Plant signalling in acute ozone exposure

Ozone risk assessment with free-air controlled exposure (FACE) experiments: A critical revisit

Since risk assessments of tropospheric ozone (O3) are crucial for agricultural and forestry sectors, there is a growing body for realistic assessments by a stomatal flux-based approach in Free-Air Controlled Exposure (FACE) facilities. Ozone risks are normally described as relative risks (RRs), which are calculated by assuming the biomass or yield at zero O3 dose as "reference". However, the estimation of the reference biomass or yield is challenging due to a lack of O3-clean-air treatment at the FACEs and the extrapolation without data in a low O3 range increases the bias for estimating the reference values. Here, we reviewed a current methodology for the risk assessment at FACEs and presented a simple and effective way ("modified Paoletti's approach") of defining RRs just using biomass or yield data with a range of expected impacts under the FACE conditions hypothesizing three possible scenarios based on prediction limits using 95% credible intervals (CI) (1. Best fit using the intercept as reference, 2. Optimistic scenario using a lower CI and 3. Worst scenario using an upper CI). As a result, O3-sensitive species show a relatively narrow effect range (optimistic vs. worst scenario) whereas a wide range of response may be possibly taken in resistant species. Showing a possible effect range allows for a comprehensive understanding of the potential risks and its uncertainties related to a species sensitivity to O3. As a supporting approach, we also recommend to use scientifically relevant tools (i.e., ethylenediurea treatments; mechanistic plant models) for strengthening the obtained results for the RRs against O3. Interestingly, the moderately sensitive or resistant species showed non-linear rather than linear dose-response relationships, suggesting a need for the flexible functional form for the risk assessment to properly describe the complex plant response such as hormesis, which depends on their plasticity to O3 stress.

A new biostimulant derived from soybean by-products enhances plant tolerance to abiotic stress triggered by ozone

BACKGROUND

Tropospheric ozone is an air pollutant that causes negative effects on vegetation, leading to significant losses in crop productivity. It is generated by chemical reactions in the presence of sunlight between primary pollutants resulting from human activity, such as nitrogen oxides and volatile organic compounds. Due to the constantly increasing emission of ozone precursors, together with the influence of a warming climate on ozone levels, crop losses may be aggravated in the future. Therefore, the search for solutions to mitigate these losses becomes a priority. Ozone-induced abiotic stress is mainly due to reactive oxygen species generated by the spontaneous decomposition of ozone once it reaches the apoplast. In this regard, compounds with antioxidant activity offer a viable option to alleviate ozone-induced damage. Using enzymatic technology, we have developed a process that enables the production of an extract with biostimulant properties from okara, an industrial soybean byproduct. The biostimulant, named as OEE (Okara Enzymatic Extract), is water-soluble and is enriched in bioactive compounds present in okara, such as isoflavones. Additionally, it contains a significant fraction of protein hydrolysates contributing to its functional effect. Given its antioxidant capacity, we aimed to investigate whether OEE could alleviate ozone-induced damage in plants. For that, pepper plants (Capsicum annuum) exposed to ozone were treated with a foliar application of OEE.

RESULTS

OEE mitigated ozone-induced damage, as evidenced by the net photosynthetic rate, electron transport rate, effective quantum yield of PSII, and delayed fluorescence. This protection was confirmed by the level of expression of genes associated with photosystem II. The beneficial effect was primarily due to its antioxidant activity, as evidenced by the lipid peroxidation rate measured through malondialdehyde content. Additionally, OEE triggered a mild oxidative response, indicated by increased activities of antioxidant enzymes in leaves (catalase, superoxide dismutase, and guaiacol peroxidase) and the oxidative stress index, providing further protection against ozone-induced stress.

CONCLUSIONS

The present results support that OEE protects plants from ozone exposure. Taking into consideration that the promotion of plant resistance against abiotic damage is an important goal of biostimulants, we assume that its use as a new biostimulant could be considered.

