Psb28 protein is indispensable for stable accumulation of PSII core complexes in Arabidopsis.

Enhancing the efficiency of photosynthesis represents a promising strategy to improve crop yields, with keeping the steady state of PSII being key to determining the photosynthetic performance. However, the mechanisms whereby the stability of PSII is maintained in oxygenic organisms remain to be explored. Here, we report that the Psb28 protein functions in regulating the homeostasis of PSII under different light conditions in Arabidopsis thaliana. The psb28 mutant is much smaller than the wild-type plants under normal growth light, which is due to its significantly reduced PSII activity. Similar defects were seen under low light and became more pronounced under photoinhibitory light. Notably, the amounts of PSII core complexes and core subunits are specifically decreased in psb28, whereas the abundance of other representative components of photosynthetic complexes remains largely unaltered. Although the PSII activity of psb28 was severely reduced when subjected to high light, its recovery from photoinactivation was not affected. By contrast, the degradation of PSII core protein subunits is dramatically accelerated in the presence of lincomycin. These results indicate that psb28 is defective in the photoprotection of PSII, which is consistent with the observation that the overall NPQ is much lower in psb28 compared to the wild type. Moreover, the Psb28 protein is associated with PSII core complexes and interacts mainly with the CP47 subunit of PSII core. Taken together, these findings reveal an important role for Psb28 in the protection and stabilization of PSII core in response to changes in light environments.

Skeletonema marinoi ecotypes show specific habitat-related responses to fluctuating light supporting high potential for growth under photobioreactor light regime.

Diatoms are a diverse group of phytoplankton usually dominating areas characterized by rapidly shifting light conditions. Because of their high growth rates and interesting biochemical profile, their biomass is considered for various commercial applications. This study aimed at identifying strains with superior growth in a photobioreactor (PBR) by screening the natural intraspecific diversity of ecotypes isolated from different habitats. We investigated the effect of PBR light fluctuating on a millisecond scale (FL, simulating the light in a PBR) on 19 ecotypes of the diatom Skeletonema marinoi isolated from the North Sea-Baltic Sea area. We compare growth, pigment ratios, phylogeny, photo-physiological variables and photoacclimation strategies between all strains and perform qPCR and absorption spectra analysis on a subset of strains. Our results show that the ecotypes responded differently to FL, and have contrasting photo-physiological and photoprotective strategies. The strains from Kattegat performed better in FL, and shared common photoacclimation and photoprotection strategies that are the results of adaptation to the specific light climate of the Kattegat area. The strains that performed better with FL conditions had a high light (HL)-acclimated phenotype coupled with unique nonphotochemical quenching features. Based on their characteristics, three strains were identified as good candidates for growth in PBRs.

Intervening dark periods negatively affect the photosynthetic performance of Chlamydomonas reinhardtii during growth under fluctuating high light.

The acclimation of the green algae Chlamydomoas reinhardtii to high light (HL) has been studied predominantly under continuous illumination of the cells. Here, we investigated the impact of fluctuating HL in alternation with either low light (LL) or darkness on photosynthetic performance and on photoprotective responses. Compared to intervening LL phases, dark phases led to (1) more pronounced reduction of the photosystem II quantum efficiency, (2) reduced degradation of the PsbS protein, (3) lower energy dissipation capacity and (4) an increased pool size of the xanthophyll cycle pigments. These characteristics indicate increased photo-oxidative stress when HL periods are interrupted by dark phases instead of LL phases. This overall trend was similar when comparing long (8 h) and short (30 min) HL phases being interrupted by long (16 h) and short (60 min) phases of dark or low light, respectively. Only the degradation of PsbS was clearly more efficient during long (16 h) LL phases when compared to short (60 min) LL phases.

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.

Contrasting effects of experiencing temporally heterogeneous light availability versus homogenous shading on plant subsequent responses to light conditions.

