Cosuppression of photosystem I subunit PSI-H in Arabidopsis thaliana. Efficient electron transfer and stability of photosystem I is dependent upon the PSI-H subunit

Maize ANT1 modulates vascular development, chloroplast development, photosynthesis, and plant growth

Arabidopsis AINTEGUMENTA (ANT), an AP2 transcription factor, is known to control plant growth and floral organogenesis. In this study, our transcriptome analysis and in situ hybridization assays of maize embryonic leaves suggested that maize ANT1 (ZmANT1) regulates vascular development. To better understand ANT1 functions, we determined the binding motif of ZmANT1 and then showed that ZmANT1 binds the promoters of millet SCR1, GNC, and AN3, which are key regulators of Kranz anatomy, chloroplast development, and plant growth, respectively. We generated a mutant with a single-codon deletion and two frameshift mutants of the ANT1 ortholog in the C4 millet Setaria viridis by the CRISPR/Cas9 technique. The two frameshift mutants displayed reduced photosynthesis efficiency and growth rate, smaller leaves, and lower grain yields than wild-type (WT) plants. Moreover, their leaves sporadically exhibited distorted Kranz anatomy and vein spacing. Conducting transcriptomic analysis of developing leaves in the WT and the three mutants we identified differentially expressed genes (DEGs) in the two frameshift mutant lines and found many down-regulated DEGs enriched in photosynthesis, heme, tetrapyrrole binding, and antioxidant activity. In addition, we predicted many target genes of ZmANT1 and chose 13 of them to confirm binding of ZmANT1 to their promoters. Based on the above observations, we proposed a model for ANT1 regulation of cell proliferation and leaf growth, vascular and vein development, chloroplast development, and photosynthesis through its target genes. Our study revealed biological roles of ANT1 in several developmental processes beyond its known roles in plant growth and floral organogenesis.

In silico structural homology modelling of EST073 motif coding protein of tea Camellia sinensis (L)

Background

Tea (Camellia sinensis (L). O. Kuntze) is known as the oldest, mild stimulating caffeine containing non-alcoholic beverage. One of the major threats in south Asian tea industry is the blister blight leaf disease (BB), caused by the fungus Exobasidium vexans Masse. SSR DNA marker EST SSR 073 is used as a molecular marker to tag blister blight disease resistance trait of tea. The amino acid sequences were derived from cDNA sequences related to EST SSR 073 of BB susceptible (TRI 2023) and BB resistant (TRI 2043) cultivars. An attempt has been made to understand the structural characteristics and variations of EST SSR 073 locus that may reveal the factors influencing the BB resistance of tea with multiple bioinformatics tools such as ORF finder, ExPasy ProtParam tools, modeler V 9.17, Rampage server, UCSF-Chimera, and HADDOCK docking server.

Results

The primary, secondary, and tertiary structures of EST SSR 073 coding protein were analyzed using the amino acid sequences of both BB resistant TRI 2043 and BB susceptible TRI 2023 tea cultivars. The coding amino acid sequences of both the cultivars were homologous to photosystem I subunit protein (PsaD I) of Pisum sativum. The predicted 3D structures of proteins were validated and considered as an acceptable overall stereochemical quality. The BB resistant protein showed CT repeat extension and did not involve in topology of the PsaD I subunit. The C terminal truncation of BB resistance caused the formation of hydrogen bonds interacting with PsaD I and other subunits of photosystem I in the modeled three-dimensional protein structure.

Conclusions

Camellia sinensis EST 073 SSR motif coding protein was identified as the PsaD I subunit of photosystem I. The exact mechanism of PsaD I conferring the resistance for blister blight in tea needs to be further investigated.

Effects of the Herbicide Imazethapyr on Photosynthesis in PGR5- and NDH-Deficient Arabidopsis thaliana at the Biochemical, Transcriptomic, and Proteomic Levels

Photosynthesis is a very important metabolic pathway for plant growth and crop yield. This report investigated the effect of the herbicide imazethapyr on photosynthesis in the Arabidopsis thaliana pnsB3 mutant (a defect in the NDH pathway) and pgr5 mutant (a defect in the PGR5 pathway) to determine which cyclic electron transport chain (CET) of the NDH and PGR5 pathways is more important for protecting the photosynthetic system under herbicide stress. The results showed that 20 μg/L imazethapyr markedly inhibited the growth of the three ecotypes of A. thaliana and produced more anthocyanins and reactive oxygen species (ROS), particularly in the pgr5 mutant. The chlorophyll fluorescence results showed that PSII was severely damaged in the pgr5 mutant. Additionally, the CET was significantly stimulated to protect the photosynthetic system from light damage in Wt and the pnsB3 mutant but not the pgr5 mutant. The real-time PCR analysis indicated that imazethapyr treatment considerably decreased the transcript levels of most photosynthesis-related genes in the three treated groups. Several genes in the PGR5 pathway were significantly induced in the pnsB3 mutant, but no genes in the NDH pathway were induced in the pgr5 mutant. The gene transcription analysis showed that the pgr5 mutant cannot compensate for the deficit in the PGR5 pathway by stimulating the NDH pathway, whereas the pnsB3 mutant can compensate for the deficit in the CET cycle by regulating the PGR5 pathway. The iTRAQ analyses also showed that the photosynthesis system, glycolysis, and TCA cycle suffered the most severe damage in the pgr5 mutant. All of these results showed that the PGR5 pathway is more critical for electron transfer around PSI than the NDH pathway to resist herbicide stress.

