Regulation of photosynthetic electron transport and photoinhibition

The operation of PEPCK increases light harvesting plasticity in C4 NAD-ME and NADP-ME photosynthetic subtypes: A theoretical study

The repeated emergence of NADP-malic enzyme (ME), NAD-ME and phosphoenolpyruvate carboxykinase (PEPCK) subtypes of C4 photosynthesis are iconic examples of convergent evolution, which suggests that these biochemistries do not randomly assemble, but are instead specific adaptations resulting from unknown evolutionary drivers. Theoretical studies that are based on the classic biochemical understanding have repeatedly proposed light-use efficiency as a possible benefit of the PEPCK subtype. However, quantum yield measurements do not support this idea. We explore this inconsistency here via an analytical model that features explicit descriptions across a seamless gradient between C4 biochemistries to analyse light harvesting and dark photosynthetic metabolism. Our simulations show that the NADP-ME subtype, operated by the most productive crops, is the most efficient. The NAD-ME subtype has lower efficiency, but has greater light harvesting plasticity (the capacity to assimilate CO2 in the broadest combination of light intensity and spectral qualities). In both NADP-ME and NAD-ME backgrounds, increasing PEPCK activity corresponds to greater light harvesting plasticity but likely imposed a reduction in photosynthetic efficiency. We draw the first mechanistic links between light harvesting and C4 subtypes, providing the theoretical basis for future investigation.

High night temperature stress on rice (Oryza sativa) - insights from phenomics to physiology. A review

Rice (Oryza sativa ) faces challenges to yield and quality due to urbanisation, deforestation and climate change, which has exacerbated high night temperature (HNT). This review explores the impacts of HNT on the physiological, molecular and agronomic aspects of rice growth. Rise in minimum temperature threatens a potential 41% reduction in rice yield by 2100. HNT disrupts rice growth stages, causing reduced seed germination, biomass, spikelet sterility and poor grain development. Recent findings indicate a 4.4% yield decline for every 1°C increase beyond 27°C, with japonica ecotypes exhibiting higher sensitivity than indica. We examine the relationships between elevated CO2 , nitrogen regimes and HNT, showing that the complexity of balancing positive CO2 effects on biomass with HNT challenges. Nitrogen enrichment proves crucial during the vegetative stage but causes disruption to reproductive stages, affecting grain yield and starch synthesis. Additionally, we elucidate the impact of HNT on plant respiration, emphasising mitochondrial respiration, photorespiration and antioxidant responses. Genomic techniques, including CRISPR-Cas9, offer potential for manipulating genes for HNT tolerance. Plant hormones and carbohydrate enzymatic activities are explored, revealing their intricate roles in spikelet fertility, grain size and starch metabolism under HNT. Gaps in understanding genetic factors influencing heat tolerance and potential trade-offs associated with hormone applications remain. The importance of interdisciplinary collaboration is needed to provide a holistic approach. Research priorities include the study of regulatory mechanisms, post-anthesis effects, cumulative HNT exposure and the interaction between climate variability and HNT impact to provide a research direction to enhance rice resilience in a changing climate.

Chloroplasts lacking class I glutaredoxins are functional but show a delayed recovery of protein cysteinyl redox state after oxidative challenge

Redox status of protein cysteinyl residues is mediated via glutathione (GSH)/glutaredoxin (GRX) and thioredoxin (TRX)-dependent redox cascades. An oxidative challenge can induce post-translational protein modifications on thiols, such as protein S-glutathionylation. Class I GRX are small thiol-disulfide oxidoreductases that reversibly catalyse S-glutathionylation and protein disulfide formation. TRX and GSH/GRX redox systems can provide partial backup for each other in several subcellular compartments, but not in the plastid stroma where TRX/light-dependent redox regulation of primary metabolism takes place. While the stromal TRX system has been studied at detail, the role of class I GRX on plastid redox processes is still unknown. We generate knockout lines of GRXC5 as the only chloroplast class I GRX of the moss Physcomitrium patens. While we find that PpGRXC5 has high activities in GSH-dependent oxidoreductase assays using hydroxyethyl disulfide or redox-sensitive GFP2 as substrates in vitro, Δgrxc5 plants show no detectable growth defect or stress sensitivity, in contrast to mutants with a less negative stromal EGSH (Δgr1). Using stroma-targeted roGFP2, we show increased protein Cys steady state oxidation and decreased reduction rates after oxidative challenge in Δgrxc5 plants in vivo, indicating kinetic uncoupling of the protein Cys redox state from EGSH. Compared to wildtype, protein Cys disulfide formation rates and S-glutathionylation levels after H2O2 treatment remained unchanged. Lack of class I GRX function in the stroma did not result in impaired carbon fixation. Our observations suggest specific roles for GRXC5 in the efficient transfer of electrons from GSH to target protein Cys as well as negligible cross-talk with metabolic regulation via the TRX system. We propose a model for stromal class I GRX function in efficient catalysis of protein dithiol/disulfide equilibria upon redox steady state alterations affecting stromal EGSH and highlight the importance of identifying in vivo target proteins of GRXC5.

Improved capacity for the repair of photosystem II via reinforcement of the translational and antioxidation systems in Synechocystis sp. PCC 6803

In the cyanobacterium Synechocystis sp. PCC 6803, translation factor EF-Tu is inactivated by reactive oxygen species (ROS) via oxidation of Cys82 and the oxidation of EF-Tu enhances the inhibition of the repair of photosystem II (PSII) by suppressing protein synthesis. In our present study, we generated transformants of Synechocystis that overexpressed a mutated form of EF-Tu, designated EF-Tu (C82S), in which Cys82 had been replaced by a Ser residue, and ROS-scavenging enzymes individually or together. Expression of EF-Tu (C82S) alone in Synechocystis enhanced the repair of PSII under strong light, with the resultant mitigation of PSII photoinhibition, but it stimulated the production of ROS. However, overexpression of superoxide dismutase and catalase, together with the expression of EF-Tu (C82S), lowered intracellular levels of ROS and enhanced the repair of PSII more significantly under strong light, via facilitation of the synthesis de novo of the D1 protein. By contrast, the activity of photosystem I was hardly affected in wild-type cells and in all the lines of transformed cells under the same strong-light conditions. Furthermore, transformed cells that overexpressed EF-Tu (C82S), superoxide dismutase, and catalase were able to survive longer under stronger light than wild-type cells. Thus, the reinforced capacity for both protein synthesis and ROS scavenging allowed both photosynthesis and cell proliferation to tolerate strong light.

