Mitochondrial redox biology and homeostasis in plants

Dynamic interplay of reactive oxygen and nitrogen species (ROS and RNS) in plant resilience: unveiling the signaling pathways and metabolic responses to biotic and abiotic stresses

KEY MESSAGE

Plants respond to environmental challenges by producing reactive species such as ROS and RNS, which play critical roles in signaling pathways that lead to adaptation and survival strategies. Understanding these pathways, as well as their detection methods and effects on plant development and metabolism, provides insight into increasing crop tolerance to combined stresses. Plants encounter various environmental stresses (abiotic and biotic) that affect plant growth and development. Plants sense biotic and abiotic stresses by producing different molecules, including reactive species, that act as signaling molecules and stimulate secondary messengers and subsequent gene transcription. Reactive oxygen and nitrogen species (ROS and RNS) are produced in both physiological and pathological conditions in the plasma membranes, chloroplasts, mitochondria, and endoplasmic reticulum. Various techniques, including spectroscopy, chromatography, and fluorescence methods, are used to detect highly reactive, short-half-life ROS and RNS either directly or indirectly. In this review, we highlight the roles of ROS and RNS in seed germination, root development, senescence, mineral nutrition, and post-harvest control. In addition, we provide information on the specialized metabolism involved in plant growth and development. Secondary metabolites, including alkaloids, flavonoids, and terpenoids, are produced in low concentrations in plants for signaling and metabolism. Strategies for improving crop performance under combined drought and pathogen stress conditions are discussed in this review.

Deciphering molecular regulation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) signalling networks in Oryza genus amid environmental stress

The Oryza genus, containing Oryza sativa L., is quintessential to sustain global food security. This genus has a lot of sophisticated molecular mechanisms to cope with environmental stress, particularly during vulnerable stages like flowering. Recent studies have found key involvements and genetic modifications that increase resilience to stress, including exogenous application of melatonin, allantoin, and trehalose as well as OsSAPK3 and OsAAI1 in the genetic realm. Due to climate change and anthropogenic reasons, there is a rise in sea level which raises a concern of salinity stress. It is tackled through osmotic adjustment and ion homeostasis, mediated by genes like P5CS, P5CR, GSH1, GSH2, and SPS, and ion transporters like NHX, NKT, and SKC, respectively. Oxidative damage is reduced by a complex action of antioxidants, scavenging RONS. A complex action of genes mediates cold stress with studies highlighting the roles of OsWRKY71, microRNA2871b, OsDOF1, and OsICE1. There is a need to research the mechanism of action of proteins like OsRbohA in ROS control and the action of regulatory genes in stress response. This is highly relevant due to the changing climate which will raise a lot of environmental changes that will adversely affect production and global food security if certain countermeasures are not taken. Overall, this study aims to unravel the molecular intricacies of ROS and RNS signaling networks in Oryza plants under stress conditions, with the ultimate goal of informing strategies for enhancing stress tolerance and crop performance in this important agricultural genus.

The Stylo Cysteine-Rich Peptide SgSnakin1 Is Involved in Aluminum Tolerance through Enhancing Reactive Oxygen Species Scavenging

Stylo (Stylosanthes spp.) is an important pasture legume with strong aluminum (Al) resistance. However, the molecular mechanisms underlying its Al tolerance remain fragmentary. Due to the incomplete genome sequence information of stylo, we first conducted full-length transcriptome sequencing for stylo root tips treated with and without Al and identified three Snakin/GASA genes, namely, SgSnakin1, SgSnakin2, and SgSnakin3. Through quantitative RT-PCR, we found that only SgSnakin1 was significantly upregulated by Al treatments in stylo root tips. Histochemical localization assays further verified the Al-enhanced expression of SgSnakin1 in stylo root tips. Subcellular localization in both tobacco and onion epidermis cells showed that SgSnakin1 localized to the cell wall. Overexpression of SgSnakin1 conferred Al tolerance in transgenic Arabidopsis, as reflected by higher relative root growth and cell vitality, as well as lower Al concentration in the roots of transgenic plants. Additionally, overexpression of SgSnakin1 increased the activities of SOD and POD and decreased the levels of O2·- and H2O2 in transgenic Arabidopsis in response to Al stress. These findings indicate that SgSnakin1 may function in Al resistance by enhancing the scavenging of reactive oxygen species through the regulation of antioxidant enzyme activities.

Enhancing systematic tolerance in Bermuda grass (Cynodon dactylon L.) through amplified alkB gene expression and bacterial-driven hydrocarbon degradation

This study aimed to access the impact of soil polluted with petroleum (5, 10 g petroleum kg-1 soil) on Bermuda grass (Cynodon dactylon L.) with and without applied bacterial inoculants (Arthrobacter oxydans ITRH49 and Pseudomonas sp. MixRI75). Both soil and seed were given bacterial inoculation. The evaluated morphological parameters of Bermuda grass were fresh and dry weight. The results demonstrated that applied bacterial inoculants enhanced 5.4%, 20%, 28% and 6.4%, 21%, and 29% shoot and root fresh/dry weights in Bermuda grass under controlled environment. The biochemical analysis of shoot and root was affected deleteriously by the 10 g petroleum kg-1 soil pollution. Microbial inoculants enhanced the activities of enzymatic (catalase, peroxidase, glutathione reductase, ascorbate peroxidase, superoxide dismutase) and non-enzymatic (ɑ-tocopherols, proline, reduced glutathione, ascorbic acid) antioxidant to mitigate the toxic effects of ROS (H2O2) under hydrocarbon stressed condition. The maximum hydrocarbon degradation (75%) was recorded by Bermuda grass at 5 g petroleum kg-1 soil contamination. Moreover, bacterial persistence and alkane hydroxylase gene (alkB) abundance and expression were observed more in the root interior than in the rhizosphere and shoot interior of Bermuda grass. Subsequently, the microbe used a biological tool to propose that the application of plant growth-promoting bacteria would be the most favorable choice in petroleum hydrocarbon polluted soil to conquer the abiotic stress in plants and the effective removal of polyaromatic hydrocarbons in polluted soil.

Effect of low oxygen stress on the metabolic responses of tomato fruit cells

Postharvest losses of fruits and vegetables can occur due to cell breakdown and browning during controlled atmosphere storage as a result of low oxygen (O2) stress. Therefore, the study was designed to better understand the underlying mechanisms of the response of isolated tomato fruit cells incubated at low O2 (hypoxic and anoxic) conditions as a model system. The O2 stress conditions used for the experiment were based on the results of the Michaelis-Menten constant (Km) of respiration. A total of 56 polar metabolites belonging mainly to different functional groups, including amino acids, organic acids, sugars and sugar alcohols, were identified using GC-MS. O2 stress stimulated the biosynthesis of most of the free amino acids while decreasing the synthesis of most of the organic acids (especially those linked to the tricarboxylic acid cycle), sugars (except for ribose) and other nitrogen-containing compounds. The down-regulation of these TCA cycle metabolites served to provide energy to ensure the survival of the cell. Increases in the sugar alcohol levels and induction of fermentative metabolism were observed under low O2 stress. By employing multivariate statistics, metabolites were identified that were essential to the oxygen stress response and establishing the correlation between metabolite abundance, oxygen levels, and incubation period were achievable. A higher correlation was observed between the O2 levels and most of the metabolites.

Impact of Heavy Metals on Cold Acclimation of Salix viminalis Roots

In nature, plants are exposed to a range of climatic conditions. Those negatively impacting plant growth and survival are called abiotic stresses. Although abiotic stresses have been extensively studied separately, little is known about their interactions. Here, we investigate the impact of long-term mild metal exposure on the cold acclimation of Salix viminalis roots using physiological, transcriptomic, and proteomic approaches. We found that, while metal exposure significantly affected plant morphology and physiology, it did not impede cold acclimation. Cold acclimation alone increased glutathione content and glutathione reductase activity. It also resulted in the increase in transcripts and proteins belonging to the heat-shock proteins and related to the energy metabolism. Exposure to metals decreased antioxidant capacity but increased catalase and superoxide dismutase activity. It also resulted in the overexpression of transcripts and proteins related to metal homeostasis, protein folding, and the antioxidant machinery. The simultaneous exposure to both stressors resulted in effects that were not the simple addition of the effects of both stressors taken separately. At the antioxidant level, the response to both stressors was like the response to metals alone. While this should have led to a reduction of frost tolerance, this was not observed. The impact of the simultaneous exposure to metals and cold acclimation on the transcriptome was unique, while at the proteomic level the cold acclimation component seemed to be dominant. Some genes and proteins displayed positive interaction patterns. These genes and proteins were related to the mitigation and reparation of oxidative damage, sugar catabolism, and the production of lignans, trehalose, and raffinose. Interestingly, none of these genes and proteins belonged to the traditional ROS homeostasis system. These results highlight the importance of the under-studied role of lignans and the ROS damage repair and removal system in plants simultaneously exposed to multiple stressors.

Unveiling the effect of gibberellin-induced iron oxide nanoparticles on bud dormancy release in sweet cherry (Prunus avium L.)

