The occurrence of a thylakoid-localized small zinc finger protein in land plants

The thylakoid membrane protein NTA1 is an assembly factor of the cytochrome b6f complex essential for chloroplast development in Arabidopsis

The cytochrome b_6f (Cyt b_6f) complex is a multisubunit protein complex in chloroplast thylakoid membranes required for photosynthetic electron transport. Here we report the isolation and characterization of the new tiny albino 1 (nta1) mutant in Arabidopsis, which has severe defects in Cyt b_6f accumulation and chloroplast development. Gene cloning revealed that the nta1 phenotype was caused by disruption of a single nuclear gene, NTA1, which encodes an integral thylakoid membrane protein conserved across green algae and plants. Overexpression of NTA1 completely rescued the nta1 phenotype, and knockout of NTA1 in wild-type plants recapitulated the mutant phenotype. Loss of NTA1 function severely impaired the accumulation of multiprotein complexes related to photosynthesis in thylakoid membranes, particularly the components of Cyt b_6f. NTA1 was shown to directly interact with four subunits (Cyt b₆/PetB, PetD, PetG, and PetN) of Cyt b_6f through the DUF1279 domain and C-terminal sequence to mediate their assembly. Taken together, our results identify NTA1 as a new and key regulator of chloroplast development that plays essential roles in assembly of the Cyt b_6f complex by interacting with multiple Cyt b_6f subunits.

Functional relationships of three NFU proteins in the biogenesis of chloroplastic iron-sulfur clusters

Iron-sulfur clusters are required in a variety of biological processes. Biogenesis of iron-sulfur clusters includes assembly of iron-sulfur clusters on scaffold complexes and transfer of iron-sulfur clusters to recipient apoproteins by iron-sulfur carriers, such as nitrogen-fixation-subunit-U (NFU)-type proteins. Arabidopsis thaliana has three plastid-targeted NFUs: NFU1, NFU2, and NFU3. We previously discovered that nfu2 -/- nfu3 -/- mutants are embryo lethal. The lack of viable nfu2 -/- nfu3 -/- mutants posed a serious challenge. To overcome this problem, we characterized nfu2-1 -/- nfu3-2+/- and nfu2-1+/- nfu3-2 -/- sesquimutants. Simultaneous loss-of-function mutations in NFU2 and NFU3 have an additive effect on the declines of 4Fe-4S-containing PSI core subunits. Consequently, the sesquimutants had much lower PSI and PSII activities, much less chlorophyll, and much smaller plant sizes, than nfu2-1 and nfu3-2 single mutants. These observations are consistent with proposed roles of NFU3 and NFU2 in the biogenesis of chloroplastic 4Fe-4S. By performing spectroscopic and in vitro reconstitution experiments, we found that NFU1 may act as a carrier for chloroplastic 4Fe-4S and 3Fe-4S clusters. In line with this hypothesis, loss-of-function mutations in NFU1 resulted in significant declines in 4Fe-4S- and 3Fe-4S-containing chloroplastic proteins. The declines of PSI activity and 4Fe-4S-containing PSI core subunits in nfu1 mutants indicate that PSI is the main target of NFU1 action. The reductions in 4Fe-4S-containing PSI core proteins and PSI activity in nfu3-2, nfu2-1, and nfu1 single mutants suggest that all three plastid-targeted NFU proteins contribute to the biogenesis of chloroplastic 4Fe-4S clusters. Although different insertion sites of T-DNA lines may cause variations in phenotypic results, mutation severity could be an indicator of the relative importance of the gene product. Our results are consistent with the hypothesis that NFU3 contributes more than NFU2 and NFU2 contributes more than NFU1 to the production of 4Fe-4S-containing PSI core subunits.

