Regulation of cyclic and linear electron flow in higher plants

Coordinated metabolic adaptation of Arabidopsis thaliana to high light

In plants, exposure to high light irradiation induces various stress responses, which entail complex metabolic rearrangements. To explore these dynamics, we conducted time‐course experiments spanning 2 min to 72 h with Arabidopsis thaliana under high and control light. Comparative metabolomics, transcriptomics, redox proteomics, and stable isotope labeling on leaf rosettes identified a series of synchronous and successive responses that provide a deeper insight into well‐orchestrated mechanisms contributing to high‐light acclimation. We observed transient transcriptome downregulation related to light harvesting and electron flow before the profound remodeling of the photosynthetic apparatus. Throughout the entire time course, redox homeostasis is tightly balanced between downregulation of production and enhanced transformation of NADPH accompanied by redistribution of reducing equivalents across several subcellular compartments. In both light conditions, C4 acids such as malate and fumarate are produced via anaplerosis. In carbon units, their accumulation in vacuoles surpasses plastidic levels of starch and intensifies notably under high light. In parallel, citrate synthesis from pyruvate is significantly hindered diurnally. Isotopic labeling in 2‐oxoglutarate and glutamate suggests a moderate de novo synthesis of C5 acids from a vacuolar citrate reservoir during the light phase while they are largely renewed during the night. In the absence of a diurnal clockwise flow through the tricarboxylic acid (TCA) cycle, increased oxidation of photorespiratory glycine takes over as a source of reductants to fuel mitochondrial ATP production. These findings, along with previous research, contribute to a model integrating redox balance and linking increased carbon assimilation and nitrogen metabolism, especially in the context of an incomplete TCA cycle.

The mechanisms of photoinhibition and repair in plants under high light conditions and interplay with abiotic stressors

This review comprehensively examines the phenomenon of photoinhibition in plants, focusing mainly on the intricate relationship between photodamage and photosystem II (PSII) repair and the role of PSII extrinsic proteins and protein phosphorylation in these processes. In natural environments, photoinhibition occurs together with a suite of concurrent stress factors, including extreme temperatures, drought and salinization. Photoinhibition, primarily caused by high irradiance, results in a critical imbalance between the rate of PSII photodamage and its repair. Central to this process is the generation of reactive oxygen species (ROS), which not only impair the photosynthetic apparatus first PSII but also play a signalling role in chloroplasts and other cellulular structures. ROS generated under stress conditions inhibit the repair of photodamaged PSII by suppressing D1 protein synthesis and affecting PSII protein phosphorylation. Furthermore, this review considers how environmental stressors exacerbate PSII damage by interfering with PSII repair primarily by reducing de novo protein synthesis. In addition to causing direct damage, these stressors also contribute to ROS production by restricting CO2 fixation, which also reduces the intensity of protein synthesis. This knowledge has significant implications for agricultural practices and crop improvement under stressful conditions.

Spliceosomal complex components are critical for adjusting the C:N balance during high-light acclimation

Plant acclimation to an ever-changing environment is decisive for growth, reproduction, and survival. Light availability limits biomass production on both ends of the intensity spectrum. Therefore, the adjustment of plant metabolism is central to high-light (HL) acclimation, and the accumulation of photoprotective anthocyanins is commonly observed. However, mechanisms and factors regulating the HL acclimation response are less clear. Two Arabidopsis mutants of spliceosome components exhibiting a pronounced anthocyanin overaccumulation in HL were isolated from a forward genetic screen for new factors crucial for plant acclimation. Time-resolved physiological, transcriptome, and metabolome analysis revealed a vital function of the spliceosome components for rapidly adjusting gene expression and metabolism. Deficiency of INCREASED LEVEL OF POLYPLOIDY1 (ILP1), NTC-RELATED PROTEIN1 (NTR1), and PLEIOTROPIC REGULATORY LOCUS1 (PRL1) resulted in a marked overaccumulation of carbohydrates and strongly diminished amino acid biosynthesis in HL. While not generally limited in N-assimilation, ilp1, ntr1, and prl1 showed higher glutamate levels and reduced amino acid biosynthesis in HL. The comprehensive analysis reveals a function of the spliceosome components in the conditional regulation of the carbon:nitrogen balance and the accumulation of anthocyanins during HL acclimation. The importance of gene expression, metabolic regulation, and re-direction of carbon towards anthocyanin biosynthesis for HL acclimation are discussed.

Differences in gas exchange, chlorophyll fluorescence, and modulated reflection of light at 820 nm between two rhododendron cultivars under aluminum stress conditions

Aluminum (Al) toxicity is an important factor restricting the normal growth of plants in acidic soil. Rhododendron (Ericaceae) can grow relatively well in acidic soil. To uncover the adaptive mechanisms of photosynthesis under Al stress, the influence of Al stress on the photosynthetic activities of Al-sensitive (Baijinpao) and Al-resistant (Kangnaixin) rhododendron cultivars was examined by measuring gas exchange, chlorophyll fluorescence, and the modulated reflection of light at 820 nm. Under Al stress conditions, the net photosynthetic rate and stomatal conductance of the rhododendron leaves decreased, whereas the intercellular CO2 concentration increased. The Al stress treatment damaged the oxygen-evolving complex of the rhododendron seedlings, while also inhibiting electron transport on the photosystem II (PSII) donor side. In addition, the exposure to Al stress restricted the oxidation of plastocyanin (PC) and the photosystem I (PSI) reaction center (P700) and led to the re-reduction of PC+ and P700+. The comparison with Kangnaixin revealed an increase in the PSII connectivity in Baijinpao. Additionally, the donor-side electron transport efficiency was more inhibited and the overall activity of PSII, PSI, and the intersystem electron transport chain decreased more extensively in Baijinpao than in Kangnaixin. On the basis of the study findings, we concluded that Al stress adversely affects photosynthesis in rhododendron seedlings by significantly decreasing the activity of PSII and PSI. Under Al stress, Kangnaixin showed stronger tolerance compared with Baijinpao.

Regulatory mechanisms used by ZmMYB39 to enhance drought tolerance in maize (Zea mays) seedlings

Drought is a significant abiotic stressor that limits maize (Zea mays L.) growth and development. Thus, enhancing drought tolerance is critical for promoting maize production. Our findings demonstrated that ZmMYB39 is an MYB transcription factor with transcriptional activation activity. Drought stress experiments involving ZmMYB39 overexpression and knockout lines indicated that ZmMYB39 positively regulated drought stress tolerance in maize. DAP-Seq, EMSA, dual-LUC, and RT-qPCR provided initial insights into the molecular regulatory mechanisms by which ZmMYB39 enhances drought tolerance in maize. ZmMYB39 directly promoted the expression of ZmP5CS1, ZmPOX1, ZmSOD2, ZmRD22, ZmNAC49, and ZmDREB2A, which are involved in stress resistance. ZmMYB39 enhanced drought tolerance by interacting with and promoting the expression of ZmFNR1, ZmHSP20, and ZmDOF6. Our study offers a theoretical basis for understanding the molecular regulatory networks involved in maize drought stress response. Furthermore, ZmMYB39 serves as a valuable genetic resource for breeding drought-resistant maize.

