Plastoquinone homoeostasis by Arabidopsis proton gradient regulation 6 is essential for photosynthetic efficiency

Arabidopsis METHYLENETETRAHYDROFOLATE REDUCTASE 2 functions independently of PENETRATION 2 during primary immunity against rice blast

Non-host resistance (NHR) is the most durable and robust form of innate immunity, with a surge of interest in its role in crop improvement. Of the NHR genes identified against rice blast, a devastating disease caused by Magnaporthe oryzae, Arabidopsis PEN2 is indispensable for pre-penetration resistance to M. oryzae, while a consortium of genes orchestrates post-penetration resistance via lesser known mechanisms. We identified M. oryzae-susceptible mosA (mthfr2 pen2-3) from a randomly mutagenized Arabidopsis pen2-3 population using forward genetics. Analysis of T-DNA-inserted mthfr2 lines and pen2-3-complemented mosA lines revealed that MTHFR2-dependent resistance to M. oryzae is independent of PEN2. MTHFR2-defective plants exhibited higher accumulation of reactive oxygen species and expression of salicylic acid-dependent defense markers. MTHFR2-ligand docking revealed that A55V non-synonymous substitution in mosA altered ligand binding efficiency. This further affected the metabolomic profile of mosA, effectively allowing in vitro germination and development of M. oryzae conidia. Moreover, the loss-of-function mutation in mthfr2 (involved in the 1C metabolic pathway) potentiated mosA immunity against Pst DC3000. In conclusion, our findings showed that MTHFR2 is a positive modulator of NHR against M. oryzae. This work documents another layer of conserved yet divergent metabolomic defense in Arabidopsis regulated by folate-mediated 1C metabolism that has the potential to revolutionize crop improvement.

Arabidopsis conditional photosynthesis mutants abc1k1 and var2 accumulate partially processed thylakoid preproteins and are defective in chloroplast biogenesis

Photosynthetic activity is established during chloroplast biogenesis. In this study we used 680 nm red light to overexcite Photosystem II and disrupt photosynthesis in two conditional mutants (var2 and abc1k1) which reversibly arrested chloroplast biogenesis. During biogenesis, chloroplasts import most proteins associated with photosynthesis. Some of these must be inserted in or transported across the thylakoid membrane into the thylakoid lumen. They are synthesized in the cytoplasm with cleavable targeting sequences and the lumenal ones have bi-partite targeting sequences (first for the chloroplast envelope, second for the thylakoid membrane). Cleavage of these peptides is required to establish photosynthesis and a critical step of chloroplast biogenesis. We employ a combination of Western blotting and mass spectrometry to analyze proteins in var2 and abc1k1. Under red light, var2 and abc1k1 accumulated incompletely cleaved processing intermediates of thylakoid proteins. These findings correlated with colorless cotyledons, and defects in both chloroplast morphology and photosynthesis. Together the results provide evidence for the requirement of active photosynthesis for processing of photosystem-associated thylakoid proteins and concomitantly progression of chloroplast biogenesis.

The conserved active site aspartate residue is required for the function of the chloroplast atypical kinase ABC1K1

Introduction

The Arabidopsis abc1k1/pgr6 (Activity of BC1 complex/proton regulation 6) mutant is characterized by photosynthetic and conditional developmental phenotypes triggered by stressful red as well as high light. The Arabidopsis ABC1-like kinases belong to the atypical kinase family and contain conserved ATP-binding and hydrolysis motifs, but their physiological requirement has never been investigated.

Methods

By mutation to asparagine, we demonstrate that the highly conserved active site aspartate residue within ATP-binding motif VIIb is required for the physiological functions of ABC1K1.

Results

Complementation of the abc1k1 knock out mutant with ABC1K1 D400N, failed to restore the wildtype phenotype.

Discussion

These results provide in vivo evidence for a critical role of the active site aspartate residue (D400) of ABC1K1.

