Two proteins homologous to PsbQ are novel subunits of the chloroplast NAD(P)H dehydrogenase

Modulation of the wheat transcriptome by TaZFP13D under well-watered and drought conditions

Maintaining global food security in the context of climate changes will be an important challenge in the next century. Improving abiotic stress tolerance of major crops such as wheat can contribute to this goal. This can be achieved by the identification of the genes involved and their use to develop tools for breeding programs aiming to generate better adapted cultivars. Recently, we identified the wheat TaZFP13D gene encoding Zinc Finger Protein 13D as a new gene improving water-stress tolerance. The current work analyzes the TaZFP13D-dependent transcriptome modifications that occur in well-watered and dehydration conditions to better understand its function during normal growth and during drought. Plants that overexpress TaZFP13D have a higher biomass under well-watered conditions, indicating a positive effect of the protein on growth. Survival rate and stress recovery after a severe drought stress are improved compared to wild-type plants. The latter is likely due the higher activity of key antioxidant enzymes and concomitant reduction of drought-induced oxidative damage. Conversely, down-regulation of TaZFP13D decreases drought tolerance and protection against drought-induced oxidative damage. RNA-Seq transcriptome analysis identified many genes regulated by TaZFP13D that are known to improve drought tolerance. The analysis also revealed several genes involved in the photosynthetic electron transfer chain known to improve photosynthetic efficiency and chloroplast protection against drought-induced ROS damage. This study highlights the important role of TaZFP13D in wheat drought tolerance, contributes to unravel the complex regulation governed by TaZFPs, and suggests that it could be a promising marker to select wheat cultivars with higher drought tolerance.

Integrative transcriptome and whole-genome bisulfite sequencing analyses of a temperature-sensitive albino tea plant cultivar

Green tea made from albino buds and leaves has a strong umami taste and aroma. The cultivar 'Zhonghuang 2' (ZH2, Camellia sinensis) is a natural mutant with young shoots that are yellow in spring and green or yellow-green in summer. However, the mechanism of leaf color change remains unclear. Here, we found that young shoots of ZH2 were yellow at low temperature (LT) and green at high temperature (HT), indicating that ZH2 is a temperature-sensitive cultivar. Transmission electron microscopy analysis showed that the grana in the chloroplasts of young shoots grown at LT were poorly stacked, which caused a lack of photoreactions and chlorophyll. RNA-seq results showed 1279 genes differentially expressed in the young shoots grown at LT compared with those at HT, including genes related to cytochrome synthesis, chloroplast development, photosynthesis, and DNA methylation. A whole-genome bisulfite sequencing assay revealed that the dynamics of DNA methylation levels in the CG, CHG, and CHH contexts decreased under LT, and the change was most obvious in the CHH context. Furthermore, 72 genes showed significant changes in both expression and DNA methylation levels, and most of them were related to cytochrome synthesis, chloroplast development, photosynthesis, transcription factors, and signaling pathways. These results demonstrate that DNA methylation is involved in the LT-regulated albino processes of ZH2. Changes in DNA methylation levels were associated with changes in gene expression levels, affecting the structure and function of chloroplasts, which may have a phenotypic impact on shoot and leaf color.

Supramolecular assembly of chloroplast NADH dehydrogenase-like complex with photosystem I from Arabidopsis thaliana

Cyclic electron transport/flow (CET/CEF) in chloroplasts is a regulatory process essential for the optimization of plant photosynthetic efficiency. A crucial CEF pathway is catalyzed by a membrane-embedded NADH dehydrogenase-like (NDH) complex that contains at least 29 protein subunits and associates with photosystem I (PSI) to form the NDH-PSI supercomplex. Here, we report the 3.9 Å resolution structure of the Arabidopsis thaliana NDH-PSI (AtNDH-PSI) supercomplex. We constructed structural models for 26 AtNDH subunits, among which 11 are unique to chloroplasts and stabilize the core part of the NDH complex. In the supercomplex, one NDH can bind up to two PSI-light-harvesting complex I (PSI-LHCI) complexes at both sides of its membrane arm. Two minor LHCIs, Lhca5 and Lhca6, each present in one PSI-LHCI, interact with NDH and contribute to supercomplex formation and stabilization. Collectively, our study reveals the structural details of the AtNDH-PSI supercomplex assembly and provides a molecular basis for further investigation of the regulatory mechanism of CEF in plants.

