Production and scavenging of reactive oxygen species in chloroplasts and their functions

Turnip mosaic virus impairs perinuclear chloroplast clustering to facilitate viral infection

Chloroplasts play crucial roles in plant defence against viral infection. We now report that chloroplast NADH dehydrogenase-like (NDH) complex M subunit gene (NdhM) was first up-regulated and then down-regulated in turnip mosaic virus (TuMV)-infected N. benthamiana. NbNdhM-silenced plants were more susceptible to TuMV, whereas overexpression of NbNdhM inhibited TuMV accumulation. Overexpression of NbNdhM significantly induced the clustering of chloroplasts around the nuclei and disturbing this clustering facilitated TuMV infection, suggesting that the clustering mediated by NbNdhM is a defence against TuMV. It was then shown that NbNdhM interacted with TuMV VPg, and that the NdhMs of different plant species interacted with the proteins of different viruses, implying that NdhM may be a common target of viruses. In the presence of TuMV VPg, NbNdhM, which is normally localized in the nucleus, chloroplasts, cell periphery and chloroplast stromules, colocalized with VPg at the nucleus and nucleolus, with significantly increased nuclear accumulation, while NbNdhM-mediated chloroplast clustering was significantly impaired. This study therefore indicates that NbNdhM has a defensive role in TuMV infection probably by inducing the perinuclear clustering of chloroplasts, and that the localization of NbNdhM is altered by its interaction with TuMV VPg in a way that promotes virus infection.

The sweetpotato β-amylase gene IbBAM1.1 enhances drought and salt stress resistance by regulating ROS homeostasis and osmotic balance

Abiotic stressors, such as drought and high salinity, seriously affect plant growth, productivity, and quality. Maintaining reactive oxygen species (ROS) homeostasis and osmotic balance plays a crucial role in abiotic stress tolerance. β-amylase (BAM) hydrolyzes α-1,4-glycosidic bonds by releasing maltose from starch in the regulation of soluble sugars. However, the function and mechanism of BAMs related to abiotic stress resistance remain unclear in sweetpotato (Ipomoea batatas (L.) Lam.). In this study, we isolated a novel β-amylase gene IbBAM1.1, which was strongly induced by PEG6000, NaCl, and maltose treatments in sweetpotato variety Yanshu25. Overexpression of IbBAM1.1 conferred enhanced tolerance to the drought and high salinity stressors in Arabidopsis thaliana. The activity of β-amylase and the degradation of starch were promoted under drought or salt stress. Accordingly, the contents of osmoprotectants, including maltose and proline were significantly higher in the transgenic lines than those in wild type (WT) plants. Less ROS, such as H₂O₂ and O₂^-, accumulated in the overexpressing lines than in WT plants. Superoxide dismutase activity was strongly enhanced and the level of malondialdehyde was lower under the drought or salt treatment in transgenic plants. Taken together, these results demonstrate that IbBAM1.1 acted as a positive regulator, at least in part, by regulating the level of osmoprotectants to balance the osmotic pressure and activate the scavenging system to maintain ROS homeostasis in the plants.

Salicylic acid: A key regulator of redox signalling and plant immunity

In plants, the reactive oxygen species (ROS) formed during normal conditions are essential in regulating several processes, like stomatal physiology, pathogen immunity and developmental signaling. However, biotic and abiotic stresses can cause ROS over-accumulation leading to oxidative stress. Therefore, a suitable equilibrium is vital for redox homeostasis in plants, and there have been major advances in this research arena. Salicylic acid (SA) is known as a chief regulator of ROS; however, the underlying mechanisms remain largely unexplored. SA plays an important role in establishing the hypersensitive response (HR) and systemic acquired resistance (SAR). This is underpinned by a robust and complex network of SA with Non-Expressor of Pathogenesis Related protein-1 (NPR1), ROS, calcium ions (Ca²⁺), nitric oxide (NO) and mitogen-activated protein kinase (MAPK) cascades. In this review, we summarize the recent advances in the regulation of ROS and antioxidant defense system signalling by SA at the physiological and molecular levels. Understanding the molecular mechanisms of how SA controls redox homeostasis would provide a fundamental framework to develop approaches that will improve plant growth and fitness, in order to meet the increasing global demand for food and bioenergy.

Overexpression of AtDREB1 and BcZAT12 genes confers drought tolerance by reducing oxidative stress in double transgenic tomato (Solanum lycopersicum L.)

Double transgenic tomato developed by AtDREB1A and BcZAT12 genes pyramiding showed significant drought tolerance by reducing oxidative stress with enhanced yield. Although a large number of efforts have been made by different researchers to develop abiotic stress tolerance tomato for improving yield using single gene, however, no reports are available which targets AtDREB1 and BcZAT12 genes together. Hence, in the present study, double transgenic plants were developed using AtDREB1 and BcZAT12 genes to improve yield potential with better drought tolerance. Double transgenic (DZ1-DZ5) tomato lines showed enhanced drought tolerance than their counterpart non-transgenic and single transgenic plants at 0, 07, 14, and 21 days of water deficit, respectively. Double transgenic plants showed increased activity of antioxidant enzymes, like catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and guaiacol peroxidase (POD), and accumulation of non-enzymatic antioxidants like ascorbic acid, glutathione as compared to non-transgenic and single transgenic. Additionally, the transcript analysis of antioxidant enzymes revealed the increased level of gene expression in double transgenic tomato lines. Developed double-transgenic tomato plants co-over-expressing both genes exhibited more enzymatic and non-enzymatic anti-oxidative activities as compared to the non-transgenic and single transgenic control, respectively. This is the preliminary report in tomato, which forms the basis for a multigene transgenic approach to cope with drought stress.

Comparative Proteomic Analysis of Grapevine Rootstock in Response to Waterlogging Stress

Waterlogging severely affects global agricultural production. Clarifying the regulatory mechanism of grapevine in response to waterlogging stress will help to improve the waterlogging tolerance of grapevine. In the present study, the physiological and proteomic responses of SO4 grapevine rootstock to different waterlogging tolerances were comparatively assayed. The results showed that the activities of SOD and POD first increased and then decreased, while the change trend of CAT and APX activities was the opposite. In addition, the MDA and H₂O₂ contents increased after waterlogging treatment, but the chlorophyll a and chlorophyll b contents decreased. A total of 5,578 grapevine proteins were identified by the use of the tandem mass tag (TMT) labeling technique. Among them, 214 (103 and 111 whose expression was upregulated and downregulated, respectively), 314 (129 and 185 whose expression was upregulated and downregulated, respectively), and 529 (248 and 281 whose expression was upregulated and downregulated, respectively) differentially expressed proteins (DEPs) were identified in T0d vs. T10d, T10d vs. T20d, and T0d vs. T20d comparison groups, respectively. Enrichment analysis showed that these DEPs were mainly involved in glutathione metabolism, carbon fixation, amino sugar and nucleotide sugar metabolism, biosynthesis of amino acids, photosynthesis, carbon metabolism, starch, and sucrose metabolism, galactose metabolism, protein processing and ribosomes. To further verify the proteomic data, the expression of corresponding genes that encode eight DEPs was confirmed by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). The results of this study presented an important step toward understanding the resistance mechanisms of grapevine in response to waterlogging stress at the proteome level.

Natural Biostimulant Attenuates Salinity Stress Effects in Chili Pepper by Remodeling Antioxidant, Ion, and Phytohormone Balances, and Augments Gene Expression

A biostimulant is any microorganism or substance used to enhance the efficiency of nutrition, tolerance to abiotic stress and/or quality traits of crops, depending on its contents from nutrients. Plant biostimulants like honey bee (HB) and silymarin (Sm) are a strategic trend for managing stressed crops by promoting nutritional and hormonal balance, regulating osmotic protectors, antioxidants, and genetic potential, reflecting plant growth and productivity. We applied diluted honey bee (HB) and silymarin-enriched honey bee (HB- Sm) as foliar nourishment to investigate their improving influences on growth, yield, nutritional and hormonal balance, various osmoprotectant levels, different components of antioxidant system, and genetic potential of chili pepper plants grown under NaCl-salinity stress (10 dS m‒1). HB significantly promoted the examined attributes and HB-Sm conferred optimal values, including growth, productivity, K+/Na+ ratio, capsaicin, and Sm contents. The antioxidative defense components were significantly better than those obtained with HB alone. Conversely, levels of oxidative stress markers (superoxide ions and hydrogen peroxide) and parameters related to membrane damage (malondialdehyde level, stability index, ionic leakage, Na+, and Cl- contents) were significantly reduced. HB-Sm significantly affects inactive gene expression, as a natural biostimulator silencing active gene expression. SCoT primers were used as proof in salt-treated or untreated chili pepper plants. There were 41 cDNA amplicons selected by SCoT-primers. Twenty of them were EcDNA amplicons (cDNA-amplicons that enhanced their genes by one or more treatments) representing 49% of all cDNA amplicons, whereas 7 amplicons for ScDNA (whose genes were silenced in one or more treatments) represented 17%, and 14 McDNA (monomorphic cDNA-amplicons with control) amplicons were represented by 34% from all cDNA amplicons. This indicates the high effect of BH-Sm treatments in expression enhancement of some inactive genes and their silenced effect for expression of some active genes, also confirming that cDNA-SCoT markers succeeded in detection of variable gene expression patterns between the untreated and treated plants. In conclusion, HB-Sm as a natural multi-biostimulator can attenuate salt stress effects in chili pepper plants by remodeling the antioxidant defense system and ameliorating plant productivity.

