Contribution of NDH-dependent cyclic electron transport around photosystem I to the generation of proton motive force in the weak mutant allele of pgr5

Dynamics and interplay of photosynthetic regulatory processes depend on the amplitudes of oscillating light

Plants have evolved multiple regulatory mechanisms to cope with natural light fluctuations. The interplay between these mechanisms leads presumably to the resilience of plants in diverse light patterns. We investigated the energy-dependent nonphotochemical quenching (qE) and cyclic electron transports (CET) in light that oscillated with a 60-s period with three different amplitudes. The photosystem I (PSI) and photosystem II (PSII) function-related quantum yields and redox changes of plastocyanin and ferredoxin were measured in Arabidopsis thaliana wild types and mutants with partial defects in qE or CET. The decrease in quantum yield of qE due to the lack of either PsbS- or violaxanthin de-epoxidase was compensated by an increase in the quantum yield of the constitutive nonphotochemical quenching. The mutant lacking NAD(P)H dehydrogenase (NDH)-like-dependent CET had a transient significant PSI acceptor side limitation during the light rising phase under high amplitude of light oscillations. The mutant lacking PGR5/PGRL1-CET restricted electron flows and failed to induce effective photosynthesis control, regardless of oscillation amplitudes. This suggests that PGR5/PGRL1-CET is important for the regulation of PSI function in various amplitudes of light oscillation, while NDH-like-CET acts' as a safety valve under fluctuating light with high amplitude. The results also bespeak interplays among multiple photosynthetic regulatory mechanisms.

Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts

The proton motive force (pmf) generated across the thylakoid membrane rotates the Fo-ring of ATP synthase in chloroplasts. The pmf comprises two components: membrane potential (∆Ψ) and proton concentration gradient (∆pH). Acidification of the thylakoid lumen resulting from ∆pH downregulates electron transport in the cytochrome b6f complex. This process, known as photosynthetic control, is crucial for protecting photosystem I (PSI) from photodamage in response to fluctuating light. To optimize the balance between efficient photosynthesis and photoprotection, it is necessary to regulate pmf. Cyclic electron transport around PSI and pseudo-cyclic electron transport involving flavodiiron proteins contribute to the modulation of pmf magnitude. By manipulating the ratio between the two components of pmf, it is possible to modify the extent of photosynthetic control without affecting the pmf size. This adjustment can be achieved by regulating the movement of ions (such as K+ and Cl-) across the thylakoid membrane. Since ATP synthase is the primary consumer of pmf in chloroplasts, its activity must be precisely regulated to accommodate other mechanisms involved in pmf optimization. Although fragments of information about each regulatory process have been accumulated, a comprehensive understanding of their interactions is lacking. Here, I summarize current knowledge of the network for pmf regulation, mainly based on genetic studies.

Critical role of cyclic electron transport around photosystem I in the maintenance of photosystem I activity

In angiosperms, cyclic electron transport around photosystem I (PSI) is mediated by two pathways that depend on the PROTON GRADIENT REGULATION 5 (PGR5) protein and the chloroplast NADH dehydrogenase-like (NDH) complex, respectively. In the Arabidopsis double mutants defective in both pathways, plant growth and photosynthesis are impaired. The pgr5-1 mutant used in the original study is a missense allele and accumulates low levels of PGR5 protein. In this study, we generated two knockout (KO) alleles, designated as pgr5-5 and pgr5-6, using the CRISPR-Cas9 technology. Although both KO alleles showed a severe reduction in P700 similar to the pgr5-1 allele, NPQ induction was less severely impaired in the KO alleles than in the pgr5-1 allele. In the pgr5-1 allele, the second mutation affecting NPQ size was mapped to ~21 cM south of the pgr5-1 locus. Overexpression of the pgr5-1 allele, encoding the glycine130-to-serine change, complemented the pgr5-5 phenotype, suggesting that the pgr5-1 mutation destabilizes PGR5 but that the mutant protein retains partial functionality. Using two KO alleles, we created the double mutants with two chlororespiratory reduction (crr) mutants defective in the NDH complex. The growth of the double mutants was notably impaired. In the double mutant seedlings that survived on the medium containing sucrose, PSI activity evaluated by the P700 oxidation was severely impaired, whereas PSII activity was only mildly impaired. Cyclic electron transport around PSI is required to maintain PSI activity.