Architecture of the chloroplast PSI-NDH supercomplex in Hordeum vulgare

The chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and has an important role in mediating photosystem I (PSI) cyclic electron transport (CET)^1-3. The NDH complex associates with PSI to form the PSI-NDH supercomplex and fulfil its function. Here, we report cryo-electron microscopy structures of a PSI-NDH supercomplex from barley (Hordeum vulgare). The structures reveal that PSI-NDH is composed of two copies of the PSI-light-harvesting complex I (LHCI) subcomplex and one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast-specific NDH subunits are presented and their exact positions as well as their interactions with other subunits in NDH are elucidated. In all, this study provides a structural basis for further investigations on the functions and regulation of PSI-NDH-dependent CET.

Identification of Micro Ribonucleic Acids and Their Targets in Response to Plasmodiophora brassicae Infection in Brassica napus

Clubroot disease, which is caused by the soil-borne pathogen Plasmodiophora brassicae War (P. brassicae), is one of the oldest and most destructive diseases of Brassica and cruciferous crops in the world. Plant microRNAs [micro ribonucleic acids (miRNAs)] play important regulatory roles in several developmental processes. Although the role of plant miRNAs in plant-microbe interaction has been extensively studied, there are only few reports on the specific functions of miRNAs in response to P. brassicae. This study investigated the roles of miRNAs and their targets during P. brassicae infection in a pair of Brassica napus near-isogenic lines (NILs), namely clubroot-resistant line 409R and clubroot-susceptible line 409S. Small RNA sequencing (sRNA-seq) and degradome-seq were performed on root samples of 409R and 409S with or without P. brassicae inoculation. sRNA-seq identified a total of 48 conserved and 72 novel miRNAs, among which 18 had a significant differential expression in the root of 409R, while only one miRNA was differentially expressed in the root of 409S after P. brassicae inoculation. The degradome-seq analysis identified 938 miRNA target transcripts, which are transcription factors, enzymes, and proteins involved in multiple biological processes and most significantly enriched in the plant hormone signal transduction pathway. Between 409R and 409S, we found eight different degradation pathways in response to P. brassicae infection, such as those related to fatty acids. By combining published transcriptome data, we identified a total of six antagonistic miRNA-target pairs in 409R that are responsive to P. brassicae infection and involved in pathways associated with root development, hypersensitive cell death, and chloroplast metabolic synthesis. Our results reveal that P. brassicae infection leads to great changes in miRNA pool and target transcripts. More interestingly, these changes are different between 409R and 409S. Clarification of the crosstalk between miRNAs and their targets may shed new light on the possible mechanisms underlying the pathogen resistance against P. brassicae.

Gene Coexpression Network Analysis Indicates that Hub Genes Related to Photosynthesis and Starch Synthesis Modulate Salt Stress Tolerance in Ulmus pumila

Ulmus pumila L. is an excellent afforestation and biofuel tree that produces high-quality wood, rich in starch. In addition, U. pumila is highly adaptable to adverse environmental conditions, which is conducive to its utilization for vegetating saline soils. However, little is known about the physiological responses and transcriptional regulatory network of U. pumila under salt stress. In this study, we exposed five main cultivars in saline-alkali land (Upu2, 5, 8, 11, and 12) to NaCl stress. Of the five cultivars assessed, Upu11 exhibited the highest salt resistance. Growth and biomass accumulation in Upu11 were promoted under low salt concentrations (<150 mM). However, after 3 months of continuous treatment with 150 mM NaCl, growth was inhibited, and photosynthesis declined. A transcriptome analysis conducted after 3 months of treatment detected 7009 differentially expressed unigenes (DEGs). The gene annotation indicated that these DEGs were mainly related to photosynthesis and carbon metabolism. Furthermore, PHOTOSYNTHETIC ELECTRON TRANSFERH (UpPETH), an important electron transporter in the photosynthetic electron transport chain, and UpWAXY, a key gene controlling amylose synthesis in the starch synthesis pathway, were identified as hub genes in the gene coexpression network. We identified 25 and 62 unigenes that may interact with PETH and WAXY, respectively. Overexpression of UpPETH and UpWAXY significantly increased the survival rates, net photosynthetic rates, biomass, and starch content of transgenic Arabidopsis plants under salt stress. Our findings clarify the physiological and transcriptional regulators that promote or inhibit growth under environmental stress. The identification of salt-responsive hub genes directly responsible for photosynthesis and starch synthesis or metabolism will provide targets for future genetic improvements.