Computational Analysis of Alternative Photosynthetic Electron Flows Linked With Oxidative Stress

During photosynthesis, organisms respond to their energy demand and ensure the supply of energy and redox equivalents that sustain metabolism. Hence, the photosynthetic apparatus can, and in fact should, be treated as an integrated supply-demand system. Any imbalance in the energy produced and consumed can lead to adverse reactions, such as the production of reactive oxygen species (ROS). Reaction centres of both photosystems are known sites of ROS production. Here, we investigate in particular the central role of Photosystem I (PSI) in this tightly regulated system. Using a computational approach we have expanded a previously published mechanistic model of C3 photosynthesis by including ROS producing and scavenging reactions around PSI. These include two water to water reactions mediated by Plastid terminal oxidase (PTOX) and Mehler and the ascorbate-glutathione (ASC-GSH) cycle, as a main non-enzymatic antioxidant. We have used this model to predict flux distributions through alternative electron pathways under various environmental stress conditions by systematically varying light intensity and enzymatic activity of key reactions. In particular, we studied the link between ROS formation and activation of pathways around PSI as potential scavenging mechanisms. This work shines light on the role of alternative electron pathways in photosynthetic acclimation and investigates the effect of environmental perturbations on PSI activity in the context of metabolic productivity.

Harnessing Chlorophyll Fluorescence for Phenotyping Analysis of Wild and Cultivated Tomato for High Photochemical Efficiency under Water Deficit for Climate Change Resilience

Fluctuations of the weather conditions, due to global climate change, greatly influence plant growth and development, eventually affecting crop yield and quality, but also plant survival. Since water shortage is one of the key risks for the future of agriculture, exploring the capability of crop species to grow with limited water is therefore fundamental. By using chlorophyll fluorescence analysis, we evaluated the responses of wild tomato accession Solanum pennellii LA0716, Solanum lycopersicum cv. Μ82, the introgression line IL12-4 (from cv. M82 Χ LA0716), and the Greek tomato cultivars cv. Santorini and cv. Zakinthos, to moderate drought stress (MoDS) and severe drought stress (SDS), in order to identify the minimum irrigation level for efficient photosynthetic performance. Agronomic traits (plant height, number of leaves and root/shoot biomass), relative water content (RWC), and lipid peroxidation, were also measured. Under almost 50% deficit irrigation, S. pennellii exhibited an enhanced photosynthetic function by displaying a hormetic response of electron transport rate (ETR), due to an increased fraction of open reaction centers, it is suggested to be activated by the low increase of reactive oxygen species (ROS). A low increase of ROS is regarded to be beneficial by stimulating defense responses and also triggering a more oxidized redox state of quinone A (QA), corresponding in S. pennellii under 50% deficit irrigation, to the lowest stomatal opening, resulting in reduction of water loss. Solanumpennellii was the most tolerant to drought, as it was expected, and could manage to have an adequate photochemical function with almost 30% water regime of well-watered plants. With 50% deficit irrigation, cv. Μ82 and cv. Santorini did not show any difference in photochemical efficiency to control plants and are recommended to be cultivated under deficit irrigation as an effective strategy to enhance agricultural sustainability under a global climate change. We conclude that instead of the previously used Fv/Fm ratio, the redox state of QA, as it can be estimated by the chlorophyll fluorescence parameter 1 - qL, is a better indicator to evaluate photosynthetic efficiency and select drought tolerant cultivars under deficit irrigation.

Wheat Leaf Antioxidative Status—Variety-Specific Mechanisms of Zinc Tolerance during Biofortification

In this study, we evaluated the leaf antioxidative responses of three wheat varieties (Srpanjka, Divana, and Simonida) treated with two different forms of zinc (Zn), Zn-sulfate and Zn-EDTA, in concentrations commonly used in agronomic biofortification. Zn concentration was significantly higher in the flag leaves of all three wheat varieties treated with Zn-EDTA compared to control and leaves treated with Zn-sulfate. Both forms of Zn increased malondialdehyde level and total phenolics content in varieties Srpanjka and Divana. Total glutathione content was not affected after the Zn treatment. Zn-sulfate increased the activities of glutathione reductase (GR) and guaiacol peroxidase (GPOD) in both Srpanjka and Divana, while glutathione S-transferase (GST) was only induced in var. Srpanjka. Chelate form of Zn increased the activities of GST and GPOD in both Simonida and Divana. Catalase activity was shown to be less sensitive to Zn treatment and was only induced in var. Srpanjka treated with Zn-EDTA where GPOD activity was not induced. Concentrations of Zn used for agronomic biofortification can induce oxidative stress in wheat leaves. The antioxidative status of wheat leaves could be a good indicator of Zn tolerance, whereas wheat genotype and chemical form of Zn are the most critical factors influencing Zn toxicity.

Modulation of Phototropin Signalosome with Artificial Illumination Holds Great Potential in the Development of Climate-Smart Crops

Changes in environmental conditions like temperature and light critically influence crop production. To deal with these changes, plants possess various photoreceptors such as Phototropin (PHOT), Phytochrome (PHY), Cryptochrome (CRY), and UVR8 that work synergistically as sensor and stress sensing receptors to different external cues. PHOTs are capable of regulating several functions like growth and development, chloroplast relocation, thermomorphogenesis, metabolite accumulation, stomatal opening, and phototropism in plants. PHOT plays a pivotal role in overcoming the damage caused by excess light and other environmental stresses (heat, cold, and salinity) and biotic stress. The crosstalk between photoreceptors and phytohormones contributes to plant growth, seed germination, photo-protection, flowering, phototropism, and stomatal opening.

Molecular genetic studies using gene targeting and synthetic biology approaches have revealed the potential role of different photoreceptor genes in the manipulation of various beneficial agronomic traits. Overexpression of PHOT2 in Fragaria ananassa leads to the increase in anthocyanin content in its leaves and fruits. Artificial illumination with blue light alone and in combination with red light influence the growth, yield, and secondary metabolite production in many plants, while in algal species, it affects growth, chlorophyll content, lipid production and also increases its bioremediation efficiency. Artificial illumination alters the morphological, developmental, and physiological characteristics of agronomic crops and algal species. This review focuses on PHOT modulated signalosome and artificial illumination-based photo-biotechnological approaches for the development of climate-smart crops.

Eutrophication on Coral Reefs: What Is the Evidence for Phase Shifts, Nutrient Limitation and Coral Bleaching

Coral reefs continue to experience extreme environmental pressure from climate change stressors, but many coral reefs are also exposed to eutrophication. It has been proposed that changes in the stoichiometry of ambient nutrients increase the mortality of corals, whereas eutrophication may facilitate phase shifts to macroalgae-dominated coral reefs when herbivory is low or absent. But are corals ever nutrient limited, and can eutrophication destabilize the coral symbiosis making it more sensitive to environmental stress because of climate change? The effects of eutrophication are confounded not just by the effects of climate change but by the presence of chemical pollutants in industrial, urban, and agricultural wastes. Because of these confounding effects, the increases in nutrients or changes in their stoichiometry in coastal environments, although they are important at the organismal and community level, cannot currently be disentangled from each other or from the more significant effects of climate change stressors on coral reefs.

The threshold between life and death in Cistus albidus L. seedlings: mechanisms underlying drought tolerance and resilience

Drought can lead to important shifts in population dynamics if it occurs during seedling establishment. With the aim of elucidating the underlying mechanisms of drought tolerance and resilience, here we monitored the survival of seedlings of the Mediterranean shrub Cistus albidus L. throughout a year growing in the natural Park of the Montserrat Mountains (Spain) and, additionally, we studied the response to severe drought and subsequent recovery after rewatering of seedlings grown in growth chambers. To find possible mechanisms explaining how seedlings respond to drought, growth and survival together with physiological-related parameters such as chlorophyll contents, vitamin E and stress-related phytohormones were measured. We found that survival decreased by 30% at the end of summer and that the main proxy of seedling survival was total chlorophyll. This proxy was further confirmed in the growth chambers, where we found that seedlings that recovered from drought had higher levels of total chlorophyll compared with the seedlings that did not recover. Furthermore, modulation of vitamin E and jasmonates contents appeared to be crucial in the drought response of C. albidus seedlings. We propose a prediction model of survival that includes total chlorophyll height, leaf mass area and maximum photosystem II efficiency with chlorophyll contents being a good long-term predictor of C. albidus seedling survival under severe stress, which, in turn, could help to better foresee population fluctuations in the field.

