PSII-LHCII megacomplexes probably located at the interface between appressed and stroma-exposed membranes. As a consequence, stipules had a lower capacity to perform state transitions and the overall thylakoid architecture was less structurally flexible and ordered than in leaflets. Yet, stipules proved to be quite efficient in regulating the redox state of the electron transport chain and more capable of inducing NPQ than leaflets. It is proposed that, in spite of a relatively static thylakoid arrangement, LHCII interaction with both photosystems in megacomplexes can contribute to a regulated electron flow.     

Structural organization of photosynthetic apparatus in agranal chloroplasts of maize

We investigated the organization of photosystem II (PSII) in agranal bundle sheath thylakoids from a C(4) plant maize. Using blue native/SDS-PAGE and single particle analysis, we show for the first time that PSII in the bundle sheath (BS) chloroplasts exists in a dimeric form and forms light-harvesting complex II (LHCII).PSII supercomplexes. We also demonstrate that a similar set of photosynthetic membrane complexes exists in mesophyll and agranal BS chloroplasts, including intact LHCI.PSI supercomplexes, PSI monomers, PSII core dimers, PSII monomers devoid of CP43, LHCII trimers, LHCII monomers, ATP synthase, and cytochrome b(6)f complex. Fluorescence functional measurements clearly indicate that BS chloroplasts contain PSII complexes that are capable of performing charge separation and are efficiently sensitized by the associated LHCII. We identified a fraction of LHCII present within BS thylakoids that is weakly energetically coupled to the PSII reaction center; however, the majority of BS LHCII is shown to be tightly connected to PSII. Overall, we demonstrate that organization of the photosynthetic apparatus in BS agranal chloroplasts of a model C(4) plant is clearly distinct from that of the stroma lamellae of the C(3) plants. In particular, supramolecular organization of the dimeric LHCII.PSII in the BS thylakoids strongly suggests that PSII in the BS agranal membranes may donate electrons to PSI. We propose that the residual PSII activity may supply electrons to poise cyclic electron flow around PSI and prevent PSI overoxidation, which is essential for the CO(2) fixation in BS cells, and hence, may optimize ATP production within this compartment.     

Light-harvesting complex II (LHCII) and its supramolecular organization in Chlamydomonas reinhardtii

LHCII is the most abundant membrane protein on earth. It participates in the first steps of photosynthesis by harvesting sunlight and transferring excitation energy to the core complex. Here we have analyzed the LHCII complex of the green alga Chlamydomonas reinhardtii and its association with the core of Photosystem II (PSII) to form multiprotein complexes. Several PSII supercomplexes with different antenna sizes have been purified, the largest of which contains three LHCII trimers (named S, M and N) per monomeric core. A projection map at a 13 Å resolution was obtained allowing the reconstruction of the 3D structure of the supercomplex. The position and orientation of the S trimer are the same as in plants; trimer M is rotated by 45° and the additional trimer (named here as LHCII-N), which is taking the position occupied in plants by CP24, is directly associated with the core. The analysis of supercomplexes with different antenna sizes suggests that LhcbM1, LhcbM2/7 and LhcbM3 are the major components of the trimers in the PSII supercomplex, while LhcbM5 is part of the “extra” LHCII pool not directly associated with the supercomplex. It is also shown that Chlamydomonas LHCII has a slightly lower Chlorophyll a/b ratio than the complex from plants and a blue shifted absorption spectrum. Finally the data indicate that there are at least six LHCII trimers per dimeric core in the thylakoid membranes, meaning that the antenna size of PSII of C. reinhardtii is larger than that of plants.     

Arrangement of Photosystem II and ATP Synthase in Chloroplast Membranes of Spinach and Pea

We used cryoelectron tomography to reveal the arrangements of photosystem II (PSII) and ATP synthase in vitreous sections of intact chloroplasts and plunge-frozen suspensions of isolated thylakoid membranes. We found that stroma and grana thylakoids are connected at the grana margins by staggered lamellar membrane protrusions. The stacking repeat of grana membranes in frozen-hydrated chloroplasts is 15.7 nm, with a 4.5-nm lumenal space and a 3.2-nm distance between the flat stromal surfaces. The chloroplast ATP synthase is confined to minimally curved regions at the grana end membranes and stroma lamellae, where it covers 20% of the surface area. In total, 85% of the ATP synthases are monomers and the remainder form random assemblies of two or more copies. Supercomplexes of PSII and light-harvesting complex II (LHCII) occasionally form ordered arrays in appressed grana thylakoids, whereas this order is lost in destacked membranes. In the ordered arrays, each membrane on either side of the stromal gap contains a two-dimensional crystal of supercomplexes, with the two lattices arranged such that PSII cores, LHCII trimers, and minor LHCs each face a complex of the same kind in the opposite membrane. Grana formation is likely to result from electrostatic interactions between these complexes across the stromal gap.     