Photosynthetic adaptation strategies in peppers under continuous lighting: insights into photosystem protection

Energy efficient lighting strategies have received increased interest from controlled environment producers. Long photoperiods (up to 24 h - continuous lighting (CL)) of lower light intensities could be used to achieve the desired daily light integral (DLI) with lower installed light capacity/capital costs and low electricity costs in regions with low night electricity prices. However, plants grown under CL tend to have higher carbohydrate and reactive oxygen species (ROS) levels which may lead to leaf chlorosis and down-regulation of photosynthesis. We hypothesize that the use of dynamic CL using a spectral change and/or light intensity change between day and night can negate CL-injury. In this experiment we set out to assess the impact of CL on pepper plants by subjecting them to white light during the day and up to 150 µmol m-2 s-1 of monochromatic blue light at night while controlling the DLI at the same level. Plants grown under all CL treatments had similar cumulative fruit number and weight compared to the 16h control indicating no reduction in production. Plants grown under CL had higher carbohydrate levels and ROS-scavenging capacity than plants grown under the 16h control. Conversely, the amount of photosynthetic pigment decreased with increasing nighttime blue light intensity. The maximum quantum yield of photosystem II (Fv/Fm), a metric often used to measure stress, was unaffected by light treatments. However, when light-adapted, the operating efficiency of photosystem II (ΦPSII) decreased and non-photochemical quenching (NPQ) increased with increasing nighttime blue light intensity. This suggests that both acclimated and instantaneous photochemistry during CL can be altered and is dependent on the nighttime light intensity. Furthermore, light-adapted chlorophyll fluorescence measurements may be more adept at detecting altered photochemical states than the conventional stress metric using dark-adapted measurements.

The cytochrome b6f complex: plastoquinol oxidation and regulation of electron transport in chloroplasts

In oxygenic photosynthetic systems, the cytochrome b6f (Cytb6f) complex (plastoquinol:plastocyanin oxidoreductase) is a heart of the hub that provides connectivity between photosystems (PS) II and I. In this review, the structure and function of the Cytb6f complex are briefly outlined, being focused on the mechanisms of a bifurcated (two-electron) oxidation of plastoquinol (PQH2). In plant chloroplasts, under a wide range of experimental conditions (pH and temperature), a diffusion of PQH2 from PSII to the Cytb6f does not limit the intersystem electron transport. The overall rate of PQH2 turnover is determined mainly by the first step of the bifurcated oxidation of PQH2 at the catalytic site Qo, i.e., the reaction of electron transfer from PQH2 to the Fe2S2 cluster of the high-potential Rieske iron-sulfur protein (ISP). This point has been supported by the quantum chemical analysis of PQH2 oxidation within the framework of a model system including the Fe2S2 cluster of the ISP and surrounding amino acids, the low-potential heme b6L, Glu78 and 2,3,5-trimethylbenzoquinol (the tail-less analog of PQH2). Other structure-function relationships and mechanisms of electron transport regulation of oxygenic photosynthesis associated with the Cytb6f complex are briefly outlined: pH-dependent control of the intersystem electron transport and the regulatory balance between the operation of linear and cyclic electron transfer chains.

Dysfunction of Chloroplast Protease Activity Mitigates pgr5 Phenotype in the Green Algae Chlamydomonas reinhardtii

Researchers have described protection mechanisms against the photoinhibition of photosystems under strong-light stress. Cyclic Electron Flow (CEF) mitigates electron acceptor-side limitation, and thus contributes to Photosystem I (PSI) protection. Chloroplast protease removes damaged protein to assist with protein turn over, which contributes to the quality control of Photosystem II (PSII). The PGR5 protein is involved in PGR5-dependent CEF. The FTSH protein is a chloroplast protease which effectively degrades the damaged PSII reaction center subunit, D1 protein. To investigate how the PSI photoinhibition phenotype in pgr5 would be affected by adding the ftsh mutation, we generated double-mutant pgr5ftsh via crossing, and its phenotype was characterized in the green algae Chlamydomonas reinhardtii. The cells underwent high-light incubation as well as low-light incubation after high-light incubation. The time course of Fv/Fm values in pgr5ftsh showed the same phenotype with ftsh1-1. The amplitude of light-induced P700 photo-oxidation absorbance change was measured. The amplitude was maintained at a low value in the control and pgr5ftsh during high-light incubation, but was continuously decreased in pgr5. During the low-light incubation after high-light incubation, amplitude was more rapidly recovered in pgr5ftsh than pgr5. We concluded that the PSI photoinhibition by the pgr5 mutation is mitigated by an additional ftsh1-1 mutation, in which plastoquinone pool would be less reduced due to damaged PSII accumulation.

Comparative transcriptomics of the chilling stress response in two Asian mangrove species, Bruguiera gymnorhiza and Rhizophora apiculata

Low temperatures largely determine the geographic limits of plant species by reducing survival and growth. Inter-specific differences in the geographic distribution of mangrove species have been associated with cold tolerance, with exclusively tropical species being highly cold-sensitive and subtropical species being relatively cold-tolerant. To identify species-specific adaptations to low temperatures, we compared the chilling stress response of two widespread Indo-West Pacific mangrove species from Rhizophoraceae with differing latitudinal range limits-Bruguiera gymnorhiza (L.) Lam. ex Savigny (subtropical range limit) and Rhizophora apiculata Blume (tropical range limit). For both species, we measured the maximum photochemical efficiency of photosystem II (Fv/Fm) as a proxy for the physiological condition of the plants and examined gene expression profiles during chilling at 15 and 5 °C. At 15 °C, B. gymnorhiza maintained a significantly higher Fv/Fm than R. apiculata. However, at 5 °C, both species displayed equivalent Fv/Fm values. Thus, species-specific differences in chilling tolerance were only found at 15 °C, and both species were sensitive to chilling at 5 °C. At 15 °C, B. gymnorhiza downregulated genes related to the light reactions of photosynthesis and upregulated a gene involved in cyclic electron flow regulation, whereas R. apiculata downregulated more RuBisCo-related genes. At 5 °C, both species repressed genes related to CO2 assimilation. The downregulation of genes related to light absorption and upregulation of genes related to cyclic electron flow regulation are photoprotective mechanisms that likely contributed to the greater photosystem II photochemical efficiency of B. gymnorhiza at 15 °C. The results of this study provide evidence that the distributional range limits and potentially the expansion rates of plant species are associated with differences in the regulation of photosynthesis and photoprotective mechanisms under low temperatures.

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.

Optimizing the electron transport chain to sustainably improve photosynthesis

Genetically improving photosynthesis is a key strategy to boosting crop production to meet the rising demand for food and fuel by a rapidly growing global population in a warming climate. Many components of the photosynthetic apparatus have been targeted for genetic modification for improving photosynthesis. Successful translation of these modifications into increased plant productivity in fluctuating environments will depend on whether the electron transport chain (ETC) can support the increased electron transport rate without risking overreduction and photodamage. At present atmospheric conditions, the ETC appears suboptimal and will likely need to be modified to support proposed photosynthetic improvements and to maintain energy balance. Here, I derive photochemical equations to quantify the transport capacity and the corresponding reduction level based on the kinetics of redox reactions along the ETC. Using these theoretical equations and measurements from diverse C3/C4 species across environments, I identify several strategies that can simultaneously increase the transport capacity and decrease the reduction level of the ETC. These strategies include increasing the abundances of reaction centers, cytochrome b6f complexes, and mobile electron carriers, improving their redox kinetics, and decreasing the fraction of secondary quinone-nonreducing photosystem II reaction centers. I also shed light on several previously unexplained experimental findings regarding the physiological impacts of the abundances of the cytochrome b6f complex and plastoquinone. The model developed, and the insights generated from it facilitate the development of sustainable photosynthetic systems for greater crop yields.