Transcriptomic and proteomic analyses of distinct Arabidopsis organs reveal high PSI-NDH complex accumulation in stems

In addition to leaves, the main site of photosynthetic reactions, active photosynthesis also takes place in stems, siliques and tree trunks. Although non-foliar photosynthesis has a marked effect on plant growth and yield, only limited information on the expression patterns of photosynthesis-related genes and the structure of photosynthetic machinery in different plant organs has been available. Here, we report the results of transcriptomic analysis of various organs of Arabidopsis thaliana and compare the gene expression profiles of young and mature leaves with a special focus on photosynthetic genes. Further, we analyzed the composition and organization of the photosynthetic electron transfer machinery in leaves, stems and green siliques at the protein level using BN-PAGE. RNA-Seq analysis revealed unique gene expression profiles in different plant organs and showed major differences in the expression of photosynthesis-related genes in young as compared to mature rosettes. Gel-based proteomic analysis of the thylakoid protein complex organization further showed that all studied plant organs contain the necessary components of the photosynthetic electron transfer chain. Intriguingly, stems accumulate high amounts of PSI-NDH complex, which has previously been implicated in cyclic electron transfer.

Comparing plastid proteomes points towards a higher plastidial redox turnover in vascular tissues than in mesophyll cells

Plastids are complex organelles that vary in size and function depending on the cell type. Accordingly, they can be referred to as amyloplasts, chloroplasts, chromoplasts, etioplasts, proplasts to only cite a few denominations. Over the past decades, methods based on density gradients and differential centrifugations have been extensively used for the purification of plastids. However, these methods need large amounts of starting material, and hardly provide a tissue-specific resolution. Here, we applied our IPTACT (Isolation of Plastids TAgged in specific Cell Types) method, which involves the biotinylation of plastids in vivo using one-shot transgenic lines expressing the TOC64 gene coupled with a biotin ligase receptor particle and the BirA biotin ligase, to isolate plastids from mesophyll and companion cells of Arabidopsis thaliana using tissue specific pCAB3 and pSUC2 promoters, respectively. Subsequently, a proteome profiling was performed, and allowed the identification of 1672 proteins, among which 1342 were predicted plastidial, and 705 were fully confirmed according to SUBA5. Interestingly, although 92% of plastidial proteins were equally distributed between the two tissues, we observed an accumulation of proteins associated with jasmonic acid biosynthesis, plastoglobuli (e.g. NDC1, VTE1, PGL34, ABC1K1) and cyclic electron flow in plastids originating from vascular tissues. Besides demonstrating the technical feasibility of isolating plastids in a tissue-specific manner, our work provides strong evidence that plastids from vascular tissue have a higher redox turnover to ensure optimal functioning, notably under high solute strength as encountered in vascular cells.

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.

Chromoplast plastoglobules recruit the carotenoid biosynthetic pathway and contribute to carotenoid accumulation during tomato fruit maturation

Tomato (Solanum lycopersicum) fruit maturation is associated with a developmental transition from chloroplasts (in mature green fruit) to chromoplasts (in red fruit). The hallmark red color of ripe tomatoes is due to carotenogenesis and accumulation of the red carotenoid lycopene inside chromoplasts. Plastoglobules (PG) are lipid droplets in plastids that are involved in diverse lipid metabolic pathways. In tomato, information on the possible role of PG in carotogenesis and the PG proteome is largely lacking. Here, we outline the role of PG in carotenogenesis giving particular attention to tomato fruit PG proteomes and metabolomes. The proteome analysis revealed the presence of PG-typical FBNs, ABC1K-like kinases, and metabolic enzymes, and those were decreased in the PG of tomato chromoplasts compared to chloroplasts. Notably, the complete β-carotene biosynthesis pathway was recruited to chromoplast PG, and the enzymes PHYTOENE SYNTHASE 1 (PSY-1), PHYTOENE DESATURASE (PDS), ZETA-CAROTENE DESATURASE (ZDS), and CAROTENOID ISOMERASE (CRTISO) were enriched up to twelvefold compared to chloroplast PG. We profiled the carotenoid and prenyl lipid changes in PG during the chloroplast to chromoplast transition and demonstrated large increases of lycopene and β-carotene in chromoplast PG. The PG proteome and metabolome are subject to extensive remodeling resulting in high accumulation of lycopene during the chloroplast-to-chromoplast transition. Overall, the results indicate that PGs contribute to carotenoid accumulation during tomato fruit maturation and suggest that they do so by functioning as a biosynthetic platform for carotenogenesis.