Fine structure of granal thylakoid membrane organization using cryo electron tomography

The architecture of grana membranes from spinach chloroplasts was studied by cryo electron tomography. Tomographic reconstructions of ice-embedded isolated grana stacks enabled to resolve features of photosystem II (PSII) in the native membrane and to assign the absolute orientation of individual membranes of granal thylakoid discs. Averaging of 3D sub-volumes containing PSII complexes provided a 3D structure of the PSII complex at 40 ? resolution. Comparison with a recently proposed pseudo-atomic model of the PSII supercomplex revealed the presence of unknown protein densities right on top of 4 light harvesting complex II (LHCII) trimers at the lumenal side of the membrane. The positions of individual dimeric PSII cores within an entire membrane layer indicates that about 23% supercomplexes must be of smaller size than full C(2)S(2)M(2) supercomplexes, to avoid overlap.     

Structural and functional differentiation of the light‐harvesting protein Lhcb4 during land plant diversification

About 475 million years ago, plants originated from an ancestral green alga and evolved first as non‐vascular and later as vascular plants, becoming the primary producers of biomass on lands. During that time, the light‐harvesting complex II (LHCII), responsible for sunlight absorption and excitation energy transfer to the photosystem II (PSII) core, underwent extensive differentiation. Lhcb4 is an ancestral LHCII that, in flowering plants, differentiated into up to three isoforms, Lhcb4.1, Lhcb4.2 and Lhcb4.3. The pivotal position of Lhcb4 in the PSII‐LHCII supercomplex (PSII‐LHCIIsc) allows functioning as linker for either S‐ or M‐trimers of LHCII to the PSII core. The increased accumulation of Lhcb4.3 observed in PSII‐LHCIIsc of plants acclimated to moderate and high light intensities induced us to investigate, whether this isoform has a preferential localization in a specific PSII‐LHCIIsc conformation that might explain its light‐dependent accumulation. In this work, by combining an improved method for separation of different forms of PSII‐LHCIIsc from thylakoids of Pisum sativum L. grown at increasing irradiances with quantitative proteomics, we assessed that Lhcb4.3 is abundant in PSII‐LHCIIsc of type C₂S₂, and, interestingly, similar results were found for the PsbR subunit. Phylogenetic comparative analysis on different taxa of the Viridiplantae lineage and structural modeling further pointed out to an effect of the evolution of different Lhcb4 isoforms on the light‐dependent modulation of the PSII‐LHCIIsc organization. This information provides new insight on the properties of the Lhcb4 and its isoforms and their role on the structure, function and regulation of PSII.     

A land plant‐specific thylakoid membrane protein contributes to photosystem II maintenance in Arabidopsis thaliana

The structure and function of photosystem II (PSII) are highly susceptible to photo‐oxidative damage induced by high‐fluence or fluctuating light. However, many of the mechanistic details of how PSII homeostasis is maintained under photoinhibitory light remain to be determined. We describe an analysis of the Arabidopsis thaliana gene At5g07020, which encodes an unannotated integral thylakoid membrane protein. Loss of the protein causes altered PSII function under high‐irradiance light, and hence it is named ‘Maintenance of PSII under High light 1’ (MPH1). The MPH1 protein co‐purifies with PSII core complexes and co‐immunoprecipitates core proteins. Consistent with a role in PSII structure, PSII complexes (supercomplexes, dimers and monomers) of the mph1 mutant are less stable in plants subjected to photoinhibitory light. Accumulation of PSII core proteins is compromised under these conditions in the presence of translational inhibitors. This is consistent with the hypothesis that the mutant has enhanced PSII protein damage rather than defective repair. These data are consistent with the distribution of the MPH1 protein in grana and stroma thylakoids, and its interaction with PSII core complexes. Taken together, these results strongly suggest a role for MPH1 in the protection and/or stabilization of PSII under high‐light stress in land plants.     

Photosystem I-LHCII megacomplexes respond to high light and aging in plants

Photosystem II is known to be a highly dynamic multi-protein complex that participates in a variety of regulatory and repair processes. In contrast, photosystem I (PSI) has, until quite recently, been thought of as relatively static. We report the discovery of plant PSI-LHCII megacomplexes containing multiple LHCII trimers per PSI reaction center. These PSI-LHCII megacomplexes respond rapidly to changes in light intensity, as visualized by native gel electrophoresis. PSI-LHCII megacomplex formation was found to require thylakoid stacking, and to depend upon growth light intensity and leaf age. These factors were, in turn, correlated with changes in PSI/PSII ratios and, intriguingly, PSI-LHCII megacomplex dynamics appeared to depend upon PSII core phosphorylation. These findings suggest new functions for PSI and a new level of regulation involving specialized subpopulations of photosystem I which have profound implications for current models of thylakoid dynamics.