PsbQ-Like Protein 3 Functions as an Assembly Factor for the Chloroplast NADH Dehydrogenase-Like Complex in Arabidopsis

Angiosperms have three PsbQ-like (PQL) proteins in addition to the PsbQ subunit of the oxygen-evolving complex of photosystem II. Previous studies have shown that two PQL proteins, PnsL2 and PnsL3, are subunits of the chloroplast NADH dehydrogenase-like (NDH) complex involved in the photosystem I (PSI) cyclic electron flow. In addition, another PsbQ homolog, PQL3, is required for the NDH activity; however, the molecular function of PQL3 has not been elucidated. Here, we show that PQL3 is an assembly factor, particularly for the accumulation of subcomplex B (SubB) of the chloroplast NDH. In the pql3 mutant of Arabidopsis thaliana, the amounts of NDH subunits in SubB, PnsB1 and PsnB4, were decreased, causing a severe reduction in the NDH-PSI supercomplex. Analysis using blue native polyacrylamide gel electrophoresis suggested that the incorporation of PnsL3 into SubB was affected in the pql3 mutant. Unlike other PsbQ homologs, PQL3 was weakly associated with thylakoid membranes and was only partially protected from thermolysin digestion. Consistent with the function as an assembly factor, PQL3 accumulated independently in other NDH mutants, such as pnsl1-3. Furthermore, PQL3 accumulated in young leaves in a manner similar to the accumulation of CRR3, an assembly factor for SubB. These results suggest that PQL3 has developed a distinct function as an assembly factor for the NDH complex during evolution of the PsbQ protein family in angiosperms.

Stepwise evolution of supercomplex formation with photosystem I is required for stabilization of chloroplast NADH dehydrogenase-like complex: Lhca5-dependent supercomplex formation in Physcomitrella patens

In angiosperms, such as Arabidopsis and barley, the chloroplast NADH dehydrogenase-like (NDH) complex associates with two copies of photosystem I (PSI) supercomplex to form an NDH-PSI supercomplex for the stabilization of the NDH complex. Two linker proteins, Lhca5 and Lhca6, are members of the light-harvesting complex I (LHCI) family and mediate this supercomplex formation. The liverwort Marchantia polymorpha has branched from the basal land plant lineage and has neither Lhca5 nor Lhca6. Consequently, the NDH complex does not form a supercomplex with PSI in this plant. The Lhca6 gene does not seem to exist also in the moss Physcomitrella patens (Physcomitrella). Conversely, the Lhca5 gene has been found in Physcomitrella, although experimental evidence is still lacking for its contribution to NDH-PSI supercomplex formation as a linker. Here, we biochemically characterized the Lhca5 knock-out mutant (lhca5) in Physcomitrella. The NDH-PSI supercomplex observed in wild-type Physcomitrella was absent in the lhca5 mutant. Lhca5 protein was detected in this NDH-PSI supercomplex. Some PSI and NDH subunits were co-immunoprecipitated with Lhca5-HA. These results indicate that the Physcomitrella gene is the functional ortholog of Lhca5 reported in Arabidopsis. Between Physcomitrella and Arabidopsis, the stromal loop region is highly conserved in Lhca5 proteins but not in other LHCI members. We found that Lhca5 contributed to the stable accumulation of the NDH complex, but part of the NDH complex was still sensitive to high light intensity, even in the wild-type. We considered that angiosperms acquired another linker protein, Lhca6, to further stabilize the NDH complex.

Heterosis-related genes under different planting densities in maize

Heterosis and increasing planting density have contributed to improving maize grain yield (GY) for several decades. As planting densities increase, the GY per plot also increases, whereas the contribution of heterosis to GY decreases. There are trade-offs between heterosis and planting density, and the transcriptional characterization of heterosis may explain the mechanism involved. In this study, 48 transcriptome libraries were sequenced from four inbred Chinese maize lines and their F1 hybrids. They were planted at densities of 45000 and 67500 plants ha-1. Maternal-effect differentially expressed genes (DEGs) played important roles in processes related to photosynthesis and carbohydrate biosynthesis and metabolism. Paternal-effect DEGs participated in abiotic/biotic stress response and plant hormone production under high planting density. Weighted gene co-expression network analysis revealed that high planting density induced heterosis-related genes regulating abiotic/biotic stress response, plant hormone biosynthesis, and ubiquitin-mediated proteolysis, but repressed other genes regulating energy formation. Under high planting density, maternal genes were mainly enriched in the photosynthesis reaction center, while paternal genes were mostly concentrated in the peripheral antenna system. Four important genes were identified in maize heterosis and high planting density, all with functions in photosynthesis, starch biosynthesis, auxin metabolism, gene silencing, and RNAi.