Adjustment of light-responsive NADP dynamics in chloroplasts by stromal pH

Cyclic electron transfer (CET) predominates when NADP+ is at basal levels, early in photosynthetic induction; however, the mechanism underlying the subsequent supply of NADP+ to fully drive steady-state linear electron transfer remains unclear. Here, we investigated whether CET is involved in de novo NADP+ supply in Arabidopsis thaliana and measured chloroplastic NADP dynamics to evaluate responsiveness to variable light, photochemical inhibitors, darkness, and CET activity. The sum of oxidized and reduced forms shows that levels of NADP and NAD increase and decrease, respectively, in response to light; levels of NADP and NAD decrease and increase in the dark, respectively. Moreover, consistent with the pH change in the stroma, the pH preference of chloroplast NAD+ phosphorylation and NADP+ dephosphorylation is alkaline and weakly acidic, respectively. Furthermore, CET is correlated with upregulation of light-responsive NADP level increases and downregulation of dark-responsive NADP level reductions. These findings are consistent with CET helping to regulate NADP pool size via stromal pH regulation under fluctuating light conditions.

Artificial photosynthetic cells with biotic-abiotic hybrid energy modules for customized CO2 conversion

Programmable artificial photosynthetic cell is the ultimate goal for mimicking natural photosynthesis, offering tunable product selectivity via reductase selection toward device integration. However, this concept is limited by the capacity of regenerating the multiple cofactors that hold the key to various reductases. Here, we report the design of artificial photosynthetic cells using biotic-abiotic thylakoid-CdTe as hybrid energy modules. The rational integration of thylakoid with CdTe quantum dots substantially enhances the regeneration of bioactive NADPH, NADH and ATP cofactors without external supplements by promoting proton-coupled electron transfer. Particularly, this approach turns thylakoid highly active for NADH regeneration, providing a more versatile platform for programming artificial photosynthetic cells. Such artificial photosynthetic cells can be programmed by coupling with diverse reductases, such as formate dehydrogenase and remodeled nitrogenase for highly selective production of formate or methane, respectively. This work opens an avenue for customizing artificial photosynthetic cells toward multifarious demands for CO2 conversion.

Changes in Activity of the Plasma Membrane H+-ATPase as a Link Between Formation of Electrical Signals and Induction of Photosynthetic Responses in Higher Plants

Action of numerous adverse environmental factors on higher plants is spatially-heterogenous; it means that induction of a systemic adaptive response requires generation and transmission of the stress signals. Electrical signals (ESs) induced by local action of stressors include action potential, variation potential, and system potential and they participate in formation of fast physiological changes at the level of a whole plant, including photosynthetic responses. Generation of these ESs is accompanied by the changes in activity of H+-ATPase, which is the main system of electrogenic proton transport across the plasma membrane. Literature data show that the changes in H+-ATPase activity and related changes in intra- and extracellular pH play a key role in the ES-induced inactivation of photosynthesis in non-irritated parts of plants. This inactivation is caused by both suppression of CO2 influx into mesophyll cells in leaves, which can be induced by the apoplast alkalization and, probably, cytoplasm acidification, and direct influence of acidification of stroma and lumen of chloroplasts on light and, probably, dark photosynthetic reactions. The ES-induced inactivation of photosynthesis results in the increasing tolerance of photosynthetic machinery to the action of adverse factors and probability of the plant survival.

Development of Modified Farquhar-von Caemmerer-Berry Model Describing Photodamage of Photosynthetic Electron Transport in C3 Plants under Different Temperatures

Photodamage of photosynthetic electron transport is a key mechanism of disruption of photosynthesis in plants under action of stressors. This means that investigation of photodamage is an important task for basic and applied investigations. However, its complex mechanisms restrict using experimental methods of investigation for this process; the development of mathematical models of photodamage and model-based analysis can be used for overcoming these restrictions. In the current work, we developed the modified Farquhar-von Caemmerer-Berry model which describes photodamage of photosynthetic electron transport in C3 plants. This model was parameterized on the basis of experimental results (using an example of pea plants). Analysis of the model showed that combined inactivation of linear electron flow and Rubisco could induce both increasing and decreasing photodamage at different magnitudes of inactivation of these processes. Simulation of photodamage under different temperatures and light intensities showed that simulated temperature dependences could be multi-phase; particularly, paradoxical increases in the thermal tolerance of photosynthetic electron transport could be observed under high temperatures (37-42 °C). Finally, it was shown that changes in temperature optimums of linear electron flow and Rubisco could modify temperature dependences of the final activity of photosynthetic electron transport under photodamage induction; however, these changes mainly stimulated its photodamage. Thus, our work provides a new theoretical tool for investigation of photodamage of photosynthetic processes in C3 plants and shows that this photodamage can be intricately dependent on parameters of changes in activities of linear electron flow and Rubisco including changes induced by temperature.

High salt-induced PSI-supercomplex is associated with high CEF and attenuation of state transitions

While PSI-driven cyclic electron flow (CEF) and assembly of thylakoid supercomplexes have been described in model organisms like Chlamydomonas reinhardtii, open questions remain regarding their contributions to survival under long-term stress. The Antarctic halophyte, C. priscuii UWO241 (UWO241), possesses constitutive high CEF rates and a stable PSI-supercomplex as a consequence of adaptation to permanent low temperatures and high salinity. To understand whether CEF represents a broader acclimation strategy to short- and long-term stress, we compared high salt acclimation between the halotolerant UWO241, the salt-sensitive model, C. reinhardtii, and a moderately halotolerant Antarctic green alga, C. sp. ICE-MDV (ICE-MDV). CEF was activated under high salt and associated with increased non-photochemical quenching in all three Chlamydomonas species. Furthermore, high salt-acclimated cells of either strain formed a PSI-supercomplex, while state transition capacity was attenuated. How the CEF-associated PSI-supercomplex interferes with state transition response is not yet known. We present a model for interaction between PSI-supercomplex formation, state transitions, and the important role of CEF for survival during long-term exposure to high salt.

Restricting electron flow at cytochrome b6f when downstream electron acceptors are severely limited

Photosynthetic organisms frequently experience abiotic stress that restricts their growth and development. Under such circumstances, most absorbed solar energy cannot be used for CO2 fixation and can cause the photoproduction of reactive oxygen species (ROS) that can damage the photosynthetic reaction centers of PSI and PSII, resulting in a decline in primary productivity. This work describes a biological "switch" in the green alga Chlamydomonas reinhardtii that reversibly restricts photosynthetic electron transport (PET) at the cytochrome b6f (Cyt b6f) complex when the capacity for accepting electrons downstream of PSI is severely limited. We specifically show this restriction in STARCHLESS6 (sta6) mutant cells, which cannot synthesize starch when they are limited for nitrogen (growth inhibition) and subjected to a dark-to-light transition. This restriction represents a form of photosynthetic control that causes diminished electron flow to PSI and thereby prevents PSI photodamage but does not appear to rely on a ΔpH. Furthermore, when electron flow is restricted, the plastid alternative oxidase (PTOX) becomes active, functioning as an electron valve that dissipates some excitation energy absorbed by PSII and allows the formation of a proton motive force (PMF) that would drive some ATP production (potentially sustaining PSII repair and nonphotochemical quenching [NPQ]). The restriction at the Cyt b6f complex can be gradually relieved with continued illumination. This study provides insights into how PET responds to a marked reduction in availability of downstream electron acceptors and the protective mechanisms involved.

An ancient metabolite damage-repair system sustains photosynthesis in plants

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the major catalyst in the conversion of carbon dioxide into organic compounds in photosynthetic organisms. However, its activity is impaired by binding of inhibitory sugars such as xylulose-1,5-bisphosphate (XuBP), which must be detached from the active sites by Rubisco activase. Here, we show that loss of two phosphatases in Arabidopsis thaliana has detrimental effects on plant growth and photosynthesis and that this effect could be reversed by introducing the XuBP phosphatase from Rhodobacter sphaeroides. Biochemical analyses revealed that the plant enzymes specifically dephosphorylate XuBP, thus allowing xylulose-5-phosphate to enter the Calvin-Benson-Bassham cycle. Our findings demonstrate the physiological importance of an ancient metabolite damage-repair system in degradation of by-products of Rubisco, and will impact efforts to optimize carbon fixation in photosynthetic organisms.