Specific toxicity of azithromycin to the freshwater microalga Raphidocelis subcapitata

Pharmaceuticals are produced to inflict a specific physiological response in organisms. However, as only partially metabolized after administration, these types of compounds can also originate harmful side effects to non-target organisms. Additionally, there is still a lack of knowledge on the toxicological effects of legacy pharmaceuticals such as the antibiotic azithromycin. This macrolide occurs at high concentrations in the aquatic environment and can constitute a threat to aquatic organisms that are at the basis of the aquatic food chain, namely microalgae. This study established a high-throughput methodology to study the toxicity of azithromycin to the freshwater microalga Raphidocelis subcapitata. Flow cytometry and pulse amplitude modulated (PAM) fluorometry were used as screening tools. General toxicity was shown by effects in growth rate, cell size, cell complexity, cell viability and cell cycle. More specific outcomes were indicated by the analysis of mitochondrial and cytoplasmatic membrane potentials, DNA content, formation of ROS and LPO, natural pigments content and photosystem II performance. The specific mode of action (MoA) of azithromycin to crucial components of microalgae cells was revealed. Azithromycin had a negative impact on the regulation of energy dissipation at the PSII centers, along with an insufficient protection by the regulatory mechanisms leading to photodamage. The blockage of photosynthetic electrons led to ROS formation and consequent oxidative damage, affecting membranes and DNA. Overall, the used methodology exhibited its high potential for detecting the toxic MoA of compounds in microalgae and should be considered for future risk assessment of pharmaceuticals.

Potassium: A track to develop salinity tolerant plants

Salinity is one of the major constraints to plant growth and development across the globe that leads to the huge crop productivity loss. Salinity stress causes impairment in plant's metabolic and cellular processes including disruption in ionic homeostasis due to excess of sodium (Na⁺) ion influx and potassium (K⁺) efflux. This condition subsequently results in a significant reduction of the cytosolic K⁺ levels, eventually inhibiting plant growth attributes. K⁺ plays a crucial role in alleviating salinity stress by recasting key processes of plants. In addition, K⁺ acquisition and retention also serve as the perquisite trait to establish salt tolerant mechanism. In addition, an intricate network of genes and their regulatory elements are involved in coordinating salinity stress responses. Furthermore, plant growth regulators (PGRs) and other signalling molecules influence K⁺-mediated salinity tolerance in plants. Recently, nanoparticles (NPs) have also been found several implications in plants with respect to their roles in mediating K⁺ homoeostasis during salinity stress in plants. The present review describes salinity-induced adversities in plants and role of K⁺ in mitigating salinity-induced damages. The review also highlights the efficacy of PGRs and other signalling molecules in regulating K⁺ mediated salinity tolerance along with nano-technological perspective for improving K⁺ mediated salinity tolerance in plants.

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

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

Microalgae biofilm formation and antioxidant responses to stress induce by Lemna minor L., Chlorella vulgaris, and Aphanizomenon flos-aquae

The study shows how microalgae biofilm formation and antioxidant responses to the production of reactive oxygen species (ROS) is alter by the presences of Lemna minor L., Chlorella vulgaris, and Aphanizomenon flos-aquae. The study involves the cultivation of the biofilm of Chlorella vulgaris and Aphanizomenon flos-aquae in three bioreactors. The condition of growth for the biofilm formation was varied across the three bioreactors to enable the dominance Chlorella vulgaris and Aphanizomenon flos-aquae in one of the bioreactors. Lemna minor L. was also introduce into one of the bioreactors to determine its effect on the biofilm formation. The result obtained shows that C. vulgaris and A. flos-aquae dominate the biofilm, resulting in a high level of H₂O₂ and O₂^- (H₂O₂ was 0.122 ± 0.052 and 0.183 ± 0.108 mmol/L in C. vulgaris and A. flos-aquae, respectively, and O₂^- was 0.261 ± 0.039 and 0.251 ± 0.148 mmol/L in C. vulgaris and A. flos-aquae, respectively). The study also revealed that the presence of L. minor L. tend to reduce the oxidative stress to the biofilm leading to low production of ROS (H₂O₂ was 0.086 ± 0.027 and 0.089 ± 0.045 mmol/L in C. vulgaris and A. flos-aquae respectively, and O₂^- was 0.185 ± 0.044 and 0.161 ± 0.065 mmol/L in C. vulgaris and A. flos-aquae respectively). The variation in the ability of the biofilm of C. vulgaris and A. flos-aquae to respond via chlorophyll, carotenoid, flavonoid, anthocyanin, superoxide dismutase, peroxidase, catalase, glutathione reductase activities, antioxidant reducing power, phosphomolybdate activity, DPPH reduction activity, H₂O₂ scavenging activity, lipid content and organic carbon also supports the fact that the presence of biomass of microalgae and aquatic macrophytes tend to affect the process of microalgae biofilm formation and the ability of the biofilm to produce antioxidant. This high nutrient utilization leads to the production of biomass which can be used for biofuel production and other biotechnological products.

Inter-domain horizontal gene transfer of nickel-binding superoxide dismutase

The ability of aerobic microorganisms to regulate internal and external concentrations of the reactive oxygen species (ROS) superoxide directly influences the health and viability of cells. Superoxide dismutases (SODs) are the primary regulatory enzymes that are used by microorganisms to degrade superoxide. SOD is not one, but three separate, non-homologous enzymes that perform the same function. Thus, the evolutionary history of genes encoding for different SOD enzymes is one of convergent evolution, which reflects environmental selection brought about by an oxygenated atmosphere, changes in metal availability, and opportunistic horizontal gene transfer (HGT). In this study, we examine the phylogenetic history of the protein sequence encoding for the nickel-binding metalloform of the SOD enzyme (SodN). The genomic potential to produce SodN is widespread among bacteria, including Actinobacteriota (Actinobacteria), Chloroflexota (Chloroflexi), Cyanobacteria, Proteobacteria, Patescibacteria, and others. The gene is also present in many archaea, with Thermoplasmatota and Nanoarchaeota representing the vast majority of archaeal sodN diversity. A comparison of organismal and SodN protein phylogenetic trees reveals several instances of HGT, including multiple inter-domain transfers of the sodN gene from the bacterial domain to the archaeal domain. Nearly half of the archaeal members with sodN live in the photic zone of the marine water column. The sodN gene is widespread and characterized by apparent vertical gene transfer in some sediment- or soil-associated lineages within the Actinobacteriota and Chloroflexota phyla, suggesting the ancestral sodN likely originated in one of these clades before expanding its taxonomic and biogeographic distribution to additional microbial groups in the surface ocean in response to decreasing iron availability. In addition to decreasing iron quotas, nickel-binding SOD has the added benefit of withstanding high reactant and product ROS concentrations without damaging the enzyme, making it particularly well suited for the modern surface ocean.

The circadian-controlled PIF8-BBX28 module regulates petal senescence in rose flowers by governing mitochondrial ROS homeostasis at night

Reactive oxygen species (ROS) are unstable reactive molecules that are toxic to cells. Regulation of ROS homeostasis is crucial to protect cells from dysfunction, senescence, and death. In plant leaves, ROS are mainly generated from chloroplasts and are tightly temporally restricted by the circadian clock. However, little is known about how ROS homeostasis is regulated in nonphotosynthetic organs, such as petals. Here, we showed that hydrogen peroxide (H2O2) levels exhibit typical circadian rhythmicity in rose (Rosa hybrida) petals, consistent with the measured respiratory rate. RNA-seq and functional screening identified a B-box gene, RhBBX28, whose expression was associated with H2O2 rhythms. Silencing RhBBX28 accelerated flower senescence and promoted H2O2 accumulation at night in petals, while overexpression of RhBBX28 had the opposite effects. RhBBX28 influenced the expression of various genes related to respiratory metabolism, including the TCA cycle and glycolysis, and directly repressed the expression of SUCCINATE DEHYDROGENASE 1, which plays a central role in mitochondrial ROS (mtROS) homeostasis. We also found that PHYTOCHROME-INTERACTING FACTOR8 (RhPIF8) could activate RhBBX28 expression to control H2O2 levels in petals and thus flower senescence. Our results indicate that the circadian-controlled RhPIF8-RhBBX28 module is a critical player that controls flower senescence by governing mtROS homeostasis in rose.