Hydrogen cyanide has been extensively used worldwide for bud dormancy break in fruit trees, consequently enhancing fruit production via expedited cultivation, especially in areas with controlled environments or warmer regions. A novel and safety nanotechnology was developed since the hazard of hydrogen cyanide for the operators and environments, there is an urgent need for the development of novel and safety approaches to replace it to break bud dormancy for fruit trees. In current study, we have systematically explored the potential of iron oxide nanoparticles, specifically α-Fe2O3, to modulate bud dormancy in sweet cherry (Prunus avium). The synthesized iron oxide nanoparticles underwent meticulous characterization and assessment using various techniques, including Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and ultraviolet-visible infrared (UV-Vis) spectroscopy. Remarkably, when applied at a concentration of 10 mg L-1 alongside gibberellin (GA4+7), these iron oxide nanoparticles exhibited a substantial 57% enhancement in bud dormancy release compared to control groups, all achieved within a remarkably short time span of 4 days. Our RNA-seq analyses further unveiled that 2757 genes within the sweet cherry buds were significantly up-regulated when treated with 10 mg L-1 α-Fe2O3 nanoparticles in combination with GA, while 4748 genes related to dormancy regulation were downregulated in comparison to the control. Moreover, we discovered an array of 58 transcription factor families among the crucial differentially expressed genes (DEGs). Through hormonal quantification, we established that the increased bud burst was accompanied by a reduced concentration of abscisic acid (ABA) at 761.3 ng/g fresh weight in the iron oxide treatment group, coupled with higher levels of gibberellins (GAs) in comparison to the control. Comprehensive transcriptomic and metabolomic analyses unveiled significant alterations in hormone contents and gene expression during the bud dormancy-breaking process when α-Fe2O3 nanoparticles were combined with GA. In conclusion, our findings provide valuable insights into the intricate molecular mechanisms underlying the impact of iron oxide nanoparticles on achieving uniform bud dormancy break in sweet cherry trees.

Mechanistic insights into hormesis induced by erythromycin in the marine alga Thalassiosira weissflogii

Erythromycin (ERY) is a typical macrolide antibiotic with large production and extensive use on a global scale. Detection of ERY in both freshwaters and coaster seawaters, as well as relatively high ecotoxicity of ERY have been documented. Notably, hormesis has been reported on several freshwater algae under ERY stress, where growth was promoted at relatively lower exposures but inhibited at higher treatment levels. On the contrary, there is limited information of ERY toxicity in marine algae, hampering the risk assessment on ERY in the coaster waters. The presence of hormesis may challenge the current concept of dose-response adopted in chemical risk assessment. Whether and how exposure to ERY can induce dose-dependent toxicity in marine algae remain virtually unknown, especially at environmentally relevant concentrations. The present study used a model marine diatom Thalassiosira weissflogii (T. weissflogii) to reveal its toxicological responses to ERY at different biological levels and decipher the underlying mechanisms. Assessment of multiple apical endpoints shows an evident growth promotion following ERY exposure at an environmentally relevant concentration (1 µg/L), associated with increased contents reactive oxygen species (ROS) and chlorophyll-a (Chl-a), activated signaling pathways related to ribosome biosynthesis and translation, and production of total soluble protein. By contrast, growth inhibition in the 750 and 2500 µg/L treatments was attributed to reduced viability, increased ROS formation, reduced content of total soluble protein, inhibited photosynthesis, and perturbed signaling pathways involved in xenobiotic metabolism, ribosome, metabolism of amino acid, and nitrogen metabolism. Measurements of multiple apical endpoints coupled with de novo transcriptomics analysis applied in the present study, a systems biology approach, can generate detailed mechanistic information of chemical toxicity including dose-response and species sensitivity difference used in environmental risk assessment.

RGA1 Negatively Regulates Thermo-tolerance by Affecting Carbohydrate Metabolism and the Energy Supply in Rice

BACKGROUND

Signal transduction mediated by heterotrimeric G proteins, which comprise the α, β, and γ subunits, is one of the most important signaling pathways in rice plants. RGA1, which encodes the Gα subunit of the G protein, plays an important role in the response to various types of abiotic stress, including salt, drought, and cold stress. However, the role of RGA1 in the response to heat stress remains unclear.

RESULTS

The heat-resistant mutant ett1 (enhanced thermo-tolerance 1) with a new allele of the RGA1 gene was derived from an ethane methyl sulfonate-induced Zhonghua11 mutant. After 45 °C heat stress treatment for 36 h and recovery for 7 d, the survival rate of the ett1 mutants was significantly higher than that of wild-type (WT) plants. The malondialdehyde content was lower, and the maximum fluorescence quantum yield of photosystem II, peroxidase activity, and hsp expression were higher in ett1 mutants than in WT plants after 12 h of exposure to 45 °C. The RNA-sequencing results revealed that the expression of genes involved in the metabolism of carbohydrate, nicotinamide adenine dinucleotide, and energy was up-regulated in ett1 under heat stress. The carbohydrate content and the relative expression of genes involved in sucrose metabolism indicated that carbohydrate metabolism was accelerated in ett1 under heat stress. Energy parameters, including the adenosine triphosphate (ATP) content and the energy charge, were significantly higher in the ett1 mutants than in WT plants under heat stress. Importantly, exogenous glucose can alleviate the damages on rice seedling plants caused by heat stress.

CONCLUSION

RGA1 negatively regulates the thermo-tolerance in rice seedling plants through affecting carbohydrate and energy metabolism.

Proteomic and Biochemical Evidence Involving Root Cell Wall Biosynthesis and Modification, Tricarboxylic Acid Cycle, and Glutathione Metabolism in Cultivar-Dependent Cd Accumulation of Water Spinach (Ipomoea aquatica)

Proteomic analysis and biochemical tests were employed to investigate the critical biological processes responsible for the different cadmium (Cd) accumulations between two water spinach (Ipomoea aquatica) cultivars, QLQ and T308. QLQ, with lower shoot Cd accumulation and translocation factor than T308, possessed higher expression of cell wall biosynthesis and modification proteins in roots, together with higher lignin and pectin contents, higher pectin methylesterase activity, and lower pectin methylation. The results demonstrated that QLQ could more effectively restrict root-to-shoot Cd translocation by compartmentalizing more Cd in root cell walls. In contrast, T308 showed higher expression of the tricarboxylic acid (TCA) cycle, glutathione (GSH) metabolism, and heavy metal transporter proteins, accompanied by higher GSH content and glutathione S-transferase (GST) and glutathione reductase (GR) activity, which accelerated Cd uptake and translocation in T308. These findings revealed several critical biological processes responsible for cultivar-dependent Cd accumulation in water spinach, which are important for elucidating Cd accumulation and transport mechanisms in different cultivars.

The Key Roles of ROS and RNS as a Signaling Molecule in Plant-Microbe Interactions

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play a pivotal role in the dynamic cell signaling systems in plants, even under biotic and abiotic stress conditions. Over the past two decades, various studies have endorsed the notion that these molecules can act as intracellular and intercellular signaling molecules at a very low concentration to control plant growth and development, symbiotic association, and defense mechanisms in response to biotic and abiotic stress conditions. However, the upsurge of ROS and RNS under stressful conditions can lead to cell damage, retarded growth, and delayed development of plants. As signaling molecules, ROS and RNS have gained great attention from plant scientists and have been studied under different developmental stages of plants. However, the role of RNS and RNS signaling in plant-microbe interactions is still unknown. Different organelles of plant cells contain the enzymes necessary for the formation of ROS and RNS as well as their scavengers, and the spatial and temporal positions of these enzymes determine the signaling pathways. In the present review, we aimed to report the production of ROS and RNS, their role as signaling molecules during plant-microbe interactions, and the antioxidant system as a balancing system in the synthesis and elimination of these species.

Modulation of Photosynthesis and ROS Scavenging Response by Beneficial Bacteria in Olea europaea Plantlets under Salt Stress Conditions

Climate change consequences for agriculture involve an increase of saline soils which results in lower crop yields due to increased oxidative stress in plants. The present study reports the use of Plant Growth Promoting Bacteria (PGPB) as a tool to modulate plant innate mechanisms of adaptation to water stress (salinity and drought) in one year-old olive plantlets var. Arbosana and Arbequina. Integration of external changes in plants involve changes in Reactive Oxygen Species (ROS) that behave as signals to trigger plant adaptative mechanisms; however, they become toxic in high concentrations. For this reason, plants are endowed with antioxidant systems to keep ROS under control. So, the working hypothesis is that specific beneficial strains will induce a systemic response able to modulate oxidative stress and improve plant adaptation to water stress. Ten strains were assayed, evaluating changes in photosynthesis, pigments, ROS scavenging enzymes and antioxidant molecules, osmolytes and malondialdehyde, as oxidative stress marker. Photosynthesis and photosynthetic pigments were the most affected variables. Despite the specific response of each variety, the favorite targets of PGPBs to improve plant fitness were photosynthetic pigments and the antioxidant pools of glutathione and ascorbate. Our results show the potential of PGPBs to improve plant fitness modulating oxidative stress.