Regulation of photosynthetic light reaction proteins via reversible phosphorylation

The regulation of photosynthesis occurs at different levels including the control of nuclear and plastid genes transcription, RNA processing and translation, protein translocation, assemblies and their post translational modifications. Out of all these, post translational modification enables rapid response of plants towards changing environmental conditions. Among all post-translational modifications, reversible phosphorylation is known to play a crucial role in the regulation of light reaction of photosynthesis. Although, phosphorylation of PS II subunits has been extensively studied but not much attention is given to other photosynthetic complexes such as PS I, Cytochrome b6f complex and ATP synthase. Phosphorylation reaction is known to protect photosynthetic apparatus in challenging environment conditions such as high light, elevated temperature, high salinity and drought. Recent studies have explored the role of photosynthetic protein phosphorylation in conferring plant immunity against the rice blast disease. The evolution of phosphorylation of different subunits of photosynthetic proteins occurred along with the evolution of plant lineage for their better adaptation to the changing environment conditions. In this review, we summarize the progress made in the research field of phosphorylation of photosynthetic proteins and highlights the missing links that need immediate attention.

Loss of a single chlorophyll in CP29 triggers re-organization of the Photosystem II supramolecular assembly

In land plants, both efficient light capture and photoprotective dissipation of chlorophyll excited states in excess require proper assembly of Photosystem II supercomplexes PSII-LHCs. These include a dimeric core moiety and a peripheral antenna system made of trimeric LHCII proteins connected to the core through monomeric LHC subunits. Regulation of light harvesting involves re-organization of the PSII supercomplex, including dissociation of its LHCII-CP24-CP29 domain under excess light. The Chl a603-a609-a616 chromophore cluster within CP29 was recently identified as responsible for the fast component of Non-Photochemical Quenching of chlorophyll fluorescence. Here, we pinpointed a chlorophyll-protein domain of CP29 involved in the macro-organization of PSII-LHCs. By complementing an Arabidopsis knock-out mutant with CP29 sequences deleted in the residue binding chlorophyll b614/b3-binding, we found that the site is promiscuous for chlorophyll a and b. By plotting NPQ amplitude vs. CP29 content we observed that quenching activity was significantly reduced in mutants compared to the wild type. Analysis of pigment-binding supercomplexes showed that the missing Chl did hamper the assembly of PSII-LHCs supercomplexes, while observation by electron microscopy of grana membranes highlighted the PSII particles were organized in two-dimensional arrays in mutant grana partitions. As an effect of such array formation electron transport rate between Q_A and Q_B reduced, likely due to restricted plastoquinone diffusion. We conclude that chlorophyll b614, rather being part of pigment cluster responsible for quenching, is needed to maintain full rate of electron flow in the thylakoids by controlling protein-protein interactions between PSII units in grana partitions.

Plastoquinone homeostasis in plant acclimation to light intensity

Arabidopsis plants were grown from seeds at different photon flux densities (PFDs) of white light ranging from 65 to 800 µmol photons m^-2 s^-1. Increasing PFD brought about a marked accumulation of plastoquinone (PQ) in leaves. However, the thylakoid photoactive PQ pool, estimated to about 700 pmol mg^-1 leaf dry weight, was independent of PFD; PQ accumulation in high light mostly occurred in the photochemically non-active pool (plastoglobules, chloroplast envelopes) which represented up to 75% of total PQ. The amounts of PSII reaction center (on a leaf dry weight basis) also were little affected by PFD during growth, leading to a constant PQ/PSII ratio at all PFDs. Boosting PQ biosynthesis by overexpression of a solanesyl diphosphate-synthesizing enzyme strongly enhanced the PQ levels, particularly at high PFDs. Again, this accumulation occurred exclusively in the non-photoactive PQ pool. Mutational suppression of the plastoglobular ABC1K1 kinase led to a selective reduction of the thylakoid PQ pool size to ca. 400 pmol mg^-1 in a large range of PFDs, which was associated with a restriction of the photosynthetic electron flow. Our results show that photosynthetic acclimation to light intensity does not involve modulation of the thylakoid PQ pool size or the amounts of PSII reaction centers. There appears to be a fixed amount of PQ molecules for optimal interaction with PSII and efficient photosynthesis, with the extra PQ molecules being stored outside the thylakoid membranes, implying a tight regulation of PQ distribution within the chloroplasts.

