Structural insights into photosynthetic cyclic electron transport

ER-plastid contact sites as molecular crossroads for plastid lipid biosynthesis

Membrane contact sites are specialized regions where organelle membranes are in close proximity, enabling lipid transfer while preserving membrane identity. In plants, ER‒chloroplast contact sites are critical for maintaining glycerolipid homeostasis. This review examines the lipid-modifying and lipid-transfer proteins/complexes involved in these processes. Key proteins at these sites, including components of the TGD and VAP27‒ORP2A complexes, as well as Sec14 proteins, facilitate lipid exchange. Additionally, the roles of lipid-modifying proteins at these contact sites are discussed. Despite significant progress, further research is needed to identify additional proteins, investigate ER‒chloroplast dynamics under stress and explore ER contact sites in non-chloroplast plastids.

Supplementary Calcium Overcomes Nocturnal Chilling-Induced Carbon Source-Sink Limitations of Cyclic Electron Transport in Peanuts

'Calcium (Ca2+) priming' is an effective strategy to restore efficient carbon assimilation with undergoing unfavourable cold stress (day/night: 25°C/8°C). However, it is unclear how exogenous calcium strengthens the cyclic electron transfer (CET) to attain optimal carbon flux. To assess the nutrient fortification role of Ca2+ (15 mM) in facilitating this process for peanuts, we added antimycin (AA, 100 μM) and rotenone (R, 100 μM) as specific inhibitors. Our results revealed that inhibiting CET caused a negative effect on photosynthesis. The Ca2+ treatment accelerated the turnover of non-structural carbohydrates, and linear electron carriers while balancing the photosystem I (PSI) bilateral redox potential. The treatment also strengthened the PROTON GRADIENT REGULATION5 (PGR5)/PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) and the NADH dehydrogenase-like (NDH)-mediated CET, with plausible crosstalk between thioredoxin (Trx) system and Ca2+ signalling, to regulate chloroplast redox homoeostasis. Specifically, exogenous Ca2+ strengthened the PGR5/PGRL1-mediated CET by providing sufficient ATP and adequate photoprotection during the long-term exposure; the NDH-mediated CET served to alleviate limitations on the PSI acceptor side by translocating protons. This study demonstrated the effectiveness of harnessing optimal nutrient supply, in the form of foliar Ca2+-based sprays to strengthen the eco-physiological resilience of peanuts against cold stress.

Loss of the cytochrome b6f subunit PetN destabilizes the complex and severely impairs state transitions in Anabaena variabilis

The cytochrome b6f complex (Cyt b6f) plays pivotal roles in both linear and cyclic electron transport of oxygenic photosynthesis in plants and cyanobacteria. The 4 large subunits of Cyt b6f are responsible for organizing the electron transfer chain within Cyt b6f and have their counterparts in the cytochrome bc1 complex in other bacteria. The 4 small subunits of Cyt b6f are unique to oxygenic photosynthesis, and their functions remain to be elucidated. Here, we report that Cyt b6f was destabilized by the loss of PetN, one of the small subunits, in a petN mutant (ΔpetN) of Anabaena variabilis ATCC 29413 and that the amount of the large subunits of Cyt b6f decreased to 20%-25% of that in the wild type (WT). The oxygen evolution activity of ΔpetN was ∼30% of that from the WT, and the activity could largely be restored by the addition of N,N,N', N'-tetramethyl-p-phenylenediamine (TMPD), which functions as an electron carrier and bypasses Cyt b6f. Both linear and cyclic electron transfer of the mutant became partially insensitive to the Cyt b6f inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone. Although the plastoquinone pool was largely reduced in ΔpetN under normal light conditions, the mutant had a substantially higher PSII/PSI ratio than the WT. State transitions in ΔpetN were abolished, as revealed by 77 K fluorescence spectra and room temperature fluorescence kinetics in the presence of TMPD. Our findings strongly suggest that Cyt b6f is required for state transitions in the cyanobacteria.