Mitochondrial Peroxiredoxin-IIF (PRXIIF) Activity and Function during Seed Aging

Mitochondria play a major role in energy metabolism, particularly in cell respiration, cellular metabolism, and signal transduction, and are also involved in other processes, such as cell signaling, cell cycle control, cell growth, differentiation and apoptosis. Programmed cell death is associated with the production of reactive oxygen species (ROS) and a concomitant decrease in antioxidant capacity, which, in turn, determines the aging of living organisms and organs and thus also seeds. During the aging process, cell redox homeostasis is disrupted, and these changes decrease the viability of stored seeds. Mitochondrial peroxiredoxin-IIF (PRXIIF), a thiol peroxidase, has a significant role in protecting the cell and sensing oxidative stress that occurs during the disturbance of redox homeostasis. Thioredoxins (TRXs), which function as redox transmitters and switch protein function in mitochondria, can regulate respiratory metabolism. TRXs serve as electron donors to PRXIIF, as shown in Arabidopsis. In contrast, sulfiredoxin (SRX) can regenerate mitochondrial PRXIIF once hyperoxidized to sulfinic acid. To protect against oxidative stress, another type of thiol peroxidases, glutathione peroxidase-like protein (GPXL), is important and receives electrons from the TRX system. They remove peroxides produced in the mitochondrial matrix. However, the TRX/PRX and TRX/GPXL systems are not well understood in mitochondria. Knowledge of both systems is important because these systems play an important role in stress sensing, response and acclimation, including redox imbalance and generation of ROS and reactive nitrogen species (RNS). The TRX/PRX and TRX/GPXL systems are important for maintaining cellular ROS homeostasis and maintaining redox homeostasis under stress conditions. This minireview focuses on the functions of PRXIIF discovered in plant cells approximately 20 years ago and addresses the question of how PRXIIF affects seed viability maintenance and aging. Increasing evidence suggests that the mitochondrial PRXIIF plays a major role in metabolic processes in seeds, which was not previously known.

Ca2+ participates in programmed cell death by modulating ROS during pollen cryopreservation

After cryopreservation, the Ca²⁺ content increased, which affected the intracellular ROS content, then participated in the occurrence of programmed cell death in pollen. Programmed cell death (PCD) is one of the reasons for the decline in pollen viability after cryopreservation. However, the role of calcium ions (Ca²⁺) in PCD during pollen cryopreservation has not been revealed in the existing studies. In this study, Paeonia lactiflora 'Fen Yu Nu' pollen was used as the research material for investigating the effects of Ca²⁺ changes on PCD indices and reactive oxygen species (ROS) during pollen cryopreservation. The results showed that after cryopreservation, with the decrease of pollen viability, the Ca²⁺ content significantly increased. The regulation of Ca²⁺ content had a significant effect on PCD indices, which showed that the Ca²⁺ carrier A23187 accelerated the decrease of mitochondrial membrane potential level and increased the activity of caspase-3-like and caspase-9-like proteases and the apoptosis rate. The expression levels of partial pro-PCD genes were upregulated, the anti-PCD gene BI-1 was downregulated, and the addition of Ca²⁺-chelating agent EGTA had the opposite effect. The addition of the Ca²⁺ carrier A23187 after cryopreservation significantly increased the ROS content of pollen, the addition of the Ca²⁺-chelating agent EGTA had the opposite effect, and Ca²⁺ regulators also had significant effects on the contents of ROS production and clearance-related substances. Ca²⁺ affected intracellular ROS content by acting on the ROS production and clearance system during the cryopreservation of pollen and is thus involved in the occurrence of PCD.

Inhibition of peroxidases and oxidoreductases is crucial for avoiding false-positive reactions in the localization of reactive oxygen species in intact barley root tips

NBT and HE may be efficiently used for the detection of superoxide, while DCDHF-DA and DHR123 for the detection of peroxynitrite in intact barley root tips, only if PRXs and oxidoreductases are inhibited to avoid false-positive reactions. Strong peroxidase (PRX) and oxidoreductase activities were observed in the barley root tips that were markedly inhibited by NaN₃. Rapid and strong nitro-blue tetrazolium chloride (NBT) reduction is associated mainly with the vital functions of root cells but not with superoxide formation. In turn, the inhibition of root surface redox activity by NaN₃ strongly reduced the formation of formazan, but its slight accumulation, observed in the root elongation zone, was a result of NADPH oxidase-mediated apoplastic superoxide formation. A longer staining time period with NBT was required for the detection of antimycin A-mediated superoxide formation inside the cells. This antimycin A-induced superoxide was clearly detectable by hydroethidine (HE) after the inhibition of PRXs by NaN₃, and it was restricted into the root transition zone. TEMPOL, a superoxide scavenger, strongly inhibited both NBT reduction and HE oxidation in the presence of NaN₃. Similarly, the DCDHF-DA and DHR123 oxidation was markedly reduced after the inhibition of apoplastic PRXs by NaN₃ and was detectable mainly in the root transition zone. This fluorescence signal was not influenced by the application of pyruvate but was strongly reduced by urea, a peroxynitrite scavenger. The presented results suggest that if the root PRXs and oxidoreductases are inhibited, both NBT and HE detect mainly superoxide, whereas both DCDHF-DA and DHR123 may be efficiently used for the detection of peroxynitrite in intact barley root tips. The inhibition of PRXs and oxidoreductases is crucial for avoiding false-positive reactions in the localization of reactive oxygen species in the intact barley root tip.

Modelling biophoton emission kinetics based on the initial intensity value in Helianthus annuus plants exposed to different types of stress

Biophoton radiation also referred to as ultra-weak photon emission (UPE) is used to denote a spontaneous and permanent photon emission associated with oxidative processes in cells and seems to universally occur in all living systems as a result of the generation of reactive oxygen species (ROS) that are produced under stress conditions. The measurement of this biophoton emission allows for a non-invasive approach in monitoring phenological stages throughout plant development which has direct relevance in agriculture research. In this study, the emission of photons emanating from sunflower (Helianthus annuus, L.) plants exposed to biotic and abiotic stress has been investigated. In healthy plants raised under controlled growth conditions UPE was low whereas in stressed individuals it considerably increased; particularly upon water stress. The kinetics of the signal is shown to reveal an exponential decay with characteristic dynamics, which appears to reflect different physiological states concomitantly setting in upon stress. The dynamics of the signal decay is shown to vary according to the type of stress applied (biotic vs. abiotic) hence suggesting a putative relationship between the kinetic traits of change in the signal intensity-decay and stress. Intriguingly, the determination of the change in the intensity of biophoton emission that ensued in a short time course was possible by using the initial biophoton emission intensity. The predictability level of the equations demonstrated the applicability of the model in a corroborative manner when employing it in independent UPE-measurements, thus permitting to forecast the intensity change in a very accurate way over a short time course. Our findings allow the notion that albeit stress confers complex and complicated changes on oxidative metabolism in biological systems, the employment of biophoton imaging offers a feasible method making it possible to monitor oxidative processes triggered by stress in a non-invasive and label-free way which has versatile applications especially in precision agriculture.

Coral symbionts evolved a functional polycistronic flavodiiron gene

Photosynthesis in cyanobacteria, green algae, and basal land plants is protected against excess reducing pressure on the photosynthetic chain by flavodiiron proteins (FLV) that dissipate photosynthetic electrons by reducing O₂. In these organisms, the genes encoding FLV are always conserved in the form of a pair of two-type isozymes (FLVA and FLVB) that are believed to function in O₂ photo-reduction as a heterodimer. While coral symbionts (dinoflagellates of the family Symbiodiniaceae) are the only algae to harbor FLV in photosynthetic red plastid lineage, only one gene is found in transcriptomes and its role and activity remain unknown. Here, we characterized the FLV genes in Symbiodiniaceae and found that its coding region is composed of tandemly repeated FLV sequences. By measuring the O₂-dependent electron flow and P700 oxidation, we suggest that this atypical FLV is active in vivo. Based on the amino-acid sequence alignment and the phylogenetic analysis, we conclude that in coral symbionts, the gene pair for FLVA and FLVB have been fused to construct one coding region for a hybrid enzyme, which presumably occurred when or after both genes were inherited from basal green algae to the dinoflagellate. Immunodetection suggested the FLV polypeptide to be cleaved by a post-translational mechanism, adding it to the rare cases of polycistronic genes in eukaryotes. Our results demonstrate that FLV are active in coral symbionts with genomic arrangement that is unique to these species. The implication of these unique features on their symbiotic living environment is discussed.