High cyclic electron transfer via the PGR5 pathway in the absence of photosynthetic control

The light reactions of photosynthesis couple electron and proton transfers across the thylakoid membrane, generating NADPH, and proton motive force (pmf) that powers the endergonic synthesis of ATP by ATP synthase. ATP and NADPH are required for CO2 fixation into carbohydrates by the Calvin-Benson-Bassham cycle. The dominant ΔpH component of the pmf also plays a photoprotective role in regulating photosystem II light harvesting efficiency through nonphotochemical quenching (NPQ) and photosynthetic control via electron transfer from cytochrome b6f (cytb6f) to photosystem I. ΔpH can be adjusted by increasing the proton influx into the thylakoid lumen via upregulation of cyclic electron transfer (CET) or decreasing proton efflux via downregulation of ATP synthase conductivity (gH+). The interplay and relative contributions of these two elements of ΔpH control to photoprotection are not well understood. Here, we showed that an Arabidopsis (Arabidopsis thaliana) ATP synthase mutant hunger for oxygen in photosynthetic transfer reaction 2 (hope2) with 40% higher proton efflux has supercharged CET. Double crosses of hope2 with the CET-deficient proton gradient regulation 5 and ndh-like photosynthetic complex I lines revealed that PROTON GRADIENT REGULATION 5 (PGR5)-dependent CET is the major pathway contributing to higher proton influx. PGR5-dependent CET allowed hope2 to maintain wild-type levels of ΔpH, CO2 fixation and NPQ, however photosynthetic control remained absent and PSI was prone to photoinhibition. Therefore, high CET in the absence of ATP synthase regulation is insufficient for PSI photoprotection.

An exploratory steady-state redox model of photosynthetic linear electron transport for use in complete modelling of photosynthesis for broad applications

A photochemical model of photosynthetic electron transport (PET) is needed to integrate photophysics, photochemistry, and biochemistry to determine redox conditions of electron carriers and enzymes for plant stress assessment and mechanistically link sun-induced chlorophyll fluorescence to carbon assimilation for remotely sensing photosynthesis. Towards this goal, we derived photochemical equations governing the states and redox reactions of complexes and electron carriers along the PET chain. These equations allow the redox conditions of the mobile plastoquinone pool and the cytochrome b₆ f complex (Cyt) to be inferred with typical fluorometry. The equations agreed well with fluorometry measurements from diverse C₃ /C₄ species across environments in the relationship between the PET rate and fraction of open photosystem II reaction centres. We found the oxidation of plastoquinol by Cyt is the bottleneck of PET, and genetically improving the oxidation of plastoquinol by Cyt may enhance the efficiency of PET and photosynthesis across species. Redox reactions and photochemical and biochemical interactions are highly redundant in their complex controls of PET. Although individual reaction rate constants cannot be resolved, they appear in parameter groups which can be collectively inferred with fluorometry measurements for broad applications. The new photochemical model developed enables advances in different fronts of photosynthesis research.

Algal PETC-Pro171-Leu suppresses electron transfer in cytochrome b6f under acidic lumenal conditions

Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH varies, cyclic electron flow (CEF) between Photosystem I and the cytochrome b6f complex (b6f) produces extra ATP. b6f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b6f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 (proton gradient regulation 1) mutant of the vascular plant Arabidopsis (Arabidopsis thaliana), which contains a Pro194Leu substitution in the b6f Rieske Iron-sulfur protein Photosynthetic Electron Transfer C (PETC) subunit. In this work, we introduced the equivalent pgr1 mutation in the green alga Chlamydomonas reinhardtii to generate PETC-P171L. Consistent with the pgr1 phenotype, PETC-P171L displayed impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provide evidence that the ΔpH component in PETC-P171L depends on oxygen availability. Only under low oxygen conditions was the ΔpH component sufficient to trigger a phenotype in algal PETC-P171L where the mutant b6f was more restricted to oxidize the plastoquinol pool and showed diminished electron flow through the b6f complex. These results demonstrate that photosynthetic control of different stringency are established in C. reinhardtii depending on the cellular metabolism, and the lumen pH-sensitive PETC-P171L was generated to read out various associated effects.

Chloroplast inner envelope protein FtsH11 is involved in the adjustment of assembly of chloroplast ATP synthase under heat stress

The maintenance of a proton gradient across the thylakoid membrane is an integral aspect of photosynthesis that is mainly established by the splitting of water molecules in photosystem II and plastoquinol oxidation at the cytochrome complex, and it is necessary for the generation of ATP in the last step of photophosphorylation. Although environmental stresses, such as high temperatures, are known to disrupt this fundamental process, only a few studies have explored the molecular mechanisms underlying proton gradient regulation during stress. The present study identified a heat-sensitive mutant that displays aberrant photosynthesis at high temperatures. This mutation was mapped to AtFtsH11, which encodes an ATP-dependent AAA-family metalloprotease. We showed that AtFtsH11 localizes to the chloroplast inner envelope membrane and is capable of degrading the ATP synthase assembly factor BFA3 under heat stress. We posit that this function limits the amount of ATP synthase integrated into the thylakoid membrane to regulate proton efflux from the lumen to the stroma. Our data also suggest that AtFtsH11 is critical in stabilizing photosystem II and cytochrome complexes at high temperatures, and additional studies can further elucidate the specific molecular functions of this critical regulator of photosynthetic thermotolerance.

Salt-responsive bermudagrass microRNAs and insights into light reaction photosynthetic performance

INTRODUCTION

Bermudagrass (Cynodon dactylon L.) is a warm-season grass with high drought and salt tolerance. However, its cultivation as a silage crop is limited by its lower forage value when compared to other C4 crops. Because of its high genetic variability in abiotic stress tolerance, bermudagrass-mediated genetic breeding offers significant promise for introducing alternative fodder crops in saline and drought-affected regions, and improved photosynthetic capacity is one way for increasing forage yield.

METHODS

Here, we used RNA sequencing to profile miRNAs in two bermudagrass genotypes with contrasting salt tolerance growing under saline conditions.

RESULTS

Putatively, 536 miRNA variants were salt-inducible, with the majority being downregulated in salt-tolerant vs sensitive varieties. Also, seven miRNAs putatively targeted 6 genes which were significantly annotated to light reaction photosynthesis. Among the microRNAs, highly abundant miRNA171f in the salt tolerant regime targeted Pentatricopeptide repeat-containing protein and dehydrogenase family 3 member F1 both annotated to electron transport and Light harvesting protein complex 1 genes annotated to light photosynthetic reaction in salt tolerant regime vs salt sensitive counterparts. To facilitate genetic breeding for photosynthetic capacity, we overexpressed miR171f in Medicago tracantula which resulted in a substantial increase in the chlorophyll transient curve, electron transport rate, quantum yield of photosystem II non photochemical quenching, NADPH and biomass accumulation under saline conditions while its targets were downregulated. At ambient light level the electron transport was negatively correlated with all parameters while the NADPH was positively associated higher dry matter in mutants.

DISCUSSION

These results demonstrate that miR171f improves photosynthetic performance and dry matter accumulation via transcriptional repression of genes in the electron transport pathway under saline conditions and thus a target for breeding.