Myxobacteria of the Cystobacterineae Suborder Are Producers of New Vitamin K2 Derived Myxoquinones

Vitamin K is an essential, lipid soluble vitamin that plays an important role in the human blood coagulation cascade as well as in the life cycle of bacteria and plants. In this study, we report the isolation and structure elucidation of unprecedented polyhydroxylated menaquinone variants named myxoquinones that are produced by myxobacteria and structurally belong to the Vitamin K family. We analyze the occurrence of myxoquinones across an LC-MS data collection from myxobacterial extracts and shed light on the distribution of myxoquinone-type biosynthetic gene clusters among publicly available myxobacterial genomes. Our findings indicate that myxoquinones are specifically produced by strains of the Cystobacterineae suborder within myxobacteria. Furthermore, bioinformatic analysis of the matching gene clusters allowed us to propose a biosynthetic model for myxoquinone formation. Due to their increased water-solubility, the myxoquinones could be a suitable starting point for the development of a better bioavailable treatment of vitamin K deficiency.

The plastoglobule-localized protein AtABC1K6 is a Mn2+-dependent kinase necessary for timely transition to reproductive growth

The Absence of bc₁ Complex (ABC1) is an ancient, atypical protein kinase family that emerged prior to the archaeal-eubacterial divergence. Loss-of-function mutants in ABC1 genes are linked to respiratory defects in microbes and humans and to compromised photosynthetic performance and stress tolerance in plants. However, demonstration of protein kinase activity remains elusive, hampering their study. Here, we investigate a homolog from Arabidopsis thaliana, AtABC1K6, and demonstrate in vitro autophosphorylation activity, which we replicate with a human ABC1 ortholog. We also show that AtABC1K6 protein kinase activity requires an atypical buffer composition, including Mn²⁺ as a divalent cation cofactor and a low salt concentration. AtABC1K6 associates with plastoglobule lipid droplets of A. thaliana chloroplasts, along with five paralogs. We show that the protein kinase activity associated with isolated A. thaliana plastoglobules was inhibited at higher salt concentrations, but could accommodate Mg²⁺ as well as Mn²⁺, indicating salt sensitivity, but not the requirement for Mn²⁺, may be a general characteristic of ABC1 proteins. Finally, loss of functional AtABC1K6 impairs the developmental transition from vegetative to reproductive growth. This phenotype was complemented by the wild-type sequence of AtABC1K6, but not by a kinase-dead point mutant in the unique Ala-triad of the ATP-binding pocket, demonstrating the physiological relevance of the protein's kinase activity. We suggest that ABC1s are bona fide protein kinases with a unique regulatory mechanism. Our results open the door to detailed functional and mechanistic studies of ABC1 proteins and plastoglobules.