Salinity Mitigates the Negative Effect of Elevated Temperatures on Photosynthesis in the C3-C4 Intermediate Species Sedobassia sedoides

The adaptation of plants to combined stresses requires unique responses capable of overcoming both the negative effects of each individual stress and their combination. Here, we studied the C3-C4 (C2) halophyte Sedobassia sedoides in response to elevated temperature (35 °C) and salinity (300 mM NaCl) as well as their combined effect. The responses we studied included changes in water-salt balance, light and dark photosynthetic reactions, the expression of photosynthetic genes, the activity of malate dehydrogenase complex enzymes, and the antioxidant system. Salt treatment led to altered water-salt balance, improved water use efficiency, and an increase in the abundance of key enzymes involved in intermediate C3-C4 photosynthesis (i.e., Rubisco and glycine decarboxylase). We also observed a possible increase in the activity of the C2 carbon-concentrating mechanism (CCM), which allowed plants to maintain high photosynthesis intensity and biomass accumulation. Elevated temperatures caused an imbalance in the dark and light reactions of photosynthesis, leading to stromal overreduction and the excessive generation of reactive oxygen species (ROS). In response, S. sedoides significantly activated a metabolic pathway for removing excess NADPH, the malate valve, which is catalyzed by NADP-MDH, without observable activation of the antioxidant system. The combined action of these two factors caused the activation of antioxidant defenses (i.e., increased activity of SOD and POX and upregulation of FDI), which led to a decrease in oxidative stress and helped restore the photosynthetic energy balance. Overall, improved PSII functioning and increased activity of PSI cyclic electron transport (CET) and C2 CCM led to an increase in the photosynthesis intensity of S. sedoides under the combined effect of salinity and elevated temperature relative to high temperature alone.

Plants cope with fluctuating light by frequency-dependent nonphotochemical quenching and cyclic electron transport

In natural environments, plants are exposed to rapidly changing light. Maintaining photosynthetic efficiency while avoiding photodamage requires equally rapid regulation of photoprotective mechanisms. We asked what the operation frequency range of regulation is in which plants can efficiently respond to varying light. Chlorophyll fluorescence, P700, plastocyanin, and ferredoxin responses of wild-types Arabidopsis thaliana were measured in oscillating light of various frequencies. We also investigated the npq1 mutant lacking violaxanthin de-epoxidase, the npq4 mutant lacking PsbS protein, and the mutants crr2-2, and pgrl1ab impaired in different pathways of the cyclic electron transport. The fastest was the PsbS-regulation responding to oscillation periods longer than 10 s. Processes involving violaxanthin de-epoxidase dampened changes in chlorophyll fluorescence in oscillation periods of 2 min or longer. Knocking out the PGR5/PGRL1 pathway strongly reduced variations of all monitored parameters, probably due to congestion in the electron transport. Incapacitating the NDH-like pathway only slightly changed the photosynthetic dynamics. Our observations are consistent with the hypothesis that nonphotochemical quenching in slow light oscillations involves violaxanthin de-epoxidase to produce, presumably, a largely stationary level of zeaxanthin. We interpret the observed dynamics of photosystem I components as being formed in slow light oscillations partially by thylakoid remodeling that modulates the redox rates.

Distinct contribution of two cyclic electron transport pathways to P700 oxidation

Cyclic electron transport (CET) around Photosystem I (PSI) acidifies the thylakoid lumen and downregulates electron transport at the cytochrome b6f complex. This photosynthetic control is essential for oxidizing special pair chlorophylls (P700) of PSI for PSI photoprotection. In addition, CET depending on the PROTON GRADIENT REGULATION 5 (PGR5) protein oxidizes P700 by moving a pool of electrons from the acceptor side of PSI to the plastoquinone pool. This model of the acceptor-side regulation was proposed on the basis of the phenotype of the Arabidopsis (Arabidopsis thaliana) pgr5-1 mutant expressing Chlamydomonas (Chlamydomonas reinhardtii) plastid terminal oxidase (CrPTOX2). In this study, we extended the research including the Arabidopsis chlororespiratory reduction 2-2 (crr2-2) mutant defective in another CET pathway depending on the chloroplast NADH dehydrogenase-like (NDH) complex. Although the introduction of CrPTOX2 did not complement the defect in the acceptor-side regulation by PGR5, the function of the NDH complex was complemented except for its reverse reaction during the induction of photosynthesis. We evaluated the impact of CrPTOX2 under fluctuating light intensity in the wild-type, pgr5-1 and crr2-2 backgrounds. In the high-light period, both PGR5- and NDH-dependent CET were involved in the induction of photosynthetic control, whereas PGR5-dependent CET preferentially contributed to the acceptor-side regulation. On the contrary, the NDH complex probably contributed to the acceptor-side regulation in the low-light period but not in the high-light period. We evaluated the sensitivity of PSI to fluctuating light and clarified that acceptor-side regulation was necessary for PSI photoprotection by oxidizing P700 under high light.