Specific substitutions of light-harvesting complex I proteins associated with photosystem I are required for supercomplex formation with chloroplast NADH dehydrogenase-like complex

In Arabidopsis, the chloroplast NADH-dehydrogenase-like (NDH) complex is sandwiched between two copies of photosystem I (PSI) supercomplex, consisting of a PSI core and four light-harvesting complex I (LHCI) proteins (PSI-LHCI) to form the NDH-PSI supercomplex. Two minor LHCI proteins, Lhca5 and Lhca6, contribute to the interaction of each PSI-LHCI copy with the NDH complex. Here, large-pore blue-native gel electrophoresis revealed that, in addition to this complex, there were at least two types of higher-order association of more LHCI copies with the NDH complex. In single-particle images, this higher-order association of PSI-LHCI preferentially occurs at the left side of the NDH complex when viewed from the stromal side, placing subcomplex A at the top (Yadav et al., Biochim. Biophys. Acta - Bioenerg., 1858, 2017, 12). The association was impaired in the lhca6 mutant but not in the lhca5 mutant, suggesting that the left copy of PSI-LHCI was linked to the NDH complex via Lhca6. From an analysis of subunit compositions of the NDH-PSI supercomplex in lhca5 and lhca6 mutants, we propose that Lhca6 substitutes for Lhca2 in the left copy of PSI-LHCI, whereas Lhca5 substitutes for Lhca4 in the right copy. In the lhca2 mutant, Lhca3 was specifically stabilized in the NDH-PSI supercomplex through heterodimer formation with Lhca6. In the left copy of PSI-LHCI, subcomplex B, Lhca6 and NdhD likely formed the core of the supercomplex interaction. In contrast, a larger protein complex, including at least subcomplexes B and L and NdhB, was needed to form the contact site with Lhca5 in the right copy of PSI-LHCI.

NDH-1 and NDH-2 Plastoquinone Reductases in Oxygenic Photosynthesis

Oxygenic photosynthesis converts solar energy into chemical energy in the chloroplasts of plants and microalgae as well as in prokaryotic cyanobacteria using a complex machinery composed of two photosystems and both membrane-bound and soluble electron carriers. In addition to the major photosynthetic complexes photosystem II (PSII), cytochrome b6f, and photosystem I (PSI), chloroplasts also contain minor components, including a well-conserved type I NADH dehydrogenase (NDH-1) complex that functions in close relationship with photosynthesis and likewise originated from the endosymbiotic cyanobacterial ancestor. Some plants and many microalgal species have lost plastidial ndh genes and a functional NDH-1 complex during evolution, and studies have suggested that a plastidial type II NADH dehydrogenase (NDH-2) complex substitutes for the electron transport activity of NDH-1. However, although NDH-1 was initially thought to use NAD(P)H as an electron donor, recent research has demonstrated that both chloroplast and cyanobacterial NDH-1s oxidize reduced ferredoxin. We discuss more recent findings related to the biochemical composition and activity of NDH-1 and NDH-2 in relation to the physiology and regulation of photosynthesis, particularly focusing on their roles in cyclic electron flow around PSI, chlororespiration, and acclimation to changing environments.

Cyclic electron flow provides acclimatory plasticity for the photosynthetic machinery under various environmental conditions and developmental stages

Photosynthetic electron flow operates in two modes, linear and cyclic. In cyclic electron flow (CEF), electrons are recycled around photosystem I. As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast. At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway. This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

Localization and functional characterization of the extrinsic subunits of photosystem II: an update

Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The extrinsic proteins of PSII shield the catalytic Mn4CaO5 cluster from exogenous reductants and serve to optimize oxygen evolution at physiological ionic conditions. These proteins include PsbO, found in all oxygenic organisms, PsbP and PsbQ, specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP in cyanobacteria. Furthermore, red algal PSII has PsbQ′ in addition to PsbO, PsbV, and PsbU, and diatoms have Psb31 in supplement to red algal-type extrinsic proteins, exemplifying the functional divergence of these proteins during evolution. This review provides an updated summary of recent findings on PSII extrinsic proteins and discusses their binding, function, and evolution within various photosynthetic organisms.