Chloroplast pH Homeostasis for the Regulation of Photosynthesis

The pH of various chloroplast compartments, such as the thylakoid lumen and stroma, is light-dependent. Light illumination induces electron transfer in the photosynthetic apparatus, coupled with proton translocation across the thylakoid membranes, resulting in acidification and alkalization of the thylakoid lumen and stroma, respectively. Luminal acidification is crucial for inducing regulatory mechanisms that protect photosystems against photodamage caused by the overproduction of reactive oxygen species (ROS). Stromal alkalization activates enzymes involved in the Calvin-Benson-Bassham (CBB) cycle. Moreover, proton translocation across the thylakoid membranes generates a proton gradient (ΔpH) and an electric potential (ΔΨ), both of which comprise the proton motive force (pmf) that drives ATP synthase. Then, the synthesized ATP is consumed in the CBB cycle and other chloroplast metabolic pathways. In the dark, the pH of both the chloroplast stroma and thylakoid lumen becomes neutral. Despite extensive studies of the above-mentioned processes, the molecular mechanisms of how chloroplast pH can be maintained at proper levels during the light phase for efficient activation of photosynthesis and other metabolic pathways and return to neutral levels during the dark phase remain largely unclear, especially in terms of the precise control of stromal pH. The transient increase and decrease in chloroplast pH upon dark-to-light and light-to-dark transitions have been considered as signals for controlling other biological processes in plant cells. Forward and reverse genetic screening approaches recently identified new plastid proteins involved in controlling ΔpH and ΔΨ across the thylakoid membranes and chloroplast proton/ion homeostasis. These proteins have been conserved during the evolution of oxygenic phototrophs and include putative photosynthetic protein complexes, proton transporters, and/or their regulators. Herein, we summarize the recently identified protein players that control chloroplast pH and influence photosynthetic efficiency in plants.

Thylakoid Protein Phosphorylation in Chloroplasts

Because of their abundance and extensive phosphorylation, numerous thylakoid proteins stand out amongst the phosphoproteins of plants and algae. In particular, subunits of light-harvesting complex II (LHCII) and of photosystem II (PSII) are dynamically phosphorylated and dephosphorylated in response to light conditions and metabolic demands. These phosphorylations are controlled by evolutionarily conserved thylakoid protein kinases and counteracting protein phosphatases, which have distinct but partially overlapping substrate specificities. The best characterized are the kinases STATE TRANSITION 7 (STN7/STT7) and STATE TRANSITION 8 (STN8), and the antagonistic phosphatases PROTEIN PHOSPHATASE 1/THYLAKOID-ASSOCIATED PHOSPHATASE 38 (PPH1/TAP38) and PHOTOSYSTEM II CORE PHOSPHATASE (PBCP). The phosphorylation of LHCII is mainly governed by STN7 and PPH1/TAP38 in plants. LHCII phosphorylation is essential for state transitions, a regulatory feedback mechanism that controls the allocation of this antenna to either PSII or PSI, and thus maintains the redox balance of the electron transfer chain. Phosphorylation of several core subunits of PSII, regulated mainly by STN8 and PBCP, correlates with changes in thylakoid architecture, the repair cycle of PSII after photodamage as well as regulation of light harvesting and of alternative routes of photosynthetic electron transfer. Other kinases, such as the PLASTID CASEIN KINASE II (pCKII), also intervene in thylakoid protein phosphorylation and take part in the chloroplast kinase network. While some features of thylakoid phosphorylation were conserved through the evolution of photosynthetic eukaryotes, others have diverged in different lineages possibly as a result of their adaptation to varied environments.

Comparative proteomics of thylakoids from Arabidopsis grown in laboratory and field conditions

Compared to controlled laboratory conditions, plant growth in the field is rarely optimal since it is frequently challenged by large fluctuations in light and temperature which lower the efficiency of photosynthesis and lead to photo-oxidative stress. Plants grown under natural conditions therefore place an increased onus on the regulatory mechanisms that protect and repair the delicate photosynthetic machinery. Yet, the exact changes in thylakoid proteome composition which allow plants to acclimate to the natural environment remain largely unexplored. Here, we use quantitative label-free proteomics to demonstrate that field-grown Arabidopsis plants incorporate aspects of both the low and high light acclimation strategies previously observed in laboratory-grown plants. Field plants showed increases in the relative abundance of ATP synthase, cytochrome b 6 f, ferredoxin-NADP+ reductases (FNR1 and FNR2) and their membrane tethers TIC62 and TROL, thylakoid architecture proteins CURT1A, CURT1B, RIQ1, and RIQ2, the minor monomeric antenna complex CP29.3, rapidly-relaxing non-photochemical quenching (qE)-related proteins PSBS and VDE, the photosystem II (PSII) repair machinery and the cyclic electron transfer complexes NDH, PGRL1B, and PGR5, in addition to decreases in the amounts of LHCII trimers composed of LHCB1.1, LHCB1.2, LHCB1.4, and LHCB2 proteins and CP29.2, all features typical of a laboratory high light acclimation response. Conversely, field plants also showed increases in the abundance of light harvesting proteins LHCB1.3 and CP29.1, zeaxanthin epoxidase (ZEP) and the slowly-relaxing non-photochemical quenching (qI)-related protein LCNP, changes previously associated with a laboratory low light acclimation response. Field plants also showed distinct changes to the proteome including the appearance of stress-related proteins ELIP1 and ELIP2 and changes to proteins that are largely invariant under laboratory conditions such as state transition related proteins STN7 and TAP38. We discuss the significance of these alterations in the thylakoid proteome considering the unique set of challenges faced by plants growing under natural conditions.