Suppression of metabolite shuttles for export of chloroplast and mitochondrial ATP and NADPH increases the cytosolic NADH:NAD+ ratio in tobacco leaves in the dark

The communication between chloroplasts and mitochondria, which depends on the inter-organellar exchange of carbon skeletons, energy, and reducing equivalents, is essential for maintaining efficient respiratory metabolism and photosynthesis. We devised a multi-transgene approach to manipulate the leaf energy and redox balance in tobacco (Nicotiana tabacum) while monitoring the in vivo cytosolic redox status of NAD(H) using the biosensor c-Peredox-mCherry. Our strategy involved altering the shuttling capacity of the chloroplast by (1) increasing the chloroplast malate valve capacity by overexpression of the chloroplast malate valve transporter pOMT from Arabidopsis (AtpOMT1) while (2) reducing the activity of the chloroplast triose-phosphate/3-phosphoglycerate shuttle by knocking down the cytosolic NAD-dependent glyceraldehyde 3-phosphate dehydrogenase (NtGAPC). This was accompanied by (3) alterations to the export of reducing equivalents in the mitochondria by knocking down the mitochondrial malate dehydrogenase (NtmMDH) and (4) an increased expression of the mitochondrial fission regulator FIS1A from Arabidopsis (AtFIS1A). The multi-transgene tobacco plants were analysed in glasshouse conditions and showed significant increases in the cytosolic NADH:NAD+ in the dark when transcript levels for NtGAPC or NtmMDH were knocked down. In addition, principal component analysis and Spearman correlation analyses showed negative correlations between average transcript levels for the gene targets and parameters related to chlorophyll fluorescence and plant growth. Our results highlight the importance of the shuttling of energy and reducing equivalents from chloroplasts and mitochondria to support photosynthesis and growth and suggest an important role for the dual 2-oxoglutarate/malate and oxaloacetate/malate transporter (pOMT).

Alleviating aluminum toxicity in plants: Implications of reactive oxygen species signaling and crosstalk with other signaling pathways

Aluminum (Al) toxicity is a major limiting factor for plant growth and productivity in acidic soil. At pH lower than 5.0 (pH < 5.0), the soluble and toxic form of Al (Al³⁺ ions) enters root cells and inhibits root growth and uptake of water and nutrients. The organic acids malate, citrate, and oxalate are secreted by the roots and chelate Al³⁺ to form a non-toxic Al-OA complex, which decreases the entry of Al³⁺ into the root cells. When Al³⁺ enters, it leads to the production of reactive oxygen species (ROS) in cells, which are toxic and cause damage to biomolecules like lipids, carbohydrates, proteins, and nucleic acids. When ROS levels rise beyond the threshold, plants activate an antioxidant defense system that comprises of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione S-transferase (GST), ascorbic acid (ASA), phenolics and alkaloids etc., which protect plant cells from oxidative damage by scavenging and neutralizing ROS. Besides, ROS also play an important role in signal transduction and influence many molecular and cellular process like hormone signaling, gene expression, cell wall modification, cell cycle, programed cell death (PCD), and development. In the present review, the mechanisms of Al-induced ROS generation, ROS signaling, and crosstalk with other signaling pathways helping to combat Al toxicity have been summarized, which will help researchers to understand the intricacies of Al-induced plant response at cellular level and plan research for developing Al-toxicity tolerant crops for sustainable agriculture in acid soil-affected regions of the world.

Mitochondria-Targeted SmsHSP24.1 Overexpression Stimulates Early Seedling Vigor and Stress Tolerance by Multi-Pathway Transcriptome-Reprogramming

Among the diverse array of heat shock proteins across the three domains of life, mitochondria-targeted small heat shock proteins (sHSPs) are evolved in the plant lineage. However, they remained mysterious and understudied. In this study, we reported a systematic study of a novel mitochondria-targeted nuclear sHSP from eggplant (Solanum melongena L.; SmsHSP24.1). Differential expression of SmsHSP24.1 indicated its positive role exerted during stress conditions. Escherichia coli-BL21 cell line overexpressing the SmsHSP24.1 showed excellent thermo-tolerance ability, tolerating up to 52°C. Spectrometry and electron microscopy revealed a multimeric structure of the protein which acted as a molecular chaperone at high temperatures. Overexpression of SmsHSP24.1 significantly enhanced resistance against heat, drought, and salt stresses and showed rapid germination in constitutively overexpressed eggplant lines. RNA-seq analysis reveals an apparent upregulation of a set of reactive oxygen species (ROS) scavenging enzymes of the glutathione (GHS) pathway and mitochondrial electron transport chain (ETC). Significant upregulation was also observed in auxin biosynthesis and cell-wall remodeling transcripts in overexpressed lines. qPCR, biochemical and physiological analysis further aligned with the finding of transcriptome analysis and suggested an essential role of SmsHSP24.1 under various stress responses and positive physiological influence on the growth of eggplants. Therefore, this gene has immense potential in engineering stress-resilient crop plants.

The knowns and unknowns of intracellular partitioning of carbon and nitrogen, with focus on the organic acid-mediated interplay between mitochondrion and chloroplast

The presence of specialized cellular compartments in higher plants express an extraordinary degree of intracellular organization, which provides efficient mechanisms to avoid misbalancing of the metabolism. This offers the flexibility by which plants can quickly acclimate to fluctuating environmental conditions. For that, a fine temporal and spatial regulation of metabolic pathways is required and involves several players e.g. organic acids. In this review we discuss different facets of the organic acid metabolism within plant cells with special focus to those related to the interactions between organic acids compartmentalization and the partitioning of carbon and nitrogen. The connections between organic acids and CO₂ assimilation, tricarboxylic acid (TCA) cycle, amino acids metabolism, and redox status are highlighted. Moreover, the key enzymes and transporters as well as their function on the coordination of interorganellar metabolic exchanges are discussed.

Mitochondrial phosphate transporter and methyltransferase genes contribute to Fusarium head blight Type II disease resistance and grain development in wheat

Fusarium head blight (FHB) is an economically important disease of wheat that results in yield loss and grain contaminated with fungal mycotoxins that are harmful to human and animal health. Herein we characterised two wheat genes involved in the FHB response in wheat: a wheat mitochondrial phosphate transporter (TaMPT) and a methyltransferase (TaSAM). Wheat has three sub-genomes (A, B, and D) and gene expression studies demonstrated that TaMPT and TaSAM homoeologs were differentially expressed in response to FHB infection and the mycotoxigenic Fusarium virulence factor deoxynivalenol (DON) in FHB resistant wheat cv. CM82036 and susceptible cv. Remus. Virus-induced gene silencing (VIGS) of either TaMPT or TaSAM enhanced the susceptibility of cv. CM82036 to FHB disease, reducing disease spread (Type II disease resistance). VIGS of TaMPT and TaSAM significantly reduced grain number and grain weight. This indicates TaSAM and TaMPT genes also contribute to grain development in wheat and adds to the increasing body of evidence linking FHB resistance genes to grain development. Hence, Fusarium responsive genes TaSAM and TaMPT warrant further study to determine their potential to enhance both disease resistance and grain development in wheat.

Nanoceria seed priming enhanced salt tolerance in rapeseed through modulating ROS homeostasis and α-amylase activities

BACKGROUND

Salinity is a big threat to agriculture by limiting crop production. Nanopriming (seed priming with nanomaterials) is an emerged approach to improve plant stress tolerance; however, our knowledge about the underlying mechanisms is limited.

RESULTS

Herein, we used cerium oxide nanoparticles (nanoceria) to prime rapeseeds and investigated the possible mechanisms behind nanoceria improved rapeseed salt tolerance. We synthesized and characterized polyacrylic acid coated nanoceria (PNC, 8.5 ± 0.2 nm, -43.3 ± 6.3 mV) and monitored its distribution in different tissues of the seed during the imbibition period (1, 3, 8 h priming). Our results showed that compared with the no nanoparticle control, PNC nanopriming improved germination rate (12%) and biomass (41%) in rapeseeds (Brassica napus) under salt stress (200 mM NaCl). During the priming hours, PNC were located mostly in the seed coat, nevertheless the intensity of PNC in cotyledon and radicle was increased alongside with the increase of priming hours. During the priming hours, the amount of the absorbed water (52%, 14%, 12% increase at 1, 3, 8 h priming, respectively) and the activities of α-amylase were significantly higher (175%, 309%, 295% increase at 1, 3, 8 h priming, respectively) in PNC treatment than the control. PNC primed rapeseeds showed significantly lower content of MDA, H2O2, and •O2- in both shoot and root than the control under salt stress. Also, under salt stress, PNC nanopriming enabled significantly higher K+ retention (29%) and significantly lower Na+ accumulation (18.5%) and Na+/K+ ratio (37%) than the control.

CONCLUSIONS

Our results suggested that besides the more absorbed water and higher α-amylase activities, PNC nanopriming improves salt tolerance in rapeseeds through alleviating oxidative damage and maintaining Na+/K+ ratio. It adds more knowledge regarding the mechanisms underlying nanopriming improved plant salt tolerance.

Exploring natural diversity reveals alleles to enhance antioxidant system in barley under salt stress

Soil salinity stress causes osmotic/ionic imbalances and induces oxidative stress that causes cellular structure damage, perturbs metabolism, antioxidant system (comprising enzymatic and non-enzymatic components) and hence inhibits plant growth performance. In this study, we used genome-wide association scan (GWAS) in 174 diverse spring barley accessions which were exposed to salt stress under field conditions at the vegetative stage to uncover the genetic basis of antioxidant components and agronomic traits. High activities of enzymatic and content of non-enzymatic antioxidants were observed under salt stress compared to control conditions. Under salt stress, all the agronomic and yield-related traits were significantly reduced. Six genomic regions were associated with antioxidants and agronomic traits under salt stress conditions which were found to be linked with candidate genes. Several significant associations were physically located inside or near genes which are potentially involved in antioxidants. Two candidate genes at 2H (40,659,364 bp) and 7H (416,743,127 bp) were found to be involved in Dihydroflavonol 4-reductase/flavanone protein and Glyceraldehyde-3-phosphate dehydrogenase, respectively. The allelic variation at SNP of BK_07 at 7H inside the GAPDH gene demonstrates a negative selection of accessions carrying A allele. This allele appears in cultivars with lower activity of enzymatic antioxidants e.g. superoxide dismutase and catalases under salt stress conditions. These accessions are predominantly two-rowed, cultivars, originated from Europe, and carrying photoperiod sensitive alleles. The detected associated molecular markers in this work are considered as an important source for selection of increased amount of antioxidant compounds in barley under stress conditions.