Impacts of high ATP supply from chloroplasts and mitochondria on the leaf metabolism of Arabidopsis thaliana

Chloroplasts and mitochondria are the major ATP producing organelles in plant leaves. Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) is a phosphatase dually targeted to the outer membranes of both organelles and it plays a role in the import of selected nuclear-encoded proteins into these two organelles. Overexpression (OE) of AtPAP2 in A. thaliana accelerates plant growth and promotes flowering, seed yield, and biomass at maturity. Measurement of ADP/ATP/NADP(+)/NADPH contents in the leaves of 20-day-old OE and wild-type (WT) lines at the end of night and at 1 and 8 h following illumination in a 16/8 h photoperiod revealed that the ATP levels and ATP/NADPH ratios were significantly increased in the OE line at all three time points. The AtPAP2 OE line is therefore a good model to investigate the impact of high energy on the global molecular status of Arabidopsis. In this study, transcriptome, proteome, and metabolome profiles of the high ATP transgenic line were examined and compared with those of WT plants. A comparison of OE and WT at the end of the night provide valuable information on the impact of higher ATP output from mitochondria on plant physiology, as mitochondrial respiration is the major source of ATP in the dark in leaves. Similarly, comparison of OE and WT following illumination will provide information on the impact of higher energy output from chloroplasts on plant physiology. OE of AtPAP2 was found to significantly affect the transcript and protein abundances of genes encoded by the two organellar genomes. For example, the protein abundances of many ribosomal proteins encoded by the chloroplast genome were higher in the AtPAP2 OE line under both light and dark conditions, while the protein abundances of multiple components of the photosynthetic complexes were lower. RNA-seq data also showed that the transcription of the mitochondrial genome is greatly affected by the availability of energy. These data reflect that the transcription and translation of organellar genomes are tightly coupled with the energy status. This study thus provides comprehensive information on the impact of high ATP level on plant physiology, from organellar biology to primary and secondary metabolism.

The PsbP and PsbQ family proteins in the photosynthetic machinery of chloroplasts

The PsbP and PsbQ proteins are extrinsic subunits of the photosystem II in eukaryotic photosynthetic organisms including higher plants, green algae and euglena. It has been suggested that PsbP and PsbQ have evolved from their cyanobacterial homologs, while considerable genetic and functional modifications have occurred to generate the eukaryote-type proteins. In addition, number of PsbP and PsbQ homologs exist in the thylakoid lumen of chloroplasts. These homologs are nuclear-encoded and likely diverged by gene duplication, and recent studies have elucidated their various functions in the photosynthetic machinery. In this short review, recent findings and new idea about these components will be discussed.

Understanding the roles of the thylakoid lumen in photosynthesis regulation

It has been known for a long time that the thylakoid lumen provides the environment for oxygen evolution, plastocyanin-mediated electron transfer, and photoprotection. More recently lumenal proteins have been revealed to play roles in numerous processes, most often linked with regulating thylakoid biogenesis and the activity and turnover of photosynthetic protein complexes, especially the photosystem II and NAD(P)H dehydrogenase-like complexes. Still, the functions of the majority of lumenal proteins in Arabidopsis thaliana are unknown. Interestingly, while the thylakoid lumen proteome of at least 80 proteins contains several large protein families, individual members of many protein families have highly divergent roles. This is indicative of evolutionary pressure leading to neofunctionalization of lumenal proteins, emphasizing the important role of the thylakoid lumen for photosynthetic electron transfer and ultimately for plant fitness. Furthermore, the involvement of anterograde and retrograde signaling networks that regulate the expression and activity of lumen proteins is increasingly pertinent. Recent studies have also highlighted the importance of thiol/disulfide modulation in controlling the functions of many lumenal proteins and photosynthetic regulation pathways.