Proximity to Photosystem II is necessary for activation of Plastid Terminal Oxidase (PTOX) for photoprotection

The Plastid Terminal Oxidase (PTOX) is a chloroplast localized plastoquinone oxygen oxidoreductase suggested to have the potential to act as a photoprotective safety valve for photosynthesis. However, PTOX overexpression in plants has been unsuccessful at inducing photoprotection, and the factors that control its activity remain elusive. Here, we show that significant PTOX activity is induced in response to high light in the model species Eutrema salsugineum and Arabidopsis thaliana. This activation correlates with structural reorganization of the thylakoid membrane. Over-expression of PTOX in mutants of Arabidopsis thaliana perturbed in thylakoid stacking also results in such activity, in contrast to wild type plants with normal granal structure. Further, PTOX activation protects against photoinhibition of Photosystem II and reduces reactive oxygen production under stress conditions. We conclude that structural re-arrangements of the thylakoid membranes, bringing Photosystem II and PTOX into proximity, are both required and sufficient for PTOX to act as a Photosystem II sink and play a role in photoprotection.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Genomics and Physiology of Chlorophyll Fluorescence Parameters in Hordeum vulgare L. under Drought and Salt Stresses

To map the genomic regions and control chlorophyll fluorescence attributes under normal, salinity-, and drought-stress conditions in barley (Hordeum vulgare L.) at the seedling stage, an experiment was conducted in 2019-2020 using 106 F8 lines resulting from the cross between Badia × Kavir. Initially, the different chlorophyll fluorescence parameters were evaluated. Under drought stress, the highest decrease was related to REo/CSm (59.56%), and the highest increase was related to dV/dto (77.17%). Also, under salinity stress, the highest decrease was related to Fv/Fo (59.56%), and the highest increase was related to DIo/RC (77.17%). Linkage maps were prepared using 152 SSR polymorphic markers, 72 ISSR alleles, 7 IRAP alleles, 29 CAAT alleles, 27 Scot alleles, and 15 iPBS alleles. The obtained map accounted for 999.2 centi-Morgans (cM) of the barley genome length (92% of the whole barley genome). The results indicated the importance of chromosomes 3, 2, and 7 in controlling ABS/CSm, Area, ETo/CSm, Fm, Fv, and ETo/RC under drought stress. qEToRCD-7, as a major QTL, controlled 18.3% of ETo/RC phenotypic variation under drought stress. Under salinity stress, the regions of chromosomes 2 and 7 (102 cM and 126 cM) controlled the parameters ABS/CSo, Fm, Fo, Fv, TRo/SCo, Area, ETo/CSm, and ETo/CSo. The results showed that chlorophyll fluorescence is an important parameter in the study of drought and salinity effects on barley. This is the first report of the investigation of changes in the genetic structure of quantitative genes controlling the fluorescence parameters associated with barley response to drought and salinity stresses in the Iranian barley RILs population. According to the obtained results, it is possible to use HVPLASC1B and EBmac0713 in normal conditions, ISSR21-2 and ISSR30-4 in drought conditions, and Bmac0047, Scot5-B, CAAT6-C, and ISSR30iPBS2076-4 in saline stress conditions to select genotypes with higher photosynthetic capacity in marker-assisted selection programs.

Evaluation of secondary sexual dimorphism of the dioecious Amaranthus palmeri under abiotic stress

The evolution of secondary sex-specific traits of dioecious species under abiotic stress conditions has received limited research, especially in the case of Amaranthus palmeri, a fast adapting and highly competing plant. Here, we have examined the interactive effects of abiotic stress on mineral accumulation, chlorophyll a and b content, and the operating capacity of Photosystem II (PSII) in both male and female A. palmeri plants grown under three different intensities of white light, and under N, K or P deficiency. Mineral profiling of the leaves and stems (with inflorescence) highlighted intra- and intersexual differences in their accumulation pattern and mineral associations. Chlorophyll a and chlorophyll b were different between the male and the female plants, being slightly lower in the latter, at high light intensity towards maturity, or under K or P deficiency. Further, slight, although statistically significant differences were recorded in the chlorophyll a/b ratio, which was lower at the higher light intensity in the female, over that in the male, plants towards maturity. Chlorophyll fluorescence parameters, i.e., steady state and maximum fluorescence increased under high light intensity, whereas the PSII operating efficiency decreased in the female plants, indicating reduced PSII capacity. Sex-specific differences in A. palmeri showed a differential response to stressful conditions because of differences in their ontogeny and physiology, and possibly due to the cost of reproduction. We suggest that the breeding system of dioecious species has weaknesses that can be used for the ecological management of dioecious weeds without relying on the use of herbicides.

Chlamydomonas: Fast tracking from genomics

Elucidating biological processes has relied on the establishment of model organisms, many of which offer advantageous features such as rapid axenic growth, extensive knowledge of their physiological features and gene content, and the ease with which they can be genetically manipulated. The unicellular green alga Chlamydomonas reinhardtii has been an exemplary model that has enabled many scientific breakthroughs over the decades, especially in the fields of photosynthesis, cilia function and biogenesis, and the acclimation of photosynthetic organisms to their environment. Here, we discuss recent molecular/technological advances that have been applied to C. reinhardtii and how they have further fostered its development as a "flagship" algal system. We also explore the future promise of this alga in leveraging advances in the fields of genomics, proteomics, imaging, and synthetic biology for addressing critical future biological issues.

Metabolic sink engineering in cyanobacteria: Perspectives and applications

Recent advances in metabolic engineering have made cyanobacteria emerge as promising and attractive microorganisms for sustainable production, by exploiting their natural capability for producing metabolites. The potential of metabolically engineered cyanobacterium would depend on its source-sink balance in the same way as other phototrophs. In cyanobacteria, the amount of light energy harvested (Source) is incompletely utilized by the cell to fix carbon (sink) resulting in wastage of the absorbed energy causing photoinhibition and cellular damage leading to lowered photosynthetic efficiency. Although regulatory pathways like photo-acclimation and photoprotective processes can be helpful unfortunately they limit the cell's metabolic capacity. This review describes approaches for source-sink balance and engineering heterologous metabolic sinks in cyanobacteria for enhanced photosynthetic efficiency. The advances for engineering additional metabolic pathways in cyanobacteria are also described which will provide a better understanding of the cyanobacterial source-sink balance and approaches for efficient cyanobacterial strains for valuable metabolites.

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.