Temporally heterogeneous environments is hypothesized to correlate with greater plasticity of plants, which has rarely been supported by direct evidence. To address this issue, we subjected three species from different ranges of habitats to a first round of alternating full light and heavy shading (temporally heterogeneous light experience), constant moderate shading and full light conditions (temporally homogeneous light experiences, control) and a second round of light-gradient treatments. We measured plant performance in a series of morphological, biomass, physiological and biochemical traits at the end of each round. Compared to constant full light experience, temporally heterogeneous light conditions induced immediate active biochemical responses (in the first round) with improved late growth in biomass (during the second round); constant moderate shading experience increased photosynthetic physiological and biomass performances of plants in early response, and decreased their late growth in biomass. The karst endemic species of Kmeria septentrionalis showed greater improvement in late growth of biomass and lower decrease in biochemical performance, due to early heterogeneous experience, compared to the non-karst species of Lithocarpus glaber and the karst adaptable species of Celtis sinensis. Results suggested plants will prefer to produce morphological and physiological responses that are less reversible and more costly in the face of more reliable environmental cues at early stage in spite of decreased future growth potential, but to produce immediate biochemical responses for higher late growth potential when early environmental cues are less reliable, to avoid the loss of high costs and low profits. Typical karst species should be more able to benefit from early temporally heterogeneous experience, due to long-term adaptation to karst habitats of high environmental heterogeneity and low resource availability.

A low-cost and realistic noisy light system for studying photosynthesis.

Unlike the light conditions commonly used to grow photosynthetic organisms in the research laboratory, the light intensity in real environments is dynamic. A simple and low-cost system is described in which a commercial dimmable LED panel is controlled to simulate a sinusoidal function representing daylight hours and overlaid with stochastic shading events. The output closely resembles light intensity measurements on Earth's surface on partly cloudy days or in lower levels of plant canopies. This tool may be useful to researchers studying photosynthetic acclimation responses.

Plant NADPH-dependent thioredoxin reductases are crucial for the metabolism of sink leaves and plant acclimation to elevated CO2.

Plants contain three NADPH-thioredoxin reductases (NTR) located in the cytosol/mitochondria (NTRA/B) and the plastid (NTRC) with important metabolic functions. However, mutants deficient in all NTRs remained to be investigated. Here, we generated and characterised the triple Arabidopsis ntrabc mutant alongside with ntrc single and ntrab double mutants under different environmental conditions. Both ntrc and ntrabc mutants showed reduced growth and substantial metabolic alterations, especially in sink leaves and under high CO2 (HC), as compared to the wild type. However, ntrabc showed higher effective quantum yield of PSII under both constant and fluctuating light conditions, altered redox states of NADH/NAD+ and glutathione (GSH/GSSG) and lower potential quantum yield of PSII in sink leaves in ambient but not high CO2 concentrations, as compared to ntrc, suggesting a functional interaction between chloroplastic and extra-chloroplastic NTRs in photosynthesis regulation depending on leaf development and environmental conditions. Our results unveil a previously unknown role of the NTR system in regulating sink leaf metabolism and plant acclimation to HC, while it is not affecting full plant development, indicating that the lack of the NTR system can be compensated, at least to some extent, by other redox mechanisms.

Maize (Zea mays L.) planted at higher density utilizes dynamic light more efficiently.

In nature, plants are exposed to a dynamic light environment. Fluctuations in light decreased the photosynthetic light utilization efficiency (PLUE) of leaves, and much more severely in C4 species than in C3 species. However, little is known about the plasticity of PLUE under dynamic light in C4 species. Present study focused on the influence of planting density to the photosynthesis under dynamic light in maize (Zea mays L.), a most important C4 crop. In addition, the molecular mechanism behind photosynthetic adaptation to planting density were also explored by quantitative proteomics analysis. Results revealed that as planting density increases, maize leaves receive less light that fluctuates more. The maize planted at high density (HD) improved the PLUE under dynamic light, especially in the middle and later growth stages. Quantitative proteomics analysis showed that the transfer of nitrogen from Rubisco to RuBP regeneration and C4 pathway related enzymes contributes to the photosynthetic adaptation to lower and more fluctuating light environment in HD maize. This study provides potential ways to further improve the light energy utilization efficiency of maize in HD.