Gaseous ozone and ozonized mist in the control of Escherichia coli on 'Rama Forte' persimmon

This study aimed to evaluate the effectiveness of using two ozone applications (gaseous and mist) as a disinfection method for fresh persimmon. To test these sanitizers, in vitro and in vivo assays were performed, and the Escherichia coli was selected because it is a pathogen that causes foodborne diseases in humans. For in vitro experiments, a plate was inoculated with Escherichia coli strain ATCC 25922 and treated. For in vivo assays, persimmon fruit surface was inoculated with the bacteria and treated. For both assays, it was used 10,15,20,30,40 and 50 μL L-1 of gaseous ozone or ozonized mist for five minutes. The results demonstrated that the gas ozone application significantly reduced the growth of E. coli on the plate surface in vitro at doses of 30, 40 and 50 μL L-1 (with 0.83, 0.89 and 0.95 log CFU mL-1, respectively). The application of ozonized mist showed a significant reduction for 50 μL L-1 (with 1.28 log CFU g-1). And, for the in vivo assays, ozonized mist significantly reduced the number of bacteria on the persimmon surface, with a 1.57 log reduction, which was the largest for 40 μL L-1. Therefore, it is possible to conclude that the ozone application can contribute to the control of microorganisms present on fruit surfaces.

Identification of cysteine-rich receptor-like kinase gene family in potato: revealed StCRLK9 in response to heat, salt and drought stresses

The investigation into cysteine-rich receptor-like kinases (CRLKs) holds pivotal significance as these conserved, upstream signalling molecules intricately regulate fundamental biological processes such as plant growth, development and stress adaptation. This study undertakes a comprehensive characterisation of CRLKs in Solanum tuberosum (potato), a staple food crop of immense economic importance. Employing comparative genomics and evolutionary analyses, we identified 10 distinct CRLK genes in potato. Further categorisation into three major groups based on sequence similarity was performed. Each CRLK member in potato was systematically named according to its chromosomal position. Multiple sequence alignment and phylogenetic analyses unveiled conserved gene structures and motifs within the same groups. The genomic distribution of CRLKs was observed across Chromosomes 2-5, 8 and 12. Gene duplication analysis highlighted a noteworthy trend, with most gene pairs exhibiting a Ka/Ks ratio greater than one, indicating positive selection of StCRLKs in potato. Salt and drought stresses significantly impacted peroxidase and catalase activities in potato seedlings. The presence of diverse cis -regulatory elements, including hormone-responsive elements, underscored their involvement in myriad biotic and abiotic stress responses. Interestingly, interactions between the phytohormone auxin and CRLK proteins unveiled a potential auxin-mediated regulatory mechanism. A holistic approach combining transcriptomics and quantitative PCR validation identified StCRLK9 as a potential candidate involved in plant response to heat, salt and drought stresses. This study lays a robust foundation for future research on the functional roles of the CRLK gene family in potatoes, offering valuable insights into their diverse regulatory mechanisms and potential applications in stress management.

Ozone stress response of leaf BVOC emission and photosynthesis in mountain birch (Betula pubescens spp. czerepanovii) depends on leaf age

Oxidative stress from ozone (O3) causes plants to alter their emission of biogenic volatile organic compounds (BVOC) and their photosynthetic rate. Stress reactions from O3 on birch trees can result in prohibited plant growth and lead to increased BVOC emission rates as well as changes in their compound blend to emit more monoterpenes (MT) and sesquiterpenes (SQT). BVOCs take part in atmospheric reactions such as enhancing the production of secondary organic aerosols (SOA). As the compound blend and emission rate change with O3 stress, this can influence the atmospheric conditions by affecting the production of SOA. Studying the stress responses of plants provides important information on how these reactions might change, which is vital to making better predictions of the future climate. In this study, measurements were taken to find out how the leaves of mature mountain birch trees (Betula pubescens ssp. czerepanovii) respond to different levels of elevated O3 exposure in situ depending on leaf age. We found that leaves from both early and late summers responded with induced SQT emission after exposure to 120 ppb O3. Early leaves were, however, more sensitive to increased O3 concentrations, with enhanced emission of green leaf volatiles (GLV) and tendencies of both induced leaf senescence as well as poor recovery in the photosynthetic rate between exposures. Late leaves had more stable photosynthetic rates throughout the experiment and responded less to exposure at different O3 levels.