Tissue-type specific accumulation of the plastoglobular proteome, transcriptional networks, and plastoglobular functions

Plastoglobules are dynamic protein-lipid microcompartments in plastids enriched for isoprenoid-derived metabolites. Chloroplast plastoglobules support formation, remodeling, and controlled dismantling of thylakoids during developmental transitions and environmental responses. However, the specific molecular functions of most plastoglobule proteins are still poorly understood. This review harnesses recent co-mRNA expression data from combined microarray and RNA-seq information in ATTED-II on an updated inventory of 34 PG proteins, as well as proteomics data across 30 Arabidopsis tissue types from ATHENA. Hierarchical clustering based on relative abundance for the plastoglobule proteins across non-photosynthetic and photosynthetic tissue types showed their coordinated protein accumulation across Arabidopsis parts, tissue types, development, and senescence. Evaluation of mRNA-based forced networks at different coefficient thresholds identified a central hub with seven plastoglobule proteins and four peripheral modules. Enrichment of specific nuclear transcription factors (e.g. Golden2-like) and support for crosstalk between plastoglobules and the plastid gene expression was observed, and specific ABC1 kinases appear part of a light signaling network. Examples of other specific findings are that FBN7b is involved with upstream steps of tetrapyrrole biosynthesis and that ABC1K9 is involved in starch metabolism. This review provides new insights into the functions of plastoglobule proteins and an improved framework for experimental studies.

Molecular changes of Arabidopsis thaliana plastoglobules facilitate thylakoid membrane remodeling under high light stress

Plants require rapid responses to adapt to environmental stresses. This includes dramatic changes in the size and number of plastoglobule lipid droplets within chloroplasts. Although the morphological changes of plastoglobules are well documented, little is known about the corresponding molecular changes. To address this gap, we have compared the quantitative proteome, oligomeric state, prenyl-lipid content and kinase activities of Arabidopsis thaliana plastoglobules under unstressed and 5-day light-stressed conditions. Our results show a specific recruitment of proteins related to leaf senescence and jasmonic acid biosynthesis under light stress, and identify nearly half of the plastoglobule proteins in high native molecular weight masses. Additionally, a specific increase in plastoglobule carotenoid abundance under the light stress was consistent with enhanced thylakoid disassembly and leaf senescence, supporting a specific role for plastoglobules in senescence and thylakoid remodeling as an intermediate storage site for photosynthetic pigments. In vitro kinase assays of isolated plastoglobules demonstrated kinase activity towards multiple target proteins, which was more pronounced in the plastoglobules of unstressed than light-stressed leaf tissue, and which was diminished in plastoglobules of the abc1k1/abc1k3 double-mutant. These results strongly suggest that plastoglobule-localized ABC1 kinases hold endogenous kinase activity, as these were the only known or putative kinases identified in the isolated plastoglobules by deep bottom-up proteomics. Collectively, our study reveals targeted changes to the protein and prenyl-lipid composition of plastoglobules under light stress that present strategies by which plastoglobules appear to facilitate stress adaptation within chloroplasts.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

Developmental acclimation of the thylakoid proteome to light intensity in Arabidopsis