Phosphorus-Induced Adaptation Mechanisms of Rye Grown on Post-Flotation Copper Tailings

Simple Summary

The mining activities for the production of copper, lead, zinc, and others are inevitably associated with the generation of an enormous volume of waste materials, i.e., tailings. The global area covered with tailings is on the order of hundreds of millions of hectares, with this being predicted to rise significantly in the coming decades. Importantly, the physicochemical properties of tailings, such as high content of heavy metals and high pH, pose a serious threat to the surrounding ecological environment. This, combined with low available nutrients, makes revegetation of degraded lands very difficult, with the number of field trials demonstrating successful revitalisation remaining very low. In this study, we investigate in rye, as a model plant, a wide array of physiological processes and their significance in determining survival on the copper tailings. We demonstrate that limitations in plant growth on such wastes is not simply related to high copper content. Rather, we present data that the low availability of phosphorus and activity of the mechanisms involved in phosphorus extraction from the rhizosphere are important determinants of the plant growth and survival rate. With these results, we make a direct and significant contribution towards meeting future demands for effective revitalisation techniques of degraded lands.

Abstract

Although a considerable effort has been made over the last decades to develop cost-effective phytotechnologies as an alternative to conventional techniques for the management of contaminated lands, successful revegetation of the tailings still represents a major challenge. Here, we evaluate the potential of rye (Secale cereale L.) for growth and survival on the tailings after copper (Cu) ore processing. Four rye varieties were cultivated in a pot experiment on the post-flotation sediment with increasing phosphorus (P) doses (22, 44, 66, 88, and 110 mg·kg⁻¹). The resistance of the studied rye genotypes to stress was assessed by observing the growth and development of plants, determining the dry mass accumulation, the Cu and P uptake and content, and a number of physiological parameters related mainly to P mobilisation. Exposure of tested rye varieties to high Cu concentrations in the tailings did not result in any significant plant mortality, with the intracellular Cu concentrations being below the critical toxic level. In contrast, the low availability of P due to alkaline properties of the tailings and the mechanisms involved in the mobilisation of sparingly soluble forms of this element (i.e., H⁺-ATPase-driven proton efflux in roots and organic acid exudation), were identified as main factor determining the level of tolerance. The efficiency of the photosynthetic activity was a key determinant for the P-mobilising capacity of rye. We further showed that rye varieties with more primitive genetic background might be potentially more suitable for growth on the post-flotation copper tailings. The results provide important and novel knowledge that will certainly support future works in developing strategies for successful revitalisation of degraded lands.

Changes in Photosynthesis Could Provide Important Insight into the Interaction between Wheat and Fungal Pathogens

Photosynthesis is a universal process for plant survival, and immune defense is also a key process in adapting to the growth environment. Various studies have indicated that these two processes are interconnected in a complex network. Photosynthesis can influence signaling pathways and provide both materials and energy for immune defense, while the immune defense process can also have feedback effects on photosynthesis. Pathogen infection inevitably leads to changes in photosynthesis parameters, including Pn, Gs, and Ci; biochemical materials such as SOD and CAT; signaling molecules such as H₂O₂ and hormones; and the expression of genes involved in photosynthesis. Some researchers have found that changes in photosynthesis activity are related to the resistance level of the host, the duration after infection, and the infection position (photosynthetic source or sink). Interactions between wheat and the main fungal pathogens, such as Puccinia striiformis, Blumeria graminis, and Fusarium graminearum, constitute an ideal study system to elucidate the relationship between changes in host photosynthesis and resistance levels, based on the accessibility of methods for artificially controlling infection and detecting changes in photosynthesis, the presence of multiple pathogens infecting different positions, and the abundance of host materials with various resistance levels. This review is written only from the perspective of plant pathologists, and after providing an overview of the available data, we generally found that changes in photosynthesis in the early stage of pathogen infection could be a causal factor influencing acquired resistance, while those in the late stage could be the result of resistance formation.

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.

Effects of Salinity Stress on Chloroplast Structure and Function

Salinity is a growing problem affecting soils and agriculture in many parts of the world. The presence of salt in plant cells disrupts many basic metabolic processes, contributing to severe negative effects on plant development and growth. This review focuses on the effects of salinity on chloroplasts, including the structures and function of these organelles. Chloroplasts house various important biochemical reactions, including photosynthesis, most of which are considered essential for plant survival. Salinity can affect these reactions in a number of ways, for example, by changing the chloroplast size, number, lamellar organization, lipid and starch accumulation, and interfering with cross-membrane transportation. Research has shown that maintenance of the normal chloroplast physiology is necessary for the survival of the entire plant. Many plant species have evolved different mechanisms to withstand the harmful effects of salt-induced toxicity on their chloroplasts and its machinery. The differences depend on the plant species and growth stage and can be quite different between salt-sensitive (glycophyte) and salt-tolerant (halophyte) plants. Salt stress tolerance is a complex trait, and many aspects of salt tolerance in plants are not entirely clear yet. In this review, we discuss the different mechanisms of salt stress tolerance in plants with a special focus on chloroplast structure and its functions, including the underlying differences between glycophytes and halophytes.

Chloroplast quality control pathways are dependent on plastid DNA synthesis and nucleotides provided by cytidine triphosphate synthase two

Reactive oxygen species (ROS) produced in chloroplasts cause oxidative damage, but also signal to initiate chloroplast quality control pathways, cell death, and gene expression. The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant produces the ROS singlet oxygen in chloroplasts that activates such signaling pathways, but the mechanisms are largely unknown. Here we characterize one fc2 suppressor mutation and map it to CYTIDINE TRIPHOSPHATE SYNTHASE TWO (CTPS2), which encodes one of five enzymes in Arabidopsis necessary for de novo cytoplasmic CTP (and dCTP) synthesis. The ctps2 mutation reduces chloroplast transcripts and DNA content without similarly affecting mitochondria. Chloroplast nucleic acid content and singlet oxygen signaling are restored by exogenous feeding of the dCTP precursor deoxycytidine, suggesting ctps2 blocks signaling by limiting nucleotides for chloroplast genome maintenance. An investigation of CTPS orthologs in Brassicaceae showed CTPS2 is a member of an ancient lineage distinct from CTPS3. Complementation studies confirmed this analysis; CTPS3 was unable to compensate for CTPS2 function in providing nucleotides for chloroplast DNA and signaling. Our studies link cytoplasmic nucleotide metabolism with chloroplast quality control pathways. Such a connection is achieved by a conserved clade of CTPS enzymes that provide nucleotides for chloroplast function, thereby allowing stress signaling to occur.

Exogenous melatonin induces drought stress tolerance by promoting plant growth and antioxidant defence system of soybean plants

Melatonin is an indolamine bioactive molecule that regulates a wide range of physiological processes during plant growth and enhances abiotic stress tolerance. Here we examined the putative role of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance in soybean seedlings. Pre-treatment of soybean seedlings with melatonin (50 and 100 µM) was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde. Interestingly, the contents of jasmonic acid and salicylic acid were significantly higher following melatonin treatment in the root zone than in foliar treatment compared with the control. The activity of major antioxidant enzymes such as superoxide dismutase, catalase, polyphenol oxidase, peroxidase and ascorbate peroxidase was stimulated by melatonin application. In addition, melatonin counteracted the drought-induced increase in proline and sugar content. These findings revealed that modifying the endogenous plant hormone content and antioxidant enzymes by melatonin application improved drought tolerance in soybean seedlings. Our findings provide evidence for the stronger physiological role of melatonin in the root zone than in leaves, which may be useful in the large-scale field level application during drought.

Narrow-Band 311 nm Ultraviolet-B Radiation Evokes Different Antioxidant Responses from Broad-Band Ultraviolet

Supplemental narrow-band 311 nm UV-B radiation was applied in order to study the effect of this specific wavelength on tobacco as a model plant. UV-B at photon fluxes varying between 2.9 and 9.9 μmol m⁻² s⁻¹ was applied to supplement 150 μmol m⁻² s⁻¹ photosynthetically active radiation (PAR) for four hours in the middle of the light period for four days. Narrow-band UV-B increased leaf flavonoid and phenolic acid contents. In leaves exposed to 311 nm radiation, superoxide dismutase activity increased, but phenolic peroxidase activity decreased, and the changes were proportional to the UV flux. Ascorbate peroxidase activities were not significantly affected. Narrow-band UV-B caused a dose-dependent linear decrease in the quantum efficiency of photosystem II, up to approximately 10% loss. A parallel decrease in non-regulated non-photochemical quenching indicates potential electron transfer to oxygen in UV-treated leaves. In addition to a flux-dependent increase in the imbalance between enzymatic H₂O₂ production and neutralization, this resulted in an approximately 50% increase in leaf H₂O₂ content under 2.9–6 μmol m⁻² s⁻¹ UV-B. Leaf H₂O₂ decreased to control levels under higher UV-B fluxes due to the onset of increased non-enzymatic H₂O₂- and superoxide-neutralizing capacities, which were not observed under lower fluxes. These antioxidant responses to 311 nm UV-B were different from our previous findings in plants exposed to broad-band UV-B. The results suggest that signaling pathways activated by 311 nm radiation are distinct from those stimulated by other wavelengths and support the heterogeneous regulation of plant UV responses.