Light Signaling-Dependent Regulation of PSII Biogenesis and Functional Maintenance

Light is a key environmental cue regulating photomorphogenesis and photosynthesis in plants. However, the molecular mechanisms underlying the interaction between light signaling pathways and photosystem function are unknown. Here, we show that various monochromatic wavelengths of light cooperate to regulate PSII function in Arabidopsis (Arabidopsis thaliana). The photoreceptors cryptochromes and phytochromes modulate the expression of HIGH CHLOROPHYLL FLUORESCENCE173 (HCF173), which is required for PSII biogenesis by regulating PSII core protein D1 synthesis mediated by the transcription factor ELONGATED HYPOCOTYL5 (HY5). HY5 directly binds to the ACGT-containing element ACE motif and G-box cis-element present in the HCF173 promoter and regulates its activity. PSII activity was decreased significantly in hy5 mutants under various monochromatic wavelengths of light. Interestingly, we demonstrate that HY5 also directly regulates the expression of the genes associated with PSII assembly and repair, including ALBINO3, HCF136, HYPERSENSITIVE TO HIGH LIGHT1, etc., which is required for the functional maintenance of PSII under photodamaging conditions. Moreover, deficiency of HY5 broadly decreases the accumulation of other photosystem proteins besides PSII proteins. Thus, our study reveals an important role of light signaling in both biogenesis and functional regulation of the photosystem and provides insight into the link between light signaling and photosynthesis in land plants.

Photosynthetic characterization of transgenic Synechocystis expressing a plant thiol/disulfide-modulating protein

A previous study showed that introducing an Arabidopsis thaliana thiol/disulfide-modulating protein, Low Quantum Yield of Photosystem II 1 (LQY1), into the cyanobacterium Synechocystis sp. PCC6803 increased the efficiency of Photosystem II (PSII) photochemistry. In the present study, the authors provided additional evidence for the role of AtLQY1 in improving PSII photochemical efficiency and cell growth. Light response curve analysis showed that AtLQY1-expressing Synechocystis grown at a moderate growth light intensity (50 µmol photons m^-2 s^-1) had higher minimal, maximal, and variable fluorescence than the empty-vector control, under a wide range of actinic light intensities. Light induction and dark recovery curves demonstrated that AtLQY1-expressing Synechocystis grown at the moderate growth light intensity had higher effective PSII quantum yield, higher photochemical quenching, lower regulated heat dissipation (non-photochemical quenching), low amounts of reduced plastoquinone, and higher amounts of oxidized plastoquinone than the empty-vector control. Furthermore, growth curve analysis indicated that AtLQY1-expressing Synechocystis grew faster than the empty-vector control at the moderate growth light intensity. These results suggest that transgenic expression of AtLQY1 in Synechocystis significantly improves PSII photochemical efficiency and overall cell growth.

Introducing an Arabidopsis thaliana Thylakoid Thiol/Disulfide-Modulating Protein Into Synechocystis Increases the Efficiency of Photosystem II Photochemistry

Photosynthetic species are subjected to a variety of environmental stresses, including suboptimal irradiance. In oxygenic photosynthetic organisms, a major effect of high light exposure is damage to the Photosystem II (PSII) reaction-center protein D1. This process even happens under low or moderate light. To cope with photodamage to D1, photosynthetic organisms evolved an intricate PSII repair and reassembly cycle, which requires the participation of different auxiliary proteins, including thiol/disulfide-modulating proteins. Most of these auxiliary proteins exist ubiquitously in oxygenic photosynthetic organisms. Due to differences in mobility and environmental conditions, land plants are subject to more extensive high light stress than algae and cyanobacteria. Therefore, land plants evolved additional thiol/disulfide-modulating proteins, such as Low Quantum Yield of PSII 1 (LQY1), to aid in the repair and reassembly cycle of PSII. In this study, we introduced an Arabidopsis thaliana homolog of LQY1 (AtLQY1) into the cyanobacterium Synechocystis sp. PCC6803 and performed a series of biochemical and physiological assays on AtLQY1-expressing Synechocystis. At a moderate growth light intensity (50 µmol photons m-2 s-1), AtLQY1-expressing Synechocystis was found to have significantly higher F v /F m , and lower nonphotochemical quenching and reactive oxygen species levels than the empty-vector control, which is opposite from the loss-of-function Atlqy1 mutant phenotype. Light response curve analysis of PSII operating efficiency and electron transport rate showed that AtLQY1-expressing Synechocystis also outperform the empty-vector control under higher light intensities. The increases in F v /F m , PSII operating efficiency, and PSII electron transport rate in AtLQY1-expressing Synechocystis under such growth conditions most likely come from an increased amount of PSII, because the level of D1 protein was found to be higher in AtLQY1-expressing Synechocystis. These results suggest that introducing AtLQY1 is beneficial to Synechocystis.