Growth, ultrastructural and physiological characteristics of Abelmoschus cytotypes under elevated ozone stress: a study on ploidy-specific responses

Tropospheric ozone (O3 ) is a significant abiotic stressor whose rising concentration negatively influences plant growth. Studies related to the differential response of Abelmoschus cytotypes to elevated O3 treatment are scarce and need further exploration to recognise the role of polyploidisation in stress tolerance. In this study, we analysed the changes in growth pattern, ultrastructure, physiology and foliar protein profile occurring under O3 stress in Abelmoschus moschatus (monoploid), Abelmoschus esculentus (diploid) and Abelmoschus caillei (triploid). Our findings showed that higher stomatal conductance in A. moschatus triggered higher O3 intake, causing damage to stomatal cells and photosynthetic pigments. Additionally, it caused a reduction in photosynthetic rates, leading to reduced plant growth, total biomass and economic yield. This O3 -induced toxicity was less in diploid and triploid cytotypes of Abelmoschus . Protein profiling by sodium dodecyl sulpate-polyacrylamide gel electrophoresis showed a significant decrease in the commonly found RuBisCO larger and smaller subunits. The decrease was more prominent in monoploid compared to diploid and triploid. This study provides crucial data for research that aim to enhance plant ability to withstand O3 induced oxidative stress. Our findings may help in developing a tolerant variety through plant breeding techniques, which will be economically more advantageous in reaching the objective of sustainable production at the high O3 levels projected under a climate change scenario.

Elevated tropospheric ozone and crop production: potential negative effects and plant defense mechanisms

Ozone (O3) levels on Earth are increasing because of anthropogenic activities and natural processes. Ozone enters plants through the leaves, leading to the overgeneration of reactive oxygen species (ROS) in the mesophyll and guard cell walls. ROS can damage chloroplast ultrastructure and block photosynthetic electron transport. Ozone can lead to stomatal closure and alter stomatal conductance, thereby hindering carbon dioxide (CO2) fixation. Ozone-induced leaf chlorosis is common. All of these factors lead to a reduction in photosynthesis under O3 stress. Long-term exposure to high concentrations of O3 disrupts plant physiological processes, including water and nutrient uptake, respiration, and translocation of assimilates and metabolites. As a result, plant growth and reproductive performance are negatively affected. Thus, reduction in crop yield and deterioration of crop quality are the greatest effects of O3 stress on plants. Increased rates of hydrogen peroxide accumulation, lipid peroxidation, and ion leakage are the common indicators of oxidative damage in plants exposed to O3 stress. Ozone disrupts the antioxidant defense system of plants by disturbing enzymatic activity and non-enzymatic antioxidant content. Improving photosynthetic pathways, various physiological processes, antioxidant defense, and phytohormone regulation, which can be achieved through various approaches, have been reported as vital strategies for improving O3 stress tolerance in plants. In plants, O3 stress can be mitigated in several ways. However, improvements in crop management practices, CO2 fertilization, using chemical elicitors, nutrient management, and the selection of tolerant crop varieties have been documented to mitigate O3 stress in different plant species. In this review, the responses of O3-exposed plants are summarized, and different mitigation strategies to decrease O3 stress-induced damage and crop losses are discussed. Further research should be conducted to determine methods to mitigate crop loss, enhance plant antioxidant defenses, modify physiological characteristics, and apply protectants.