Composition and physiological function of the chloroplast NADH dehydrogenase-like complex in Marchantia polymorpha

The chloroplast NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport and chloro-respiration and consists of five sub-omplexes, which in angiosperms further associate with photosystem I (PSI) to form a super-complex. In Marchantia polymorpha, 11 plastid-encoded subunits and all the nuclear-encoded subunits of the A, B, membrane and ferredoxin-binding sub-complexes are conserved. However, it is unlikely that the genome of this liverwort encodes Lhca5 and Lhca6, both of which mediate NDH-PSI super-complex formation. It is also unlikely that the subunits of the lumen sub-complex, PnsL1-L4, are encoded by the genome. Consistent with this in silico prediction, the results of blue-native gel electrophoresis showed that NDH subunits were detected in a protein complex with lower molecular mass in Marchantia than the NDH-PSI super-complex in Arabidopsis. Using the plastid transformation technique, we knocked out the ndhB gene in Marchantia. Although the wild-type genome copies were completely segregated out, the ΔndhB lines grew like the wild-type photoautotrophically. A post-illumination transient increase in chlorophyll fluorescence, which reflects NDH activity in vivo in angiosperms, was absent in the thalli of the ΔndhB lines. In ruptured chloroplasts, antimycin A-insensitive, and ferredoxin-dependent plastoquinone reduction was impaired, suggesting that chloroplast NDH mediates similar electron transport in Marchantia and Arabidopsis, despite its possible difference in structure. As in angiosperms, linear electron transport was not strongly affected in the ΔndhB lines. However, the plastoquinone pool was slightly more reduced at low light intensity, suggesting that chloroplast NDH functions in redox balancing of the inter system, especially under low light conditions.

Lycopene cyclase paralog CruP protects against reactive oxygen species in oxygenic photosynthetic organisms

In photosynthetic organisms, carotenoids serve essential roles in photosynthesis and photoprotection. A previous report designated CruP as a secondary lycopene cyclase involved in carotenoid biosynthesis [Maresca J, et al. (2007) Proc Natl Acad Sci USA 104:11784-11789]. However, we found that cruP KO or cruP overexpression plants do not exhibit correspondingly reduced or increased production of cyclized carotenoids, which would be expected if CruP was a lycopene cyclase. Instead, we show that CruP aids in preventing accumulation of reactive oxygen species (ROS), thereby reducing accumulation of β-carotene-5,6-epoxide, a ROS-catalyzed autoxidation product, and inhibiting accumulation of anthocyanins, which are known chemical indicators of ROS. Plants with a nonfunctional cruP accumulate substantially higher levels of ROS and β-carotene-5,6-epoxide in green tissues. Plants overexpressing cruP show reduced levels of ROS, β-carotene-5,6-epoxide, and anthocyanins. The observed up-regulation of cruP transcripts under photoinhibitory and lipid peroxidation-inducing conditions, such as high light stress, cold stress, anoxia, and low levels of CO(2), fits with a role for CruP in mitigating the effects of ROS. Phylogenetic distribution of CruP in prokaryotes showed that the gene is only present in cyanobacteria that live in habitats characterized by large variation in temperature and inorganic carbon availability. Therefore, CruP represents a unique target for developing resilient plants and algae needed to supply food and biofuels in the face of global climate change.

Structure of the chloroplast NADH dehydrogenase-like complex: nomenclature for nuclear-encoded subunits

The chloroplast NADH dehydrogenase-like complex (NDH) was first discovered based on its similarity to complex I in respiratory electron transport, and is involved in electron transport from photoproduced stromal reductants such as NADPH and ferredoxin to the intersystem plastoqunone pool. However, a recent study suggested that it is a ferredoxin-dependent plastoquinone reductase rather than an NAD(P)H dehydrogenase. Furthermore, recent advances in subunit analysis of NDH have revealed the presence of a novel hydrophilic subcomplex on the stromal side of the thylakoid membrane, as well as an unexpected lumenal subcomplex. This review discusses these new studies on the structure of NDH, and proposes a unified nomenclature for newly discovered NDH subunits.