High-resolution cryo-EM structures of plant cytochrome b6f at work

Plants use solar energy to power cellular metabolism. The oxidation of plastoquinol and reduction of plastocyanin by cytochrome b₆f (Cyt b₆f) is known as one of the key steps of photosynthesis, but the catalytic mechanism in the plastoquinone oxidation site (Q_p) remains elusive. Here, we describe two high-resolution cryo-EM structures of the spinach Cyt b₆f homodimer with endogenous plastoquinones and in complex with plastocyanin. Three plastoquinones are visible and line up one after another head to tail near Q_p in both monomers, indicating the existence of a channel in each monomer. Therefore, quinones appear to flow through Cyt b₆f in one direction, transiently exposing the redox-active ring of quinone during catalysis. Our work proposes an unprecedented one-way traffic model that explains efficient quinol oxidation during photosynthesis and respiration.

Phytotoxic Action of Silver Nanoparticles on Lemna minor: Multi-Parameter Analysis of Different Physiological Processes

Considering the widespread use of silver nanoparticles (AgNPs) and their consequent build-up in waterways, there is a concern about the hazardous effect of AgNPs for aquatic ecosystems. The aim of this study was to clarify the mechanism of the action of AgNPs on duckweed (Lemna minor L.) by evaluating multiple parameters in different physiological processes. Duckweed was treated with AgNPs in a concentration range of 0.5 to 5 mg/L over a 7-day period. The analysis revealed that the AgNP-treated duckweed accumulated Ag in accordance with increasing AgNP concentrations. Furthermore, higher concentrations (2 and 5 mg/L) of AgNPs negatively affected N, P and especially K and Mg levels in the plant tissue. Accordingly, the plant growth and photosynthetic parameters were more inhibited in response to higher concentrations of AgNPs. Nanosilver significantly increased the generation of ROS at higher concentrations, although lipid peroxidation was significant even at the lowest concentration of AgNPs. However, defense mechanisms were able to counteract AgNP-induced oxidative stress and balance the intracellular redox status, as evidenced by increased activities of the main detoxification enzymes. With this experimental setting, AgNPs exhibited a relatively weak phytotoxicity at 0.5 and 1 mg/L; nevertheless, silver in a nano form poses a hazard for plants, considering its continuous release into aquatic environments.

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.

Development of Two-Dimensional Model of Photosynthesis in Plant Leaves and Analysis of Induction of Spatial Heterogeneity of CO2 Assimilation Rate under Action of Excess Light and Drought

Photosynthesis is a key process in plants that can be strongly affected by the actions of environmental stressors. The stressor-induced photosynthetic responses are based on numerous and interacted processes that can restrict their experimental investigation. The development of mathematical models of photosynthetic processes is an important way of investigating these responses. Our work was devoted to the development of a two-dimensional model of photosynthesis in plant leaves that was based on the Farquhar-von Caemmerer-Berry model of CO₂ assimilation and descriptions of other processes including the stomatal and transmembrane CO₂ fluxes, lateral CO₂ and HCO₃^- fluxes, transmembrane and lateral transport of H⁺ and K⁺, interaction of these ions with buffers in the apoplast and cytoplasm, light-dependent regulation of H⁺-ATPase in the plasma membrane, etc. Verification of the model showed that the simulated light dependences of the CO₂ assimilation rate were similar to the experimental ones and dependences of the CO₂ assimilation rate of an average leaf CO₂ conductance were also similar to the experimental dependences. An analysis of the model showed that a spatial heterogeneity of the CO₂ assimilation rate on a leaf surface should be stimulated under an increase in light intensity and a decrease in the stomatal CO₂ conductance or quantity of the open stomata; this prediction was supported by the experimental verification. Results of the work can be the basis of the development of new methods of the remote sensing of the influence of abiotic stressors (at least, excess light and drought) on plants.

Enhanced abundance and activity of the chloroplast ATP synthase in rice through the overexpression of the AtpD subunit

ATP, produced by the light reactions of photosynthesis, acts as the universal cellular energy cofactor fuelling all life processes. Chloroplast ATP synthase produces ATP using the proton motive force created by solar energy-driven thylakoid electron transport reactions. Here we investigate how increasing abundance of ATP synthase affects leaf photosynthesis and growth of rice, Oryza sativa variety Kitaake. We show that overexpression of AtpD, the nuclear-encoded subunit of the chloroplast ATP synthase, stimulates both abundance of the complex, confirmed by immunodetection of thylakoid complexes separated by Blue Native-PAGE, and ATP synthase activity, detected as higher proton conductivity of the thylakoid membrane. Plants with increased AtpD content had higher CO2 assimilation rates when a stepwise increase in CO2 partial pressure was imposed on leaves at high irradiance. Fitting of the CO2 response curves of assimilation revealed that plants overexpressing AtpD had a higher electron transport rate (J) at high CO2, despite having wild-type-like abundance of the cytochrome b6f complex. A higher maximum carboxylation rate (Vcmax) and lower cyclic electron flow detected in transgenic plants both pointed to an increased ATP production compared with wild-type plants. Our results present evidence that the activity of ATP synthase modulates the rate of electron transport at high CO2 and high irradiance.

High efficient cyclic electron flow and functional supercomplexes in Chlamydomonas cells

A very high rate for cyclic electron flow (CEF) around PSI (~180 s^-1 or 210 s^-1 in minimum medium or in the presence of a carbon source respectively) is measured in the presence of methyl viologen (MV) in intact cells of Chlamydomonas reinhardtii under anaerobic conditions. The observation of an efficient CEF in the presence of methyl viologen is in agreement with the previous results reports of Asada et al. in broken chloroplasts (Plant Cell Physiol. 31(4) (1990) 557-564). From the analysis of the P700 and PC absorbance changes, we propose that a confinement between 2 PC molecules, 1 PSI and 1 cytb₆f corresponding to a functional supercomplex is responsible for these high rates of CEF. Supercomplex formation is also observed in the absence of methyl viologen, but with lower maximal CEF rate (about 100 s^-1) suggesting that this compound facilitates the mediation of electron transfer from PSI acceptors to the stromal side of cytb₆f. Further analysis of CEF in mutants of Chlamydomonas defective in state transitions shows the requirement of a kinase-driven transition to state 2 to establish this functional supercomplex configuration. However, a movement of the LHCII antennae is not involved in this process. We discuss the possible involvement of auxiliary proteins, among which is a small cytb₆f-associated polypeptide, the PETO protein, which is one of the targets of the STT7 kinase.

Granal thylakoid structure and function: explaining an enduring mystery of higher plants

In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi-media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome  b₆ f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon-reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well-known but seemingly unconnected phenomena of thylakoid structure and function in higher plants.

Cryo-EM structures of the Synechocystis sp. PCC 6803 cytochrome b6f complex with and without the regulatory PetP subunit

In oxygenic photosynthesis, the cytochrome b₆f (cytb₆f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb₆f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb₆f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown. An apparent key player in the regulatory balance between LET and CET in cyanobacteria is PetP, a ∼10 kDa protein that is also found in red algae but not in green algae and plants. Here, we used cryogenic electron microscopy to determine the structure of the Synechocystis sp. PCC 6803 cytb₆f complex in the presence and absence of PetP. Our structures show that PetP interacts with the cytoplasmic side of cytb₆f, displacing the C-terminus of the PetG subunit and shielding the C-terminus of cytochrome b₆, which binds the heme c_n cofactor that is suggested to mediate CET. The structures also highlight key differences in the mode of plastoquinone binding between cyanobacterial and plant cytb₆f complexes, which we suggest may reflect the unique combination of photosynthetic and respiratory electron transfer in cyanobacterial thylakoid membranes. The structure of cytb₆f from a model cyanobacterial species amenable to genetic engineering will enhance future site-directed mutagenesis studies of structure-function relationships in this crucial ET complex.