Photosynthetic acclimation, the ability to adjust the composition of the thylakoid membrane to optimise the efficiency of electron transfer to the prevailing light conditions, is crucial to plant fitness in the field. While much is known about photosynthetic acclimation in Arabidopsis, to date there has been no study that combines both quantitative label-free proteomics and photosynthetic analysis by gas exchange, chlorophyll fluorescence and P700 absorption spectroscopy. Using these methods we investigated how the levels of 402 thylakoid proteins, including many regulatory proteins not previously quantified, varied upon long-term (weeks) acclimation of Arabidopsis to low (LL), moderate (ML) and high (HL) growth light intensity and correlated these with key photosynthetic parameters. We show that changes in the relative abundance of cytb6 f, ATP synthase, FNR2, TIC62 and PGR6 positively correlate with changes in estimated PSII electron transfer rate and CO2 assimilation. Improved photosynthetic capacity in HL grown plants is paralleled by increased cyclic electron transport, which positively correlated with NDH, PGRL1, FNR1, FNR2 and TIC62, although not PGR5 abundance. The photoprotective acclimation strategy was also contrasting, with LL plants favouring slowly reversible non-photochemical quenching (qI), which positively correlated with LCNP, while HL plants favoured rapidly reversible quenching (qE), which positively correlated with PSBS. The long-term adjustment of thylakoid membrane grana diameter positively correlated with LHCII levels, while grana stacking negatively correlated with CURT1 and RIQ protein abundance. The data provide insights into how Arabidopsis tunes photosynthetic electron transfer and its regulation during developmental acclimation to light intensity.

Plastoquinone In and Beyond Photosynthesis

Plastoquinone-9 (PQ-9) is an essential component of photosynthesis that carries electrons in the linear and alternative electron transport chains, and is also a redox sensor that regulates state transitions and gene expression. However, a large fraction of the PQ pool is located outside the thylakoid membranes, in the plastoglobules and the chloroplast envelopes, reflecting a wider range of functions beyond electron transport. This review describes new functions of PQ in photoprotection, as a potent antioxidant, and in chloroplast metabolism as a cofactor in the biosynthesis of chloroplast metabolites. It also focuses on the essential need for tight environmental control of PQ biosynthesis and for active exchange of this compound between the thylakoid membranes and the plastoglobules. Through its multiple functions, PQ connects photosynthesis with metabolism, light acclimation, and stress tolerance.

Mutation of the Atypical Kinase ABC1K3 Partially Rescues the PROTON GRADIENT REGULATION 6 Phenotype in Arabidopsis thaliana

Photosynthesis is an essential pathway providing the chemical energy and reducing equivalents that sustain higher plant metabolism. It relies on sunlight, which is an inconstant source of energy that fluctuates in both intensity and spectrum. The fine and rapid tuning of the photosynthetic apparatus is essential to cope with changing light conditions and increase plant fitness. Recently PROTON GRADIENT REGULATION 6 (PGR6-ABC1K1), an atypical plastoglobule-associated kinase, was shown to regulate a new mechanism of light response by controlling the homeostasis of photoactive plastoquinone (PQ). PQ is a crucial electron carrier existing as a free neutral lipid in the photosynthetic thylakoid membrane. Perturbed homeostasis of PQ impairs photosynthesis and plant acclimation to high light. Here we show that a homologous kinase, ABC1K3, which like PGR6-ABC1K1 is associated with plastoglobules, also contributes to the homeostasis of the photoactive PQ pool. Contrary to PGR6-ABC1K1, ABC1K3 disfavors PQ availability for photosynthetic electron transport. In fact, in the abc1k1/abc1k3 double mutant the pgr6(abc1k1) the photosynthetic defect seen in the abc1k1 mutant is mitigated. However, the PQ concentration in the photoactive pool of the double mutant is comparable to that of abc1k1 mutant. An increase of the PQ mobility, inferred from the kinetics of its oxidation in dark, contributes to the mitigation of the pgr6(abc1k1) photosynthetic defect. Our results also demonstrate that ABC1K3 contributes to the regulation of other mechanisms involved in the adaptation of the photosynthetic apparatus to changes in light quality and intensity such as the induction of thermal dissipation and state transitions. Overall, we suggests that, besides the absolute concentration of PQ, its mobility and exchange between storage and active pools are critical for light acclimation in plants.