A New Light on Photosystem II Maintenance in Oxygenic Photosynthesis

Life on earth is sustained by oxygenic photosynthesis, a process that converts solar energy, carbon dioxide, and water into chemical energy and biomass. Sunlight is essential for growth and productivity of photosynthetic organisms. However, exposure to an excessive amount of light adversely affects fitness due to photooxidative damage to the photosynthetic machinery, primarily to the reaction center of the oxygen-evolving photosystem II (PSII). Photosynthetic organisms have evolved diverse photoprotective and adaptive strategies to avoid, alleviate, and repair PSII damage caused by high-irradiance or fluctuating light. Rapid and harmless dissipation of excess absorbed light within antenna as heat, which is measured by chlorophyll fluorescence as non-photochemical quenching (NPQ), constitutes one of the most efficient protective strategies. In parallel, an elaborate repair system represents another efficient strategy to maintain PSII reaction centers in active states. This article reviews both the reaction center-based strategy for robust repair of photodamaged PSII and the antenna-based strategy for swift control of PSII light-harvesting (NPQ). We discuss evolutionarily and mechanistically diverse strategies used by photosynthetic organisms to maintain PSII function for growth and productivity under static high-irradiance light or fluctuating light environments. Knowledge of mechanisms underlying PSII maintenance would facilitate bioengineering photosynthesis to enhance agricultural productivity and sustainability to feed a growing world population amidst climate change.

A chloroplast thylakoid lumen protein is required for proper photosynthetic acclimation of plants under fluctuating light environments

Despite our increasingly sophisticated understanding of mechanisms ensuring efficient photosynthesis under laboratory-controlled light conditions, less is known about the regulation of photosynthesis under fluctuating light. This is important because-in nature-photosynthetic organisms experience rapid and extreme changes in sunlight, potentially causing deleterious effects on photosynthetic efficiency and productivity. Here we report that the chloroplast thylakoid lumenal protein MAINTENANCE OF PHOTOSYSTEM II UNDER HIGH LIGHT 2 (MPH2; encoded by At4g02530) is required for growth acclimation of Arabidopsis thaliana plants under controlled photoinhibitory light and fluctuating light environments. Evidence is presented that mph2 mutant light stress susceptibility results from a defect in photosystem II (PSII) repair, and our results are consistent with the hypothesis that MPH2 is involved in disassembling monomeric complexes during regeneration of dimeric functional PSII supercomplexes. Moreover, mph2-and previously characterized PSII repair-defective mutants-exhibited reduced growth under fluctuating light conditions, while PSII photoprotection-impaired mutants did not. These findings suggest that repair is not only required for PSII maintenance under static high-irradiance light conditions but is also a regulatory mechanism facilitating photosynthetic adaptation under fluctuating light environments. This work has implications for improvement of agricultural plant productivity through engineering PSII repair.

An Organelle RNA Recognition Motif Protein Is Required for Photosystem II Subunit psbF Transcript Editing

Loss-of-function mutations in ORGANELLE RNA RECOGNITION MOTIF PROTEIN6 (ORRM6) result in the near absence of RNA editing of psbF-C77 and the reduction in accD-C794 editing in Arabidopsis (Arabidopsis thaliana). The orrm6 mutants have decreased levels of photosystem II (PSII) proteins, especially PsbF, lower PSII activity, pale green pigmentation, smaller leaf and plant sizes, and retarded growth. Stable expression of ORRM6 rescues the orrm6 editing defects and mutant phenotype. Unlike ORRM1, the other known ORRM plastid editing factor, ORRM6, does not contain RNA editing interacting protein/multiple organellar RNA editing factor (RIP/MORF) boxes, which are required for ORRM1 to interact with site-specific pentatricopeptide repeat protein editing factors. ORRM6 interacts with RIP1/MORF8, RIP2/MORF2, and RIP9/MORF9, known components of RNA editosomes. While some plastid RRM proteins are involved in other forms of RNA processing and translation, the primary function of ORRM6 is evidently to mediate psbF-C77 editing, like the essential site-specific pentatricopeptide repeat protein LOW PSII ACCUMULATION66. Stable expression in the orrm6 mutants of a nucleus-encoded, plastid-targeted PsbF protein from a psbF gene carrying a T at nucleotide 77 significantly increases leaf and plant sizes, chlorophyll content, and PSII activity. These transformants demonstrate that plastid RNA editing can be bypassed through the expression of nucleus-encoded, edited forms of plastid genes.