An Insight Into the Mechanism of Plant Organelle Genome Maintenance and Implications of Organelle Genome in Crop Improvement: An Update

Besides the nuclear genome, plants possess two small extra chromosomal genomes in mitochondria and chloroplast, respectively, which contribute a small fraction of the organelles’ proteome. Both mitochondrial and chloroplast DNA have originated endosymbiotically and most of their prokaryotic genes were either lost or transferred to the nuclear genome through endosymbiotic gene transfer during the course of evolution. Due to their immobile nature, plant nuclear and organellar genomes face continuous threat from diverse exogenous agents as well as some reactive by-products or intermediates released from various endogenous metabolic pathways. These factors eventually affect the overall plant growth and development and finally productivity. The detailed mechanism of DNA damage response and repair following accumulation of various forms of DNA lesions, including single and double-strand breaks (SSBs and DSBs) have been well documented for the nuclear genome and now it has been extended to the organelles also. Recently, it has been shown that both mitochondria and chloroplast possess a counterpart of most of the nuclear DNA damage repair pathways and share remarkable similarities with different damage repair proteins present in the nucleus. Among various repair pathways, homologous recombination (HR) is crucial for the repair as well as the evolution of organellar genomes. Along with the repair pathways, various other factors, such as the MSH1 and WHIRLY family proteins, WHY1, WHY2, and WHY3 are also known to be involved in maintaining low mutation rates and structural integrity of mitochondrial and chloroplast genome. SOG1, the central regulator in DNA damage response in plants, has also been found to mediate endoreduplication and cell-cycle progression through chloroplast to nucleus retrograde signaling in response to chloroplast genome instability. Various proteins associated with the maintenance of genome stability are targeted to both nuclear and organellar compartments, establishing communication between organelles as well as organelles and nucleus. Therefore, understanding the mechanism of DNA damage repair and inter compartmental crosstalk mechanism in various sub-cellular organelles following induction of DNA damage and identification of key components of such signaling cascades may eventually be translated into strategies for crop improvement under abiotic and genotoxic stress conditions. This review mainly highlights the current understanding as well as the importance of different aspects of organelle genome maintenance mechanisms in higher plants.

Mitochondrial ATP synthase subunit d, a component of the peripheral stalk, is essential for growth and heat stress tolerance in Arabidopsis thaliana

As rapid changes in climate threaten global crop yields, an understanding of plant heat stress tolerance is increasingly relevant. Heat stress tolerance involves the coordinated action of many cellular processes and is particularly energy demanding. We acquired a knockout mutant and generated knockdown lines in Arabidopsis thaliana of the d subunit of mitochondrial ATP synthase (gene name: ATPQ, AT3G52300, referred to hereafter as ATPd), a subunit of the peripheral stalk, and used these to investigate the phenotypic significance of this subunit in normal growth and heat stress tolerance. Homozygous knockout mutants for ATPd could not be obtained due to gametophytic defects, while heterozygotes possess no visible phenotype. Therefore, we used RNA interference to create knockdown plant lines for further studies. Proteomic analysis and blue native gels revealed that ATPd downregulation impairs only subunits of the mitochondrial ATP synthase (complex V). Knockdown plants were more sensitive to heat stress, had abnormal leaf morphology, and were severely slow growing compared to wild type. These results indicate that ATPd plays a crucial role in proper function of the mitochondrial ATP synthase holoenzyme, which, when reduced, leads to wide-ranging defects in energy-demanding cellular processes. In knockdown plants, more hydrogen peroxide accumulated and mitochondrial dysfunction stimulon (MDS) genes were activated. These data establish the essential structural role of ATPd and support the importance of complex V in normal plant growth, and provide new information about its requirement for heat stress tolerance.

Exogenous Putrescine Enhances Salt Tolerance and Ginsenosides Content in Korean Ginseng (Panax ginseng Meyer) Sprouts

The effect of exogenously applied putrescine (Put) on salt stress tolerance was investigated in Panax ginseng. Thirty-day-old ginseng sprouts were grown in salinized nutrient solution (150 mM NaCl) for five days, while the control sprouts were grown in nutrients solution. Putrescine (0.3, 0.6, and 0.9 mM) was sprayed on the plants once at the onset of salinity treatment, whereas control plants were sprayed with water only. Ginseng seedlings tested under salinity exhibited reduced plant growth and biomass production, which was directly interlinked with reduced chlorophyll and chlorophyll fluorescence due to higher reactive oxygen species (hydrogen peroxide; H2O2) and lipid peroxidation (malondialdehyde; MDA) production. Application of Put enhanced accumulation of proline, total soluble carbohydrate, total soluble sugar and total soluble protein. At the same time, activities of antioxidant enzymes like superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase in leaves, stems, and roots of ginseng seedlings were increased. Such modulation of physio-biochemical processes reduced the level of H2O2 and MDA, which indicates a successful adaptation of ginseng seedlings to salinity stress. Moreover, protopanaxadiol (PPD) ginsenosides enhanced by both salinity stress and exogenous Put treatment. On the other hand, protopanaxatriol (PPT) ginsenosides enhanced in roots and reduced in leaves and stems under salinity stress condition. In contrast, they enhanced by exogenous Put application in all parts of the plants for most cases, also evidenced by principal component analysis. Collectively, our findings provide an important prospect for the use of Put in modulating salinity tolerance and ginsenosides content in ginseng sprouts.

Mitochondrial heat-shock cognate protein 70 contributes to auxin-mediated embryo development

In Arabidopsis thaliana, mitochondrial-localized heat-shock cognate protein 70-1 (mtHSC70-1) plays an important role in vegetativegrowth. However, whether mtHSC70-1 affects reproductive growth remains unknown. Here, we found that the mtHSC70-1 gene was expressed in the provascular cells of the embryo proper from the early heart stage onward during embryogenesis. Phenotypic analyses of mthsc70-1 mutants revealed that mtHSC70 deficiency leads to defective embryo development and that this effect is mediated by auxin. In addition to a dwarf phenotype, the mthsc70-1 mutant displayed defects in flower morphology, anther development, and embryogenesis. At early developmental stages, the mthsc70-1 embryos exhibited abnormal cell divisions in both embryo proper and suspensor cells. From heart stage onward, they displayed an abnormal shape such as with no or very small cotyledon protrusions, had aberrant number of cotyledons, or were twisted. These embryo defects were associated with reduced or ectopic expression of auxin responsive reporter DR5rev:GFP. Consistently, the expression of auxin biosynthesis and polar auxin transport genes were markedly altered in mthsc70-1. On the other hand, mitochondrial retrograde regulation (MRR) was enhanced in mthsc70-1. Treatment of wild-type plants with an inhibitor that activates mitochondrial retrograde signaling reduced the expression level of auxin biosynthesis and polar auxin transport genes and induced phenotypes similar to those of mthsc70-1. Taken together, our data reveal that loss of function of mtHSC70-1 induces MRR, which inhibits auxin biosynthesis and polar auxin transport, leading to abnormal auxin gradients and defective embryo development.

A basic helix-loop-helix transcription factor CabHLH113 positively regulate pepper immunity against Ralstonia solanacearum

Pepper's (Capsicum annum) response to bacterial pathogen Ralstonia solanacearm inoculation (RSI) and abiotic stresses is known to be synchronized by transcriptional network; however, related molecular mechanisms need extensive experimentation. We identified and characterized functions of CabHLH113 -a basic helix-loop-helix transcription factor-in pepper immunity to R. solanacearum infection. The RSI and foliar spray of phytohormones, including salicylic acid (SA), methyl jasmonate (MeJA), ethylene (ETH), and absicic acid (ABA) induced transcription of CabHLH113 in pepper. Loss of function of CabHLH113 by virus-induced-gene-silencing (VIGS) compromised defense of pepper plants against RSI and suppressed relative expression levels of immunity-associated marker genes, i.e., CaPR1, CaNPR1, CaDEF1, CaHIR1 and CaABR1. Pathogen growth was significantly increased after loss of function of CabHLH113 compared with un-silenced plants with remarkable increase in pepper susceptibility. Besides, transiently over-expression of CabHLH113 induced HR-like cell death, H₂O₂ accumulation and up-regulation of defense-associated marker genes, e.g. CaPR1, CaNPR1, CaDEF1, CaHIR1 and CaABR1. Additionally, transient over-expression of CabHLH113 enhanced the transcriptional levels of CaWRKY6, CaWRKY27 and CaWRKY40. Conversely, transient over-expression of CaWRKY6, CaWRKY27 and CaWRKY40 enhanced the transcriptional levels of CabHLH113. Collectively, our results indicate that newly characterized CabHLH113 has novel defense functions in pepper immunity against RSI via triggering HR-like cell death and cellular levels of defense linked genes.