Antimycin A induces light hypersensitivity of PSII in the presence of quinone QB-site binding herbicides

Photosynthetic electron transport consists of linear electron flow and 2 cyclic electron flow (CEF) pathways around PSI (CEF-PSI). PROTON GRADIENT REGULATION 5 (PGR5)-dependent CEF-PSI is thought to be the major CEF-PSI pathway and an important regulator of photosynthetic electron transfer. Antimycin A (AA) is commonly recognized as an inhibitor of PGR5-dependent CEF-PSI in photosynthesis. Although previous findings imply that AA may also affect PSII, which does not participate in CEF-PSI, these "secondary effects" tend to be neglected, and AA is often used for inhibition of PGR5-dependent CEF-PSI as if it were a specific inhibitor. Here, we investigated the direct effects of AA on PSII using isolated spinach (Spinacia oleracea) PSII membranes and thylakoid membranes isolated from spinach, Arabidopsis thaliana (wild-type Columbia-0 and PGR5-deficient mutant pgr5hope1), and Chlamydomonas reinhardtii. Measurements of quinone QA- reoxidation kinetics showed that AA directly affects the acceptor side of PSII and inhibits electron transport within PSII. Furthermore, repetitive Fv/Fm measurements revealed that, in the presence of quinone QB-site binding inhibitors, AA treatment results in severe photodamage even from a single-turnover flash. The direct effects of AA on PSII are nonnegligible, and caution is required when using AA as an inhibitor of PGR5-dependent CEF-PSI. Meanwhile, we found that the commercially available compound AA3, which is a component of the AA complex, inhibits PGR5-dependent CEF-PSI without having notable effects on PSII. Thus, we propose that AA3 should be used instead of AA for physiological studies of the PGR5-dependent CEF-PSI.

Variable cyanobacterial death modes caused by ciprofloxacin in the aquatic environment: Prioritizing antibiotic-photosynthetic protein interactions for risk assessment

Antibiotics continuously discharged into the aquatic environment pose threats to phototrophs via high-affinity binding to photosynthetic apparatuses and interfering with their energy metabolism and growth. However, studies attributed the sublethal effects of antibiotics on phototrophs to damaging photosystem (PS) II (PSII) proteins while neglecting PSI proteins as potential targets. Herein, we report that frequently detected ciprofloxacin (CIP) with concentrations of 3-8 μg/L was lethal to Microcystis aeruginosa, the widely distributed phytoplankton in freshwater, via damaging DNA. Besides, CIP damages on different photosynthetic proteins at different exposure levels were evidenced to influence the cyanobacterial death phenotypes. In detail, CIP at 3 μg/L bound to PSII D1 protein exclusively, activating the tricarboxylic acid cycle for energy and proline catabolism. This favored the execution of apoptosis-like regulated cell death (RCD). However, CIP at 8 μg/L exhibited additional binding to the PSI iron-sulfur reaction center, apart from PSII, inducing carbon and arginine starvation. This shifted the RCD from apoptosis-like RCD to mazEF-mediated RCD. Furthermore, microcystin-LR risks were elevated after CIP exposure with enhanced microcystin-LR release and biosynthesis for apoptosis-like and mazEF-mediated RCD, respectively. Thus, the present study underscores the intricate interactions between antibiotics and different photosynthetic apparatuses, which alter antibiotic lethal effects at different exposure levels. This could provide new perspectives on the risk assessment and prediction of antibiotics from the standpoint of chemical-photosynthesis interactions.

The ascorbate peroxidase-related protein: insights into its functioning in Chlamydomonas and Arabidopsis

We review the newly classified ascorbate peroxidase-related (APX-R) proteins, which do not use ascorbate as electron donor to scavenge H2O2. We summarize recent discoveries on the function and the characterization of the APX-R protein of the green unicellular alga Chlamydomonas reinhardtii and the land plant Arabidopsis thaliana. Additionally, we conduct in silico analyses on the conserved MxxM motif, present in most of the APX-R protein in different organisms, which is proposed to bind copper. Based on these analyses, we discuss the similarities between the APX-R and the class III peroxidases.

A pgr5 suppressor screen uncovers two distinct suppression mechanisms and links cytochrome b6f complex stability to PGR5

PROTON GRADIENT REGULATION5 (PGR5) is thought to promote cyclic electron flow, and its deficiency impairs photosynthetic control and increases photosensitivity of photosystem (PS) I, leading to seedling lethality under fluctuating light (FL). By screening for Arabidopsis (Arabidopsis thaliana) suppressor mutations that rescue the seedling lethality of pgr5 plants under FL, we identified a portfolio of mutations in 12 different genes. These mutations affect either PSII function, cytochrome b6f (cyt b6f) assembly, plastocyanin (PC) accumulation, the CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE1 (cFBP1), or its negative regulator ATYPICAL CYS HIS-RICH THIOREDOXIN2 (ACHT2). The characterization of the mutants indicates that the recovery of viability can in most cases be explained by the restoration of PSI donor side limitation, which is caused by reduced electron flow to PSI due to defects in PSII, cyt b6f, or PC. Inactivation of cFBP1 or its negative regulator ACHT2 results in increased levels of the NADH dehydrogenase-like complex. This increased activity may be responsible for suppressing the pgr5 phenotype under FL conditions. Plants that lack both PGR5 and DE-ETIOLATION-INDUCED PROTEIN1 (DEIP1)/NEW TINY ALBINO1 (NTA1), previously thought to be essential for cyt b6f assembly, are viable and accumulate cyt b6f. We suggest that PGR5 can have a negative effect on the cyt b6f complex and that DEIP1/NTA1 can ameliorate this negative effect.