Arabidopsis OSMOTIN 34 Functions in the ABA Signaling Pathway and Is Regulated by Proteolysis

Plants have evolutionarily established resistance responses to a variety of abiotic stress conditions, in which ABA mediates the integrated regulation of these stress responses. Numerous proteins function at the transcription level or at the protein level when contributing to controls of the ABA signaling process. Although osmotin is identified as a salt-inducible protein, its function in the abiotic stress response is yet to be elucidated. To examine the role of Arabidopsis OSMOTIN 34 (OSM34) in the ABA signaling pathway, a deletion mutant osm34 generated by a CRISPR/Cas9 system and the double mutant osm34 osml (osmotin 34-like) were analyzed for various ABA responses. Both osm34 and osm34 osml showed reduced levels of ABA responses in seeds and leaves. Moreover, proline level and expression of the proline biosynthesis gene P5CS1 was significantly reduced in osm34 osml. Interestingly, OSM34 binds to SKP2A, an F-Box protein whose transcription is induced by ABA. The protein stability of OSM34 was determined to be under the control of the 26S proteasome. In conclusion, our data suggest that OSM34 functions as a positive regulator in the generation of ABA responses and is under post-translational control.

The core autophagy machinery is not required for chloroplast singlet oxygen-mediated cell death in the Arabidopsis thaliana plastid ferrochelatase two mutant

BACKGROUND

Chloroplasts respond to stress and changes in the environment by producing reactive oxygen species (ROS) that have specific signaling abilities. The ROS singlet oxygen (¹O₂) is unique in that it can signal to initiate cellular degradation including the selective degradation of damaged chloroplasts. This chloroplast quality control pathway can be monitored in the Arabidopsis thaliana mutant plastid ferrochelatase two (fc2) that conditionally accumulates chloroplast ¹O₂ under diurnal light cycling conditions leading to rapid chloroplast degradation and eventual cell death. The cellular machinery involved in such degradation, however, remains unknown. Recently, it was demonstrated that whole damaged chloroplasts can be transported to the central vacuole via a process requiring autophagosomes and core components of the autophagy machinery. The relationship between this process, referred to as chlorophagy, and the degradation of ¹O₂-stressed chloroplasts and cells has remained unexplored.

RESULTS

To further understand ¹O₂-induced cellular degradation and determine what role autophagy may play, the expression of autophagy-related genes was monitored in ¹O₂-stressed fc2 seedlings and found to be induced. Although autophagosomes were present in fc2 cells, they did not associate with chloroplasts during ¹O₂ stress. Mutations affecting the core autophagy machinery (atg5, atg7, and atg10) were unable to suppress ¹O₂-induced cell death or chloroplast protrusion into the central vacuole, suggesting autophagosome formation is dispensable for such ¹O₂-mediated cellular degradation. However, both atg5 and atg7 led to specific defects in chloroplast ultrastructure and photosynthetic efficiencies, suggesting core autophagy machinery is involved in protecting chloroplasts from photo-oxidative damage. Finally, genes predicted to be involved in microautophagy were shown to be induced in stressed fc2 seedlings, indicating a possible role for an alternate form of autophagy in the dismantling of ¹O₂-damaged chloroplasts.

CONCLUSIONS

Our results support the hypothesis that ¹O₂-dependent cell death is independent from autophagosome formation, canonical autophagy, and chlorophagy. Furthermore, autophagosome-independent microautophagy may be involved in degrading ¹O₂-damaged chloroplasts. At the same time, canonical autophagy may still play a role in protecting chloroplasts from ¹O₂-induced photo-oxidative stress. Together, this suggests chloroplast function and degradation is a complex process utilizing multiple autophagy and degradation machineries, possibly depending on the type of stress or damage incurred.

Protein Carbonylation: Emerging Roles in Plant Redox Biology and Future Prospects

Plants are sessile in nature and they perceive and react to environmental stresses such as abiotic and biotic factors. These induce a change in the cellular homeostasis of reactive oxygen species (ROS). ROS are known to react with cellular components, including DNA, lipids, and proteins, and to interfere with hormone signaling via several post-translational modifications (PTMs). Protein carbonylation (PC) is a non-enzymatic and irreversible PTM induced by ROS. The non-enzymatic feature of the carbonylation reaction has slowed the efforts to identify functions regulated by PC in plants. Yet, in prokaryotic and animal cells, studies have shown the relevance of protein carbonylation as a signal transduction mechanism in physiological processes including hydrogen peroxide sensing, cell proliferation and survival, ferroptosis, and antioxidant response. In this review, we provide a detailed update on the most recent findings pertaining to the role of PC and its implications in various physiological processes in plants. By leveraging the progress made in bacteria and animals, we highlight the main challenges in studying the impacts of carbonylation on protein functions in vivo and the knowledge gap in plants. Inspired by the success stories in animal sciences, we then suggest a few approaches that could be undertaken to overcome these challenges in plant research. Overall, this review describes the state of protein carbonylation research in plants and proposes new research avenues on the link between protein carbonylation and plant redox biology.

EGY3 mediates chloroplastic ROS homeostasis and promotes retrograde signaling in response to salt stress in Arabidopsis

The chloroplast is the main organelle for stress-induced production of reactive oxygen species (ROS). However, how chloroplastic ROS homeostasis is maintained under salt stress is largely unknown. We show that EGY3, a gene encoding a chloroplast-localized protein, is induced by salt and oxidative stresses. The loss of EGY3 function causes stress hypersensitivity while EGY3 overexpression increases the tolerance to both salt and chloroplastic oxidative stresses. EGY3 interacts with chloroplastic Cu/Zn-SOD2 (CSD2) and promotes CSD2 stability under stress conditions. In egy3-1 mutant plants, the stress-induced CSD2 degradation limits H2O2 production in chloroplasts and impairs H2O2-mediated retrograde signaling, as indicated by the decreased expression of retrograde-signal-responsive genes required for stress tolerance. Both exogenous application of H2O2 (or APX inhibitor) and CSD2 overexpression can rescue the salt-stress hypersensitivity of egy3-1 mutants. Our findings reveal that EGY3 enhances the tolerance to salt stress by promoting the CSD2 stability and H2O2-mediated chloroplastic retrograde signaling.

Leaf Age-Dependent Photosystem II Photochemistry and Oxidative Stress Responses to Drought Stress in Arabidopsis thaliana Are Modulated by Flavonoid Accumulation

We investigated flavonoid accumulation and lipid peroxidation in young leaves (YL) and mature leaves (ML) of Arabidopsis thaliana plants, whose watering stopped 24 h before sampling, characterized as onset of drought stress (OnDS), six days before sampling, characterized as mild drought stress (MiDS), and ten days before sampling, characterized as moderate drought stress (MoDS). The response to drought stress (DS) of photosystem II (PSII) photochemistry, in both leaf types, was evaluated by estimating the allocation of absorbed light to photochemistry (Φ_PSII), to heat dissipation by regulated non-photochemical energy loss (Φ_NPQ) and to non-regulated energy dissipated in PSII (Φ_NO). Young leaves were better protected at MoDS than ML leaves, by having higher concentration of flavonoids that promote acclimation of YL PSII photochemistry to MoDS, showing lower lipid peroxidation and excitation pressure (1 - q_p). Young leaves at MoDS possessed lower 1 - q_p values and lower excess excitation energy (EXC), not only compared to MoDS ML, but even to MiDS YL. They also possessed a higher capacity to maintain low Φ_NO, suggesting a lower singlet oxygen (¹O₂) generation. Our results highlight that leaves of different developmental stage may display different responses to DS, due to differential accumulation of metabolites, and imply that PSII photochemistry in Arabidopsis thaliana may not show a dose dependent DS response.

Synergistic effects of contaminants in Lombardy waters

Quantifying synergistic environmental effects in water contamination is still an open issue. Here, we have analyzed geolocalized data of pollutants recorded in 2018 in surface and groundwater of Lombardy, one of the areas with the highest agricultural production rates, not only in Italy, but also in Europe. Both herbicides and insecticides are present at concentration levels above the legal limit, mainly in surface waters. Geolocalized analysis allows us to identify interesting areas particularly affected by a combination of multiple pesticides. We thus investigated possible synergistic effects of these compounds on the environment, using the alga C. reinhardtii as a biosensor. Our results show that exposure for 7 days to four compounds, that we found present together at high concentration in surface waters, was able to induce a stress in the algae, as indicated by the presence of palmelloids. Our work results in a pipeline that could easily be exported to monitor other territories in Italy and abroad.