Signaling pathways involved in microbial indoor air pollutant 3-methyl-1-butanol in the induction of stomatal closure in Arabidopsis

Indoor air pollution is a global problem and one of the main stress factors that has negative effects on plant and human health. 3-methyl-1-butanol (3MB), an indoor air pollutant, is a microbial volatile organic compound (mVOC) commonly found in damp indoor dwellings. In this study, we reported that 1 mg/L of 3MB can elicit a significant reduction in the stomatal aperture ratio in Arabidopsis and tobacco. Our results also showed that 3MB enhances the reactive oxygen species (ROS) production in guard cells of wild-type Arabidopsis after 24 h exposure. Further investigation of 24 h 3MB fumigation of rbohD, the1-1, mkk1, mkk3, and nced3 mutants revealed that ROS production, cell wall integrity, MAPK kinases cascade, and phytohormone abscisic acid are all involved in the process of 3MB-induced stomatal. Our findings proposed a mechanism by which 3MB regulates stomatal closure in Arabidopsis. Understanding the mechanisms by which microbial indoor air pollutant induces stomatal closure is critical for modulating the intake of harmful gases from indoor environments into leaves. Investigations into how stomata respond to the indoor mVOC 3MB will shed light on the plant's "self-defense" system responding to indoor air pollution.

Ectomycorrhizal symbiosis prepares its host locally and systemically for abiotic cue signaling

Tree growth and survival are dependent on their ability to perceive signals, integrate them, and trigger timely and fitted molecular and growth responses. While ectomycorrhizal symbiosis is a predominant tree-microbe interaction in forest ecosystems, little is known about how and to what extent it helps trees cope with environmental changes. We hypothesized that the presence of Laccaria bicolor influences abiotic cue perception by Populus trichocarpa and the ensuing signaling cascade. We submitted ectomycorrhizal or non-ectomycorrhizal P. trichocarpa cuttings to short-term cessation of watering or ozone fumigation to focus on signaling networks before the onset of any physiological damage. Poplar gene expression, metabolite levels, and hormone levels were measured in several organs (roots, leaves, mycorrhizas) and integrated into networks. We discriminated the signal responses modified or maintained by ectomycorrhization. Ectomycorrhizas buffered hormonal changes in response to short-term environmental variations systemically prepared the root system for further fungal colonization and alleviated part of the root abscisic acid (ABA) signaling. The presence of ectomycorrhizas in the roots also modified the leaf multi-omics landscape and ozone responses, most likely through rewiring of the molecular drivers of photosynthesis and the calcium signaling pathway. In conclusion, P. trichocarpa-L. bicolor symbiosis results in a systemic remodeling of the host's signaling networks in response to abiotic changes. In addition, ectomycorrhizal, hormonal, metabolic, and transcriptomic blueprints are maintained in response to abiotic cues, suggesting that ectomycorrhizas are less responsive than non-mycorrhizal roots to abiotic challenges.

Crosstalk and trade-offs: Plant responses to climate change-associated abiotic and biotic stresses

As sessile organisms, plants are constantly challenged by a dynamic growing environment. This includes fluctuations in temperature, water availability, light levels, and changes in atmospheric constituents such as carbon dioxide (CO2 ) and ozone (O3 ). In concert with changes in abiotic conditions, plants experience changes in biotic stress pressures, including plant pathogens and herbivores. Human-induced increases in atmospheric CO2 levels have led to alterations in plant growth environments that impact their productivity and nutritional quality. Additionally, it is predicted that climate change will alter the prevalence and virulence of plant pathogens, further challenging plant growth. A knowledge gap exists in the complex interplay between plant responses to biotic and abiotic stress conditions. Closing this gap is crucial for developing climate resilient crops in the future. Here, we briefly review the physiological responses of plants to elevated CO2 , temperature, tropospheric O3 , and drought conditions, as well as the interaction of these abiotic stress factors with plant pathogen pressure. Additionally, we describe the crosstalk and trade-offs involved in plant responses to both abiotic and biotic stress, and outline targets for future work to develop a more sustainable future food supply considering future climate change.