Photosynthesis. Structural basis for energy transfer pathways in the plant PSI-LHCI supercomplex

Photosynthesis converts solar energy to chemical energy by means of two large pigment-protein complexes: photosystem I (PSI) and photosystem II (PSII). In higher plants, the PSI core is surrounded by a large light-harvesting complex I (LHCI) that captures sunlight and transfers the excitation energy to the core with extremely high efficiency. We report the structure of PSI-LHCI, a 600-kilodalton membrane protein supercomplex, from Pisum sativum (pea) at a resolution of 2.8 angstroms. The structure reveals the detailed arrangement of pigments and other cofactors—especially within LHCI—as well as numerous specific interactions between the PSI core and LHCI. These results provide a firm structural basis for our understanding on the energy transfer and photoprotection mechanisms within the PSI-LHCI supercomplex.

Quantitative proteomic analysis of the rice (Oryza sativa L.) salt response

Salt stress is one of most serious limiting factors for crop growth and production. An isobaric Tags for Relative and Absolute Quantitation (iTRAQ) approach was used to analyze proteomic changes in rice shoots under salt stress in this study. A total of 56 proteins were significantly altered and 16 of them were enriched in the pathways of photosynthesis, antioxidant and oxidative phosphorylation. Among these 16 proteins, peroxiredoxin Q and photosystem I subunit D were up-regulated, while thioredoxin M-like, thioredoxin x, thioredoxin peroxidase, glutathione S-transferase F3, PSI subunit H, light-harvesting antenna complex I subunits, chloroplast chaperonin, vacuolar ATP synthase subunit H, and ATP synthase delta chain were down-regulated. Moreover, physiological data including total antioxidant capacity, peroxiredoxin activity, chlorophyll a/b content, glutathione S-transferase activity, reduced glutathione content and ATPase activity were consistent with changes in the levels of these proteins. The levels of the mRNAs encoding these proteins were also analyzed by real-time quantitative reverse transcription PCR, and approximately 86% of the results were consistent with the iTRAQ data. Importantly, our data suggest the important role of PSI in balancing energy supply and ROS generation under salt stress. This study provides information for an improved understanding of the function of photosynthesis and PSI in the salt-stress response of rice.

Molecular mechanism of photosystem I assembly in oxygenic organisms

Photosystem I, an integral membrane and multi-subunit complex, catalyzes the oxidation of plastocyanin and the reduction of ferredoxin by absorbed light energy. Photosystem I participates in photosynthetic acclimation processes by being involved in cyclic electron transfer and state transitions for sustaining efficient photosynthesis. The photosystem I complex is highly conserved from cyanobacteria to higher plants and contains the light-harvesting complex and the reaction center complex. The assembly of the photosystem I complex is highly complicated and involves the concerted assembly of multiple subunits and hundreds of cofactors. A suite of regulatory factors for the assembly of photosystem I subunits and cofactors have been identified that constitute an integrative network regulating PSI accumulation. This review aims to discuss recent findings in the field relating to how the photosystem I complex is assembled in oxygenic organisms. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

The effect of Silicon on photosynthesis and expression of its relevant genes in rice (Oryza sativa L.) under high-zinc stress

The main objectives of this study were to elucidate the roles of silicon (Si) in alleviating the effects of 2 mM zinc (high Zn) stress on photosynthesis and its related gene expression levels in leaves of rice (Oryza sativa L.) grown hydroponically with high-Zn stress. The results showed that photosynthetic parameters, including net photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO2 concentration, chlorophyll concentration and the chlorophyll fluorescence, were decreased in rice exposed to high-Zn treatment. The leaf chloroplast structure was disordered under high-Zn stress, including uneven swelling, disintegrated and missing thylakoid membranes, and decreased starch granule size and number, which, however, were all counteracted by the addition of 1.5 mM Si. Furthermore, the expression levels of Os08g02630 (PsbY), Os05g48630 (PsaH), Os07g37030 (PetC), Os03g57120 (PetH), Os09g26810 and Os04g38410 decreased in Si-deprived plants under high-Zn stress. Nevertheless, the addition of 1.5 mM Si increased the expression levels of these genes in plants under high-Zn stress at 72 h, and the expression levels were higher in Si-treated plants than in Si-deprived plants. Therefore, we conclude that Si alleviates the Zn-induced damage to photosynthesis in rice. The decline of photosynthesis in Zn-stressed rice was attributed to stomatal limitation, and Si activated and regulated some photosynthesis-related genes in response to high-Zn stress, consequently increasing photosynthesis.