Dynamic response of photorespiration in fluctuating light environments.

Photorespiration is a dynamic process that is intimately linked to photosynthetic carbon assimilation. There is a growing interest in understanding carbon assimilation during dynamic conditions, but the role of photorespiration under such conditions is unclear. In this review, we discuss recent work relevant to the function of photorespiration under dynamic conditions, with a special focus on light transients. This work reveals that photorespiration is a fundamental component of the light induction of assimilation where variable diffusive processes limit CO2 exchange with the atmosphere. Additionally, metabolic interactions between photorespiration and the C3 cycle may help balance fluxes under dynamic light conditions. We further discuss how the energy demands of photorespiration present special challenges to energy balancing during dynamic conditions. We finish the review with an overview of why regulation of photorespiration may be important under dynamic conditions to maintain appropriate fluxes through metabolic pathways related to photorespiration such as nitrogen and one-carbon metabolism.

Dynamic growth QTL action in diverse light environments: characterization of light regime-specific and stable QTL in Arabidopsis.

Plant growth is a complex process affected by a multitude of genetic and environmental factors and their interactions. To identify genetic factors influencing plant performance under different environmental conditions, vegetative growth was assessed in Arabidopsis thaliana cultivated under constant or fluctuating light intensities, using high-throughput phenotyping and genome-wide association studies. Daily automated non-invasive phenotyping of a collection of 382 Arabidopsis accessions provided growth data during developmental progression under different light regimes at high temporal resolution. Quantitative trait loci (QTL) for projected leaf area, relative growth rate, and PSII operating efficiency detected under the two light regimes were predominantly condition-specific and displayed distinct temporal activity patterns, with active phases ranging from 2 d to 9 d. Eighteen protein-coding genes and one miRNA gene were identified as potential candidate genes at 10 QTL regions consistently found under both light regimes. Expression patterns of three candidate genes affecting projected leaf area were analysed in time-series experiments in accessions with contrasting vegetative leaf growth. These observations highlight the importance of considering both environmental and temporal patterns of QTL/allele actions and emphasize the need for detailed time-resolved analyses under diverse well-defined environmental conditions to effectively unravel the complex and stage-specific contributions of genes affecting plant growth processes.

Light-harvesting complex stress-related proteins play crucial roles in the acclimation of Physcomitrella patens under fluctuating light conditions.

Photosynthetic organisms have evolved photoprotective mechanisms to acclimate to light intensity fluctuations in their natural growth environments. Photosystem (PS) II subunit S (PsbS) and light-harvesting complex (LHC) stress-related proteins (LhcSR) are essential for triggering photoprotection in vascular plants and green algae, respectively. The activity of both proteins is strongly enhanced in the moss Physcomitrella patens under high-light conditions. However, their role in regulating photosynthesis acclimation in P. patens under fluctuating light (FL) conditions is still unknown. Here, we compare the responses of wild-type (WT) P. patens and mutants lacking PsbS (psbs KO) or LhcSR1 and 2 (lhcsr KO) to FL conditions in which the low-light phases were periodically interrupted with high-light pulses. lhcsr KO mutant showed a strong reduction in growth with respect to WT and psbs KO under FL conditions. The lack of LhcSR not only decreased the level of non-photochemical quenching, resulting in an over-reduced plastoquinone pool, but also significantly increased the PSI acceptor limitation values with respect to WT and psbs KO under FL conditions. Moreover, in lhcsr KO mutant, the abundance of PSI core and PSI-LHCI complex decreased greatly under FL conditions compared with the WT and psbs KO. We proposed that LhcSR in P. patens play a crucial role in moss acclimation to dynamic light changes.

Towards improved dynamic photosynthesis in C3 crops by utilizing natural genetic variation.