Structure, function, and assembly of PSI in thylakoid membranes of vascular plants

The photosynthetic apparatus is formed by thylakoid membrane-embedded multiprotein complexes that carry out linear electron transport in oxygenic photosynthesis. The machinery is largely conserved from cyanobacteria to land plants, and structure and function of the protein complexes involved are relatively well studied. By contrast, how the machinery is assembled in thylakoid membranes remains poorly understood. The complexes participating in photosynthetic electron transfer are composed of many proteins, pigments, and redox-active cofactors, whose temporally and spatially highly coordinated incorporation is essential to build functional mature complexes. Several proteins, jointly referred to as assembly factors, engage in the biogenesis of these complexes to bring the components together in a step-wise manner, in the right order and time. In this review, we focus on the biogenesis of the terminal protein supercomplex of the photosynthetic electron transport chain, PSI, in vascular plants. We summarize our current knowledge of the assembly process and the factors involved and describe the challenges associated with resolving the assembly pathway in molecular detail.

Chloroplast ATP synthase: From structure to engineering

F-type ATP synthases are extensively researched protein complexes because of their widespread and central role in energy metabolism. Progress in structural biology, proteomics, and molecular biology has also greatly advanced our understanding of the catalytic mechanism, post-translational modifications, and biogenesis of chloroplast ATP synthases. Given their critical role in light-driven ATP generation, tailoring the activity of chloroplast ATP synthases and modeling approaches can be applied to modulate photosynthesis. In the future, advances in genetic manipulation and protein design tools will significantly expand the scope for testing new strategies in engineering light-driven nanomotors.

A Mathematical Model of Diel Activity and Long Time Survival in Phototrophic Mixed-Species Subaerial Biofilms

Subaerial biofilms (SAB) are intricate microbial communities living on terrestrial surfaces, of interest in a variety of contexts including cultural heritage preservation, microbial ecology, biogeochemical cycling, and biotechnology. Here we propose a mathematical model aimed at better understanding the interplay between cyanobacteria and heterotrophic bacteria, common microbial SAB constituents, and their mutual dependence on local environmental conditions. SABs are modeled as thin mixed biofilm-liquid water layers sitting on stone. A system of ordinary differential equations regulates the dynamics of key SAB components: cyanobacteria, heterotrophs, polysaccharides and decayed biomass, as well as cellular levels of organic carbon, nitrogen and energy. These components are interconnected through a network of energetically dominant metabolic pathways, modeled with limitation terms reflecting the impact of biotic and abiotic factors. Daily cylces of temperature, humidity, and light intensity are considered as input model variables that regulate microbial activity by influencing water availability and metabolic kinetics. Relevant physico-chemical processes, including pH regulation, further contribute to a description of the SAB ecology. Numerical simulations explore the dynamics of SABs in a real-world context, revealing distinct daily activity periods shaped by water activity and light availability, as well as longer time scale survivability conditions. Results also suggest that heterotrophs could play a substantial role in decomposing non-volatile carbon compounds and regulating pH, thus influencing the overall composition and stability of the biofilm.