Integrative regulatory network of plant thylakoid energy transduction

Highly flexible regulation of photosynthetic light reactions in plant chloroplasts is a prerequisite to provide sufficient energy flow to downstream metabolism and plant growth, to protect light reactions against photodamage, and to ensure controlled cellular signaling from the chloroplast to the nucleus. Such comprehensive regulation occurs via the control of excitation energy transfer to and between the two photosystems (PSII and PSI), of the electrochemical gradient across the thylakoid membrane (ΔpH), and of electron transfer from PSII to PSI electron acceptors. In this opinion article, we propose that these regulatory mechanisms, functioning at different levels of photosynthetic energy conversion, might be interconnected and describe how the concomitant and integrated function of these mechanisms might enable plants to acclimate to a full array of environmental changes.

Changes induced by the Pepper mild mottle tobamovirus on the chloroplast proteome of Nicotiana benthamiana

We have analyzed the chloroplast proteome of Nicotiana benthamiana using two-dimensional gel electrophoresis and mass spectrometry followed by a database search. In order to improve the resolution of the two-dimensional electrophoresis gels, we have made separate maps for the low and the high pH range. At least 200 spots were detected. We identified 72 polypeptides, some being isoforms of different multiprotein families. In addition, changes in this chloroplast proteome induced by the infection with the Spanish strain of the Pepper mild mottle virus were investigated. Viral infection induced the down-regulation of several chloroplastidic proteins involved in both the photosynthetic electron-transport chain and the Benson-Calvin cycle.

A cytoplasmically inherited barley mutant is defective in photosystem I assembly due to a temperature-sensitive defect in ycf3 splicing

A cytoplasmically inherited chlorophyll-deficient mutant of barley (Hordeum vulgare) termed cytoplasmic line 3 (CL3), displaying a viridis (homogeneously light-green colored) phenotype, has been previously shown to be affected by elevated temperatures. In this article, biochemical, biophysical, and molecular approaches were used to study the CL3 mutant under different temperature and light conditions. The results lead to the conclusion that an impaired assembly of photosystem I (PSI) under higher temperatures and certain light conditions is the primary cause of the CL3 phenotype. Compromised splicing of ycf3 transcripts, particularly at elevated temperature, resulting from a mutation in a noncoding region (intron 1) in the mutant ycf3 gene results in a defective synthesis of Ycf3, which is a chaperone involved in PSI assembly. The defective PSI assembly causes severe photoinhibition and degradation of PSII.

Arabidopsis STN7 kinase provides a link between short- and long-term photosynthetic acclimation

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

AtCYP38 ensures early biogenesis, correct assembly and sustenance of photosystem II

AtCYP38 is a thylakoid lumen protein comprising the immunophilin domain and the phosphatase inhibitor module. Here we show the association of AtCYP38 with the photosystem II (PSII) monomer complex and address its functional role using AtCYP38-deficient mutants. The dynamic greening process of etiolated leaves failed in the absence of AtCYP38, due to specific problems in the biogenesis of PSII complexes. Also the development of leaves under short-day conditions was severely disturbed. Detailed biophysical and biochemical analysis of mature AtCYP38-deficient plants from favorable growth conditions (long photoperiod) revealed: (i) intrinsic malfunction of PSII, which (ii) occurred on the donor side of PSII and (iii) was dependent on growing light intensity. AtCYP38 mutant plants also showed decreased accumulation of PSII, which was shown not to originate from impaired D1 synthesis or assembly of PSII monomers, dimers and supercomplexes as such but rather from the incorrect fine-tuning of the oxygen-evolving side of PSII. This, in turn, rendered PSII centers extremely susceptible to photoinhibition. AtCYP38 deficiency also drastically decreased the in vivo phosphorylation of PSII core proteins, probably related to the absence of the AtCYP38 phosphatase inhibitor domain. It is proposed that during PSII assembly AtCYP38 protein guides the proper folding of D1 (and CP43) into PSII, thereby enabling the correct assembly of the water-splitting Mn(4)-Ca cluster even with high turnover of PSII.

Proteomic analysis of photosystem I components from different plant species

In this study, the photosystem I (PSI) highly hydrophobic proteins present within stroma lamellae of the thylakoid membrane were separated by RP-HPLC and identified either by in-solution trypsin digestion peptide fragment fingerprinting or by the close correspondence between the intact mass measurements (IMMs) and those expected from the DNA sequence. Protein identification performed by MS/MS was as reliable as IMMs. Thus, IMM is an easy and valid method for identifying proteins that have no PTMs. This paper reports the M(r) for all PSI proteins in ten different species, including those whose genes have not yet been cloned. Lhca5 was revealed unequivocally in four species, corroborating that it is indeed a protein belonging to the light-harvesting antenna of PSI. In all species examined, the product of the Lhca6 gene has never been revealed. Concerning core proteins, Psa-O has been revealed in three species; isoforms of Psa-D and Psa-E have been found in both monocots and dicots. Small proteins like Psa-I and Psa-J are well separated and identified. RP-HPLC produces reliable fingerprints and reveals that the relative amounts of PSI proteins appear to be markedly different.