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.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

Carbon and Nitrogen Allocation between the Sink and Source Leaf Tissue in Response to the Excess Excitation Energy Conditions

Plants are inevitably exposed to extreme climatic conditions that lead to a disturbed balance between the amount of absorbed energy and their ability to process it. Variegated leaves with photosynthetically active green leaf tissue (GL) and photosynthetically inactive white leaf tissue (WL) are an excellent model system to study source-sink interactions within the same leaf under the same microenvironmental conditions. We demonstrated that under excess excitation energy (EEE) conditions (high irradiance and lower temperature), regulated metabolic reprogramming in both leaf tissues allowed an increased consumption of reducing equivalents, as evidenced by preserved maximum efficiency of photosystem II (Ф_PSII) at the end of the experiment. GL of the EEE-treated plants employed two strategies: (i) the accumulation of flavonoid glycosides, especially cyanidin glycosides, as an alternative electron sink, and (ii) cell wall stiffening by cellulose, pectin, and lignin accumulation. On the other hand, WL increased the amount of free amino acids, mainly arginine, asparagine, branched-chain and aromatic amino acids, as well as kaempferol and quercetin glycosides. Thus, WL acts as an important energy escape valve that is required in order to maintain the successful performance of the GL sectors under EEE conditions. Finally, this role could be an adaptive value of variegation, as no consistent conclusions about its ecological benefits have been proposed so far.

Phycobilisome light-harvesting efficiency in natural populations of the marine cyanobacteria Synechococcus increases with depth

Cyanobacteria of the genus Synechococcus play a key role as primary producers and drivers of the global carbon cycle in temperate and tropical oceans. Synechococcus use phycobilisomes as photosynthetic light-harvesting antennas. These contain phycoerythrin, a pigment-protein complex specialized for absorption of blue light, which penetrates deep into open ocean water. As light declines with depth, Synechococcus photo-acclimate by increasing both the density of photosynthetic membranes and the size of the phycobilisomes. This is achieved with the addition of phycoerythrin units, as demonstrated in laboratory studies. In this study, we probed Synechococcus populations in an oligotrophic water column habitat at increasing depths. We observed morphological changes and indications for an increase in phycobilin content with increasing depth, in summer stratified Synechococcus populations. Such an increase in antenna size is expected to come at the expense of decreased energy transfer efficiency through the antenna, since energy has a longer distance to travel. However, using fluorescence lifetime depth profile measurement approach, which is applied here for the first time, we found that light-harvesting quantum efficiency increased with depth in stratified water column. Calculated phycobilisome fluorescence quantum yields were 3.5% at 70 m and 0.7% at 130 m. Under these conditions, where heat dissipation is expected to be constant, lower fluorescence yields correspond to higher photochemical yields. During winter-mixing conditions, Synechococcus present an intermediate state of light harvesting, suggesting an acclimation of cells to the average light regime through the mixing depth (quantum yield of ~2%). Given this photo-acclimation strategy, the primary productivity attributed to marine Synechococcus should be reconsidered.

Probing the population of the cyanobacterium Synechococcus in an oligotrophic water column habitat at increasing depths reveals that light-harvesting quantum efficiency increases with depth.

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.

Redox post-translational modifications and their interplay in plant abiotic stress tolerance

The impact of climate change entails a progressive and inexorable modification of the Earth's climate and events such as salinity, drought, extreme temperatures, high luminous intensity and ultraviolet radiation tend to be more numerous and prolonged in time. Plants face their exposure to these abiotic stresses or their combination through multiple physiological, metabolic and molecular mechanisms, to achieve the long-awaited acclimatization to these extreme conditions, and to thereby increase their survival rate. In recent decades, the increase in the intensity and duration of these climatological events have intensified research into the mechanisms behind plant tolerance to them, with great advances in this field. Among these mechanisms, the overproduction of molecular reactive species stands out, mainly reactive oxygen, nitrogen and sulfur species. These molecules have a dual activity, as they participate in signaling processes under physiological conditions, but, under stress conditions, their production increases, interacting with each other and modifying and-or damaging the main cellular components: lipids, carbohydrates, nucleic acids and proteins. The latter have amino acids in their sequence that are susceptible to post-translational modifications, both reversible and irreversible, through the different reactive species generated by abiotic stresses (redox-based PTMs). Some research suggests that this process does not occur randomly, but that the modification of critical residues in enzymes modulates their biological activity, being able to enhance or inhibit complete metabolic pathways in the process of acclimatization and tolerance to the exposure to the different abiotic stresses. Given the importance of these PTMs-based regulation mechanisms in the acclimatization processes of plants, the present review gathers the knowledge generated in recent years on this subject, delving into the PTMs of the redox-regulated enzymes of plant metabolism, and those that participate in the main stress-related pathways, such as oxidative metabolism, primary metabolism, cell signaling events, and photosynthetic metabolism. The aim is to unify the existing information thus far obtained to shed light on possible fields of future research in the search for the resilience of plants to climate change.

β-Cyclocitral: Emerging Bioactive Compound in Plants

β-cyclocitral (βCC), a main apocarotenoid of β-carotene, increases plants’ resistance against stresses. It has recently appeared as a novel bioactive composite in a variety of organisms from plants to animals. In plants, βCC marked as stress signals that accrue under adverse ecological conditions. βCC regulates nuclear gene expression through several signaling pathways, leading to stress tolerance. In this review, an attempt has been made to summarize the recent findings of the potential role of βCC. We emphasize the βCC biosynthesis, signaling, and involvement in the regulation of abiotic stresses. From this review, it is clear that discussing compound has great potential against abiotic stress tolerance and be used as photosynthetic rate enhancer. In conclusion, this review establishes a significant reference base for future research.

Metabolic, physiological and anatomical responses of soybean plants under water deficit and high temperature condition

Water deficit (WD) combined with high temperature (HT) is the major factor limiting agriculture worldwide, and it is predicted to become worse according to the current climate change scenario. It is thus important to understand how current cultivated crops respond to these stress conditions. Here we investigated how four soybean cultivars respond to WD and HT isolated or in combination at metabolic, physiological, and anatomical levels. The WD + HT increased the level of stress in soybean plants when compared to plants under well-watered (WW), WD, or HT conditions. WD + HT exacerbates the increases in ascorbate peroxidase activity, which was associated with the greater photosynthetic rate in two cultivars under WD + HT. The metabolic responses to WD + HT diverge substantially from plants under WW, WD, or HT conditions. Myo-inositol and maltose were identified as WD + HT biomarkers and were connected to subnetworks composed of catalase, amino acids, and both root and leaf osmotic potentials. Correlation-based network analyses highlight that the network heterogeneity increased and a higher integration among metabolic, physiological, and morphological nodes is observed under stress conditions. Beyond unveiling biochemical and metabolic WD + HT biomarkers, our results collectively highlight that the mechanisms behind the acclimation to WD + HT cannot be understood by investigating WD or HT stress separately.