Under field environments, fluctuating light conditions induce dynamic photosynthesis, which affects carbon gain by crop plants. Elucidating the natural genetic variations among untapped germplasm resources and their underlying mechanisms can provide an effective strategy to improve dynamic photosynthesis and, ultimately, improve crop yields through molecular breeding approaches. In this review, we first overview two processes affecting dynamic photosynthesis, namely (i) biochemical processes associated with CO2 fixation and photoprotection and (ii) gas diffusion processes from the atmosphere to the chloroplast stroma. Next, we review the intra- and interspecific variations in dynamic photosynthesis in relation to each of these two processes. It is suggested that plant adaptations to different hydrological environments underlie natural genetic variation explained by gas diffusion through stomata. This emphasizes the importance of the coordination of photosynthetic and stomatal dynamics to optimize the balance between carbon gain and water use efficiency under field environments. Finally, we discuss future challenges in improving dynamic photosynthesis by utilizing natural genetic variation. The forward genetic approach supported by high-throughput phenotyping should be introduced to evaluate the effects of genetic and environmental factors and their interactions on the natural variation in dynamic photosynthesis.

NaCl affects photosynthetic and stomatal dynamics by osmotic effects and reduces photosynthetic capacity by ionic effects in tomato.

NaCl stress affects stomatal behavior and photosynthesis by a combination of osmotic and ionic components, but it is unknown how these components affect stomatal and photosynthetic dynamics. Tomato (Solanum lycopersicum) plants were grown in a reference nutrient solution [control; electrical conductivity (EC)=2.3 dS m-1], a solution containing additional macronutrients (osmotic effect; EC=12.6 dS m-1), or a solution with additional 100 mM NaCl (osmotic and ionic effects; EC=12.8 dS m-1). Steady-state and dynamic photosynthesis, and leaf biochemistry, were characterized throughout leaf development. The osmotic effect decreased steady-state stomatal conductance while speeding up stomatal responses to light intensity shifts. After 19 d of treatment, photosynthetic induction was reduced by the osmotic effect, which was attributable to lower initial stomatal conductance due to faster stomatal closing under low light. Ionic effects of NaCl were barely observed in dynamic stomatal and photosynthetic behavior, but led to a reduction in leaf photosynthetic capacity, CO2 carboxylation rate, and stomatal conductance in old leaves after 26 d of treatment. With increasing leaf age, rates of light-triggered stomatal movement and photosynthetic induction decreased across treatments. We conclude that NaCl impacts dynamic stomatal and photosynthetic kinetics by osmotic effects and reduces photosynthetic capacity by ionic effects.

Insights on the regulation of photosynthesis in pea leaves exposed to oscillating light.

Plants growing in nature often experience fluctuating irradiance. However, in the laboratory, the dynamics of photosynthesis are usually explored by instantaneously exposing dark-adapted plants to constant light and examining the dark-to-light transition, which is a poor approximation of natural phenomena. With the aim creating a better approximation, we exposed leaves of pea (Pisum sativum) to oscillating light and measured changes in the functioning of PSI and PSII, and of the proton motive force at the thylakoid membrane. We found that the dynamics depended on the oscillation period, revealing information about the underlying regulatory networks. As demonstrated for a selected oscillation period of 60 s, the regulation tries to keep the reaction centers of PSI and PSII open. We present an evaluation of the data obtained, and discuss the involvement of particular processes in the regulation of photosynthesis. The forced oscillations provided an information-rich fingerprint of complex regulatory networks. We expect future progress in understanding these networks from experiments involving chemical interventions and plant mutants, and by using mathematical modeling and systems identification and control tools.

A Changing Light Environment Induces Significant Lateral CO2 Diffusion within Maize Leaves.