Structure, function and regulation of plant photosystem I

Photosystem I (PSI) is a multisubunit protein complex located in the thylakoid membranes of green plants and algae, where it initiates one of the first steps of solar energy conversion by light-driven electron transport. In this review, we discuss recent progress on several topics related to the functioning of the PSI complex, like the protein composition of the complex in the plant Arabidopsis thaliana, the function of these subunits and the mechanism by which nuclear-encoded subunits can be inserted into or transported through the thylakoid membrane. Furthermore, the structure of the native PSI complex in several oxygenic photosynthetic organisms and the role of the chlorophylls and carotenoids in the antenna complexes in light harvesting and photoprotection are reviewed. The special role of the 'red' chlorophylls (chlorophyll molecules that absorb at longer wavelength than the primary electron donor P700) is assessed. The physiology and mechanism of the association of the major light-harvesting complex of photosystem II (LHCII) with PSI during short term adaptation to changes in light quality and quantity is discussed in functional and structural terms. The mechanism of excitation energy transfer between the chlorophylls and the mechanism of primary charge separation is outlined and discussed. Finally, a number of regulatory processes like acclimatory responses and retrograde signalling is reviewed with respect to function of the thylakoid membrane. We finish this review by shortly discussing the perspectives for future research on PSI.

Knock-out of the chloroplast-encoded PSI-J subunit of photosystem I in Nicotiana tabacum

The plastid-encoded psaJ gene encodes a hydrophobic low-molecular-mass subunit of photosystem I (PSI) containing one transmembrane helix. Homoplastomic transformants with an inactivated psaJ gene were devoid of PSI-J protein. The mutant plants were slightly smaller and paler than wild-type because of a 13% reduction in chlorophyll content per leaf area caused by an approximately 20% reduction in PSI. The amount of the peripheral antenna proteins, Lhca2 and Lhca3, was decreased to the same level as the core subunits, but Lhca1 and Lhca4 were present in relative excess. The functional size of the PSI antenna was not affected, suggesting that PSI-J is not involved in binding of light-harvesting complex I. The specific PSI activity, measured as NADP(+) photoreduction in vitro, revealed a 55% reduction in electron transport through PSI in the mutant. No significant difference in the second-order rate constant for electron transfer from reduced plastocyanin to oxidized P700 was observed in the absence of PSI-J. Instead, a large fraction of PSI was found to be inactive. Immunoblotting analysis revealed a secondary loss of the luminal PSI-N subunit in PSI particles devoid of PSI-J. Presumably PSI-J affects the conformation of PSI-F, which in turn affects the binding of PSI-N. This together renders a fraction of the PSI particles inactive. Thus, PSI-J is an important subunit that, together with PSI-F and PSI-N, is required for formation of the plastocyanin-binding domain of PSI. PSI-J is furthermore important for stability or assembly of the PSI complex.

The PSI-O subunit of plant photosystem I is involved in balancing the excitation pressure between the two photosystems

PSI-O is a subunit of photosystem I in eukaryotes. The function of PSI-O was characterized in Arabidopsis plants using RNA interference. Several transformants with the psaO-RNAi construct were generated, and a high proportion of the plants contained only very little or virtually no residual PSI-O. Plants lacking PSI-O have a 50% reduction in state transitions indicating a role for PSI-O in the balancing of excitation energy between the two photosystems. PSI-H and -L have been shown previously to be involved in state transitions, and immunoblot analysis revealed that plants devoid of PSI-L or -H also have 80-90% reduction in the abundance of PSI-O. In contrast, down-regulation of PSI-O has no negative effect on the content of PSI-H and -L. The interaction between PSI-O and the PSI-L was confirmed by chemical cross-linking. A model of PSI is proposed in which PSI-L as the most ancient subunit is closest to the reaction center, and PSI-O is positioned close to PSI-L on the PSI-H/-L/-I side of the PSI complex. PSI-H, -L, -O, and possibly -I are all involved in forming a domain in PSI that is involved in the interaction with light-harvesting complex II.

Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

In Arabidopsis thaliana, the D-subunit of photosystem I (PSI-D) is encoded by two functional genes, PsaD1 and PsaD2, which are highly homologous. Knock-out alleles for each of the loci have been identified by a combination of forward and reverse genetics. The double mutant psad1-1 psad2-1 is seedling-lethal, high-chlorophyll-fluorescent and deficient for all tested PSI subunits, indicating that PSI-D is essential for photosynthesis. In addition, psad1-1 psad2-1 plants show a defect in the accumulation of thylakoid multiprotein complexes other than PSI. Of the single-gene mutations, psad2 plants behave like wild-type (WT) plants, whereas psad1-1 markedly affects the accumulation of PsaD mRNA and protein, and photosynthetic electron flow. Additional effects of the psad1-1 mutation include a decrease in growth rate under greenhouse conditions and downregulation of the mRNA expression of most genes involved in the light phase of photosynthesis. In the same mutant, a marked decrease in the levels of PSI and PSII polypeptides is evident, as well as a light-green leaf coloration and increased photosensitivity. Increased dosage of PsaD2 in the psad1-1 background restores the WT phenotype, indicating that PSI-D1 and PSI-D2 have redundant functions.