Chloroplastic iron-sulfur scaffold protein NFU3 is essential to overall plant fitness

A previous study showed that Nitrogen-Fixing-subunit-U-type protein NFU3 may act an iron-sulfur scaffold protein in the assembly and transfer of 4Fe-4S and 3Fe-4S clusters in the chloroplast. Examples of 4Fe-4S and 3Fe-4S-requiring proteins and complexes include Photosystem I (PSI), NAD(P)H dehydrogenase, and ferredoxin-dependent glutamine oxoglutarate aminotransferases. In this paper, the authors provided additional evidence for the role of NFU3 in 4Fe-4S and 3Fe-4S cluster assembly and transfer, as well as its role in overall plant fitness. Confocal microscopic analysis of the fluorescently-tagged NFU3 protein confirmed the chloroplast localization of the NFU3 protein. Detailed analysis of chlorophyll fluorescence data revealed that a substantial increase in minimal fluorescence is the primary contributor to the decrease in PSII maximum photochemical efficiency observed in the nfu3 mutants. The substantial increase in minimal fluorescence in the nfu3 mutants is probably the result of an impaired PSI function, blockage of electron flow from PSII to PSI, and over-accumulation of reduced plastoquinone at the acceptor side of PSII. Analyses of seed morphology and germination showed that NFU3 is essential to seed development and germination, in addition to plant growth, development, and flowering. In summary, NFU3 has wide-ranging effects on many biologic processes and is therefore important to overall plant fitness. NFU3 may exert these effects by modulating the availability of 4Fe-4S and 3Fe-4S clusters to 4Fe-4S and 3Fe-4S-requiring proteins and complexes involved in various biologic processes.

A Nitrogen-Fixing Subunit Essential for Accumulating 4Fe-4S-Containing Photosystem I Core Proteins

Nitrogen-fixation-subunit-U (NFU)-type proteins have been shown to be involved in the biogenesis of iron-sulfur clusters. We investigated the molecular function of a chloroplastic NFU-type iron-sulfur scaffold protein, NFU3, in Arabidopsis (Arabidopsis thaliana) using genetics approaches. Loss-of-function mutations in the NFU3 gene caused yellow pigmentation in leaves, reductions in plant size, leaf size, and growth rate, delay in flowering and seeding, and decreases in seed production. Biochemical and physiological analyses indicated that these defects are due to the substantial reductions in the abundances of 4Fe-4S-containing photosystem I (PSI) core subunits PsaA (where Psa stands for PSI), PsaB, and PsaC and a nearly complete loss of PSI activity. In addition to the substantial decreases in the amounts of PSI core proteins, the content of 3Fe-4S-containing ferredoxin-dependent glutamine oxoglutarate aminotransferases declined significantly in the nfu3 mutants. Furthermore, the absorption spectrum of the recombinant NFU3 protein showed features characteristic of 4Fe-4S and 3Fe-4S clusters, and the in vitro reconstitution experiment indicated an iron-sulfur scaffold function of NFU3. These data demonstrate that NFU3 is involved in the assembly and transfer of 4Fe-4S and 3Fe-4S clusters and that NFU3 is required for the accumulation of 4Fe-4S- and 3Fe-4S-containing proteins, especially 4Fe-4S-containing PSI core subunits, in the Arabidopsis chloroplast.