A leaf structure with high porosity is beneficial for lateral CO2 diffusion inside the leaves. However, the leaf structure of maize is compact, and it has long been considered that lateral CO2 diffusion is restricted. Moreover, lateral CO2 diffusion is closely related to CO2 pressure differences (ΔCO2). Therefore, we speculated that enlarging the ΔCO2 between the adjacent regions inside maize leaves may result in lateral diffusion when the diffusion resistance is kept constant. Thus, the leaf structure and gas exchange of maize (C4), cotton (C3), and other species were explored. The results showed that maize and sorghum leaves had a lower mesophyll porosity than cotton and cucumber leaves. Similar to cotton, the local photosynthetic induction resulted in an increase in the ΔCO2 between the local illuminated and the adjacent unilluminated regions, which significantly reduced the respiration rate of the adjacent unilluminated region. Further analysis showed that when the adjacent region in the maize leaves was maintained under a steady high light, the photosynthesis induction in the local regions not only gradually reduced the ΔCO2 between them but also progressively increased the steady photosynthetic rate in the adjacent region. Under field conditions, the ΔCO2, respiration, and photosynthetic rate of the adjacent region were also markedly changed by fluctuating light in local regions in the maize leaves. Consequently, we proposed that enlarging the ΔCO2 between the adjacent regions inside the maize leaves results in the lateral CO2 diffusion and supports photosynthesis in adjacent regions to a certain extent under fluctuating light.

Quantitative proteomics reveals redox-based functional regulation of photosynthesis under fluctuating light in plants.

Photosynthesis involves a series of redox reactions and is the major source of reactive oxygen species in plant cells. Fluctuating light (FL) levels, which occur commonly in natural environments, affect photosynthesis; however, little is known about the specific effects of FL on the redox regulation of photosynthesis. Here, we performed global quantitative mapping of the Arabidopsis thaliana cysteine thiol redox proteome under constant light and FL conditions. We identified 8857 redox-switched thiols in 4350 proteins, and 1501 proteins that are differentially modified depending on light conditions. Notably, proteins related to photosynthesis, especially photosystem I (PSI), are operational thiol-switching hotspots. Exposure of wild-type A. thaliana to FL resulted in decreased PSI abundance, stability, and activity. Interestingly, in response to PSI photodamage, more of the PSI assembly factor PSA3 dynamically switches to the reduced state. Furthermore, the Cys199 and Cys200 sites in PSA3 are necessary for its full function. Moreover, thioredoxin m (Trx m) proteins play roles in redox switching of PSA3, and are required for PSI activity and photosynthesis. This study thus reveals a mechanism for redox-based regulation of PSI under FL, and provides insight into the dynamic acclimation of photosynthesis in a changing environment.

Overproduction of PGR5 enhances the electron sink downstream of photosystem I in a C4 plant, Flaveria bidentis.

C4 plants can fix CO2 efficiently using CO2 -concentrating mechanisms (CCMs), but they require additional ATP. To supply the additional ATP, C4 plants operate at higher rates of cyclic electron transport around photosystem I (PSI), in which electrons are transferred from ferredoxin to plastoquinone. Recently, it has been reported that the NAD(P)H dehydrogenase-like complex (NDH) accumulated in the thylakoid membrane in leaves of C4 plants, making it a candidate for the additional synthesis of ATP used in the CCM. In addition, C4 plants have higher levels of PROTON GRADIENT REGULATION 5 (PGR5) expression, but it has been unknown how PGR5 functions in C4 photosynthesis. In this study, PGR5 was overexpressed in a C4 dicot, Flaveria bidentis. In PGR5-overproducing (OP) lines, PGR5 levels were 2.3- to 3.0-fold greater compared with wild-type plants. PGR5-like PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1), which cooperates with PGR5, increased with PGR5. A spectroscopic analysis indicated that in the PGR5-OP lines, the acceptor side limitation of PSI was reduced in response to a rapid increase in photon flux density. Although it did not affect CO2 assimilation, the overproduction of PGR5 contributed to an enhanced electron sink downstream of PSI.

Alterations in stomatal response to fluctuating light increase biomass and yield of rice under drought conditions.