Enhancement of photosynthesis efficiency and yield of strawberry (Fragaria ananassa Duch.) plants via LED systems

Due to advances in the industrial development of light-emitting diodes (LEDs), much research has been conducted in recent years to get a better understanding of how plants respond to these light sources. This study investigated the effects of different LED-based light regimes on strawberry plant development and performance. The photosynthetic pigment content, biochemical constituents, and growth characteristics of strawberry plants were investigated using a combination of different light intensities (150, 200, and 250 μmol m^-2 s^-1), qualities (red, green, and blue LEDs), and photoperiods (14/10 h, 16/8 h, and 12/12 h light/dark cycles) compared to the same treatment with white fluorescent light. Plant height, root length, shoot fresh and dry weight, chlorophyll a, total chlorophyll/carotenoid content, and most plant yield parameters were highest when illuminated with LM7 [intensity (250 μmol m^-2 s^-1) + quality (70% red/30% blue LED light combination) + photoperiod (16/8 h light/dark cycles)]. The best results for the effective quantum yield of PSII photochemistry Y(II), photochemical quenching coefficient (qP), and electron transport ratio (ETR) were obtained with LM8 illumination [intensity (250 μmol m^-2 s^-1) + quality (50% red/20% green/30% blue LED light combination) + photoperiod (12 h/12 h light/dark cycles)]. We conclude that strawberry plants require prolonged and high light intensities with a high red-light component for maximum performance and biomass production.

Separate Effects of Foliar Applied Selenate and Zinc Oxide on the Accumulation of Macrominerals, Macronutrients and Bioactive Compounds in Two Pea (Pisum sativum L.) Seed Varieties

Selenium (Se) and zinc (Zn) are important cofactors for antioxidant enzymes. Foliar Se/Zn application is a highly efficient strategy of plant biofortification. However, its effects on the accumulation of macrominerals, macronutrients and bioactive compounds in the pea plant (Pisum sativum L.) have been poorly investigated. A two-year pot experiment was performed to study responses of two pea varieties (Ambassador, Premium) to foliar-applied sodium selenate (0/50/100 g Se/ha) and zinc oxide (0/375/750 g Zn/ha) at the flowering stage. Concentrations of Ca, Mg, K, Na, soluble solids (SSC), protein, chlorophyll a and b, total chlorophyll, total carotenoids and total condensed tannins (TCT) were determined in seeds. Mg concentration in Ambassador and chlorophyll a concentration in Premium were positively affected, in part, by selenate and zinc oxide, respectively. Selenate and zinc oxide increased, in part, protein concentration in Premium. Highest protein concentration was found in Premium treated with 375 g Zn/ha (27.6% DW) vs. the control (26.6% DW). Significant (all p < 0.001) positive correlations were found, among others, between concentrations of Zn and Mg (r2 = 0.735) and between Zn and protein (r2 = 0.437) for Ambassador, and between Mg and protein (r2 = 0.682), between Zn and Mg (r2 = 0.807), as well as between Zn and protein (r2 = 0.884) for Premium. TCT significantly (all p < 0.05) and positively correlated with SSC (r2 = 0.131), chlorophyll b (r2 = 0.128) and total chlorophyll (r2 = 0.109) for Ambassador. This study provides new nutritional data on Se/Zn biofortified peas, important for improving agronomic biofortification of pea plants.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

The physiological basis for estimating photosynthesis from Chla fluorescence

Solar-induced Chl fluorescence (SIF) offers the potential to curb large uncertainties in the estimation of photosynthesis across biomes and climates, and at different spatiotemporal scales. However, it remains unclear how SIF should be used to mechanistically estimate photosynthesis. In this study, we built a quantitative framework for the estimation of photosynthesis, based on a mechanistic light reaction model with the Chla fluorescence of Photosystem II (SIF_PSII ) as an input (MLR-SIF). Utilizing 29 C₃ and C₄ plant species that are representative of major plant biomes across the globe, we confirmed the validity of this framework at the leaf level. The MLR-SIF model is capable of accurately reproducing photosynthesis for all C₃ and C₄ species under diverse light, temperature, and CO₂ conditions. We further tested the robustness of the MLR-SIF model using Monte Carlo simulations, and found that photosynthesis estimates were much less sensitive to parameter uncertainties relative to the conventional Farquhar, von Caemmerer, Berry (FvCB) model because of the additional independent information contained in SIF_PSII . Once inferred from direct observables of SIF, SIF_PSII provides 'parameter savings' to the MLR-SIF model, compared to the mechanistically equivalent FvCB model, and thus avoids the uncertainties arising as a result of imperfect model parameterization. Our findings set the stage for future efforts to employ SIF mechanistically to improve photosynthesis estimates across a variety of scales, functional groups, and environmental conditions.

Enhanced production of microalgae-originated photosensitizer by integrating photosynthetic electrons extraction and antibiotic induction towards photocatalytic degradation of antibiotic: A novel complementary treatment process for antibiotic removal from effluent of conventional biological wastewater treatment

Antibiotic residues in effluents from bio-treated wastewaters are mainly responsible for the spread of antibiotic resistance genes in the environment. Conventional physicochemical treatments are thought to be unsustainable due to high energy consumption, large consumption of chemicals and environmental unfriendly processing step. In this study, a novel approach by integrating photosynthetic electrons extraction from microalgae with antibiotic induction was used to enhance the production of microalgae-originated photosensitizer for photolytic removal of antibiotic residues in effluents from conventional bio-treated wastewaters. Results showed that the accumulation of photoactive substances in extracellular polymeric substance (EPS) of chlorella vulgaris was positively related to the amounts of photosynthetic electrons extracted by the electrode which is a potential-dependent process and can be further enhanced by tetracycline (TC) induction. The protein and humic acid which are considered two main photoactive substances in EPS produced at 0.6 V accumulated to a high level of 320 and 24 μg/cm³ and were further increased to 380 and 48 μg/cm³ when TC was added which were 4.7 and 6.4-folds higher than that produced at potential free in the absence of TC. The EPS produced at 0.6 and 0.8 V led to 1.34 and 1.53-fold acceleration in photosensitized degradation of TC compared to that of EPS free in secondary effluent of municipal wastewater treatment plant. The complex heterocyclic ring structure of TC was broken down into simple monocyclic aromatic compounds, indicating a marked reduction in biotoxicity and recalcitrance. The hydroxyl radical played a main role for the photolysis of TC followed by singlet oxygen. This technology provides a new alternative to conventional physicochemical treatment as complementary treatment processes for biological wastewater treatment in terms of antibiotics removal.