Structural insights into photosynthetic cyclic electron transport

During photosynthesis, light energy is utilized to drive sophisticated biochemical chains of electron transfers, converting solar energy into chemical energy that feeds most life on earth. Cyclic electron transfer/flow (CET/CEF) plays an essential role in efficient photosynthesis, as it balances the ATP/NADPH ratio required in various regulatory and metabolic pathways. Photosystem I, cytochrome b₆f, and NADH dehydrogenase (NDH) are large multisubunit protein complexes embedded in the thylakoid membrane of the chloroplast and key players in NDH-dependent CEF pathway. Furthermore, small mobile electron carriers serve as shuttles for electrons between these membrane protein complexes. Efficient electron transfer requires transient interactions between these electron donors and acceptors. Structural biology has been a powerful tool to advance our knowledge of this important biological process. A number of structures of the membrane-embedded complexes, soluble electron carrier proteins, and transient complexes composed of both have now been determined. These structural data reveal detailed interacting patterns of these electron donor-acceptor pairs, thus allowing us to visualize the different parts of the electron transfer process. This review summarizes the current state of structural knowledge of three membrane complexes and their interaction patterns with mobile electron carrier proteins.

Inactivation of photosynthetic cyclic electron transports upregulates photorespiration for compensation of efficient photosynthesis in Arabidopsis

Plants have multiple mechanisms to maintain efficient photosynthesis. Photosynthetic cyclic electron transports around photosystem I (CET), which includes the PGR5/PGRL1 and NDH pathways, and photorespiration play a crucial role in photosynthetic efficiency. However, how these two mechanisms are functionally linked is not clear. In this study, we revealed that photorespiration could compensate for the function of CET in efficient photosynthesis by comparison of the growth phenotypes, photosynthetic properties monitored with chlorophyll fluorescence parameters and photosynthetic oxygen evolution in leaves and photorespiratory activity monitored with the difference of photosynthetic oxygen evolution rate under high and low concentration of oxygen conditions between the deleted mutant PGR5 or PGRL1 under NDH defective background (pgr5 crr2 or pgrl1a1b crr2). Both CET mutants pgr5 crr2 and pgrl1a1b crr2 displayed similar suppression effects on photosynthetic capacities of light reaction and growth phenotypes under low light conditions. However, the total CET activity and photosynthetic oxygen evolution of pgr5 crr2 were evidently lower than those of pgrl1a1b crr2, accompanied by the upregulation of photorespiratory activity under low light conditions, resulting in severe suppression of photosynthetic capacities of light reaction and finally photodamaged phenotype under high light or fluctuating light conditions. Based on these findings, we suggest that photorespiration compensates for the loss of CET functions in the regulation of photosynthesis and that coordination of both mechanisms is essential for maintaining the efficient operation of photosynthesis, especially under stressed conditions.

Adenylates regulate Arabidopsis plastidial thioredoxin activities through the binding of a CBS domain protein