A virus-derived siRNA activates plant immunity by interfering with ROS scavenging

Virus-derived small interference RNAs (vsiRNAs) not only suppress virus infection in plants via induction of RNA silencing but also enhance virus infection by regulating host defensive gene expression. However, the underlying mechanisms that control vsiRNA-mediated host immunity or susceptibility remain largely unknown. In this study, we generated several transgenic wheat lines using four artificial microRNA expression vectors carrying vsiRNAs from Wheat yellow mosaic virus (WYMV) RNA1. Laboratory and field tests showed that two transgenic wheat lines expressing amiRNA1 were highly resistant to WYMV infection. Further analyses showed that vsiRNA1 could modulate the expression of a wheat thioredoxin-like gene (TaAAED1), which encodes a negative regulator of reactive oxygen species (ROS) production in the chloroplast. The function of TaAAED1 in ROS scavenging could be suppressed by vsiRNA1 in a dose-dependent manner. Furthermore, transgenic expression of amiRNA1 in wheat resulted in broad-spectrum disease resistance to Chinese wheat mosaic virus, Barley stripe mosaic virus, and Puccinia striiformis f. sp. tritici infection, suggesting that vsiRNA1 is involved in wheat immunity via ROS signaling. Collectively, these findings reveal a previously unidentified mechanism underlying the arms race between viruses and plants.

Chitosan-Induced Activation of the Antioxidant Defense System Counteracts the Adverse Effects of Salinity in Durum Wheat

The present study was carried out with the aim of (i) evaluating the effect of chitosan (CTS) on the growth of durum wheat under salinity and (ii) examining CTS-regulated mechanisms of salinity tolerance associated with the antioxidant defense system. To achieve these goals, durum wheat seedlings were treated with CTS at different molecular weight, low (L-CTS, 50-190 kDa), medium (M-CTS, 190-310 kDa) and high (H-CTS, 310-375 kDa). The results obtained show that exposure to 200 mM NaCl reduced the shoot and the root dried biomass by 38% and 59%, respectively. The growth impairment induced by salinity was strongly correlated with an increase in the superoxide anion production (5-fold), hydrogen peroxide content (2-fold) and malondialdehyde (MDA) content (4-fold). Seedlings responded to the oxidative stress triggered by salinity with an increase in the total phenolic content (TPC), total flavonoid content (TFC) and total antioxidant activity (TAA) by 67%, 51% and 32%, respectively. A salt-induced increase in the activity of the antioxidant enzymes superoxide dismutase and catalase (CAT) of 89% and 86%, respectively, was also observed. Treatment of salt-stressed seedlings with exogenous CTS significantly promoted seedling growth, with the strongest effects observed for L-CTS and M-CTS, which increased the shoot biomass of stressed seedlings by 32% and 44%, respectively, whereas the root dried biomass increased by 87% and 64%, respectively. L-CTS and M-CTS treatments also decreased the superoxide anion production (57% and 59%, respectively), the hydrogen peroxide content (35% and 38%, respectively) and the MDA content (48% and 56%, respectively) and increased the TPC (23% and 14%, respectively), the TFC (19% and 10%, respectively), the TAA (up to 10% and 7%, respectively) and the CAT activity (29% and 20%, respectively). Overall, our findings indicate that CTS exerts its protective role against the oxidative damages induced by salinity by enhancing the antioxidant defense system. L-CTS and M-CTS were the most effective in alleviating the adverse effect of NaCl, thus demonstrating that the CTS action is strictly related to its molecular weight.

Genotype- and tissue-specific physiological and biochemical changes of two chickpea (Cicer arietinum) varieties following a rapid dehydration

In nature, plants may suffer rapid dehydration (RD), which causes significant loss of the annual global chickpea production. Thus, ascertaining more knowledge concerning the RD-tolerance mechanisms in chickpea is crucial for developing high drought-tolerant varieties to assure sustainable chickpea production under sudden water deficit. Here, we focused on genotype-driven variation in leaf relative water content (RWC) and associated differences in RD-responsive physiological and biochemical attributes in roots and leaves of two chickpea varieties, FLIP00-21C and FLIP02-89C, subjected to well-watered and RD conditions. FLIP00-21C showed higher RD-tolerance than FLIP02-89C, evident by its higher leaf RWC during RD. Consistently, FLIP00-21C exhibited lower membrane injury due to lower hydrogen peroxide (H₂ O₂ ) accumulation than FLIP02-89C during RD, indicating reduced RD-induced oxidative damage. Under RD conditions, total phenolics in roots and flavonoids in roots and leaves increased more in FLIP02-89C compared to FLIP00-21C; however, the increased activities of superoxide dismutase and H₂ O₂ -scavenging enzymes were more properly coordinated in FLIP00-21C than in FLIP02-89C, which might contribute to more efficient antioxidant defense in FLIP00-21C than in FLIP02-89C. The higher leaf RWC of FLIP00-21C versus FLIP02-89C under RD might be associated with greater increases in the levels of the osmo-regulators proline and total free amino acids (TFAAs) in FLIP00-21C than in FLIP02-89C. Collectively, the higher RD-tolerance of FLIP00-21C is mainly associated with the maintenance of higher RWC, stronger antioxidant defense, and greater accumulation of proline and TFAAs. These traits could be useful for evaluating the drought-tolerance of chickpea varieties and further marker-assisted breeding approaches for improvement of chickpea productivity.

Sodium sequestration confers salinity tolerance in an ancestral wild rice

Wild rice Oryza rufipogon, a progenitor of cultivated rice Oryza sativa L., possesses superior salinity tolerance and is a potential donor for breeding salinity tolerance traits in rice. However, a mechanistic basis of salinity tolerance in this donor species has not been established. Here, we examined salinity tolerance from the early vegetative stage to maturity in O. rufipogon in comparison with a salt-susceptible (Koshihikari) and a salt-tolerant (Reiziq) variety of O. sativa. We assessed their phylogeny and agronomical traits, photosynthetic performance, ion contents, as well as gene expression in response to salinity stress. Salt-tolerant O. rufipogon exhibited efficient leaf photosynthesis and less damage to leaf tissues during the course of salinity treatment. In addition, O. rufipogon showed a significantly higher tissue Na⁺ accumulation that is achieved by vacuolar sequestration compared to the salt tolerant O. sativa indica subspecies. These findings are further supported by the upregulation of genes involved with ion transport and sequestration (e.g. high affinity K⁺ transporter 1;4 [HKT1;4], Na⁺ /H⁺ exchanger 1 [NHX1] and vacuolar H⁺ -ATPase c [VHA-c]) in salt-tolerant O. rufipogon as well as by the close phylogenetic relationship of key salt-responsive genes in O. rufipogon to these in salt-tolerant wild rice species such as O. coarctata. Thus, the high accumulation of Na⁺ in the leaves of O. rufipogon acts as a cheap osmoticum to minimize the high energy cost of osmolyte biosynthesis and excessive reactive oxygen species production. These mechanisms demonstrated that O. rufipogon has important traits that can be used for improving salinity tolerance in cultivated rice.

Abiotic stress-induced anthocyanins in plants: Their role in tolerance to abiotic stresses

Abiotic stresses, such as heat, drought, salinity, low temperature, and heavy metals, inhibit plant growth and reduce crop productivity. Abiotic stresses are becoming increasingly extreme worldwide due to the ongoing deterioration of the global climate and the increase in agrochemical utilization and industrialization. Plants grown in fields are affected by one or more abiotic stresses. The consequent stress response of plants induces reactive oxygen species (ROS), which are then used as signaling molecules to activate stress-tolerance mechanism. However, under extreme stress conditions, ROS are overproduced and cause oxidative damage to plants. In such conditions, plants produce anthocyanins after ROS signaling via the transcription of anthocyanin biosynthesis genes. These anthocyanins are then utilized in antioxidant activities by scavenging excess ROS for their sustainability. In this review, we discuss the physiological, biochemical, and molecular mechanisms underlying abiotic stress-induced anthocyanins in plants and their role in abiotic stress tolerance. In addition, we highlight the current progress in the development of anthocyanin-enriched transgenic plants and their ability to increase abiotic stress tolerance. Overall, this review provides valuable information that increases our understanding of the mechanisms by which anthocyanins respond to abiotic stress and protect plants against it. This review also provides practical guidance for plant biologists who are engineering stress-tolerant crops using anthocyanin biosynthesis or regulatory genes.