Micronutrient deficiency-induced oxidative stress in plants

Micronutrients like iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), nickel (Ni), and molybdenum (Mo) perform significant roles in the regulation of plant metabolism, growth, and development. Micronutrients, namely Fe, Zn, Cu, Mn, and Ni, are involved in oxidative stress and antioxidant defense as they are cofactors or activators of various antioxidant enzymes, viz., superoxide dismutase (Fe, Cu/Zn, Mn, and Ni), catalase (Fe), and ascorbate peroxidase (Fe). An effort has been made to incorporate recent advances along with classical work done on the micronutrient deficiency-induced oxidative stress and associated antioxidant responses of plants. Deficiency of a micronutrient produces ROS in the cellular compartments. Enzymatic and non-enzymatic antioxidant defense systems are often modulated by micronutrient deficiency to regulate redox balance and scavenge deleterious ROS for the safety of cellular constituents. ROS can strike cellular constituents such as lipids, proteins, and nucleic acids and can destruct cellular membranes and proteins. ROS might act as a signaling molecule and activate the antioxidant proteins by interacting with signaling partners such as respiratory burst oxidase homolog (RBOH), G-proteins, Ca2+, mitogen activated protein kinases (MAPKs), and various transcription factors (TFs). Opinions on probable ROS signaling under micronutrient deficiency have been described in this review. However, further research is required to decipher micronutrient deficiency-induced ROS generation, perception, and associated downstream signaling events, leading to the development of antioxidant responses in plants.

Alien emergent aquatic plants develop better ciprofloxacin tolerance and metabolic capacity than one native submerged species

Antibiotic pollution and biological invasion pose significant risks to freshwater biodiversity and ecosystem health. However, few studies have compared the ecological adaptability and ciprofloxacin (CIPR) degradation potential between alien and native macrophytes. We examined growth, physiological response, and CIPR accumulation, translocation and metabolic abilities of two alien plants (Eichhornia crassipes and Myriophyllum aquaticum) and one native submerged species (Vallisneria natans) exposed to CIPR at 0, 1 and 10 mg/L. We found that E. crassipes and M. aquaticum's growth were unaffected by CIPR while V. natans was significantly hindered under the 10 mg/L treatment. CIPR significantly decreased the maximal quantum yield of PSII, actual quantum yield of PSII and relative electron transfer rate in E. crassipes and V. natans but didn't impact these photosynthetic characteristics in M. aquaticum. All the plants can accumulate, translocate and metabolize CIPR. M. aquaticum and E. crassipes in the 10 mg/L treatment group showed greater CIPR accumulation potential than V. natans indicated by higher CIPR contents in their roots. The oxidative cleavage of the piperazine ring acts as a key pathway for these aquatic plants to metabolize CIPR and the metabolites mainly distributed in plant roots. M. aquaticum and E. crassipes showed a higher production of CIPR metabolites compared to V. natans, with M. aquaticum exhibiting the strongest CIPR metabolic ability, as indicated by the most extensive structural breakdown of CIPR and the largest number of potential metabolic pathways. Taken together, alien species outperformed the native species in ecological adaptability, CIPR accumulation and metabolic capacity. These findings may shed light on the successful invasion mechanisms of alien aquatic species under antibiotic pressure and highlight the potential ecological impacts of alien species, particularly M. aquaticum. Additionally, the interaction of antibiotic contamination and invasion might further challenge the native submerged macrophytes and pose greater risks to freshwater ecosystems.

Transcriptomic analysis reveals the regulatory mechanisms of messenger RNA (mRNA) and long non-coding RNA (lncRNA) in response to waterlogging stress in rye (Secale cereale L.)

BACKGROUND

Waterlogging stress (WS) negatively impacts crop growth and productivity, making it important to understand crop resistance processes and discover useful WS resistance genes. In this study, rye cultivars and wild rye species were subjected to 12-day WS treatment, and the cultivar Secale cereale L. Imperil showed higher tolerance. Whole transcriptome sequencing was performed on this cultivar to identify differentially expressed (DE) messenger RNAs (DE-mRNAs) and long non-coding RNAs (DE-lncRNAs) involved in WS response.

RESULTS

Among the 6 species, Secale cereale L. Imperil showed higher tolerance than wild rye species against WS. The cultivar effectively mitigated oxidative stress, and regulated hydrogen peroxide and superoxide anion. A total of 728 DE-mRNAs and 60 DE-lncRNAs were discovered. Among these, 318 DE-mRNAs and 32 DE-lncRNAs were upregulated, and 410 DE-mRNAs and 28 DE-lncRNAs were downregulated. GO enrichment analysis discovered metabolic processes, cellular processes, and single-organism processes as enriched biological processes (BP). For cellular components (CC), the enriched terms were membrane, membrane part, cell, and cell part. Enriched molecular functions (MF) terms were catalytic activity, binding, and transporter activity. LncRNA and mRNA regulatory processes were mainly related to MAPK signaling pathway-plant, plant hormone signal transduction, phenylpropanoid biosynthesis, anthocyanin biosynthesis, glutathione metabolism, ubiquitin-mediated proteolysis, ABC transporter, Cytochrome b6/f complex, secondary metabolite biosynthesis, and carotenoid biosynthesis pathways. The signalling of ethylene-related pathways was not mainly dependent on AP2/ERF and WRKY transcription factors (TF), but on other factors. Photosynthetic activity was active, and carotenoid levels increased in rye under WS. Sphingolipids, the cytochrome b6/f complex, and glutamate are involved in rye WS response. Sucrose transportation was not significantly inhibited, and sucrose breakdown occurs in rye under WS.