Cystathionine-β-synthase (CBS) domains are found in proteins of all living organisms and have been proposed to play a role as energy sensors regulating protein activities through their adenosyl ligand binding capacity. In plants, members of the CBSX protein family carry a stand-alone pair of CBS domains. In Arabidopsis (Arabidopsis thaliana), CBSX1 and CBSX2 are targeted to plastids where they have been proposed to regulate thioredoxins (TRXs). TRXs are ubiquitous cysteine thiol oxido-reductases involved in the redox-based regulation of numerous enzymatic activities as well as in the regeneration of thiol-dependent peroxidases. In Arabidopsis, 10 TRX isoforms have been identified in plastids and divided into five sub-types. Here, we show that CBSX2 specifically inhibits the activities of m-type TRXs toward two chloroplast TRX-related targets. By testing activation of NADP-malate dehydrogenase and reduction of 2-Cys peroxiredoxin, we found that TRXm1/2 inhibition by CBSX2 was alleviated in the presence of AMP or ATP. We also determined, by pull-down assays, a direct interaction of CBSX2 with reduced TRXm1 and m2 that was abolished in the presence of adenosyl ligands. In addition, we report that, compared with wild-type plants, the Arabidopsis T-DNA double mutant cbsx1 cbsx2 exhibits growth and chlorophyll accumulation defects in cold conditions, suggesting a function of plastidial CBSX proteins in plant stress adaptation. Together, our results show an energy-sensing regulation of plastid TRX m activities by CBSX, possibly allowing a feedback regulation of ATP homeostasis via activation of cyclic electron flow in the chloroplast, to maintain a high energy level for optimal growth.

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.

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.

Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato

Fluctuating light (FL) is a typical natural light stress that can cause photodamage to photosystem I (PSI). However, the effect of growth light on FL-induced PSI photoinhibition remains controversial. Plants grown under high light enhance photorespiration to sustain photosynthesis, but the contribution of photorespiration to PSI photoprotection under FL is largely unknown. In this study, we examined the photosynthetic performance under FL in tomato (Lycopersicon esculentum) plants grown under high light (HL-plants) and moderate light (ML-plants). After an abrupt increase in illumination, the over-reduction of PSI was lowered in HL-plants, resulting in a lower FL-induced PSI photoinhibition. HL-plants displayed higher capacities for CO2 fixation and photorespiration than ML-plants. Within the first 60 s after transition from low to high light, PSII electron transport was much higher in HL-plants, but the gross CO2 assimilation rate showed no significant difference between them. Therefore, upon a sudden increase in illumination, the difference in PSII electron transport between HL- and ML-plants was not attributed to the Calvin–Benson cycle but was caused by the change in photorespiration. These results indicated that the higher photorespiration in HL-plants enhanced the PSI electron sink downstream under FL, which mitigated the over-reduction of PSI and thus alleviated PSI photoinhibition under FL. Taking together, we here for the first time propose that photorespiration acts as a safety valve for PSI photoprotection under FL.

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.

Current Knowledge on Mechanisms Preventing Photosynthesis Redox Imbalance in Plants

Photosynthesis includes a set of redox reactions that are the source of reducing power and energy for the assimilation of inorganic carbon, nitrogen and sulphur, thus generating organic compounds, and oxygen, which supports life on Earth. As sessile organisms, plants have to face continuous changes in environmental conditions and need to adjust the photosynthetic electron transport to prevent the accumulation of damaging oxygen by-products. The balance between photosynthetic cyclic and linear electron flows allows for the maintenance of a proper NADPH/ATP ratio that is adapted to the plant’s needs. In addition, different mechanisms to dissipate excess energy operate in plants to protect and optimise photosynthesis under adverse conditions. Recent reports show an important role of redox-based dithiol–disulphide interchanges, mediated both by classical and atypical chloroplast thioredoxins (TRXs), in the control of these photoprotective mechanisms. Moreover, membrane-anchored TRX-like proteins, such as HCF164, which transfer electrons from stromal TRXs to the thylakoid lumen, play a key role in the regulation of lumenal targets depending on the stromal redox poise. Interestingly, not all photoprotective players were reported to be under the control of TRXs. In this review, we discuss recent findings regarding the mechanisms that allow an appropriate electron flux to avoid the detrimental consequences of photosynthesis redox imbalances.

Roles of alternative electron flows in response to excess light in Ginkgo biloba

Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O₂ mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba, FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba, FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light.

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.