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.

Assessing and modeling nitrite inhibition in microalgae-bacteria consortia for wastewater treatment by means of photo-respirometric and chlorophyll fluorescence techniques

Total nitrite (TNO₂ = HNO₂ + NO^-₂) accumulation due to the activity of ammonia-oxidizing bacteria (AOB) was monitored in microalgae-bacteria consortia, and the inhibitory effect of nitrite/free nitrous acid (NO₂-N/FNA) on microalgae photosynthesis and inhibition mechanism was studied. A culture of Scenedesmus was used to run two sets of batch reactors at different pH and TNO₂ concentrations to evaluate the toxic potential of NO₂-N and FNA. Photo-respirometric tests showed that NO₂-N accumulation has a negative impact on net oxygen production rate (OPR_NET). Chlorophyll a fluorescence analysis was used to examine the biochemical effects of NO₂-N stress and the mechanism of NO₂-N inhibition. The electron transport rate (ETR), non-photochemical quenching (NPQ), and JIP-test revealed that the electron transport chain between Photosystems II and I (PS II and PS I) was hindered at NO₂-N concentrations above 25 g N m^-3. Electron acceptor Q_A was not able to reoxidize and could not transfer electrons to the next electron acceptor, Q_B, accumulating P₆₈₀⁺ (excited PS II reaction center) and limiting oxygen production. A semi-continuous reactor containing a Scenedesmus culture was monitored by photo-respirometry tests and Chlorophyll a fluorescence to calibrate NO₂-N inhibition (5-35 g N m^-3). Non-competitive inhibition and Hill-type models were compared to select the best-fitting inhibition equations. Inhibition was correctly modeled by the Hill-type model and a half inhibition constant (K_I) for OPR_NET, NPQ, maximum photosynthetic rate (ETR_MAX) and the performance index PI_ABS was 23.7 ± 1.2, 26.36 ± 1.10, 39 ± 2 and 26.5 ± 0.4, respectively.

A System Dynamics Approach to Model Photosynthesis at Leaf Level Under Fluctuating Light

Photosynthesis has been mainly studied under steady-state conditions even though this assumption results inadequate for assessing the biochemical responses to rapid variations occurring in natural environments. The combination of mathematical models with available data may enhance the understanding of the dynamic responses of plants to fluctuating environments and can be used to make predictions on how photosynthesis would respond to non-steady-state conditions. In this study, we present a leaf level System Dynamics photosynthesis model based and validated on an experiment performed on two soybean varieties, namely, the wild type Eiko and the chlorophyll-deficient mutant MinnGold, grown in constant and fluctuating light conditions. This mutant is known to have similar steady-state photosynthesis compared to the green wild type, but it is found to have less biomass at harvest. It has been hypothesized that this might be due to an unoptimized response to non-steady-state conditions; therefore, this mutant seems appropriate to investigate dynamic photosynthesis. The model explained well the photosynthetic responses of these two varieties to fluctuating and constant light conditions and allowed to make relevant conclusions on the different dynamic responses of the two varieties. Deviations between data and model simulations are mostly evident in the non-photochemical quenching (NPQ) dynamics due to the oversimplified combination of PsbS- and zeaxanthin-dependent kinetics, failing in finely capturing the NPQ responses at different timescales. Nevertheless, due to its simplicity, the model can provide the basis of an upscaled dynamic model at a plant level.

Photosystem II photoinhibition and photoprotection in a lycophyte, Selaginella martensii

The Lycophyte Selaginella martensii efficiently acclimates to diverse light environments, from deep shade to full sunlight. The plant does not modulate the abundance of the Light Harvesting Complex II, mostly found as a free trimer, and does not alter the maximum capacity of thermal dissipation (NPQ). Nevertheless, the photoprotection is expected to be modulatable upon long-term light acclimation to preserve the photosystems (PSII, PSI). The effects of long-term light acclimation on PSII photoprotection were investigated using the chlorophyll fluorometric method known as "photochemical quenching measured in the dark" (qP_d ). Singularly high-qP_d values at relatively low irradiance suggest a heterogeneous antenna system (PSII antenna uncoupling). The extent of antenna uncoupling largely depends on the light regime, reaching the highest value in sun-acclimated plants. In parallel, the photoprotective NPQ (pNPQ) increased from deep-shade to high-light grown plants. It is proposed that the differences in the long-term modulation in the photoprotective capacity are proportional to the amount of uncoupled LHCII. In deep-shade plants, the inconsistency between invariable maximum NPQ and lower pNPQ is attributed to the thermal dissipation occurring in the PSII core.

Different Inhibitory Effects of Erythromycin and Chlortetracycline on Early Growth of Brassica campestris Seedlings

Veterinary antibiotics, including erythromycin (Ery) and chlortetracycline (CTC), are often detected in agricultural land. Although these contaminants affect plant growth and development, their effects on crops remain elusive. In this study, the effects of Ery and CTC on plant growth were investigated and compared by analyzing transcript abundance in Brassica campestris seedlings. Treatment with Ery and/or CTC reduced chlorophyll content in leaves and photosynthetic efficiency. Examination of the chloroplast ultrastructure revealed the presence of abnormally shaped plastids in response to Ery and CTC treatments. The antibiotics produced similar phenotypes of lower accumulation of photosynthetic genes, including RBCL and LHCB1.1. Analysis of the transcript levels revealed that Ery and CTC differentially down-regulated genes involved in the tetrapyrrole biosynthetic pathway and primary root growth. In the presence of Ery and CTC, chloroplasts were undeveloped and photosynthesis efficiency was reduced. These results suggest that both Ery and CTC individually affect gene expression and influence plant physiological activity, independently of one another.