CONCLUSIONS

This study investigated the expression levels and regulatory functions of mRNAs and lncRNAs in 12-day waterlogged rye seedlings. The findings shed light on the genes that play a significant role in rye ability to withstand WS. The findings from this study will serve as a foundation for further investigations into the mRNA and lncRNA WS responses in rye.

Linking the key physiological functions of essential micronutrients to their deficiency symptoms in plants

In this review, we untangle the physiological key functions of the essential micronutrients and link them to the deficiency responses in plants. Knowledge of these responses at the mechanistic level, and the resulting deficiency symptoms, have improved over the last decade and it appears timely to review recent insights for each of them. A proper understanding of the links between function and symptom is indispensable for an accurate and timely identification of nutritional disorders, thereby informing the design and development of sustainable fertilization strategies. Similarly, improved knowledge of the molecular and physiological functions of micronutrients will be important for breeding programmes aiming to develop new crop genotypes with improved nutrient-use efficiency and resilience in the face of changing soil and climate conditions.

Uncovering the photosystem I assembly pathway in land plants

Photosystem I (PSI) is one of two large pigment-protein complexes responsible for converting solar energy into chemical energy in all oxygenic photosynthetic organisms. The PSI supercomplex consists of the PSI core complex and peripheral light-harvesting complex I (LHCI) in eukaryotic photosynthetic organisms. However, how the PSI complex assembles in land plants is unknown. Here we describe PHOTOSYSTEM I BIOGENESIS FACTOR 8 (PBF8), a thylakoid-anchored protein in Arabidopsis thaliana that is required for PSI assembly. PBF8 regulates two key consecutive steps in this process, the building of two assembly intermediates comprising eight or nine subunits, by interacting with PSI core subunits. We identified putative PBF8 orthologues in charophytic algae and land plants but not in Cyanobacteria or Chlorophyta. Our data reveal the major PSI assembly pathway in land plants. Our findings suggest that novel assembly mechanisms evolved during plant terrestrialization to regulate PSI assembly, perhaps as a means to cope with terrestrial environments.

Enhanced Reduction of Ferredoxin in PGR5-Deficient Mutant of Arabidopsis thaliana Stimulated Ferredoxin-Dependent Cyclic Electron Flow around Photosystem I

The molecular entity responsible for catalyzing ferredoxin (Fd)-dependent cyclic electron flow around photosystem I (Fd-CEF) remains unidentified. To reveal the in vivo molecular mechanism of Fd-CEF, evaluating ferredoxin reduction-oxidation kinetics proves to be a reliable indicator of Fd-CEF activity. Recent research has demonstrated that the expression of Fd-CEF activity is contingent upon the oxidation of plastoquinone. Moreover, chloroplast NAD(P)H dehydrogenase does not catalyze Fd-CEF in Arabidopsis thaliana. In this study, we analyzed the impact of reduced Fd on Fd-CEF activity by comparing wild-type and pgr5-deficient mutants (pgr5hope1). PGR5 has been proposed as the mediator of Fd-CEF, and pgr5hope1 exhibited a comparable CO2 assimilation rate and the same reduction-oxidation level of PQ as the wild type. However, P700 oxidation was suppressed with highly reduced Fd in pgr5hope1, unlike in the wild type. As anticipated, the Fd-CEF activity was enhanced in pgr5hope1 compared to the wild type, and its activity further increased with the oxidation of PQ due to the elevated CO2 assimilation rate. This in vivo research clearly demonstrates that the expression of Fd-CEF activity requires not only reduced Fd but also oxidized PQ. Importantly, PGR5 was found to not catalyze Fd-CEF, challenging previous assumptions about its role in this process.