Critical Role of NdhA in the Incorporation of the Peripheral Arm into the Membrane-Embedded Part of the Chloroplast NADH Dehydrogenase-Like Complex

The chloroplast NADH dehydrogenase-like (NDH) complex mediates ferredoxin-dependent plastoquinone reduction in the thylakoid membrane. In angiosperms, chloroplast NDH is composed of five subcomplexes and further forms a supercomplex with photosystem I (PSI). Subcomplex A (SubA) mediates the electron transport and consists of eight subunits encoded by both plastid and nuclear genomes. The assembly of SubA in the stroma has been extensively studied, but it is unclear how SubA is incorporated into the membrane-embedded part of the NDH complex. Here, we isolated a novel Arabidopsis mutant chlororespiratory reduction 16 (crr16) defective in NDH activity. CRR16 encodes a chloroplast-localized P-class pentatricopeptide repeat protein conserved in angiosperms. Transcript analysis of plastid-encoded ndh genes indicated that CRR16 was responsible for the efficient splicing of the group II intron in the ndhA transcript, which encodes a membrane-embedded subunit localized to the connecting site between SubA and the membrane subcomplex (SubM). To analyze the roles of NdhA in the assembly and stability of the NDH complex, the homoplastomic knockout plant of ndhA (ΔndhA) was generated in tobacco (Nicotiana tabacum). Biochemical analyses of crr16 and ΔndhA plants indicated that NdhA was essential for stabilizing SubA and SubE but not for the accumulation of the other three subcomplexes. Furthermore, the crr16 mutant accumulated the SubA assembly intermediates in the stroma more than that in the wild type. These results suggest that NdhA biosynthesis is essential for the incorporation of SubA into the membrane-embedded part of the NDH complex at the final assembly step of the NDH-PSI supercomplex.

The Complementary Roles of Chloroplast Cyclic Electron Transport and Mitochondrial Alternative Oxidase to Ensure Photosynthetic Performance

Chloroplasts use light energy and a linear electron transport (LET) pathway for the coupled generation of NADPH and ATP. It is widely accepted that the production ratio of ATP to NADPH is usually less than required to fulfill the energetic needs of the chloroplast. Left uncorrected, this would quickly result in an over-reduction of the stromal pyridine nucleotide pool (i.e., high NADPH/NADP⁺ ratio) and under-energization of the stromal adenine nucleotide pool (i.e., low ATP/ADP ratio). These imbalances could cause metabolic bottlenecks, as well as increased generation of damaging reactive oxygen species. Chloroplast cyclic electron transport (CET) and the chloroplast malate valve could each act to prevent stromal over-reduction, albeit in distinct ways. CET avoids the NADPH production associated with LET, while the malate valve consumes the NADPH associated with LET. CET could operate by one of two different pathways, depending upon the chloroplast ATP demand. The NADH dehydrogenase-like pathway yields a higher ATP return per electron flux than the pathway involving PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1). Similarly, the malate valve could couple with one of two different mitochondrial electron transport pathways, depending upon the cytosolic ATP demand. The cytochrome pathway yields a higher ATP return per electron flux than the alternative oxidase (AOX) pathway. In both Arabidopsis thaliana and Chlamydomonas reinhardtii, PGR5/PGRL1 pathway mutants have increased amounts of AOX, suggesting complementary roles for these two lesser-ATP yielding mechanisms of preventing stromal over-reduction. These two pathways may become most relevant under environmental stress conditions that lower the ATP demands for carbon fixation and carbohydrate export.

Elevated CO2 Concentration Alters Photosynthetic Performances under Fluctuating Light in Arabidopsis thaliana

In view of the current and expected future rise in atmospheric CO₂ concentrations, we examined the effect of elevated CO₂ on photoinhibition of photosystem I (PSI) under fluctuating light in Arabidopsis thaliana. At 400 ppm CO₂, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO₂ conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO₂, indicating that elevated atmospheric CO₂ facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO₂ than in 400 ppm CO₂, suggesting that elevated atmospheric CO₂ mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO₂ does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO₂ fixation. Therefore, elevated CO₂ mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO₂ can have large effects on thylakoid reactions under fluctuating light.

Light Quality-Dependent Regulation of Non-Photochemical Quenching in Tomato Plants

Simple Summary

Photosynthetic organisms, such as land plants, evolved to utilize available light and to use its energy to assimilate carbon dioxide and produce carbohydrates. However, the light intensity often exceeds the ability of plants to successfully utilize absorbed energy, thus inducing stress, manifested by an increased radical concentration inside plant cells and disruption of the inner structures, and consequently decreased plant yield. Plants solve this problem by using a mechanism termed non-photochemical quenching, by which they can dissipate the energy not used in photosynthesis. Modern agriculture, however, also involves indoor plant farming. For indoor plant farming LED-based lighting systems, with non-saturating light intensities, are suitable based on their restricted energy consumption. However, the composition of applied light should first be optimized to maximize its utilization. Our study examined the influence of monochromatic LEDs (red, green, and blue) on the photoprotective and photosynthetic properties of tomato plants. We indicate that monochromatic green light could be considered an important component of lighting systems to alleviate energy dissipation, while blue light enhances photosynthetic efficiency. Our study not only proves the crucial importance of spectrum optimization but also provides evidence that different light wavelengths modify photosynthetic and photoprotective properties.