Light-harvesting complex II binds to several small subunits of photosystem I

Mobile light-harvesting complex II (LHCII) is implicated in the regulation of excitation energy distribution between Photosystem I (PSI) and Photosystem II (PSII) during state transitions. To investigate how LHCII interacts with PSI during state transitions, PSI was isolated from Arabidopsis thaliana plants treated with PSII or PSI light. The PSI preparations were made using digitonin. Chemical cross-linking using dithio-bis(succinimidylpropionate) followed by diagonal electrophoresis and immunoblotting showed that the docking site of LHCII (Lhcb1) on PSI is comprised of the PSI-H, -L, and -I subunits. This was confirmed by the lack of energy transfer from LHCII to PSI in the digitonin-PSI isolated from plants lacking PSI-H and -L. Digitonin-PSI was purified further to obtain an LHCII.PSI complex, and two to three times more LHCII was associated with PSI in the wild type in State 2 than in State 1. Lhcb1 was also associated with PSI from plants lacking PSI-K, but PSI from PSI-H, -L, or -O mutants contained only about 30% of Lhcb1 compared with the wild type. Surprisingly, a significant fraction of the LHCII bound to PSI in State 2 was not phosphorylated. Cross-linking prior to sucrose gradient purification resulted in copurification of phosphorylated LHCII in the wild type, but not with PSI from the PSI-H, -L, and -O mutants. The data suggest that migration of LHCII during state transitions cannot be explained sufficiently by different affinity of phosphorylated and unphosphorylated LHCII for PSI but is likely to involve structural changes in thylakoid organization.

Arabidopsis thaliana plants lacking the PSI-D subunit of photosystem I suffer severe photoinhibition, have unstable photosystem I complexes, and altered redox homeostasis in the chloroplast stroma

The PSI-D subunit of photosystem I is a hydrophilic subunit of about 18 kDa, which is exposed to the stroma and has an important function in the docking of ferredoxin to photosystem I. We have used an antisense approach to obtain Arabidopsis thaliana plants with only 5-60% of PSI-D. No plants were recovered completely lacking PSI-D, suggesting that PSI-D is essential for a functional PSI in plants. Plants with reduced amounts of PSI-D showed a similar decrease in all other subunits of PSI including the light harvesting complex, suggesting that in the absence of PSI-D, PSI cannot be properly assembled and becomes degraded. Plants with reduced amounts of PSI-D became light-stressed even in low light although they exhibited high non-photochemical quenching (NPQ). The high NPQ was generated by upregulating the level of violaxanthin de-epoxidase and PsbS, which are both essential components of NPQ. Interestingly, the lack of PSI-D affected the redox state of thioredoxin. During the normal light cycle thioredoxin became increasingly oxidized, which was observed as decreasing malate dehydrogenase activity over a 4-h light period. This result shows that photosynthesis was close to normal the first 15 min, but after 2-4 h photoinhibition dominated as the stroma progressively became less reduced. The change in the thiol disulfide redox state might be fatal for the PSI-D-less plants, because reduction of thioredoxin is one of the main switches for the initiation of CO2 assimilation and photoprotection upon light exposure.

From Genes to Photosynthesis in Arabidopsis thaliana

Although photosynthesis in higher plants is of cyanobacterial descent, it differs strikingly in organization and regulation from the prokaryotic process. Genomics, proteomics, and comparative genome analysis are now providing powerful new tools for the molecular dissection of photosynthesis in higher plants. Mutant screens and reverse genetics identify an increasing number of gene-function relationships that have a bearing on photosynthesis, revealing a marked interdependency between photosynthesis and other cellular processes. Photosynthesis-related functions are mostly located in the chloroplast, but can also be located in other compartments of the plant cell. The analysis by DNA-array hybridization of mRNA expression patterns both in the chloroplast and the nucleus, under various environmental conditions and/or in different genetic backgrounds that affect the function of the plastid, is rapidly improving our understanding of how photosynthesis is regulated, and it reveals that plastid-to-nucleus signaling plays a central role in its control.

Protein-protein and protein-function relationships in Arabidopsis photosystem I: cluster analysis of PSI polypeptide levels and photosynthetic parameters in PSI mutants

In flowering plants, photosystem I (PSI) mediates electron transport across the thylakoid membrane and contains at least 14 proteins. The availability of co-suppression and/or mutant lines deficient for individual PSI polypeptides in Arabidopsis thaliana allows one to assign functions to PSI subunits. We have performed cluster analysis on an extensive set of data on PSI polypeptide levels in ten different PSI mutants. This type of analysis serves to group proteins that exhibit similar changes in amount in different genotypes, and also identifies genotypes which show similar PSI compositions. The interdependence of levels of PSI-C, -D and -E, of -H and -L, and of Lhca2 and 3, which was previously proposed based on the study of single genotypes or on cross-linking experiments, was confirmed by our analyses. In addition, the levels of the lumenal subunits F and N are found to be interdependent. The incorporation of photosynthetic parameters into the cluster analysis revealed that the level of photosynthetic state transitions correlates with the abundance of PSI-H in all 8 genotypes tested, supporting the hypothesis that PSI-H serves as a docking site for LHCII during state transitions.