Chloroplasts are key players to cope with light and temperature stress

Under natural environmental conditions, changes in light intensity and temperature are closely interwoven, and of all organelles, only chloroplasts react strongly upon alterations of these two parameters. We review increasing evidence indicating that changes in chloroplast metabolism are critical for the comprehensive cellular answer in a challenging environment. This cellular answer starts with rapid modifications of thylakoid-located processes, followed by modifications in the stroma and transport activities across the chloroplast envelope. We propose that the 'modulators' involved contribute to plant stress tolerance and that deciphering of their characteristics is essential to understand 'acclimation'. Especially in times of climatic changes, we must gain knowledge on physiological reactions that might become instrumental for directed breeding strategies aiming to develop stress-tolerant crop plants.

Regulation of Chloroplast ATP Synthase Modulates Photoprotection in the CAM Plant Vanilla planifolia

Generally, regulation of cyclic electron flow (CEF) and chloroplast ATP synthase play key roles in photoprotection for photosystems I and II (PSI and PSII) in C3 and C4 plants, especially when CO₂ assimilation is restricted. However, how CAM plants protect PSI and PSII when CO₂ assimilation is restricted is largely known. In the present study, we measured PSI, PSII, and electrochromic shift signals in the CAM plant Vanilla planifolia. The quantum yields of PSI and PSII photochemistry largely decreased in the afternoon compared to in the morning, indicating that CO₂ assimilation was strongly restricted in the afternoon. Meanwhile, non-photochemical quenching (NPQ) in PSII and the donor side limitation of PSI (Y(ND)) significantly increased to protect PSI and PSII. Under such conditions, proton gradient (∆pH) across the thylakoid membranes largely increased and CEF was slightly stimulated, indicating that the increased ∆pH was not caused by the regulation of CEF. In contrast, the activity of chloroplast ATP synthase (g_H⁺) largely decreased in the afternoon. At a given proton flux, the decreasing g_H⁺ increased ∆pH and thus contributed to the enhancement of NPQ and Y(ND). Therefore, in the CAM plant V. planifolia, the ∆pH-dependent photoprotective mechanism is mainly regulated by the regulation of g_H⁺ rather than CEF when CO₂ assimilation is restricted.

High levels of anoxygenic photosynthesis revealed by dual-frequency Fourier photoacoustics in Ailanthus altissima leaves

In contrast to oxygenic photosynthesis, true anoxygenic photosynthesis is not associated with O2 evolution originated from water photolysis but still converts light energy to that of the phosphoanhydride bonds of ATP. In a narrow sense, anoxygenic photosynthesis is mainly known as to be related to the purple and green sulfur bacteria, but in a broad sense, it also occurs in the vascular plants. The portion of photosynthetic water photolysis that is compensated by the processes of O2 uptake (respiration, photorespiration, Mehler cycle, etc.) may be referred to as 'quasi' anoxygenic photosynthesis. Photoacoustic method allows for the separate detection of photolytic O2 at frequencies of measuring light about 20-40Hz, whereas at 250-400Hz, it detects the photochemical energy storage. We have developed a fast-Fourier transform photoacoustic method enabling measurements of both these signals simultaneously in one sample. This method allows to calculate oxygenic coefficients, which reflect the part of photochemically stored light energy that is used for the water photolysis. We show that the true anoxygenic photosynthesis in Ailanthus altissima Mill. leaves reached very high levels under low light, under moderate light at the beginning of the 1-h period, and at the end of the 40-min period under saturating light.

Change in the photochemical and structural organization of thylakoids from pea (Pisum sativum) under salt stress

Salt can induce adverse effects, primarily on the photosynthetic process, ultimately influencing plant productivity. Still, the impact of salt on the photosynthesis process in terms of supercomplexes organization of thylakoid structure and function is not understood in Pea (Pisum sativum). To understand the structure and function in the leaves and thylakoids under salt (NaCl) treatment, we used various biophysical and biochemical techniques like infrared gas analyzer, chlorophyll a fluorescence, circular dichroism, electron microscopy, blue native gels, and western blots. The net photosynthetic rate, transpiration rate, and stomatal conductance were reduced significantly, whereas the water use efficiency was enhanced remarkably under high salt conditions (200 mM NaCl). The photochemical efficiency of both photosystem (PS) I and II was reduced in high salt by inhibiting their donor and acceptor sides. Interestingly the non-photochemical quenching (NPQ) is reduced in high salt; however, the non-regulated energy dissipation (NO) of PSII increased, leading to inactivation of PSII. The obtained results exhibit inhibition of NAD(P)H dehydrogenase (NDH) mediated pathway-dependent cyclic electron transport under salinity caused a decrease in proton motive force of ΔpH and Δψ. Further, the electron micrographs show the disorganization of grana thylakoids under salt stress. Furthermore, the macro-organization and supercomplexes of thylakoids were significantly affected by high salt. Specifically, the mega complexes, PSII-LHCII, PSI-LHCI, and NDH complexes were notably reduced, ultimately altering the electron transport. The reaction center proteins of oxygen-evolving complexes, D1 and D2 proteins were affected to high salt indicating changes in photochemical activities.

Inference of photosynthetic capacity parameters from chlorophyll a fluorescence is affected by redox state of PSII reaction centers

Solar-induced chlorophyll fluorescence (SIF) has been used to infer photosynthetic capacity parameters (e.g., the maximum carboxylation rate V_cmax , and the maximum electron transport rate J_max ). However, the precise mechanism and practical utility of such approach under dynamic environments remain unclear. We used the balance between the light and carbon reactions to derive theoretical equations relating chlorophyll a fluorescence (ChlF) emission and photosynthetic capacity parameters, and formulated testable hypotheses regarding the dynamic relationships between the true total ChlF emitted from PSII (SIF_PSII ) and V_cmax and J_max . We employed concurrent measurements of gas exchanges and ChlF parameters for 15 species from six biomes to test the formulated hypotheses across species, temperatures, and limitation state of carboxylation. Our results revealed that SIF_PSII alone is incapable of informing the variations in V_cmax and J_max across species, even when SIF_PSII is determined under the same environmental conditions. In contrast, the product of SIF_PSII and the fraction of open PSII reactions q_L , which indicates the redox state of PSII, is a strong predictor of both V_cmax and J_max , although their precise relationships vary somewhat with environmental conditions. Our findings suggest the redox state of PSII strongly influences the relationship between SIF_PSII and V_cmax and J_max .

Glycinebetaine mitigates tomato chilling stress by maintaining high-cyclic electron flow rate of photosystem I and stability of photosystem II

Glycinebetaine alleviates chilling stress by protecting photosystems I and II in BADH-transgenic and GB-treated tomato plants, which can be an effective strategy for improving crop chilling tolerance. Tomato (Solanum lycopersicum) is one of the most cultivated vegetables in the world, but is highly susceptible to chilling stress and does not naturally accumulate glycinebetaine (GB), one of the most effective stress protectants. The protective mechanisms of GB on photosystem I (PSI) and photosystem II (PSII) against chilling stress, however, remain poorly understood. Here, we address this problem through exogenous GB application and generation of transgenic tomatoes (Moneymaker) with a gene encoding betaine aldehyde dehydrogenase (BADH), which is the key enzyme in the synthesis of GB, from spinach. Our results demonstrated that GB can protect chloroplast ultramicrostructure, alleviate PSII photoinhibition and maintain PSII stability under chilling stress. More importantly, GB increased the electron transfer between Q_A and Q_B and the redox potential of Q_B and maintained a high rate of cyclic electron flow around PSI, contributing to reduced production of reactive oxygen species, thereby mitigating PSI photodamage under chilling stress. Our results highlight the novel roles of GB in enhancing chilling tolerance via the protection of PSI and PSII in BADH transgenic and GB-treated tomato plants under chilling stress. Thus, introducing GB-biosynthetic pathway into tomato and exogenous GB application are effective strategies for improving chilling tolerance.