The DnaJ-Like Zinc Finger Domain Protein PSA2 Affects Light Acclimation and Chloroplast Development in Arabidopsis thaliana

The biosynthesis of chlorophylls and carotenoids and the assembly of thylakoid membranes are critical for the photoautotrophic growth of plants. Different factors are involved in these two processes. In recent years, members of the DnaJ-like zinc finger domain proteins have been found to take part in the biogenesis and/or the maintenance of plastids. One member of this family of proteins, PSA2, was recently found to localize to the thylakoid lumen and regulate the accumulation of photosystem I. In this study, we report that the silencing of PSA2 in Arabidopsis thaliana resulted in variegated leaves and retarded growth. Although both chlorophylls and total carotenoids decreased in the psa2 mutant, violaxanthin, and zeaxanthin accumulated in the mutant seedlings grown under growth condition. Lower levels of non-photochemical quenching and electron transport rate were also found in the psa2 mutant seedlings under growth condition compared with those of the wild-type plants, indicating an impaired capability to acclimate to normal light irradiance when PSA2 was silenced. Moreover, we also observed an abnormal assembly of grana thylakoids and poorly developed stroma thylakoids in psa2 chloroplasts. Taken together, our results demonstrate that PSA2 is a member of the DnaJ-like zinc finger domain protein family that affects light acclimation and chloroplast development.

Identification and Roles of Photosystem II Assembly, Stability, and Repair Factors in Arabidopsis

Photosystem II (PSII) is a multi-component pigment-protein complex that is responsible for water splitting, oxygen evolution, and plastoquinone reduction. Components of PSII can be classified into core proteins, low-molecular-mass proteins, extrinsic oxygen-evolving complex (OEC) proteins, and light-harvesting complex II proteins. In addition to these PSII subunits, more than 60 auxiliary proteins, enzymes, or components of thylakoid protein trafficking/targeting systems have been discovered to be directly or indirectly involved in de novo assembly and/or the repair and reassembly cycle of PSII. For example, components of thylakoid-protein-targeting complexes and the chloroplast-vesicle-transport system were found to deliver PSII subunits to thylakoid membranes. Various auxiliary proteins, such as PsbP-like (Psb stands for PSII) and light-harvesting complex-like proteins, atypical short-chain dehydrogenase/reductase family proteins, and tetratricopeptide repeat proteins, were discovered to assist the de novo assembly and stability of PSII and the repair and reassembly cycle of PSII. Furthermore, a series of enzymes were discovered to catalyze important enzymatic steps, such as C-terminal processing of the D1 protein, thiol/disulfide-modulation, peptidylprolyl isomerization, phosphorylation and dephosphorylation of PSII core and antenna proteins, and degradation of photodamaged PSII proteins. This review focuses on the current knowledge of the identities and molecular functions of different types of proteins that influence the assembly, stability, and repair of PSII in the higher plant Arabidopsis thaliana.

Genome-Wide Identification of QTL for Seed Yield and Yield-Related Traits and Construction of a High-Density Consensus Map for QTL Comparison in Brassica napus

Seed yield (SY) is the most important trait in rapeseed, is determined by multiple seed yield-related traits (SYRTs) and is also easily subject to environmental influence. Many quantitative trait loci (QTLs) for SY and SYRTs have been reported in Brassica napus; however, no studies have focused on seven agronomic traits simultaneously affecting SY. Genome-wide QTL analysis for SY and seven SYRTs in eight environments was conducted in a doubled haploid population containing 348 lines. Totally, 18 and 208 QTLs for SY and SYRTs were observed, respectively, and then these QTLs were integrated into 144 consensus QTLs using a meta-analysis. Three major QTLs for SY were observed, including cqSY-C6-2 and cqSY-C6-3 that were expressed stably in winter cultivation area for 3 years and cqSY-A2-2 only expressed in spring rapeseed area. Trait-by-trait meta-analysis revealed that the 144 consensus QTLs were integrated into 72 pleiotropic unique QTLs. Among them, all the unique QTLs affected SY, except for uq.A6-1, including uq.A2-3, uq.C1-2, uq.C1-3, uq.C6-1, uq.C6-5, and uq.C6-6 could also affect more than two SYRTs. According to the constructed high-density consensus map and QTL comparison from literatures, 36 QTLs from five populations were co-localized with QTLs identified in this study. In addition, 13 orthologous genes were observed, including five each gene for SY and thousand seed weight, and one gene each for biomass yield, branch height, and plant height. The genomic information of these QTLs will be valuable in hybrid cultivar breeding and in analyzing QTL expression in different environments.