Plants impaired in state transitions can to a large degree compensate for their defect

Arabidopsis thaliana plants lacking the PSI-H or PSI-L subunit of photosystem I have been shown to be severely affected in their ability to perform state transitions, but no visual phenotype was observed when these plants were grown under different light quantities and qualities. However, the chloroplasts in the PSI-H- and PSI-L-less plants contained fewer and more extended grana stacks. The plants lacking PSI-H or PSI-L were characterised with respect to their photosynthetic performance. Wild-type plants adjusted the non-photochemical fluorescence quenching to maintain constant levels of PSII quantum yield and reduction of the plastoquinone pool. In contrast, the plants deficient in state transitions had a more reduced plastoquinone pool and consequently, a less efficient PSII-photochemistry under growth-light conditions and in state 2. The maximal photosynthetic capacity and the quantum efficiency of oxygen evolution were diminished by 8-14% in the PSI-H-less plants. Under growth-light conditions, the stroma was similarly reduced in the PSI-H-less plants and the rate of cyclic electron transport was unchanged. Pigment analysis showed that the xanthophyll cycle was not upregulated in order to compensate for the lack of state transitions. In general, the plants lacking PSI-H and PSI-L showed a decreased ability to optimise photosynthesis according to the light conditions.

The location of plastocyanin in vascular plant photosystem I

We have studied the binding sites of the electron donor and acceptor proteins of vascular plant photosystem I by electron microscopy/crystallography. Previously, we identified the binding site for the electron acceptor (ferredoxin). In this paper we complete these studies with the characterization of the electron donor (plastocyanin) binding site. After cross-linking, plastocyanin is detected using Fourier difference analysis of two dimensionally ordered arrays of photosystem I located at the periphery of chloroplast grana. Plastocyanin binds in a small cavity on the lumenal surface of photosystem I, close to the center and with a slight bias toward the PsaL subunit of the complex. The recent release of the full coordinates for the cyanobacterial photosystem I reaction center has allowed a detailed comparison between the structures of the eukaryotic and prokaryotic systems. This reveals a very close homology, which is particularly striking for the lumenal side of photosystem I.

Single and double knockouts of the genes for photosystem I subunits G, K, and H of Arabidopsis. Effects on photosystem I composition, photosynthetic electron flow, and state transitions

Photosystem I (PSI) of higher plants contains 18 subunits. Using Arabidopsis En insertion lines, we have isolated knockout alleles of the genes psaG, psaH2, and psaK, which code for PSI-G, -H, and -K. In the mutants psak-1 and psag-1.4, complete loss of PSI-K and -G, respectively, was confirmed, whereas the residual H level in psah2-1.4 is due to a second gene encoding PSI-H, psaH1. Double mutants, lacking PSI-G, and also -K, or a fraction of -H, together with the three single mutants were characterized for their growth phenotypes and PSI polypeptide composition. In general, the loss of each subunit has secondary, in some cases additive, effects on the abundance of other PSI polypeptides, such as D, E, H, L, N, and the light-harvesting complex I proteins Lhca2 and 3. In the G-less mutant psag-1.4, the variation in PSI composition suggests that PSI-G stabilizes the PSI-core. Levels of light-harvesting complex I proteins in plants, which lack simultaneously PSI-G and -K, indicate that PSI subunits other than G and K can also bind Lhca2 and 3. In the same single and double mutants, psag-1.4, psak-1, psah2-1.4, psag-1.4/psah2-1.4, and psag-1.4/psak-1 photosynthetic electron flow and excitation energy quenching were analyzed to address the roles of the various subunits in P700 reduction (mediated by PSI-F and -N) and oxidation (PSI-E), and state transitions (PSI-H). Based on the results, we also suggest for PSI-K a role in state transitions.

Double mutation cpSRP43--/cpSRP54-- is necessary to abolish the cpSRP pathway required for thylakoid targeting of the light-harvesting chlorophyll proteins

Biochemical and genetic studies have established that the light-harvesting chlorophyll proteins (LHCPs) of the photosystems use the cpSRP (chloroplast signal recognition particle) pathway for their targeting to thylakoids. Previous analyses of single cpSRP mutants, chaos and ffc, deficient in cpSRP43 and cpSRP54, respectively, have revealed that half of the LHCPs are still integrated into the thylakoid membranes. Surprisingly, the effects of both mutations are additive in the double mutant ffc/chaos described here. This mutant has pale yellow leaves at all stages of growth and drastically reduced levels of all the LHCPs except Lhcb 4. Although the chloroplasts have a normal shape, the thylakoid structure is affected by the mutation, probably as a consequence of reduction of all the LHCPs. ELIPs (early light-inducible proteins), nuclear-encoded proteins related to the LHCP family and inducible by light stress, were also drastically reduced in the double mutant. However, proteins targeted by other chloroplastic targeting pathways (DeltapH, Sec and spontaneous pathways) accumulated to similar levels in the wild-type and the double mutant. Therefore, the near total loss of LHCPs and ELIPs in the double mutant suggests that cpSRP is the predominant, if not exclusive, targeting pathway for these proteins. Phenotypic analysis of the double mutant, compared to the single mutants, suggests that the cpSRP subunits cpSRP43 and cpSRP54 contribute to antenna targeting in an independent but additive way.