Abstract

Photosynthetic pigments of plants capture light as a source of energy for photosynthesis. However, the amount of energy absorbed often exceeds its utilization, thus causing damage to the photosynthetic apparatus. Plants possess several mechanisms to minimize such risks, including non-photochemical quenching (NPQ), which allows them to dissipate excess excitation energy in the form of harmless heat. However, under non-stressful conditions in indoor farming, it would be favorable to restrict the NPQ activity and increase plant photosynthetic performance by optimizing the light spectrum. Towards this goal, we investigated the dynamics of NPQ, photosynthetic properties, and antioxidant activity in the leaves of tomato plants grown under different light qualities: monochromatic red (R), green (G), or blue (B) light (L) at 80 µmol m⁻² s⁻¹ and R:G:B = 1:1:1 (referred to as the white light (WL)) at 120 µmol m⁻² s⁻¹. The results confirm that monochromatic BL increased the quantum efficiency of PSII and photosynthetic pigments accumulation. The RL and BL treatments enhanced the NPQ amplitude and showed negative effects on antioxidant enzyme activity. In contrast, plants grown solely under GL or WL presented a lower amplitude of NPQ due to the reduced accumulation of NPQ-related proteins, photosystem II (PSII) subunit S (PsbS), PROTON GRADIENT REGULATION-LIKE1 (PGRL1), cytochrome b₆f subunit f (cytf) and violaxanthin de-epoxidase (VDE). Additionally, we noticed that plants grown under GL or RL presented an increased rate of lipid peroxidation. Overall, our results indicate the potential role of GL in lowering the NPQ amplitude, while the role of BL in the RGB spectrum is to ensure photosynthetic performance and photoprotective properties.

A Holistic Approach to Study Photosynthetic Acclimation Responses of Plants to Fluctuating Light

Plants in natural environments receive light through sunflecks, the duration and distribution of these being highly variable across the day. Consequently, plants need to adjust their photosynthetic processes to avoid photoinhibition and maximize yield. Changes in the composition of the photosynthetic apparatus in response to sustained changes in the environment are referred to as photosynthetic acclimation, a process that involves changes in protein content and composition. Considering this definition, acclimation differs from regulation, which involves processes that alter the activity of individual proteins over short-time periods, without changing the abundance of those proteins. The interconnection and overlapping of the short- and long-term photosynthetic responses, which can occur simultaneously or/and sequentially over time, make the study of long-term acclimation to fluctuating light in plants challenging. In this review we identify short-term responses of plants to fluctuating light that could act as sensors and signals for acclimation responses, with the aim of understanding how plants integrate environmental fluctuations over time and tailor their responses accordingly. Mathematical modeling has the potential to integrate physiological processes over different timescales and to help disentangle short-term regulatory responses from long-term acclimation responses. We review existing mathematical modeling techniques for studying photosynthetic responses to fluctuating light and propose new methods for addressing the topic from a holistic point of view.

The Significance of Chloroplast NAD(P)H Dehydrogenase Complex and Its Dependent Cyclic Electron Transport in Photosynthesis

Chloroplast NAD(P)H dehydrogenase (NDH) complex, a multiple-subunit complex in the thylakoid membranes mediating cyclic electron transport, is one of the most important alternative electron transport pathways. It was identified to be essential for plant growth and development during stress periods in recent years. The NDH-mediated cyclic electron transport can restore the over-reduction in stroma, maintaining the balance of the redox system in the electron transfer chain and providing the extra ATP needed for the other biochemical reactions. In this review, we discuss the research history and the subunit composition of NDH. Specifically, the formation and significance of NDH-mediated cyclic electron transport are discussed from the perspective of plant evolution and physiological functionality of NDH facilitating plants' adaptation to environmental stress. A better understanding of the NDH-mediated cyclic electron transport during photosynthesis may offer new approaches to improving crop yield.