Drought tolerance in selected aerobic and upland rice varieties is driven by different metabolic and antioxidative responses

Sugar-mediated osmotic acclimation and a strong antioxidative response reduce drought-induced biomass loss at the vegetative stage in rice. A clear understanding of the physiological and biochemical adaptations to water limitation in upland and aerobic rice can help to identify the mechanisms underlying their tolerance to low water availability. In this study, three indica rice varieties-IR64 (lowland), Apo (aerobic), and UPL Ri-7 (upland)-, that are characterized by contrasting levels of drought tolerance, were exposed to drought at the vegetative stage. Drought-induced changes in biomass, leaf metabolites and oxidative stress markers/enzyme activities were analyzed in each variety at multiple time points. The two drought-tolerant varieties, Apo and UPL Ri-7 displayed a reduced water use in contrast to the susceptible variety IR64 that displayed high water consumption and consequent strong leaf dehydration upon drought treatment. A sugar-mediated osmotic acclimation in UPL Ri-7 and a strong antioxidative response in Apo were both effective in limiting the drought-induced biomass loss in these two varieties, while biomass loss was high in IR64, also after recovery. A qualitative comparison of these results with the ones of a similar experiment conducted in the field at the reproductive stage showed that only Apo, which also in this stage showed the highest antioxidant power, was able to maintain a stable grain yield under stress. Our results show that different metabolic and antioxidant adaptations confer drought tolerance to aerobic and upland rice varieties in the vegetative stage. The effectiveness of these adaptations differs between developmental stages. Unraveling the genetic control of these mechanisms might be exploited in breeding for new rice varieties adapted to water-limited environments.

Effect of long-term heat stress on grain yield, pollen grain viability and germinability in bread wheat (Triticum aestivum L.) under field conditions

Frequent episodes of heat threaten sustainable agriculture in Egypt. This study is an urgent call to select tolerant genotypes of heat and discover the predicted screening phenotypic parameters. Here, twenty spring wheat genotypes were exposed to heat stress under field conditions for screening heat tolerance. Stress environments were simulated by delaying the sowing date by 53 and 58 days than the normal environments for two successive seasons. Stressed plants received the highest peak of heat during the reproductive growth stage. Eight phenotypic parameters were measured to evaluate genotype tolerance. Mean performance, reduction percentage/trait, and heat susceptibility index parameters were calculated. Additionally, the pollen grain viability during spike emergence and the germinability of producing grains were investigated. Results demonstrated: (1) Highly significant differences (P < 0.01) between genotypes, treatments and genotypes by treatments in grain yield and other traits in both studied seasons, (2) significant reduction in all studied traits compared to the non-stress environment, (3) the overall yield reduction, based on grain yield/m², was 40.17, 41.19 % in the first and second seasons, respectively, and the most tolerant genotypes were Masr2, Sids1, Giza 171 and Line 9, (4) limited impact of heat has detected on pollen grains viability and germinability, and (5) grain yield as a selection criterion for heat stress remains the most reliable yardstick.

Induction of a Compensatory Photosynthetic Response Mechanism in Tomato Leaves upon Short Time Feeding by the Chewing Insect Spodoptera exigua

Simple Summary

Insects such as beet armyworm (Spodoptera exigua) can cause extensive damage to tomato plants (Solanum lycopersicum). Tomato photosynthesis was clearly reduced directly at S. exigua feeding spots. However, neighboring zones and the rest of the leaf compensated through increased light energy use in photosystem II, possibly trigged by singlet oxygen from the feeding zone. Three hours after feeding, whole-leaf photosynthetic efficiency was as before feeding, demonstrating the compensatory ability. Thus, chlorophyll fluorescence imaging analysis could contribute to understanding the effects of herbivory on photosynthesis at a detailed spatial and temporal pattern.

Abstract

In addition to direct tissue consumption, herbivory may affect other important plant processes. Here, we evaluated the effects of short-time leaf feeding by Spodoptera exigua larvae on the photosynthetic efficiency of tomato plants, using chlorophyll a fluorescence imaging analysis. After 15 min of feeding, the light used for photochemistry at photosystem II (PSII) (Φ_PSII), and the regulated heat loss at PSII (Φ_NPQ) decreased locally at the feeding zones, accompanied by increased non-regulated energy losses (Φ_NO) that indicated increased singlet oxygen (¹O₂) formation. In contrast, in zones neighboring the feeding zones and in the rest of the leaf, Φ_PSII increased due to a decreased Φ_NPQ. This suggests that leaf areas not directly affected by herbivory compensate for the photosynthetic losses by increasing the fraction of open PSII reaction centers (q_p) and the efficiency of these centers (Fv’/Fm’), because of decreased non-photochemical quenching (NPQ). This compensatory reaction mechanism may be signaled by singlet oxygen formed at the feeding zone. PSII functionality at the feeding zones began to balance with the rest of the leaf 3 h after feeding, in parallel with decreased compensatory responses. Thus, 3 h after feeding, PSII efficiency at the whole-leaf level was the same as before feeding, indicating that the plant managed to overcome the feeding effects with no or minor photosynthetic costs.

PREMATURE SENESCENCE LEAF 50 Promotes Heat Stress Tolerance in Rice (Oryza sativa L.)

BACKGROUND

Heat stress is a major environmental factor that could induce premature leaf senescence in plants. So far, a few rice premature senescent leaf mutants have been reported to involve in heat tolerance.

FINDINGS

We identified a premature senescence leaf 50 (psl50) mutant that exhibited a higher heat susceptibility with decreased survival rate, over-accumulated hydrogen peroxide (H2O2) content and increased cell death under heat stress compared with the wild-type. The causal gene PREMATURE SENESCENCE LEAF 50 (PSL50) was isolated by using initial map-based resequencing (IMBR) approach, and we found that PSL50 promoted heat tolerance probably by acting as a modulator of H2O2 signaling in response to heat stress in rice (Oryza sativa L.).

CONCLUSIONS

PSL50 negatively regulates heat-induced premature leaf senescence in rice.

Mutations of the Genomes Uncoupled 4 Gene Cause ROS Accumulation and Repress Expression of Peroxidase Genes in Rice

The Genomes Uncoupled 4 (GUN4) is one of the retrograde signaling genes in Arabidopsis and its orthologs have been identified in oxygenic phototrophic organisms from cyanobacterium to higher plants. GUN4 is involved in tetrapyrrole biosynthesis and its mutation often causes chlorophyll-deficient phenotypes with increased levels of reactive oxygen species (ROS), hence it has been speculated that GUN4 may also play a role in photoprotection. However, the biological mechanism leading to the increased ROS accumulation in gun4 mutants remains largely unknown. In our previous studies, we generated an epi-mutant allele of OsGUN4 (gun4 epi ), which downregulated its expression to ∼0.5% that of its wild-type (WT), and a complete knockout allele gun4-1 due to abolishment of its translation start site. In the present study, three types of F2 plant derived from a gun4-1/gun4 epi cross, i.e., gun4-1/gun4-1, gun4-1/gun4 epi and gun4 epi /gun4 epi were developed and used for further investigation by growing them under photoperiodic condition (16 h/8 h light/dark) with low light (LL, 100 μmol photons m-2 s-1) or high light (HL, 1000 μmol photons m-2 s-1). The expression of OsGUN4 was light responsive and had two peaks in the daytime. gun4-1/gun4-1-F2 seeds showed defective germination and died within 7 days. Significantly higher levels of ROS accumulated in all types of OsGUN4 mutants than in WT plants under both the LL and HL conditions. A comparative RNA-seq analysis of WT variety LTB and its gun4 epi mutant HYB led to the identification of eight peroxidase (PRX)-encoding genes that were significantly downregulated in HYB. The transcription of these eight PRX genes was restored in transgenic HYB protoplasts overexpressing OsGUN4, while their expression was repressed in LTB protoplasts transformed with an OsGUN4 silencing vector. We conclude that OsGUN4 is indispensable for rice, its expression is light- and oxidative-stress responsive, and it plays a role in ROS accumulation via its involvement in the transcriptional regulation of PRX genes.

Identification of Glutathione S-Transferase Genes in Hami Melon (Cucumis melo var. saccharinus) and Their Expression Analysis Under Cold Stress

As a group of multifunctional enzymes, glutathione S-transferases (GSTs) participate in oxidative stress resistance and cellular detoxification. Here, we identified 39 CmGST genes with typical binding sites from the Hami melon genome, and they can be classified into seven subfamilies. Their molecular information, chromosomal locations, phylogenetic relationships, synteny relationships, gene structures, protein-protein interactions, structure of 3-D models, and expression levels under cold stress were analyzed. Expression analysis indicates that cold-tolerant Jia Shi-310 (JS) had higher GST enzyme activities and expression levels of 28 stress-related genes under cold stress. Some CmGSTs belonging to Tau, Phi, and DHAR classes play significant roles under cold stress, and they could be regarded as candidate genes for further studies. The present study systematically investigated the characterization of the Hami melon GST gene family, extending our understanding of Hami melon GST mediated stress-response mechanisms in this worldwide fruit.