Photosystem I activity is increased in the absence of the PSI-G subunit

PSI-G is a subunit of photosystem I in eukaryotes. The function of PSI-G was characterized in Arabidopsis plants transformed with a psaG cDNA in antisense orientation. Several plants with significantly decreased PSI-G protein content were identified. Plants with reduced PSI-G content were indistinguishable from wild type when grown under optimal conditions, despite a 40% reduction of photosystem I. This decrease of photosystem I was correlated with a similar reduction in state transitions. Surprisingly, the reduced photosystem I content was compensated for by a more effective photosystem I because the light-dependent reduction of NADP(+) in vitro was 48% higher. Photosystem I antenna size determined from flash-induced P700 absorption changes did not reveal any significant effect on the size of the photosystem I antenna in the absence of PSI-G, whereas a 17% reduction was seen in the absence of PSI-K. However, nondenaturing green gels revealed that the interaction between photosystem I and the light-harvesting complex I was less stable in the absence of PSI-G. Thus, PSI-G plays a role in stabilizing the binding of the peripheral antenna. The increased activity in the absence of PSI-G suggests that PSI-G could have an important role in regulation of photosystem I.

PSI-O, a new 10-kDa subunit of eukaryotic photosystem I

A novel polypeptide with an apparent molecular mass of 9 kDa was detected after sodium dodecyl sulphate-polyacrylamide gel electrophoresis of Arabidopsis photosystem I (PSI) and was N-terminally sequenced. Corresponding cDNA clones encode a precursor protein of 140 amino acid residues which was imported into isolated intact chloroplasts and processed to the mature protein, designated PSI-O. The mature protein has two transmembrane helices and a calculated mass of 10104 Da. The PSI-O protein was also shown to be present in PSI isolated from barley and spinach, and was essentially absent in chloroplast grana. Expressed sequences encoding similar proteins are available from many species of plants and green algae.

Daddy, where did (PS)I come from?

The reacton centre I (RCI)-type photosystems from plants, cyano-, helio- and green sulphur bacteria are compared and the essential properties of an archetypal RCI are deduced. Species containing RCI-type photosystems most probably cluster together on a common branch of the phylogenetic tree. The predicted branching order is green sulphur, helio- and cyanobacteria. Striking similarities between RCI- and RCII-type photosystems recently became apparent in the three-dimensional structures of photosystem I (PSI), PSII and RCII. The phylogenetic relationship between all presently known photosystems is analysed suggesting (a) RCI as the ancestral photosystem and (b) the descendence of PSII from RCI via gene duplication and gene splitting. An evolutionary model trying to rationalise available data is presented.

Role of subunits in eukaryotic Photosystem I

Photosystem I (PSI) of eukaryotes has a number of features that distinguishes it from PSI of cyanobacteria. In plants, the PSI core has three subunits that are not found in cyanobacterial PSI. The remaining 11 subunits of the core are conserved but several of the subunits have a different role in eukaryotic PSI. A distinguishing feature of eukaryotic PSI is the membrane-imbedded peripheral antenna. Light-harvesting complex I is composed of four different subunits and is specific for PSI. Light-harvesting complex II can be associated with both PSI and PSII. Several of the core subunits interact with the peripheral antenna proteins and are important for proper function of the peripheral antenna. The review describes the role of the different subunits in eukaryotic PSI. The emphasis is on features that are different from cyanobacterial PSI.

Balance of power: a view of the mechanism of photosynthetic state transitions

Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes. Plants can balance the distribution of absorbed light energy between the two photosystems. When photosystem II is favoured, a mobile pool of light harvesting complex II moves from photosystem II to photosystem I. This short-term and reversible redistribution is known as a state transition. It is associated with changes in the phosphorylation of light harvesting complex II but the regulation is complex. Redistribution of energy during state transitions depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvesting complex II-photosystem I complexes.

PHOTOSYSTEM I: Function and Physiology

Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in the metabolic networks and physiological responses in plants.

Green plant photosystem I binds light-harvesting complex I on one side of the complex

We report a structural characterization by electron microscopy of green plant photosystem I solubilized by the mild detergent n-dodecyl-alpha-D-maltoside. It is shown by immunoblotting that the isolated complexes contain all photosystem I core proteins and all peripheral light-harvesting proteins. The electron microscopic analysis is based on a large data set of 14 000 negatively stained single-particle projections and reveals that most of the complexes are oval-shaped monomers. The monomers have a tendency to associate into artificial dimers, trimers, and tetramers in which the monomers are oppositely oriented. Classification of the dimeric complexes suggests that some of the monomers lack a part of the peripheral antenna. On the basis of a comparison with projections from trimeric photosystem I complexes from cyanobacteria, we conclude that light-harvesting complex I only binds to the core complex at the side of the photosystem I F/J subunits and does not cause structural hindrances for the type of trimerization observed in cyanobacterial photosystem I.

The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis

Photosynthesis in plants involves two photosystems responsible for converting light energy into redox processes. The photosystems, PSI and PSII, operate largely in series, and therefore their excitation must be balanced in order to optimize photosynthetic performance. When plants are exposed to illumination favouring either PSII or PSI they can redistribute excitation towards the light-limited photosystem. Long-term changes in illumination lead to changes in photosystem stoichiometry. In contrast, state transition is a dynamic mechanism that enables plants to respond rapidly to changes in illumination. When PSII is favoured (state 2), the redox conditions in the thylakoids change and result in activation of a protein kinase. The kinase phosphorylates the main light-harvesting complex (LHCII) and the mobile antenna complex is detached from PSII. It has not been clear if attachment of LHCII to PSI in state 2 is important in state transitions. Here we show that in the absence of a specific PSI subunit, PSI-H, LHCII cannot transfer energy to PSI, and state transitions are impaired.