The acceleration of stomatal closure upon high to low light transition could improve plant water use efficiency and drought tolerance. Herein, using genome-wide association study, we showed that the genetic variation in OsNHX1 was strongly associated with the changes in τcl , the time constant of stomatal closure, in 206 rice accessions. OsNHX1 overexpression in rice resulted in a decrease in τcl , and an increase in biomass, grain yield under drought. Conversely, OsNHX1 knockout by CRISPR/CAS9 shows opposite trends for these traits. We further found three haplotypes spanning the OsNHX1 promoter and CDS regions. Two among them, HapII and HapIII, were found to be associated with a high and low τcl , respectively. A near-isogenic line (NIL, S464) was developed through replacing the genomic region harboring HapII (~10 kb) from MH63 (recipient) rice cultivar by the same sized genomic region containing Hap III from 02428 (donor). Compared with MH63, S464 shows a reduction by 35% in τcl and an increase by 40% in the grain yield under drought. However, under normal conditions, S464 maintains closely similar grain yield as MH63. The global distribution of the two OsNHX1 haplotypes is associated with the local precipitation. Taken together, the natural variation in OsNHX1 could be utilized to manipulate the stomatal dynamics for an improved rice drought tolerance.

Photosynthesis acclimation under severely fluctuating light conditions allows faster growth of diatoms compared with dinoflagellates.

BACKGROUND: Diatoms contribute 20% of the global primary production and are adaptable in dynamic environments. Diatoms always bloom earlier in the annual phytoplankton succession instead of dinoflagellates. However, how diatoms acclimate to a dynamic environment, especially under changing light conditions, remains unclear. RESULTS: We compared the growth and photosynthesis under fluctuating light conditions of red tide diatom Skeletonema costatum, red tide dinoflagellate Amphidinium carterae, Prorocentrum donghaiense, Karenia mikimotoi, model diatom Phaeodactylum tricornutum, Thalassiosira pseudonana and model dinoflagellate Dinophycae Symbiodinium. Diatoms grew faster and maintained a consistently higher level of photosynthesis. Diatoms were sensitive to the specific inhibitor of Proton Gradient Regulation 5 (PGR5) depending photosynthetic electron flow, which is a crucial mechanism to protect their photosynthetic apparatus under fluctuating light. In contrast, the dinoflagellates were not sensitive to this inhibitor. Therefore, we investigate how PGR5 functions under light fluctuations in the model diatom P. tricornutum by knocking down and overexpressing PGR5. Overexpression of PGR5 reduced the photosystem I acceptor side limitation (Y (NA)) and increased growth rate under severely fluctuating light in contrast to the knockdown of PGR5. CONCLUSION: Diatoms acclimatize to fluctuating light conditions better than dinoflagellates. PGR5 in diatoms can regulate their photosynthetic electron flow and accelerate their growth under severe light fluctuation, supporting fast biomass accumulation under dynamic environments in pioneer blooms.

Leaf photosynthetic and anatomical insights into mechanisms of acclimation in rice in response to long-term fluctuating light.

Long-term fluctuating light (FL) conditions are very common in natural environments. The physiological and biochemical mechanisms for acclimation to FL differ between species. However, most of the current conclusions regarding acclimation to FL were made based on studies in algae or Arabidopsis thaliana. It is still unclear how rice (Oryza sativa L.) integrate multiple physiological changes to acclimate to long-term FL. In this study, we found that rice growth was repressed under long-term FL. By systematically measuring phenotypes and physiological parameters, we revealed that: (a) under short-term FL, photosystem I (PSI) was inhibited, while after 1-7 days of long-term FL, both PSI and PSII were inhibited. Higher acceptor-side limitation in electron transport and higher overall nonphotochemical quenching (NPQ) explained the lower efficiencies of PSI and PSII, respectively. (b) An increase in pH differences across the thylakoid membrane and a decrease in thylakoid proton conductivity revealed a reduction of ATP synthase activity. (c) Using electron microscopy, we showed a decrease in membrane stacking and stomatal opening after 7 days of FL treatment. Taken together, our results show that electron flow, ATP synthase activity and NPQ regulation are the major processes determining the growth performance of rice under long-term FL conditions.