NADH dehydrogenase-like complex L subunit improves salt tolerance by enhancing photosynthetic electron transport

Cyclic electron transport (CET) around photosystem I (PSI) mediated by the NADH dehydrogenase-like (NDH) complex is closely related to plant salt tolerance. However, whether overexpression of a core subunit of the NDH complex affects the photosynthetic electron transport under salt stress is currently unclear. Here, we expressed the NDH complex L subunit (Ndhl) genes ZmNdhl1 and ZmNdhl2 from C4 plant maize (Zea mays) or OsNdhl from C3 plant rice (Oryza sativa) using a constitutive promoter in rice. Transgenic rice lines expressing ZmNdhl1, ZmNdhl2, or OsNdhl displayed enhanced salt tolerance, as indicated by greater plant height, dry weight, and leaf relative water content, as well as lower malondialdehyde content compared to wild-type plants under salt stress. Fluorescence parameters such as post-illumination rise (PIR), the prompt chlorophyll a fluorescence transient (OJIP), modulated 820-nm reflection (MR), and delayed chlorophyll a fluorescence (DF) remained relatively normal in transgenic plants during salt stress. These results indicate that expression of ZmNdhl1, ZmNdhl2, or OsNdhl increases cyclic electron transport activity, slows down damage to linear electron transport, alleviates oxidative damage to the PSI reaction center and plastocyanin, and reduces damage to electron transport on the receptor side of PSI in rice leaves under salt stress. Thus, expression of Ndhl genes from maize or rice improves salt tolerance by enhancing photosynthetic electron transport in rice. Maize and rice Ndhl genes played a similar role in enhancing salinity tolerance and avoiding photosynthetic damage.

Comparative study of the toxicity mechanisms of quinolone antibiotics on soybean seedlings: Insights from molecular docking and transcriptomic analysis

The ecological effects of quinolone antibiotics (QNs) on non-target organisms have received widespread attention. The toxicological mechanisms of three common QNs, that is, enrofloxacin, levofloxacin, and ciprofloxacin, on soybean seedlings were investigated in this study. Enrofloxacin and levofloxacin caused significant growth inhibition, ultrastructural alterations, photosynthetic suppression, and stimulation of the antioxidant system, with levofloxacin exhibiting the strongest toxic effects. Ciprofloxacin (<1 mg·L-1) did not have a significant effect on the soybean seedlings. As the concentrations of enrofloxacin and levofloxacin increased, antioxidant enzyme activities, malondialdehyde content, and hydrogen peroxide levels also increased. Meanwhile, the chlorophyll content and chlorophyll fluorescence parameters decreased, indicating that the plants underwent oxidative stress and photosynthesis was suppressed. The cellular ultrastructure was also disrupted, which was manifested by swollen chloroplasts, increased starch granules, disintegration of plastoglobules, and mitochondrial degradation. The molecular docking results suggested that the QNs have an affinity for soybean target protein receptors (4TOP, 2IUJ, and 1FHF), with levofloxacin having the highest binding energy (-4.97, -3.08, -3.8, respectively). Transcriptomic analysis has shown that genes were upregulated under the enrofloxacin and levofloxacin treatments were mainly involved in ribosome metabolism and processes to synthesize oxidative stress-related proteins. Downregulated genes in the levofloxacin treatment were primarily enriched in photosynthesis-related pathways, indicating that levofloxacin significantly inhibited gene expression for photosynthesis. Genes expression level by quantitative real-time PCR analysis was consistent with the transcriptomic results. This study confirmed the toxic effect of QNs on soybean seedlings, and provided new insights into the environmental risks of antibiotics.

NDH-1L with a truncated NdhM subunit is unstable under stress conditions in cyanobacteria

Cyanobacterial NdhM, an oxygenic photosynthesis-specific NDH-1 subunit, has been found to be essential for the formation of a large complex of NDH-1 (NDH-1L). The cryo-electron microscopic (cryo-EM) structure of NdhM from Thermosynechococcus elongatus showed that the N-terminus of NdhM contains three β-sheets, while two α-helixes are present in the middle and C-terminal part of NdhM. Here, we obtained a mutant of the unicellular cyanobacterium Synechocystis 6803 expressing a C-terminal truncated NdhM subunit designated NdhMΔC. Accumulation and activity of NDH-1 were not affected in NdhMΔC under normal growth conditions. However, the NDH-1 complex with truncated NdhM is unstable under stress. Immunoblot analyses showed that the assembly process of the cyanobacterial NDH-1L hydrophilic arm was not affected in the NdhMΔC mutant even under high temperature. Thus, our results indicate that NdhM can bind to the NDH-1 complex without its C-terminal α-helix, but the interaction is weakened. NDH-1L with truncated NdhM is more prone to dissociation, and this is particularly evident under stress conditions.