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

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

Silencing of the tomato sugar partitioning affecting protein (SPA) modifies sink strength through a shift in leaf sugar metabolism

Limitations in our understanding about the mechanisms that underlie source-sink assimilate partitioning are increasingly becoming a major hurdle for crop yield enhancement via metabolic engineering. By means of a comprehensive approach, this work reports the functional characterization of a DnaJ chaperone related-protein (named as SPA; sugar partition-affecting) that is involved in assimilate partitioning in tomato plants. SPA protein was found to be targeted to the chloroplast thylakoid membranes. SPA-RNAi tomato plants produced more and heavier fruits compared with controls, thus resulting in a considerable increment in harvest index. The transgenic plants also displayed increased pigment levels and reduced sucrose, glucose and fructose contents in leaves. Detailed metabolic and enzymatic activities analyses showed that sugar phosphate intermediates were increased while the activity of phosphoglucomutase, sugar kinases and invertases was reduced in the photosynthetic organs of the silenced plants. These changes would be anticipated to promote carbon export from foliar tissues. The combined results suggested that the tomato SPA protein plays an important role in plastid metabolism and mediates the source-sink relationships by affecting the rate of carbon translocation to fruits.

HYPERSENSITIVE TO HIGH LIGHT1 interacts with LOW QUANTUM YIELD OF PHOTOSYSTEM II1 and functions in protection of photosystem II from photodamage in Arabidopsis

Under high-irradiance conditions, plants must efficiently protect photosystem II (PSII) from damage. In this study, we demonstrate that the chloroplast protein HYPERSENSITIVE TO HIGH LIGHT1 (HHL1) is expressed in response to high light and functions in protecting PSII against photodamage. Arabidopsis thaliana hhl1 mutants show hypersensitivity to high light, drastically decreased PSII photosynthetic activity, higher nonphotochemical quenching activity, a faster xanthophyll cycle, and increased accumulation of reactive oxygen species following high-light exposure. Moreover, HHL1 deficiency accelerated the degradation of PSII core subunits under high light, decreasing the accumulation of PSII core subunits and PSII-light-harvesting complex II supercomplex. HHL1 primarily localizes in the stroma-exposed thylakoid membranes and associates with the PSII core monomer complex through direct interaction with PSII core proteins CP43 and CP47. Interestingly, HHL1 also directly interacts, in vivo and in vitro, with LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), which functions in the repair and reassembly of PSII. Furthermore, the hhl1 lqy1 double mutants show increased photosensitivity compared with single mutants. Taken together, these results suggest that HHL1 forms a complex with LQY1 and participates in photodamage repair of PSII under high light.

Operation of trans-thylakoid thiol-metabolizing pathways in photosynthesis

Thiol oxidation to disulfides and the reverse reaction, i.e., disulfide reduction to free thiols, are under the control of catalysts in vivo. Enzymatically assisted thiol-disulfide chemistry is required for the biogenesis of all energy-transducing membrane systems. However, until recently, this had only been demonstrated for the bacterial plasma membrane. Long considered to be vacant, the thylakoid lumen has now moved to the forefront of photosynthesis research with the realization that its proteome is far more complicated than initially anticipated. Several lumenal proteins are known to be disulfide bonded in Arabidopsis, highlighting the importance of sulfhydryl oxidation in the thylakoid lumen. While disulfide reduction in the plastid stroma is known to activate several enzymatic activities, it appears that it is the reverse reaction, i.e., thiol oxidation that is required for the activity of several lumen-resident proteins. This paradigm for redox regulation in the thylakoid lumen has opened a new frontier for research in the field of photosynthesis. Of particular significance in this context is the discovery of trans-thylakoid redox pathways controlling disulfide bond formation and reduction, which are required for photosynthesis.