Acetaminophen Induces an Antioxidative Response in Lettuce Plants

Contaminants of environmental concern, like pharmaceuticals, are being detected in increasing amounts in soils and irrigation waters and can thus be taken up by plants. In this work, the uptake of acetaminophen (ACT) by lettuce plants was evaluated through a hydroponic experiment at different concentrations (0, 0.1, 1 and 5 mg L⁻¹ ACT). The pathways related to oxidative stress induced by ACT were studied in lettuce leaves and roots at 1, 8 and 15 days after exposure. Stress indicators such as hydrogen peroxide and malondialdehyde (MDA) contents were analyzed, revealing increases in plants contaminated with ACT in comparison to control, confirming the occurrence of oxidative stress, with the exception of MDA in leaves. The enzymatic activities of catalase, superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase and glutathione peroxidase, directly involved in the antioxidative system, showed significant differences when compared to control plants, and, depending on the enzyme and the tissue, different trends were observed. Glutathione reductase revealed a decrease in contaminated leaves, which may imply a specific impact of ACT in the glutathione cycle. Significant increases were found in the anthocyanin content of leaves, both with exposure time and ACT concentration, indicating an antioxidative response induced by ACT contamination.

Using sodium percarbonate to suppress vertically distributed filamentous cyanobacteria while maintaining the stability of microeukaryotic communities in drinking water reservoirs

The increasing frequency and intensity of blooms of toxin- and taste & odour-producing filamentous cyanobacteria in water sources is a growing global issue. Compared to the common spherical Microcystis genus, the removal of filamentous cyanobacteria is more difficult in drinking water treatment plants; hence, abatement and control of the occurrence and proliferation of harmful filamentous cyanobacteria within drinking water sources is important for water supply. In this study, the solid sodium percarbonate (SPC), Na₂CO₃·1.5H₂O₂, was used as an algaecide to eliminate the cyanobacteria distributed throughout the water column in the surface and bottom layer of a reservoir serving as a drinking water source. Results showed that although the oxidation capacity of SPC was higher in the surface water due to the higher light intensity than in the bottom water, 3.0 mg/L SPC can still suppress the harmful cyanobacteria in the bottom water after 36 h because the carbonate ion generated by SPC decomposition can act as an activator of H₂O₂ to generate many reactive oxygen species - including superoxide radicals, carbonate radical anions, and hydroxyl radicals - even in the light-limited environment. The obtained inactivation rates for the main cyanobacteria in this reservoir followed the order: Pseudanabaena limnetica > Raphidiopsis curvata > Cylindrospermopsis raciborskii. 3.0 mg/L SPC has a slight impact on microeukaryotic communities according to the 18S rRNA gene sequencing, while 6.0 mg/L SPC changed the composition of eukaryotic phytoplankton and zooplankton clearly. Eukaryotic co-occurrence networks showed that although the network of eukaryotic plankton in treated surface water was more compact and clustered, stability of microeukaryotes in the treated surface water was lower than for the treated bottom water, owing to the higher oxidation capacity of SPC in the surface water. The results above not only have important implications for full-scale control of harmful cyanobacteria in drinking water sources, especially filamentous cyanobacteria with vertical distributions, but also help to ensure the health and stability of the whole aquatic ecosystem.

Comparative proteomic approach to study the salinity effect on the growth of two contrasting quinoa genotypes

The aim of this study was to investigate the effect of NaCl salinity (0, 100 and 300 mM) on the individual response of the quinoa varieties Kcoito (Altiplano Ecotype) and UDEC-5 (Sea-level Ecotype) with physiological and proteomic approaches. Leaf protein profile was performed using two dimensional gel electrophoresis (2-DE). UDEC-5 showed an enhanced capacity to withstand salinity stress compared to Kcoito. In response to salinity, we detected overall the following differences between both genotypes: Toxicity symptoms, plant growth performance, photosynthesis performance and intensity of ROS-defense. We found a mirroring of these differences in the proteome of each genotype. Among the 700 protein spots reproducibly detected, 24 exhibited significant abundance variations between samples. These proteins were involved in energy and carbon metabolism, photosynthesis, ROS scavenging and detoxification, stress defense and chaperone functions, enzyme activation and ATPases. A specific set of proteins predominantly involved in photosynthesis and ROS scavenging showed significantly higher abundance under high salinity (300 mM NaCl). The adjustment was accompanied by a stimulation of various metabolic pathways to balance the supplementary demand for energy or intermediates. However, the more salt-resistant genotype UDEC-5 presented a beneficial and significantly higher expression of nearly all stress-related altered enzymes than Kcoito.

Comprehensive Phosphoproteomic Analysis of Nostoc flagelliforme in Response to Dehydration Provides Insights into Plant ROS Signaling Transduction

Terrestrial cyanobacteria, originated from aquatic cyanobacteria, exhibit a unique mechanism for drought adaptation during long-term evolution. To elucidate this diverse adaptive mechanism exhibited by terrestrial cyanobacteria from the post-translation modification aspect, we performed a global phosphoproteome analysis on the abundance of phosphoproteins in response to dehydration using Nostoc flagelliforme, a kind of terrestrial cyanobacteria having strong ecological adaptability to xeric environments. A total of 329 phosphopeptides from 271 phosphoproteins with 1168 phosphorylation sites were identified. Among these, 76 differentially expressed phosphorylated proteins (DEPPs) were identified for each dehydration treatment (30, 75, and 100% water loss), compared to control. The identified DEPPs were functionally categorized to be mainly involved in a two-component signaling pathway, photosynthesis, energy and carbohydrate metabolism, and an antioxidant system. We concluded that protein phosphorylation modifications related to the reactive oxygen species (ROS) signaling pathway might play an important role in coordinating enzyme activity involved in the antioxidant system in N. flagelliforme to adapt to dehydration stress. This study provides deep insights into the extensive modification of phosphorylation in terrestrial cyanobacteria using a phosphoproteomic approach, which may help to better understand the role of protein phosphorylation in key cellular mechanisms in terrestrial cyanobacteria in response to dehydration.

The flexibility of metabolic interactions between chloroplasts and mitochondria in Nicotiana tabacum leaf

To examine the effect of mitochondrial function on photosynthesis, wild-type and transgenic Nicotiana tabacum with varying amounts of alternative oxidase (AOX) were treated with different respiratory inhibitors. Initially, each inhibitor increased the reduction state of the chloroplast electron transport chain, most severely in AOX knockdowns and least severely in AOX overexpressors. This indicated that the mitochondrion was a necessary sink for photo-generated reductant, contributing to the 'P700 oxidation capacity' of photosystem I. Initially, the Complex III inhibitor myxothiazol and the mitochondrial ATP synthase inhibitor oligomycin caused an increase in photosystem II regulated non-photochemical quenching not evident with the Complex III inhibitor antimycin A (AA). This indicated that the increased quenching depended upon AA-sensitive cyclic electron transport (CET). Following 12 h with oligomycin, the reduction state of the chloroplast electron transport chain recovered in all plant lines. Recovery was associated with large increases in the protein amount of chloroplast ATP synthase and mitochondrial uncoupling protein. This increased the capacity for photophosphorylation in the absence of oxidative phosphorylation and enabled the mitochondrion to act again as a sink for photo-generated reductant. Comparing the AA and myxothiazol treatments at 12 h showed that CET optimized photosystem I quantum yield, depending upon the P700 oxidation capacity. When this capacity was too high, CET drew electrons away from other sinks, moderating the P700+ amount. When P700 oxidation capacity was too low, CET acted as an electron overflow, moderating the amount of reduced P700. This study reveals flexible chloroplast-mitochondrion interactions able to overcome lesions in energy metabolism.

Genome-Wide Identification of PRP Genes in Apple Genome and the Role of MdPRP6 in Response to Heat Stress

Plant proline-rich proteins (PRPs) are cell wall proteins that occur in the plant kingdom and are involved in plant development and stress response. In this study, 9 PRP genes were identified from the apple genome and a comprehensive analysis of the PRP family was conducted, including gene structures, phylogenetic analysis, chromosome mapping, and so on. The expression of MdPRPs varied among tissues and in response to different types of stresses. MdPRP4 and MdPRP7 were induced by five detected stress treatments, including heat, drought, abscisic acid, cold, and salt; the expression patterns of the others varied under different types of stress. Subcellular localization showed that MdPRPs mainly functioned in the cytoplasm, except for MdPRP1 and MdPRP5, which also functioned in the nucleus. When MdPRP6 was overexpressed in tobacco, the transgenic plants showed higher tolerance to high temperature (48 °C) compared with wild-type (WT) plants. The transgenic plants showed milder wilting, a lower accumulation of electrolyte leakage, MDA and ROS, and a higher level of chlorophyll and SOD and POD activity, indicating that MdPRP6 may be an important gene in apples for heat stress tolerance. Overall, this study suggested that MdPRPs are critically important for the ability of apple responses to stresses.