Application of Zinc Fertilizer and Mycorrhizal Inoculation on Physio-Biochemical Parameters of Wheat Grown under Water-Stressed Environment

Drought stress and poor zinc (Zn) are major constraints for commercial agriculture. Their detrimental effects significantly decrease crop’s growth and yield. Less water uptake disturbs the metabolic processes in plants. However, the deficiency of Zn leads to the inactivation of many enzymes. It is well documented that cereal crops, especially wheat, are susceptible to drought and Zn deficiency. Scientists suggest the supplementation of Zn along bio-fertilizers for the sustainable management of these issues. That is why the current experiment was conducted to explore the best combination of Zn and bio-fertilizer for wheat. There were two different recommended concentrations of Zn sulfate (Zinc level 1 (Zn1) = 20 and Zinc level 2 (Zn2) = 40 kg ha−1) applied under normal irrigation (75% field capacity = FC) and severe drought stress (40% FC). Sole and combined inoculation of arbuscular mycorrhizal fungi (AM) with Zn1 and Zn2 was also performed. Osmotic stress (40% FC) significantly decreased the examined growth parameters. It also significantly enhanced antioxidant and oxidative indicators in wheat. A significant increase in root fresh weight, root dry weight, and shoot length while a significant decrease in EL, SOD, POD over the control validated the efficacious role of Zn2 + AM. It is concluded that Zn2 + AM can improve wheat root fresh weight and root length wheat under 40% FC. Under different climatic zones, wheat varieties, and soil types, more investigations are recommended to declare Zn2 + AM as the best amendment for improving wheat growth attributes under osmotic stress.

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+.

Photosynthetic performance of rocket (Eruca sativa. Mill.) grown under different regimes of light intensity, quality, and photoperiod

In recent years, much effort has been devoted to understanding the response of plants to various light sources, largely due to advances in industry light-emitting diodes (LEDs). In this study, the effect of different light modes on rocket (Eruca sativa. Mill.) photosynthetic performance and other physiological traits was evaluated using an orthogonal design based on a combination between light intensity, quality, and photoperiod factors. Some morphological and biochemical parameters and photosynthetic efficiency of the plants were analyzed. Plants grew in a closed chamber where three light intensities (160, 190, and 220 μmol m-2 s-1) provided by LEDs with a combination of different ratios of red, green, and blue (R:G:B- 7:0:3, 3:0:7, and 5:2:3) and three different photoperiods (light/dark -10/14 h, 12/12 h, and 14/10 h) were used and compared with white fluorescent light (control). This experimental setup allowed us to study the effect of 9 light modes (LM) compared to white light. The analyzes performed showed that the highest levels of chlorophyll a, chlorophyll b, and carotenoids occurred under LM4, LM3, and LM1, respectively. Chlorophyll a fluorescence measurement showed that the best effective quantum yield of PSII photochemistry Y(II), non-photochemical quenching (NPQ), photochemical quenching coefficient (qP), and electron transport ratio (ETR) were obtained under LM2. The data showed that the application of R7:G0:B3 light mode with a shorter photoperiod than 14/10 h (light/dark), regardless of the light intensity used, resulted in a significant increase in growth as well as higher photosynthetic capacity of rocket plants. Since, a clear correlation between the studied traits under the applied light modes was not found, more features should be studied in future experiments.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H⁺-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K⁺ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

Reactive oxygen species and organellar signaling

The evolution of photosynthesis and its associated metabolic pathways has been crucial to the successful establishment of plants, but has also challenged plant cells in the form of production of reactive oxygen species (ROS). Intriguingly, multiple forms of ROS are generated in virtually every plant cell compartment through diverse pathways. As a result, a sophisticated network of ROS detoxification and signaling that is simultaneously tailored to individual organelles and safeguards the entire cell is necessary. Here we take an organelle-centric view on the principal sources and sinks of ROS across the plant cell and provide insights into the ROS-induced organelle to nucleus retrograde signaling pathways needed for operational readjustments during environmental stresses.

Hydrogen sulfide (H2S) underpins the beneficial silicon effects against the copper oxide nanoparticles (CuO NPs) phytotoxicity in Oryza sativa seedlings

Nanoparticle-pollution has associated severe negative effects on crop productivity. Hence, methods are needed to alleviate nano-toxicity in crop plants. The present study aims to evaluate if the exogenous hydrogen sulfide (H₂S) application in combination with silicon (Si) could palliate the harmful effects of copper oxide nanoparticles (CuO NPs). Fifteen day-old rice (Oryza sativa L.) seedlings were used as a model plant. The results indicate that simultaneous exogenous addition of 10 μM Si and 100 μM NaHS (as an H₂S donor) provided tolerance and enhanced defence mechanism of the rice seedlings against 100 μM CuO NPs. Thus, it was observed in terms of their growth, photosynthetic pigments, antioxidant enzyme activities, the content of non-enzymatic components, chlorophyll fluorescence and up-regulation of antioxidant genes. Si and NaHS stimulated gene expression of silicon (Lsi1 and Lsi2) and auxin (PIN5 and PIN10) transporters. Taken together, data indicate that H₂S underpins the beneficial Si effects in rice seedlings against the oxidative stress triggers by CuO NPs, and stimulation of enzymatic components of the ascorbate-glutathione cycle being the main factor for the beneficial effects triggered by the couple of Si and H₂S. Therefore, it could be concluded that the simultaneous application of Si and H₂S promote the resilience of the rice seedlings against the oxidative stress induced by CuO NPs.

Dissipation of Light Energy Absorbed in Excess: The Molecular Mechanisms

Light is essential for photosynthesis. Nevertheless, its intensity widely changes depending on time of day, weather, season, and localization of individual leaves within canopies. This variability means that light collected by the light-harvesting system is often in excess with respect to photon fluence or spectral quality in the context of the capacity of photosynthetic metabolism to use ATP and reductants produced from the light reactions. Absorption of excess light can lead to increased production of excited, highly reactive intermediates, which expose photosynthetic organisms to serious risks of oxidative damage. Prevention and management of such stress are performed by photoprotective mechanisms, which operate by cutting down light absorption, limiting the generation of redox-active molecules, or scavenging reactive oxygen species that are released despite the operation of preventive mechanisms. Here, we describe the major physiological and molecular mechanisms of photoprotection involved in the harmless removal of the excess light energy absorbed by green algae and land plants. In vivo analyses of mutants targeting photosynthetic components and the enhanced resolution of spectroscopic techniques have highlighted specific mechanisms protecting the photosynthetic apparatus from overexcitation. Recent findings unveil a network of multiple interacting elements, the reaction times of which vary from a millisecond to weeks, that continuously maintain photosynthetic organisms within the narrow safety range between efficient light harvesting and photoprotection.