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

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

PTOX-dependent safety valve does not oxidize P700 during photosynthetic induction in the Arabidopsis pgr5 mutant

Plastid terminal oxidase (PTOX) accepts electrons from plastoquinol to reduce molecular oxygen to water. We introduced the gene encoding Chlamydomonas reinhardtii (Cr)PTOX2 into the Arabidopsis (Arabidopsis thaliana) wild-type (WT) and proton gradient regulation5 (pgr5) mutant defective in cyclic electron transport around photosystem I (PSI). The accumulation of CrPTOX2 only mildly affected photosynthetic electron transport in the WT background during steady-state photosynthesis but partly complemented the induction of nonphotochemical quenching (NPQ) in the pgr5 background. During the induction of photosynthesis by actinic light (AL) of 130 µmol photons m-2 s-1, the high level of PSII yield (Y(II)) was induced immediately after the onset of AL in WT plants accumulating CrPTOX2. NPQ was more rapidly induced in the transgenic plants than in WT plants. P700 was also oxidized immediately after the onset of AL. Although CrPTOX2 does not directly induce a proton concentration gradient (ΔpH) across the thylakoid membrane, the coupled reaction of PSII generated ΔpH to induce NPQ and the downregulation of the cytochrome b6f complex. Rapid induction of Y(II) and NPQ was also observed in the pgr5 plants accumulating CrPTOX2. In contrast to the WT background, P700 was not oxidized in the pgr5 background. Although the thylakoid lumen was acidified by CrPTOX2, PGR5 was essential for oxidizing P700. In addition to acidification of the thylakoid lumen to downregulate the cytochrome b6f complex (donor-side regulation), PGR5 may be required for draining electrons from PSI by transferring them to the plastoquinone pool. We propose a reevaluation of the contribution of this acceptor-side regulation by PGR5 in the photoprotection of PSI.

Maintaining the Chloroplast Redox Balance through the PGR5-Dependent Pathway and the Trx System Is Required for Light-Dependent Activation of Photosynthetic Reactions

Light-dependent activation of chloroplast enzymes is required for the rapid induction of photosynthesis after a shift from dark to light. The thioredoxin (Trx) system plays a central role in this process. In chloroplasts, the Trx system consists of two pathways: the ferredoxin (Fd)/Trx pathway and the nicotinamide adenine dinucleotide phosphate (NADPH)-Trx reductase C (NTRC) pathway. In Arabidopsis (Arabidopsis thaliana) mutants defective in either pathway, the photoreduction of thiol enzymes was impaired, resulting in decreased carbon fixation. The close relationship between the Fd/Trx pathway and proton gradient regulation 5 (PGR5)-dependent photosystem I cyclic electron transport (PSI CET) in the induction of photosynthesis was recently elucidated. However, how the PGR5-dependent pathway is involved in the NTRC pathway is unclear, although NTRC has been suggested to physically interact with PGR5. In this study, we analyzed Arabidopsis mutants lacking either the PGR5 or the chloroplast NADH dehydrogenase-like complex (NDH)-dependent PSI CET pathway in the ntrc mutant background. The ntrc pgr5 double mutant suppressed both the growth defects and the high non-photochemical quenching phenotype of the ntrc mutant when grown under long-day conditions. By contrast, the inactivation of NDH activity with the chlororespiratory reduction 2-2 mutant failed to suppress either phenotype. We discovered that the phenotypic rescue of ntrc by pgr5 is caused by the partial restoration of Trx-dependent reduction of thiol enzymes. These results suggest that electron partitioning to the PGR5-dependent pathway and the Trx system needs to be properly regulated for the activation of the Calvin-Benson-Bassham cycle enzymes during the induction of photosynthesis.

Electron and proton transport in wheat exposed to salt stress: is the increase of the thylakoid membrane proton conductivity responsible for decreasing the photosynthetic activity in sensitive genotypes?

Effects of salinity caused by 150 mM NaCl on primary photochemical reactions and some physiological and biochemical parameters (K⁺/Na⁺ ratio, soluble sugars, proline, MDA) have been studied in five Triticum aestivum L. genotypes with contrasting salt tolerance. It was found that 150 mM NaCl significantly decreased the photosynthetic efficiency of two sensitive genotypes. The K⁺/Na⁺ ratio decreased in all genotypes exposed to salinity stress when compared with the control. Salinity stress also caused lipid peroxidation and accumulation of soluble sugars and proline. The amounts of soluble sugars and proline were higher in tolerant genotypes than sensitive ones, and lipid peroxidation was higher in sensitive genotypes. The noninvasive measurements of photosynthesis-related parameters indicated the genotype-dependent effects of salinity stress on the photosynthetic apparatus. The significant decrease of chlorophyll content (SPAD values) or adverse effects on photosynthetic functions at the PSII level (measured by the chlorophyll fluorescence parameters) were observed in the two sensitive genotypes only. Although the information obtained by different fast noninvasive techniques were consistent, the correlation analyses identified the highest correlation of the noninvasive records with MDA, K⁺/Na⁺ ratio, and free proline content. The lower correlation levels were found for chlorophyll content (SPAD) and F_v/F_m values derived from chlorophyll fluorescence. Performance index (PI_abs) derived from fast fluorescence kinetics, and F₇₃₅/F₆₈₅ ratio correlated well with MDA and Na⁺ content. The most promising were the results of linear electron flow measured by MultispeQ sensor, in which we found a highly significant correlation with all parameters assessed. Moreover, the noninvasive simultaneous measurements of chlorophyll fluorescence and electrochromic band shift using this sensor indicated the apparent proton leakage at the thylakoid membranes resulting in a high proton conductivity (gH⁺), present in sensitive genotypes only. The possible consequences for the photosynthetic functions and the photoprotection are discussed.

Glycinebetaine: a versatile protectant to improve rice performance against aluminium stress by regulating aluminium uptake and translocation

Glycinebetaine alleviates the detrimental effects of aluminium stress by regulating aluminium uptake and translocation, maintaining PSII activity, and activating the oxidative defence, thereby maintaining the growth and development of rice. Aluminium (Al) toxicity is one of the primary growth-limiting factors that limits plant growth and crop productivity in acidic soils. Rice (Oryza sativa L.) plants are susceptible to Al stress and do not naturally accumulate glycinebetaine (GB), one of the most effective protectants. Therefore, the objective of this study was to investigate whether exogenous GB can ameliorate the detrimental effects of Al stress on rice plants. Our results showed that the growth, development and biomass of rice were clearly inhibited under Al stress. However, exogenous GB application increased rice shoot growth and photosynthetic pigments contents, maintained photosystem II (PSII) activity, and activated the antioxidant defence system under Al stress. More importantly, GB may mediate the expression of Al uptake- and translocation-related genes, including OsALS1, OsNrat1, OsSTAR1 and OsSTAR2, and the galacturonic acid contents in rice roots under Al stress. Therefore, our findings highlight exogenous GB application is a valid approach to effectively combat Al toxicity by regulating physiological and biochemical processes in crops.