Recent Developments in Enzymatic Antioxidant Defence Mechanism in Plants with Special Reference to Abiotic Stress

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.

Mitochondrial movement during its association with chloroplasts in Arabidopsis thaliana

Plant mitochondria move dynamically inside cells and this movement is classified into two types: directional movement, in which mitochondria travel long distances, and wiggling, in which mitochondria travel short distances. However, the underlying mechanisms and roles of both types of mitochondrial movement, especially wiggling, remain to be determined. Here, we used confocal laser-scanning microscopy to quantitatively characterize mitochondrial movement (rate and trajectory) in Arabidopsis thaliana mesophyll cells. Directional movement leading to long-distance migration occurred at high speed with a low angle-change rate, whereas wiggling leading to short-distance migration occurred at low speed with a high angle-change rate. The mean square displacement (MSD) analysis could separate these two movements. Directional movement was dependent on filamentous actin (F-actin), whereas mitochondrial wiggling was not, but slightly influenced by F-actin. In mesophyll cells, mitochondria could migrate by wiggling, and most of these mitochondria associated with chloroplasts. Thus, mitochondria migrate via F-actin-independent wiggling under the influence of F-actin during their association with chloroplasts in Arabidopsis.

Oikawa et al. investigate the rate and trajectory of mitochondria in Arabidopsis thaliana mesophyll cells, using confocal laser-scanning microscopy. They find that mitochondria migrate via wiggling during their association with chloroplasts, providing insights into how mitochondria-chloroplast interaction affects the movement of mitochondria.

The flavonoid 3'-hydroxylase gene from the Antarctic moss Pohlia nutans is involved in regulating oxidative and salt stress tolerance

Flavonoids are the important secondary metabolites. They are thought to play an important role in plant adaptation to terrestrial environment. However, the downstream branching pathway of flavonoids in bryophytes, which are the most ancient of terrestrial plants, remains unclear. Here, we cloned a flavonoid 3'-hydroxylase gene (PnF3'H) from the Antarctic moss Pohlia nutans and studied its function in plant stress tolerance. The Arabidopsis with overexpressing PnF3'H (AtOE) were constructed. The AtOE plants had more lateral roots and higher activities of antioxidant enzymes than the wild-type plants under oxidative stress. Meanwhile, the gene expression levels of reactive oxygen species (ROS) scavengers (i.e., AtCAT3, AtFeSOD1, and AtCu-ZnSOD3) were upregulated in the AtOE plants, and the transcription levels of ROS producing enzyme genes were significantly downregulated. The AtOE plans showed increased sensitivity to NaCl stress or abscisic acid (ABA) treatment during seed germination and early root development. Furthermore, several stress-resistant genes in the ABA signaling pathway were also downregulated in the AtOE plants when compared with the wild-type plants. These results suggested that PnF3'H participates in regulating the oxidative tolerance and ABA sensitivity to enable P. nutans to adapt to polar environments.

Role of mitochondrial alternative oxidase in the regulation of cellular homeostasis during development of photosynthetic function in greening leaves

Here, recent publications on the role of mitochondrial non-phosphorylating pathways (NPhPs) in the electron transport chain during the de-etiolation of wheat leaves are reviewed. Among NPhPs, the alternative oxidase (AOX) pathway is the most effective pathway in maintaining cellular redox and energy balance, especially under stress conditions, including light stress. AOX is considered to dissipate excess reductants produced in the chloroplasts, and thereby prevent photooxidation. However, when etiolated wheat plants were exposed to a physiologically relevant light level, AOX was rapidly induced and increased, although the etioplasts did not produce excess reductants and have their own strong photoprotective mechanisms. The present study provides further insights into the role of AOX in greening cells and highlights the importance of AOX in the integration of cellular signalling pathways.

Recapitulation of the Function and Role of ROS Generated in Response to Heat Stress in Plants

In natural ecosystems, plants are constantly exposed to changes in their surroundings as they grow, caused by a lifestyle that requires them to live where their seeds fall. Thus, plants strive to adapt and respond to changes in their exposed environment that change every moment. Heat stress that naturally occurs when plants grow in the summer or a tropical area adversely affects plants' growth and poses a risk to plant development. When plants are subjected to heat stress, they recognize heat stress and respond using highly complex intracellular signaling systems such as reactive oxygen species (ROS). ROS was previously considered a byproduct that impairs plant growth. However, in recent studies, ROS gained attention for its function as a signaling molecule when plants respond to environmental stresses such as heat stress. In particular, ROS, produced in response to heat stress in various plant cell compartments such as mitochondria and chloroplasts, plays a crucial role as a signaling molecule that promotes plant growth and triggers subsequent downstream reactions. Therefore, this review aims to address the latest research trends and understandings, focusing on the function and role of ROS in responding and adapting plants to heat stress.

Redox regulation of NADP-malate dehydrogenase is vital for land plants under fluctuating light environment

Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts. These redox switches function to activate or deactivate enzymes during light-dark transitions and have the function of fine-tuning their activities according to the intensity of light. Accordingly, many studies on chloroplast redox regulation have been conducted under the hypothesis that "fine regulation of the activities of these enzymes is crucial for efficient photosynthesis." However, the impact of the regulatory system on plant metabolism is still unclear. To test this hypothesis, we here studied the impact of the ablation of a redox switch in chloroplast NADP-malate dehydrogenase (MDH). By genome editing, we generated a mutant plant whose MDH lacks one of its redox switches and is active even in dark conditions. Although NADPH consumption by MDH in the dark is expected to be harmful to plant growth, the mutant line did not show any phenotypic differences under standard long-day conditions. In contrast, the mutant line showed severe growth retardation under short-day or fluctuating light conditions. These results indicate that thiol-switch redox regulation of MDH activity is crucial for maintaining NADPH homeostasis in chloroplasts under these conditions.

Drought stress-induced physiological mechanisms, signaling pathways and molecular response of chloroplasts in common vegetable crops

Drought stress is one of the most adverse abiotic stresses that hinder plants' growth and productivity, threatening sustainable crop production. It impairs normal growth, disturbs water relations and reduces water-use efficiency in plants. However, plants have evolved many physiological and biochemical responses at the cellular and organism levels, in order to cope with drought stress. Photosynthesis, which is considered one of the most crucial biological processes for survival of plants, is greatly affected by drought stress. A gradual decrease in CO₂ assimilation rates, reduced leaf size, stem extension and root proliferation under drought stress, disturbs plant water relations, reducing water-use efficiency, disrupts photosynthetic pigments and reduces the gas exchange affecting the plants adversely. In such conditions, the chloroplast, organelle responsible for photosynthesis, is found to counteract the ill effects of drought stress by its critical involvement as a sensor of changes occurring in the environment, as the first process that drought stress affects is photosynthesis. Beside photosynthesis, chloroplasts carry out primary metabolic functions such as the biosynthesis of starch, amino acids, lipids, and tetrapyroles, and play a central role in the assimilation of nitrogen and sulfur. Because the chloroplasts are central organelles where the photosynthetic reactions take place, modifications in their physiology and protein pools are expected in response to the drought stress-induced variations in leaf gas exchanges and the accumulation of ROS. Higher expression levels of various transcription factors and other proteins including heat shock-related protein, LEA proteins seem to be regulating the heat tolerance mechanisms. However, several aspects of plastid alterations, following a water deficit environment are still poorly characterized. Since plants adapt to various stress tolerance mechanisms to respond to drought stress, understanding mechanisms of drought stress tolerance in plants will lead toward the development of drought tolerance in crop plants. This review throws light on major droughts stress-induced molecular/physiological mechanisms in response to severe and prolonged drought stress and addresses the molecular response of chloroplasts in common vegetable crops. It further highlights research gaps, identifying unexplored domains and suggesting recommendations for future investigations.

UVC light modulates vitamin C and phenolic biosynthesis in acerola fruit: role of increased mitochondria activity and ROS production

The effects of ultraviolet-C light (UVC) on vitamin C and phenolic compounds in acerola during postharvest storage were investigated in order to elucidate the mechanism inducing the antioxidant systems. The fruits, stored at 10 °C for 7 days after a hormetic UVC irradiation (two pulses of 0.3 J/cm²), showed significantly less degradation of vitamin C and phenolic compounds than the control without the UVC challenge. UVC activated the L-galactono-1,4-lactone dehydrogenase (GalDH), a key enzyme for vitamin C biosynthesis, and altered the composition of phenolic compounds, through phenolic biosynthesis, in acerola during postharvest storage. UVC also induced reactive oxygen species (ROS) productions at immediate (day 0) and late (day 7) times during postharvest storage through the mitochondrial electron transport chain and NADPH oxidase, respectively. Results suggest that UVC helps in the retention of vitamin C and phenolic content in acerola by altering ascorbic acid and phenolic metabolism through an increase in mitochondrial activity and a ROS-mediated mechanism. Data showed the beneficial effects of UVC on maintenance of nutraceutical quality in acerola during postharvest storage and supplied new insights into understanding the mechanism by which UVC irradiation enhance the antioxidant system in fruits.