Thermal Analysis of Stomatal Response under Salinity and High Light

A non-destructive thermal imaging method was used to study the stomatal response of salt-treated Arabidopsis thaliana plants to excessive light. The plants were exposed to different levels of salt concentrations (0, 75, 150, and 220 mM NaCl). Time-dependent thermograms showed the changes in the temperature distribution over the lamina and provided new insights into the acute light-induced temporary response of Arabidopsis under short-term salinity. The initial response of plants, which was associated with stomatal aperture, revealed an exponential growth in temperature kinetics. Using a single-exponential function, we estimated the time constants of thermal courses of plants exposed to acute high light. The saline-induced impairment in stomatal movement caused the reduced stomatal conductance and transpiration rate. Limited transpiration of NaCl-treated plants resulted in an increased rosette temperature and decreased thermal time constants as compared to the controls. The net CO₂ assimilation rate decreased for plants exposed to 220 mM NaCl; in the case of 75 mM NaCl treatment, an increase was observed. A significant decline in the maximal quantum yield of photosystem II under excessive light was noticeable for the control and NaCl-treated plants. This study provides evidence that thermal imaging as a highly sensitive technique may be useful for analyzing the stomatal aperture and movement under dynamic environmental conditions.

Plant defence mechanisms against mycotoxin Fumonisin B1

Fumonisin B1 (FB1) is the most harmful mycotoxin which prevails in several crops and affects the growth and yield as well. Hence, keeping the alarming consequences of FB1 under consideration, there is still a need to seek other more reliable approaches and scientific knowledge for FB1-induced cell death and a comprehensive understanding of the mechanisms of plant defence strategies. FB1-induced disturbance in sphingolipid metabolism initiates programmed cell death (PCD) through various modes such as the elevated generation of reactive oxygen species, lipid peroxidation, cytochrome c release from the mitochondria, and activation of specific proteases and nucleases causing DNA fragmentation. There is a close interaction between sphingolipids and defence phytohormones in response to FB1 exposure regulating PCD and defence. In this review, the model plant Arabidopsis and various crops have been presented with different levels of susceptibility and resistivity exposed to various concentration of FB1. In addition to this, regulation of PCD and defence mechanisms have been also demonstrated at the physiological, biochemical and molecular levels to help the understanding of the role and function of FB1-inducible molecules and genes and their expressions in plants against pathogen attacks which could provide molecular and biochemical markers for the detection of toxin exposure.

Genome-wide identification of heat shock factors and heat shock proteins in response to UV and high intensity light stress in lettuce

BACKGROUND

Heat shock factors (Hsfs) and Heat shock proteins (Hsps) belong to an essential group of molecular regulators involved in controlling cellular processes under normal and stress conditions. The role of Hsfs and Hsps is well known in model plant species under diverse stress conditions. While plants Hsfs are vital components of the signal transduction response to maintain cellular homeostasis, Hsps function as chaperones helping to maintain folding of damaged and newly formed proteins during stress conditions. In lettuce (Lactuca sativa), a highly consumed vegetable crop grown in the field and in hydroponic systems, the role of these gene families in response to artificial light is not well characterized.

RESULTS

Using a genome-wide analysis approach, we identified 32 Hsfs and 22 small heat shock proteins (LsHsps) in lettuce, some of which do not have orthologs in Arabidopsis, poplar, and rice. LsHsp60s, LsHsp90s, and LsHsp100s are highly conserved among dicot and monocot species. Surprisingly, LsHsp70s have three times more members than Arabidopsis and two times more than rice. Interestingly, the lettuce genome triplication did not contribute to the increased number of LsHsp70s genes. The large number of LsHsp70s was the result of genome tandem duplication. Chromosomal distribution analysis shows larger tandem repeats of LsHsp70s genes in Chr1, Chr7, Chr8, and Chr9. At the transcriptional level, some genes of the LsHsfs, LsHsps, LsHsp60s, and LsHsp70s families were highly responsive to UV and high intensity light stress, in contrast to LsHsp90s and LsHsp100s which did not respond to a light stimulus.

CONCLUSIONS

Our genome-wide analysis provides a detailed identification of Hsfs and Hsps in lettuce. Chromosomal location and syntenic region analysis together with our transcriptional analysis under different light conditions provide candidate genes for breeding programs aiming to produce lettuce varieties able to grow healthy under hydroponic systems that use artificial light.

Frozen in the dark: interplay of night-time activity of xanthophyll cycle, xylem attributes, and desiccation tolerance in fern resistance to winter

While most ferns avoid freezing as they have a tropical distribution or shed their fronds, wintergreen species in temperate and boreoalpine ecosystems have to deal with sub-zero temperatures. Increasing evidence has revealed overlapping mechanisms of desiccation and freezing tolerance in angiosperms, but the physiological mechanisms behind freezing tolerance in ferns are far from clear. We evaluated photochemical and hydraulic parameters in five wintergreen fern species differing in their ability to tolerate desiccation. We assessed frond freezing tolerance, ice nucleation temperature and propagation pattern, and xylem anatomical traits. Dynamics of photochemical performance and xanthophyll cycle were evaluated during freeze-thaw events under controlled conditions and, in selected species, in the field. Only desiccation-tolerant species, which possessed a greater fraction of narrow tracheids (<18 μm) than sensitive species, tolerated freezing. Frond freezing occurred in the field at -3.4 ± 0.9 °C (SD) irrespective of freezing tolerance, freezable water content, or tracheid properties. Even in complete darkness, maximal photochemical efficiency of photosystem II was down-regulated concomitantly with zeaxanthin accumulation in response to freezing. This was reversible upon re-warming only in tolerant species. Our results suggest that adaptation for freezing tolerance is associated with desiccation tolerance through complementary xylem properties (which may prevent risk of irreversible cavitation) and effective photoprotection mechanisms. The latter includes de-epoxidation of xanthophylls in darkness, a process evidenced for the first time directly in the field.

ROS Homeostasis and Plant Salt Tolerance: Plant Nanobiotechnology Updates

Salinity is an issue impairing crop production across the globe. Under salinity stress, besides the osmotic stress and Na+ toxicity, ROS (reactive oxygen species) overaccumulation is a secondary stress which further impairs plant performance. Chloroplasts, mitochondria, the apoplast, and peroxisomes are the main ROS generation sites in salt-stressed plants. In this review, we summarize ROS generation, enzymatic and non-enzymatic antioxidant systems in salt-stressed plants, and the potential for plant biotechnology to maintain ROS homeostasis. Overall, this review summarizes the current understanding of ROS homeostasis of salt-stressed plants and highlights potential applications of plant nanobiotechnology to enhance plant tolerance to stresses.