Supercomplex formation with photosystem I is required for the stabilization of the chloroplast NADH dehydrogenase-like complex in Arabidopsis

In higher plants, the chloroplast NADH dehydrogenase-like complex (NDH) interacts with photosystem I (PSI) to form the NDH-PSI supercomplex via two minor light-harvesting complex I (LHCI) proteins, Lhca5 and Lhca6. Previously, we showed that in lhca5 and lhca6, NDH still associates with PSI to form smaller versions of the NDH-PSI supercomplex, although their molecular masses are far smaller than that of the full-size NDH-PSI supercomplex. In this study, we show that the NDH complex is present in the monomeric form in Arabidopsis (Arabidopsis thaliana) lhca5 lhca6, implying that NDH interacts with multiple copies of PSI. NDH subunit levels were slightly reduced in immature leaves and more drastically (approximately 50%) in mature leaves of the lhca5 lhca6 double mutant compared with the wild type. Chlorophyll fluorescence analyses detected NDH activity of lhca5 lhca6, suggesting that the supercomplex formation is not essential for NDH activity. However, the severe phenotypes of the lhca5 lhca6 proton gradient regulation5 triple mutant in both plant growth rate and photosynthesis suggest that the function of NDH was impaired in this mutant in vivo. Accumulation of NDH subunits was drastically reduced in lhca5 lhca6 when the light intensity was shifted from 50 to 500 μmol photons m(-2) s(-1). Furthermore, the half-life of NDH subunits, especially that of NDH18, was shorter in monomeric NDH than in the NDH-PSI supercomplex under the high-light conditions. We propose that NDH-PSI supercomplex formation stabilizes NDH and that the process is especially required under stress conditions.

Molecular functions of PsbP and PsbQ proteins in the photosystem II supercomplex

The PsbP and PsbQ proteins are extrinsic subunits of the photosystem II (PSII) supercomplex, which are found in green plants including higher plants and green algae. These proteins are thought to have evolved from their cyanobacterial homologs; cyanoP and cyanoQ respectively. It has been suggested that the functions of PsbP and PsbQ have largely changed from those of cyanoP and cyanoQ. In addition, multiple isoforms and homologs of PsbP and PsbQ were found in green plants, indicating that the acquisition of PsbP and PsbQ in PSII is not a direct path but a result of intensive functional divergence during evolution from cyanobacterial endosymbiont to chloroplast. In this review, we highlight newly introduced topics related to the functions and structures of both PsbP and PsbQ proteins. The present data suggest that PsbP together with PsbQ have specific and important roles in coordinating the activity of the donor and acceptor sides of PSII and stabilizing the active form of the PSII-light-harvesting complex II (LHCII) supercomplex.

Structure and biogenesis of the chloroplast NAD(P)H dehydrogenase complex

Eleven genes (ndhA-ndhK) encoding proteins homologous to the subunits of bacterial and mitochondrial NADH dehydrogenase (complex I) were found in the plastid genome of most land plants. These genes encode subunits of the chloroplast NAD(P)H dehydrogenase (NDH) complex involved in photosystem I (PSI) cyclic electron transport and chlororespiration. Although the chloroplast NDH is believed to be closely and functionally related to the cyanobacterial NDH-1L complex, extensive proteomic, genetic and bioinformatic studies have discovered many novel subunits that are specific to higher plants. On the basis of extensive mutant characterization, the chloroplast NDH complex is divided into four parts, the A, B, membrane and lumen subcomplexes, of which subunits in the B and lumen subcomplexes are specific to higher plants. These results suggest that the structure of NDH has been drastically altered during the evolution of land plants. Furthermore, chloroplast NDH interacts with multiple copies of PSI to form the unique NDH-PSI supercomplex. Two minor light-harvesting-complex I (LHCI) proteins, Lhca5 and Lhca6, are required for the specific interaction between NDH and PSI. The evolution of chloroplast NDH in land plants may be required for development of the function of NDH to alleviate oxidative stress in chloroplasts. In this review, we summarize recent progress on the subunit composition and structure of the chloroplast NDH complex, as well as the information on some factors involved in its assembly. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Molecular functions of oxygen-evolving complex family proteins in photosynthetic electron flow

Oxygen-evolving complex (OEC) protein is the original name for membrane-peripheral subunits of photosystem (PS) II. Recently, multiple isoforms and homologs for OEC proteins have been identified in the chloroplast thylakoid lumen, indicating that functional diversification has occurred in the OEC family. Gene expression profiles suggest that the Arabidopsis OEC proteins are roughly categorized into three groups: the authentic OEC group, the stress-responsive group, and the group including proteins related to the chloroplast NAD(P)H dehydrogenase (NDH) complex involved in cyclic electron transport around PSI. Based on the above gene expression profiles, molecular functions of the OEC family proteins are discussed together with our current knowledge about their functions.