Responses to Airborne Ozone and Soilborne Metal Pollution in Afforestation Plants with Different Life Forms

With the current increases in environmental stress, understanding species-specific responses to multiple stress agents is needed. This science is especially important for managing ecosystems that are already confronted with considerable pollution. In this study, responses to ozone (O3, ambient daily course values + 20 ppb) and mixed metal contamination in soils (MC, cadmium/copper/lead/zinc = 25/1100/2500/1600 mg kg-1), separately and in combination, were evaluated for three plant species (Picea abies, Acer pseudoplatanus, Tanacetum vulgare) with different life forms and ecological strategies. The two treatments elicited similar stress reactions, as shown by leaf functional traits, gas exchange, tannin, and nutrient markers, irrespective of the plant species and life form, whereas the reactions to the treatments differed in magnitude. Visible and microscopic injuries at the organ or cell level appeared along the penetration route of ozone and metal contamination. At the whole plant level, the MC treatment caused more severe injuries than the O3 treatment and few interactions were observed between the two stress factors. Picea trees, with a slow-return strategy, showed the highest stress tolerance in apparent relation to an enhancement of conservative traits and an exclusion of stress agents. The ruderal and more acquisitive Tanacetum forbs translocated large amounts of contaminants above ground, which may be of concern in a phytostabilisation context. The deciduous Acer trees-also with an acquisitive strategy-were most sensitive to both stress factors. Hence, species with slow-return strategies may be of particular interest for managing metal-polluted sites in the current context of multiple stressors and for safely confining soil contaminants below ground.

Phytoremediation as a potential technique for vehicle hazardous pollutants around highways

With the synchronous development of highway construction and the urban economy, automobiles have entered thousands of households as essential means of transportation. This paper reviews the latest research progress in using phytoremediation technology to remediate the environmental pollution caused by automobile exhaust in recent years, including the prospects for stereoscopic forestry. Currently, most automobiles on the global market are internal combustion vehicles using fossil energy sources as the primary fuel, such as gasoline, diesel, and liquid or compressed natural gas. The composition of vehicle exhaust is relatively complex. When it enters the atmosphere, it is prone to a series of chemical reactions to generate various secondary pollutants, which are very harmful to human beings, plants, animals, and the eco-environment. Despite improving the automobile fuel quality and installing exhaust gas purification devices, helping to reduce air pollution, the treatment costs of these approaches are expensive and cannot achieve zero emissions of automobile exhaust pollutants. The purification of vehicle exhaust by plants is a crucial way to remediate the environmental pollution caused by automobile exhaust and improve the environment along the highway by utilizing the ecosystem's self-regulating ability. Therefore, it has become a global trend to use phytoremediation technology to restore the automobile exhaust pollution. Now, there is no scientific report or systematic review about how plants absorb vehicle pollutants. The screening and configuration of suitable plant species is the most crucial aspect of successful phytoremediation. The mechanisms of plant adsorption, metabolism, and detoxification are reviewed in this paper to address the problem of automobile exhaust pollution.

Response of tropical trees to elevated Ozone: a Free Air Ozone Enrichment study

Tropospheric ozone (O₃) has become one of the main urban air pollutants. In the present study, we assessed impact of ambient and future ground-level O₃ on nine commonly growing urban tree species under Free Air Ozone Enrichment (FAOE) condition. During the study period, mean ambient and elevated ozone (EO₃) concentrations were 48.59 and 69.62 ppb, respectively. Under EO₃ treatment, stomatal density (SD) significantly decreased and guard cell length (GCL) increased in Azadirachta indica, Bougainvillea spectabilis, Plumeria rubra, Saraca asoca and Tabernaemontana divaricata, while SD increased and GCL decreased in Ficus benghalensis and Terminalia arjuna. Proline levels increased in all the nine plant species under EO₃ condition. EO₃ significantly reduced photosynthetic rate, stomatal conductance (gs), and transpiration rates (E). Only A. indica and N. indicum showed higher gs and E under EO₃ treatment. Water use efficiency (WUE) significantly increased in F. benghalensis and decreased in A. indica and T. divaricata. Air Pollution Tolerance Index (APTI) significantly increased in Ficus religiosa and S. asoca whereas it decreased in B. spectabilis and A. indica. Of all the plant species B. spectabilis and A. indica were the most sensitive to EO₃ (high g_s and less ascorbic acid content) while S. asoca and F. religiosa were the most tolerant (lowg_s and more ascorbic acid content). The sensitivity of urban tree species to EO₃ is a cause of concern and should be considered for future urban forestry programmes. Our study should guide more such studies to identify tolerant trees for urban air pollution abatement.