Keep Calm and Survive: Adaptation Strategies to Energy Crisis in Fruit Trees under Root Hypoxia

Plants are permanently facing challenges imposed by the environment which, in the context of the current scenario of global climate change, implies a constant process of adaptation to survive and even, in the case of crops, at least maintain yield. O₂ deficiency at the rhizosphere level, i.e., root hypoxia, is one of the factors with the greatest impact at whole-plant level. At cellular level, this O₂ deficiency provokes a disturbance in the energy metabolism which has notable consequences on the yield of plant crops. In this sense, although several physiological studies describe processes involved in plant adaptation to root hypoxia in woody fruit trees, with emphasis on the negative impacts on photosynthetic rate, there are very few studies that include -omics strategies for specifically understanding these processes in the roots of such species. Through a de novo assembly approach, a comparative transcriptome study of waterlogged Prunus spp. genotypes contrasting in their tolerance to root hypoxia was revisited in order to gain a deeper insight into the reconfiguration of pivotal pathways involved in energy metabolism. This re-analysis describes the classically altered pathways seen in the roots of woody fruit trees under hypoxia, but also routes that link them to pathways involved with nitrogen assimilation and the maintenance of cytoplasmic pH and glycolytic flow. In addition, the effects of root hypoxia on the transcription of genes related to the mitochondrial oxidative phosphorylation system, responsible for providing adenosine triphosphate (ATP) to the cell, are discussed in terms of their roles in the energy balance, reactive oxygen species (ROS) metabolism and aerenchyma formation. This review compiles key findings that help to explain the trait of tolerance to root hypoxia in woody fruit species, giving special attention to their strategies for managing the energy crisis. Finally, research challenges addressing less-explored topics in recovery and stress memory in woody fruit trees are pointed out.

Subcellular compartmentalization of the plant antioxidant system: an integrated overview

The antioxidant system (AOS) maintains the optimal concentration of reactive oxygen species (ROS) in a cell and protects it against oxidative stress. In plants, the AOS consists of seven main classes of antioxidant enzymes, low-molecular antioxidants (e.g., ascorbate, glutathione, and their oxidized forms) and thioredoxin/glutaredoxin systems which can serve as reducing agents for antioxidant enzymes. The number of genes encoding AOS enzymes varies between classes, and same class enzymes encoded by different gene copies may have different subcellular localizations, functional loads and modes of evolution. These facts hereafter reinforce the complex nature of AOS regulation and functioning. Further studies can describe new trends in the behavior and functioning of systems components, and provide new fundamental knowledge about systems regulation. The system is revealed to have a lot of interactions and interplay pathways between its components at the subcellular level (antioxidants, enzymes, ROS level, and hormonal and transcriptional regulation). These facts should be taken into account in further studies during the AOS modeling by describing the main pathways of generating and utilizing ROS, as well as the associated signaling processes and regulation of the system on cellular and organelle levels, which is a complicated and ambitious task. Another objective for studying the phenomenon of the AOS is related to the influence of cell dynamics and circadian rhythms on it. Therefore, the AOS requires an integrated and multi-level approach to study. We focused this review on the existing scientific background and experimental data used for the systems biology research of the plant AOS.

Transcriptomic analysis of poco1, a mitochondrial pentatricopeptide repeat protein mutant in Arabidopsis thaliana

BACKGROUND

Flowering is a crucial stage during plant development. Plants may respond to unfavorable conditions by accelerating reproductive processes like flowering. In a recent study, we showed that PRECOCIOUS1 (POCO1) is a mitochondrial pentatricopeptide repeat (PPR) protein involved in flowering time and abscisic acid (ABA) signaling in Arabidopsis thaliana. Here, we use RNA-seq data to investigate global gene expression alteration in the poco1 mutant.

RESULTS

RNA-seq analysis was performed during different developmental stages for wild-type and poco1 plants. The most profound differences in gene expression were found when wild-type and poco1 plants of the same developmental stage were compared. Coverage analysis confirmed the T-DNA insertion in POCO1, which was concomitant with truncated transcripts. Many biological processes were found to be enriched. Several flowering-related genes such as FLOWERING LOCUS T (FT), which may be involved in the early-flowering phenotype of poco1, were differentially regulated. Numerous ABA-associated genes, including the core components of ABA signaling such as ABA receptors, protein phosphatases, protein kinases, and ABA-responsive element (ABRE) binding proteins (AREBs)/ABRE-binding factors (ABFs) as well as important genes for stomatal function, were mostly down-regulated in poco1. Drought and oxidative stress-related genes, including ABA-induced stress genes, were differentially regulated. RNA-seq analysis also uncovered differentially regulated genes encoding various classes of transcription factors and genes involved in cellular signaling. Furthermore, the expression of stress-associated nuclear genes encoding mitochondrial proteins (NGEMPs) was found to be altered in poco1. Redox-related genes were affected, suggesting that the redox state in poco1 might be altered.

CONCLUSION

The identification of various enriched biological processes indicates that complex regulatory mechanisms underlie poco1 development. Differentially regulated genes associated with flowering may contribute to the early-flowering phenotype of poco1. Our data suggest the involvement of POCO1 in the early ABA signaling process. The down-regulation of many ABA-related genes suggests an association of poco1 mutation with the ABA signaling deficiency. This condition further affects the expression of many stress-related, especially drought-associated genes in poco1, consistent with the drought sensitivity of poco1. poco1 mutation also affects the expression of genes associated with the cellular regulation, redox, and mitochondrial perturbation.

Malate Circulation: Linking Chloroplast Metabolism to Mitochondrial ROS

In photosynthetic cells, chloroplasts and mitochondria are the sites of the core redox reactions underpinning energy metabolism. Such reactions generate reactive oxygen species (ROS) when oxygen is partially reduced. ROS signaling leads to responses by cells which enable them to adjust to changes in redox status. Recent studies in Arabidopsis thaliana reveal that chloroplast NADH can be used to generate malate which is exported to the mitochondrion where its oxidation regenerates NADH. Oxidation of this NADH produces mitochondrial ROS (mROS) which can activate signaling systems to modulate energy metabolism, and in certain cases can lead to programmed cell death (PCD). We propose the term 'malate circulation' to describe such redistribution of reducing equivalents to mediate energy homeostasis in the cell.

Citrate valve integrates mitochondria into photosynthetic metabolism

While in heterotrophic cells and in darkness mitochondria serve as main producers of energy, during photosynthesis this function is transferred to chloroplasts and the main role of mitochondria in bioenergetics turns to be the balance of the level of phosphorylation of adenylates and of reduction of pyridine nucleotides to avoid over-energization of the cell and optimize major metabolic fluxes. This is achieved via the establishment and regulation of local equilibria of the tricarboxylic acid (TCA) cycle enzymes malate dehydrogenase and fumarase in one branch and aconitase and isocitrate dehydrogenase in another branch. In the conditions of elevation of redox level, the TCA cycle is transformed into a non-cyclic open structure (hemicycle) leading to the export of the tricarboxylic acid (citrate) to the cytosol and to the accumulation of the dicarboxylic acids (malate and fumarate). While the buildup of NADPH in chloroplasts provides operation of the malate valve leading to establishment of NADH/NAD⁺ ratios in different cell compartments, the production of NADH by mitochondria drives citrate export by establishing conditions for the operation of the citrate valve. The latter regulates the intercompartmental NADPH/NADP⁺ ratio and contributes to the biosynthesis of amino acids and other metabolic products during photosynthesis.

From sunscreens to medicines: Can a dissipation hypothesis explain the beneficial aspects of many plant compounds?

Medicine has utilised plant‐based treatments for millennia, but precisely how they work is unclear. One approach is to use a thermodynamic viewpoint that life arose by dissipating geothermal and/or solar potential. Hence, the ability to dissipate energy to maintain homeostasis is a fundamental principle in all life, which can be viewed as an accretion system where layers of complexity have built upon core abiotic molecules. Many of these compounds are chromophoric and are now involved in multiple pathways. Plants have further evolved a plethora of chromophoric compounds that can not only act as sunscreens and redox modifiers, but also have now become integrated into a generalised stress adaptive system. This could be an extension of the dissipative process. In animals, many of these compounds are hormetic, modulating mitochondria and calcium signalling. They can also display anti‐pathogen effects. They could therefore modulate bioenergetics across all life due to the conserved electron transport chain and proton gradient. In this review paper, we focus on well‐described medicinal compounds, such as salicylic acid and cannabidiol and suggest, at least in animals, their activity reflects their evolved function in plants in relation to stress adaptation, which itself evolved to maintain dissipative homeostasis.

Inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under thiocyanate exposure

Repression of the electron transport in mitochondria can result in an increase of reactive oxygen species (ROS) in plant cells. This study was to clarify inhibition of the mitochondrial respiratory components (Complex I and Complex III) as stimuli to induce oxidative damage in Oryza sativa L. under exogenous SCN^- exposure with special emphasis on lipid peroxidation, protein modification, and DNA damage at the biochemical and molecular levels. Our results showed that enzymatic activity and gene expression of cytochrome c reductase (Complex III) in roots and shoots of rice seedlings were significantly repressed by SCN^- exposure, where significant inhibition of NADH dehydrogenase (Complex I) was only detected in shoots, suggesting that Complex III was the main target attacked by SCN^- ligand in rice roots, and both components were arrested in shoots. ROS analysis in tissues indicated that SCN^- exposure caused significant accumulation of H₂O₂ and O₂^-•, increased malondialdehyde (MDA) and carbonyl content in rice materials in a dose-dependent manner. Similarly, a remarkable elevation of electrolyte leakage was observed in rice tissue samples. The comet assay indicated a positive correlation between DNA damage and external SCN^- exposure. In conclusion, oxidative burst generated from the inhibitions of the electron transport in mitochondria in rice seedlings under SCN^- exposure can cause lipid peroxidation, protein modification and DNA damage, eventually decreasing fresh weight of rice seedlings.

Influence of different seed priming techniques on oxidative and antioxidative responses during the germination of Oryza sativa varieties

NaCl and PEG stresses have negative impacts on seed germination and early seedling establishment in Oryza sativa. The present study was designed to ascertain the influence of different priming techniques (Hydro priming-HyP, Halo priming-HP, UV-B priming-UP) in enhancing oxidative and anti-oxidative mechanisms during seed germination phase in response to NaCl and PEG stresses tolerance of three rice varieties (Neeraja, Vaisakh and Vyttila 6). NaCl and PEG stresses caused delayed germination rate, enhanced reactive oxygen species content and thereby increased lipid peroxidation rate. Different priming techniques significantly hastened the metabolites/non enzymatic antioxidant contents (total sugars, total phenolics, free amino acids, proline, ascorbate and glutathione) as well as activities of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase), and thus reduced oxidative stress damages caused by NaCl and PEG stresses in rice seedlings. Seed priming techniques imparted abiotic stress tolerance not only to sensitive varieties but also additional tolerance potential to tolerant varieties. All three priming techniques protects the plants from toxicity caused by NaCl and PEG stresses but halo priming had proved to be more successful.

Photosynthesis is sensitive to nitric oxide and respiration sensitive to hydrogen peroxide: Studies with pea mesophyll protoplasts

Reports on the effect of nitric oxide (NO) or reactive oxygen species (ROS) on photosynthesis and respiration in leaf tissues are intriguing; therefore, the effects of exogenous addition of sodium nitroprusside (SNP, releases NO) or H2O2 on the photosynthetic O2 evolution and respiratory O2 uptake by mesophyll protoplasts in pea (Pisum sativum) were evaluated in the present study. Low concentrations of SNP or H2O2 were used to minimize nonspecific effects. The effects of NO or H2O2 on respiration and photosynthesis were different. The presence of NO decreased the rate of photosynthesis but caused a marginal stimulation of dark respiration. Conversely, externally administered H2O2 drastically decreased the rate of respiration but only slightly decreased photosynthesis. The PS I activity was more sensitive to NO than PS II. On the other hand, 100 μM H2O2 had no effect on the photochemical reactions of either PS I or PS II. The sensitivity of photosynthesis to antimycin A or SHAM (reflecting the interplay between chloroplasts and mitochondria) was not affected by NO. By contrast, H2O2 markedly decreased the sensitivity of photosynthesis to antimycin A and SHAM. It can be concluded that chloroplasts are the primary targets of NO, while mitochondria are the primary targets of ROS in plant cells. We propose that H2O2 can be an important signal to modulate the crosstalk between chloroplasts and mitochondria.

Efficient Photosynthetic Functioning of Arabidopsis thaliana Through Electron Dissipation in Chloroplasts and Electron Export to Mitochondria Under Ammonium Nutrition

An improvement in photosynthetic rate promotes the growth of crop plants. The sink-regulation of photosynthesis is crucial in optimizing nitrogen fixation and integrating it with carbon balance. Studies on these processes are essential in understanding growth inhibition in plants with ammonium ( NH 4 + ) syndrome. Hence, we sought to investigate the effects of using nitrogen sources with different states of reduction (during assimilation of NO 3 - versus NH 4 + ) on the photosynthetic performance of Arabidopsis thaliana. Our results demonstrated that photosynthetic functioning during long-term NH 4 + nutrition was not disturbed and that no indication of photoinhibition of PSII was detected, revealing the robustness of the photosynthetic apparatus during stressful conditions. Based on our findings, we propose multiple strategies to sustain photosynthetic activity during limited reductant utilization for NH 4 + assimilation. One mechanism to prevent chloroplast electron transport chain overreduction during NH 4 + nutrition is for cyclic electron flow together with plastid terminal oxidase activity. Moreover, redox state in chloroplasts was optimized by a dedicated type II NAD(P)H dehydrogenase. In order to reduce the amount of energy that reaches the photosynthetic reaction centers and to facilitate photosynthetic protection during NH 4 + nutrition, non-photochemical quenching (NPQ) and ample xanthophyll cycle pigments efficiently dissipate excess excitation. Additionally, high redox load may be dissipated in other metabolic reactions outside of chloroplasts due to the direct export of nucleotides through the malate/oxaloacetate valve. Mitochondrial alternative pathways can downstream support the overreduction of chloroplasts. This mechanism correlated with the improved growth of A. thaliana with the overexpression of the alternative oxidase 1a (AOX1a) during NH 4 + nutrition. Most remarkably, our findings demonstrated the capacity of chloroplasts to tolerate NH 4 + syndrome instead of providing redox poise to the cells.

Effects of Salicylic Acid on the Metabolism of Mitochondrial Reactive Oxygen Species in Plants

Different abiotic and biotic stresses lead to the production and accumulation of reactive oxygen species (ROS) in various cell organelles such as in mitochondria, resulting in oxidative stress, inducing defense responses or programmed cell death (PCD) in plants. In response to oxidative stress, cells activate various cytoprotective responses, enhancing the antioxidant system, increasing the activity of alternative oxidase and degrading the oxidized proteins. Oxidative stress responses are orchestrated by several phytohormones such as salicylic acid (SA). The biomolecule SA is a key regulator in mitochondria-mediated defense signaling and PCD, but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in mitochondrial ROS metabolism is summarized to gain a better understanding of SA-regulated processes at the subcellular level in plant defense responses.

Adaptation of Forest Trees to Rapidly Changing Climate

Climate change leads to global drought-induced stress and increased plant mortality. Tree species living in rapidly changing climate conditions are exposed to danger and must adapt to new climate conditions to survive. Trees respond to changes in the environment in numerous ways. Physiological modulation at the seed stage, germination strategy and further development are influenced by many different factors. We review forest abiotic threats (such as drought and heat), including biochemical responses of plants to stress, and biotic threats (pathogens and insects) related to global warming. We then discus the varied adaptations of tree species to changing climate conditions such as seed resistance to environmental stress, improved by an increase in temperature, affinity to specific fungal symbionts, a wide range of tolerance to abiotic environmental conditions in the offspring of populations occurring in continental climate, and germination strategies closely linked to the ecological niche of the species. The existing studies do not clearly indicate whether tree adaptations are shaped by epigenetics or phenology and do not define the role of phenotypic plasticity in tree development. We have created a juxtaposition of literature that is useful in identifying the factors that play key roles in these processes. We compare scientific evidence that species distribution and survival are possible due to phenotypic plasticity and thermal memory with studies that testify that trees’ phenology depends on phylogenesis, but this issue is still open. It is possible that studies in the near future will bring us closer to understanding the mechanisms through which trees adapt to stressful conditions, especially in the context of epigenetic memory in long-lived organisms, and allow us to minimize the harmful effects of climatic events by predicting tree species’ responses or by developing solutions such as assisted migration to mitigate the consequences of these phenomena.

Modulation of (Homo)Glutathione Metabolism and H2O2 Accumulation during Soybean Cyst Nematode Infections in Susceptible and Resistant Soybean Cultivars

In plant immune responses, reactive oxygen species (ROS) act as signaling molecules that activate defense pathways against pathogens, especially following resistance (R) gene-mediated pathogen recognition. Glutathione (GSH), an antioxidant and redox regulator, participates in the removal of hydrogen peroxide (H2O2). However, the mechanism of GSH-mediated H2O2 generation in soybeans (Glycine max (L.) Merr.) that are resistant to the soybean cyst nematode (SCN; Heterodera glycines Ichinohe) remains unclear. To elucidate this underlying relationship, the feeding of race 3 of H. glycines with resistant cultivars, Peking and PI88788, was compared with that on a susceptible soybean cultivar, Williams 82. After 5, 10, and 15 days of SCN infection, we quantified γ-glutamylcysteine (γ-EC) and (homo)glutathione ((h)GSH), and a gene expression analysis showed that GSH metabolism in resistant cultivars differed from that in susceptible soybean roots. ROS accumulation was examined both in resistant and susceptible roots upon SCN infection. The time of intense ROS generation was related to the differences of resistance mechanisms in Peking and PI88788. ROS accumulation that was caused by the (h)GSH depletion-arrested nematode development in susceptible Williams 82. These results suggest that (h)GSH metabolism in resistant soybeans plays a key role in the regulation of ROS-generated signals, leading to resistance against nematodes.