Down-regulation of the PSI-F subunit of photosystem I (PSI) in Arabidopsis thaliana. The PSI-F subunit is essential for photoautotrophic growth and contributes to antenna function

The PSI-F subunit of photosystem I is a transmembrane protein with a large lumenal domain. The role of PSI-F was investigated in Arabidopsis plants transformed with an antisense construct of the psaF cDNA. Several plant lines with reduced amounts of the PSI-F subunit were generated. Many of the transgenic plants died, apparently because they were unable to survive without the PSI-F subunit. Plants with 5% of PSI-F were capable of photoautotrophic growth but were much smaller than wild-type plants. The plants suffered severely under normal growth conditions but recovered somewhat in the dark indicating chronic photoinhibition. Photosystem I lacking PSI-F was less stable, and the stromal subunits PSI-C, PSI-D, and PSI-E were present in lower amounts than in wild type. The lack of PSI-F resulted in an inability of light-harvesting complex I-730 to transfer energy to the P700 reaction center. In thylakoids deficient in PSI-F, the steady state NADP(+) reduction rate was only 10% of the wild-type levels indicating a lower efficiency in oxidation of plastocyanin. Surprisingly, the lack of PSI-F also gave rise to disorganization of the thylakoids. The strict arrangement in grana and stroma lamellae was lost, and instead a network of elongated and distorted grana was observed.

The PSI-K subunit of photosystem I is involved in the interaction between light-harvesting complex I and the photosystem I reaction center core

PSI-K is a subunit of photosystem I. The function of PSI-K was characterized in Arabidopsis plants transformed with a psaK cDNA in antisense orientation, and several lines without detectable PSI-K protein were identified. Plants without PSI-K have a 19% higher chlorophyll a/b ratio and 19% more P700 than wild-type plants. Thus, plants without PSI-K compensate by making more photosystem I. The photosystem I electron transport in vitro is unaffected in the absence of PSI-K. Light response curves for oxygen evolution indicated that the photosynthetic machinery of PSI-K-deficient plants have less capacity to utilize light energy. Plants without PSI-K have less state 1-state 2 transition. Thus, the redistribution of absorbed excitation energy between the two photosystems is reduced. Low temperature fluorescence emission spectra revealed a 2-nm blue shift in the long wavelength emission in plants lacking PSI-K. Furthermore, thylakoids and isolated PSI without PSI-K had 20-30% less Lhca2 and 30-40% less Lhca3, whereas Lhca1 and Lhca4 were unaffected. During electrophoresis under mildly denaturing conditions, all four Lhca subunits were partially dissociated from photosystem I lacking PSI-K. The observed effects demonstrate that PSI-K has a role in organizing the peripheral light-harvesting complexes on the core antenna of photosystem I.

Chloroplast site-directed mutagenesis of photosystem I in Chlamydomonas: electron transfer reactions and light sensitivity

The photosystem I (PSI) complex is a multisubunit protein-pigment complex embedded in the thylakoid membrane which acts as a light-driven plastocyanin/cytochrome c(6)-ferredoxin oxido-reductase. The use of chloroplast transformation and site-directed mutagenesis coupled with the biochemical and biophysical analysis of mutants of the green alga Chlamydomonas reinhardtii with specific amino acid changes in several subunits of PSI has provided new insights into the structure-function relationship of this important photosynthetic complex. In particular, this molecular-genetic analysis has identified key residues of the reaction center polypeptides of PSI which are the ligands of some of the redox cofactors and it has also provided important insights into the orientation of the terminal electron acceptors of this complex. Finally this analysis has also shown that mutations affecting the donor side of PSI are limiting for overall electron transfer under high light and that electron trapping within the terminal electron acceptors of PSI is highly deleterious to the cells.

Specific and heritable genetic interference by double-stranded RNA in Arabidopsis thaliana

We investigated the potential of double-stranded RNA interference (RNAi) with gene activity in Arabidopsis thaliana. To construct transformation vectors that produce RNAs capable of duplex formation, gene-specific sequences in the sense and antisense orientations were linked and placed under the control of a strong viral promoter. When introduced into the genome of A. thaliana by Agrobacterium-mediated transformation, double-stranded RNA-expressing constructs corresponding to four genes, AGAMOUS (AG), CLAVATA3, APETALA1, and PERIANTHIA, caused specific and heritable genetic interference. The severity of phenotypes varied between transgenic lines. In situ hybridization revealed a correlation between a declining AG mRNA accumulation and increasingly severe phenotypes in AG (RNAi) mutants, suggesting that endogenous mRNA is the target of double-stranded RNA-mediated genetic interference. The ability to generate stably heritable RNAi and the resultant specific phenotypes allows us to selectively reduce gene function in A. thaliana.