Heat Shock Tolerance in Deschampsia antarctica Desv. Cultivated in vitro Is Mediated by Enzymatic and Non-enzymatic Antioxidants

Deschampsia antarctica Desv, is the most successful colonizing species of a cold continent. In recent years due to climate change, the frequency of heat waves has increased in Antarctica, registering anomalous high temperatures during the summer of 2020. However, the populations of D. antarctica are responding positively to these events, increasing in number and size throughout the Antarctic Peninsula. In this work, the physiological and biochemical responses of D. antarctica plants grown in vitro (15 ± 1°C) and plants subjected to two heat shock treatments (23 and 35°C) were evaluated. The results obtained show that D. antarctica grown in vitro is capable of tolerating heat shock treatments; without showing visible damage to its morphology, or changes in its oxidative state and photosynthetic performance. These tolerance responses are primarily mediated by the efficient role of enzymatic and non-enzymatic antioxidant systems that maintain redox balance at higher temperatures. It is postulated that these mechanisms also operate in plants under natural conditions when exposed to environmental stresses.

Signaling from Plastid Genome Stability Modulates Endoreplication and Cell Cycle during Plant Development

Plastid-nucleus genome coordination is crucial for plastid activity, but the mechanisms remain unclear. By treating Arabidopsis plants with the organellar genome-damaging agent ciprofloxacin, we found that plastid genome instability can alter endoreplication and the cell cycle. Similar results are observed in the plastid genome instability mutants of reca1why1why3. Cell division and embryo development are disturbed in the reca1why1why3 mutant. Notably, SMR5 and SMR7 genes, which encode cell-cycle kinase inhibitors, are upregulated in plastid genome instability plants, and the mutation of SMR7 can restore the endoreplication and growth phenotype of reca1why1why3 plants. Furthermore, we establish that the DNA damage response transcription factor SOG1 mediates the alteration of endoreplication and cell cycle triggered by plastid genome instability. Finally, we demonstrate that reactive oxygen species produced in plastids are important for plastid-nucleus genome coordination. Our findings uncover a molecular mechanism for the coordination of plastid and nuclear genomes during plant growth and development.

Development of photosynthetic carbon fixation model using multi-excitation wavelength fast repetition rate fluorometry in Lake Biwa

Direct measurements of gross primary productivity (GPP) in the water column are essential, but can be spatially and temporally restrictive. Fast repetition rate fluorometry (FRRf) is a bio-optical technique based on chlorophyll a (Chl-a) fluorescence that can estimate the electron transport rate (ETR_PSII) at photosystem II (PSII) of phytoplankton in real time. However, the derivation of phytoplankton GPP in carbon units from ETR_PSII remains challenging because the electron requirement for carbon fixation (Ф_e,C), which is mechanistically 4 mol e⁻ mol C⁻¹ or above, can vary depending on multiple factors. In addition, FRRf studies are limited in freshwater lakes where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust Ф_e,C model for freshwater ecosystems using simultaneous measurements of ETR_PSII by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the ¹³C method. The study was conducted in oligotrophic and mesotrophic parts of Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETR_PSII of cyanobacteria. The apparent range of Ф_e,C in the phytoplankton community was 1.1–31.0 mol e⁻ mol C⁻¹ during the study period. A generalised linear model showed that the best fit including 12 physicochemical and biological factors explained 67% of the variance in Ф_e,C. Among all factors, water temperature was the most significant, while photosynthetically active radiation intensity was not. This study quantifies the in situ FRRf method in a freshwater ecosystem, discusses core issues in the methodology to calculate Ф_e,C, and assesses the applicability of the method for lake GPP prediction.

The heterologous expression of a plastocyanin in the diatom Phaeodactylum tricornutum improves cell growth under iron-deficient conditions

We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single-mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo-plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron-deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over-reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II-also associated to a reduced oxidative stress-a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c₆ , under iron-limiting conditions.

Excited-State Dynamics of [Ru(S-Sbpy)(bpy)2]2+ to Form Long-Lived Localized Triplet States

The novel photosensitizer [Ru(^S–Sbpy)(bpy)₂]²⁺ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or ^S–Sbpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectroscopy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur-decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spin–orbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the ^S–Sbpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via S–S bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy)₃]²⁺. This structural relaxation upon localization of the charge on ^S–Sbpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength.

We investigate selective excitation in two types of ligands present in [Ru(^S−Sbpy)(bpy)₂]²⁺. Femtosecond transient absorption spectroscopy identifies the formation of the same final state within 80 fs for both excitation wavelengths. Surface hopping simulations reveal ultrafast mixing of singlet and triplet states within less than 50 fs. Energy lowering due to S−S bond cleavage is identified as the driving factor for convergence to a single final state.

Rhizobium-Linked Nutritional and Phytochemical Changes Under Multitrophic Functional Contexts in Sustainable Food Systems

Rhizobia are bacteria that exhibit both endophytic and free-living lifestyles. Endophytic rhizobial strains are widely known to infect leguminous host plants, while some do infect non-legumes. Infection of leguminous roots often results in the formation of root nodules. Associations between rhizobia and host plants may result in beneficial or non-beneficial effects. Such effects are linked to various biochemical changes that have far-reaching implications on relationships between host plants and the dependent multitrophic biodiversity. This paper explores relationships that exist between rhizobia and various plant species. Emphasis is on nutritional and phytochemical changes that occur in rhizobial host plants, and how such changes affect diverse consumers at different trophic levels. The purpose of this paper is to bring into context various aspects of such interactions that could improve knowledge on the application of rhizobia in different fields. The relevance of rhizobia in sustainable food systems is addressed in context.

Rational design of artificial redox-mediating systems toward upgrading photobioelectrocatalysis

Photobioelectrocatalysis has recently attracted particular research interest owing to the possibility to achieve sunlight-driven biosynthesis, biosensing, power generation, and other niche applications. However, physiological incompatibilities between biohybrid components lead to poor electrical contact at the biotic-biotic and biotic-abiotic interfaces. Establishing an electrochemical communication between these different interfaces, particularly the biocatalyst-electrode interface, is critical for the performance of the photobioelectrocatalytic system. While different artificial redox mediating approaches spanning across interdisciplinary research fields have been developed in order to electrically wire biohybrid components during bioelectrocatalysis, a systematic understanding on physicochemical modulation of artificial redox mediators is further required. Herein, we review and discuss the use of diffusible redox mediators and redox polymer-based approaches in artificial redox-mediating systems, with a focus on photobioelectrocatalysis. The future possibilities of artificial redox mediator system designs are also discussed within the purview of present needs and existing research breadth.