Diurnal Response of Photosystem I to Fluctuating Light Is Affected by Stomatal Conductance

Upon a sudden transition from low to high light, electrons transported from photosystem II (PSII) to PSI should be rapidly consumed by downstream sinks to avoid the over-reduction of PSI. However, the over-reduction of PSI under fluctuating light might be accelerated if primary metabolism is restricted by low stomatal conductance. To test this hypothesis, we measured the effect of diurnal changes in stomatal conductance on photosynthetic regulation under fluctuating light in tomato (Solanum lycopersicum) and common mulberry (Morus alba). Under conditions of high stomatal conductance, we observed PSI over-reduction within the first 10 s after transition from low to high light. Lower stomatal conductance limited the activity of the Calvin–Benson–Bassham cycle and aggravated PSI over-reduction within 10 s after the light transition. We also observed PSI over-reduction after transition from low to high light for 30 s at the low stomatal conductance typical of the late afternoon, indicating that low stomatal conductance extends the period of PSI over-reduction under fluctuating light. Therefore, diurnal changes in stomatal conductance significantly affect the PSI redox state under fluctuating light. Moreover, our analysis revealed an unexpected inhibition of cyclic electron flow by the severe over-reduction of PSI seen at low stomatal conductance. In conclusion, stomatal conductance can have a large effect on thylakoid reactions under fluctuating light.

Improving water deficit tolerance of Salvia officinalis L. using putrescine

To study the effects of foliar application of putrescine (distilled water (0), 0.75, 1.5, and 2.25 mM) and water deficit stress (20%, 40%, 60%, and 80% available soil water depletion (ASWD)) on the physiological, biochemical, and molecular attributes of Salvia officinalis L., a factorial experiment was performed in a completely randomized design with three replications in the growth chamber. The results of Real-Time quantitative polymerase chain reaction (qRT-PCR) analysis showed that putrescine concentration, irrigation regime, and the two-way interaction between irrigation regime and putrescine concentration significantly influenced cineole synthase (CS), sabinene synthase (SS), and bornyl diphosphate synthase (BPPS) relative expression. The highest concentration of 1,8-cineole, camphor, α-thujone, β-thujone, CS, SS, and BPPS were obtained in the irrigation regime of 80% ASWD with the application of 0.75 mM putrescine. There was high correlation between expression levels of the main monoterpenes synthase and the concentration of main monoterpenes. The observed correlation between the two enzyme activities of ascorbate peroxidase (APX) and catalase (CAT) strongly suggests they have coordinated action. On the other hand, the highest peroxidase (PO) and superoxide dismutase (SOD) concentrations were obtained with the application of 0.75 mM putrescine under the irrigation regime of 40% ASWD. Putrescine showed a significant increase in LAI and RWC under water deficit stress. There was an increasing trend in endogenous putrescine when putrescine concentration was increased in all irrigation regimes. Overall, the results suggest that putrescine may act directly as a stress-protecting compound and reduced H₂O₂ to moderate the capacity of the antioxidative system, maintain the membrane stability, and increase secondary metabolites under water deficit stress.

Physiological and proteomic analyses revealed the response mechanisms of two different drought-resistant maize varieties

BACKGROUND

Drought stress severely limits maize seedling growth and crop yield. Previous studies have elucidated the mechanisms by which maize acquires drought resistance and contends with water deficiency. However, the link between the physiological and molecular variations among maize cultivars are unknown. Here, physiological and proteomic analyses were conducted to compare the stress responses of two maize cultivars with contrasting drought stress tolerance.

RESULTS

The physiological analysis showed that the drought-tolerant SD609 maize variety maintains relatively high photochemical efficiency by enhancing its protective cyclic electron flow (CEF) mechanism and antioxidative enzymes activities. Proteomics analysis revealed that 198 and 102 proteins were differentially expressed in SD609 and the drought-sensitive SD902 cultivar, respectively. GO and KEGG enrichments indicated that SD609 upregulated proteins associated with photosynthesis, antioxidants/detoxifying enzymes, molecular chaperones and metabolic enzymes. Upregulation of the proteins related to PSII repair and photoprotection improved photochemical capacity in SD609 subjected to moderate drought stress. In SD902, however, only the molecular chaperones and sucrose synthesis pathways were induced and they failed to protect the impaired photosystem. Further analysis demonstrated that proteins related to the electron transport chain (ETC) and redox homeostasis as well as heat shock proteins (HSPs) may be important in protecting plants from drought stress.

CONCLUSIONS

Our experiments explored the mechanism of drought tolerance and clarified the interconnections between the physiological and proteomic factors contributing to it. In summary, our findings aid in further understanding of the drought tolerance mechanisms in maize.

Pre-illumination at high light significantly alleviates the over-reduction of photosystem I under fluctuating light

Photosystem I (PSI) is the primary target of photoinhibition under fluctuating light (FL). In angiosperms, cyclic electron flow (CEF) around PSI is thought to be the main player protecting PSI under FL. The activation of CEF is linked to the thylakoid stacking, which is in turn affected by light intensity. However, it is unknown how pre-illumination affects the CEF activation and PSI redox state under FL. To address this question, we conducted a spectroscopic analysis under FL in Bletilla striata and Morus alba after pre-illumination at moderate light (ML, 611 μmol photons m^-2 s^-1) or high light (HL, 1455 μmol photons m^-2 s^-1). Our results indicated that both species displayed a transient over-reduction of PSI after a transition from low to high light, but the extent of PSI over-reduction under FL was largely alleviated by pre-illumination at HL when compared with pre-illumination under ML. Furthermore, pre-illumination at HL accelerated the activation rate of CEF but did not affect the activation of non-photochemical quenching and linear electron flow from photosystem II under FL. Therefore, such increased CEF activity by pre-illumination under HL alleviated PSI over-reduction under FL by facilitating the electron sink downstream of PSI. Taking together, pre-illumination at HL protects PSI under FL through acceptor-side regulation.

Installing a Green Engine To Drive an Enzyme Cascade: A Light-Powered In Vitro Biosystem for Poly(3-hydroxybutyrate) Synthesis

Many existing in vitro biosystems harness power from the chemical energy contained in substrates and co-substrates, and light or electric energy provided from abiotic parts, leading to a compromise in atom economy, incompatibility between biological and abiotic parts, and most importantly, incapability to spatiotemporally co-regenerate ATP and NADPH. In this study, we developed a light-powered in vitro biosystem for poly(3-hydroxybutyrate) (PHB) synthesis using natural thylakoid membranes (TMs) to regenerate ATP and NADPH for a five-enzyme cascade. Through effective coupling of cofactor regeneration and mass conversion, 20 mM PHB was yielded from 50 mM sodium acetate with a molar conversion efficiency of carbon of 80.0 % and a light-energy conversion efficiency of 3.04 %, which are much higher than the efficiencies of similar in vitro PHB synthesis biosystems. This suggests the promise of installing TMs as a green engine to drive more enzyme cascades.

Down-Regulation of Photosynthetic Electron Transport and Decline in CO2 Assimilation under Low Frequencies of Pulsed Lights

The decline in CO2 assimilation in leaves exposed to decreasing frequencies of pulsed light is well characterized, in contrast to the regulation of photosynthetic electron transport under these conditions. Thus, we exposed sunflower leaves to pulsed lights of different frequencies but with the same duty ratio (25%) and averaged light intensity (575 μmoles photons m−2 s−1). The rates of net photosynthesis Pn were constant from 125 to 10 Hz, and declined by 70% from 10 to 0.1 Hz. This decline coincided with (1) a marked increase in nonphotochemical quenching (NPQ), and (2) the completion after 25 ms of illumination of the first phase of P700 photooxidation, the primary electron donor of PSI. Under longer light pulses (<5 Hz), there was a slower and larger P700 photooxidation phase that could be attributed to the larger NPQ and to a resistance of electron flow on the PSI donor side indicated by 44% slower kinetics of a P700+ dark reduction. In addition, at low frequencies, the decrease in quantum yield of photochemistry was 2.3-times larger for PSII than for PSI. Globally, our results indicate that the decline in CO2 assimilation at 10 Hz and lower frequencies coincide with the formation of NPQ and a restriction of electron flows toward PSI, favoring the accumulation of harmless P700+.