Phytocyanin-encoding genes confer enhanced ozone tolerance in Arabidopsis thaliana

Ozone is a phytotoxic air pollutant that has various damaging effects on plants, including chlorosis and growth inhibition. Although various physiological and genetic studies have elucidated some of the mechanisms underlying plant ozone sensitivity and lesion development, our understanding of plant response to this gas remains incomplete. Here, we show evidence for the involvement of certain apoplastic proteins called phytocyanins, such as AtUC5, that protect against ozone damage. Two representative ozone-inducible responses, chlorosis and stomatal closure, were suppressed in AtUC5-overexpressing plants. Analysis of transgenic plants expressing a chimeric protein composed of AtUC5 fused to green fluorescent protein indicated that this fusion protein localises to the apoplast of plant cells where it appears to suppress early responses to ozone damage such as generation or signalling of reactive oxygen species. Moreover, yeast two-hybrid analyses suggest that AtUC5 may physically interact with stress-related proteins such as copper amine oxidase and late embryogenesis abundant protein-like protein. In addition to AtUC5, other examined phytocyanins such as AtUC6 and AtSC3 could confer ozone tolerance to plants when overexpressed in A. thaliana, suggesting that these proteins act together to protect plants against oxidative stress factors.

Ozone and nitrogen dioxide regulate similar gene expression responses in Arabidopsis but natural variation in the extent of cell death is likely controlled by different genetic loci

High doses of ozone (O3) and nitrogen dioxide (NO2) cause damage and cell death in plants. These two gases are among the most harmful air pollutants for ecosystems and therefore it is important to understand how plant resistance or sensitivity to these gases work at the molecular level and its genetic control. We compared transcriptome data from O3 and NO2 fumigations to other cell death related treatments, as well as individual marker gene transcript level in different Arabidopsis thaliana accessions. Our analysis revealed that O3 and NO2 trigger very similar gene expression responses that include genes involved in pathogen resistance, cell death and ethylene signaling. However, we also identified exceptions, for example RBOHF encoding a reactive oxygen species producing RESPIRATORY BURST OXIDASE PROTEIN F. This gene had increased transcript levels by O3 but decreased transcript levels by NO2, showing that plants can identify each of the gases separately and activate distinct signaling pathways. To understand the genetics, we conducted a genome wide association study (GWAS) on O3 and NO2 tolerance of natural Arabidopsis accessions. Sensitivity to both gases seem to be controlled by several independent small effect loci and we did not find an overlap in the significantly associated regions. Further characterization of the GWAS candidate loci identified new regulators of O3 and NO2 induced cell death including ABH1, a protein that functions in abscisic acid signaling, mRNA splicing and miRNA processing. The GWAS results will facilitate further characterization of the control of programmed cell death and differences between oxidative and nitrosative stress in plants.

Nocturnal pollutant uptake contributes significantly to the total stomatal uptake of Mangifera indica

DO₃SE (Deposition of Ozone for Stomatal Exchange), is a dry deposition model, designed to assess tropospheric ozone risk to vegetation, and is based on two alternative algorithms to estimate stomatal conductance: multiplicative and photosynthetic. The multiplicative model has been argued to perform better for leaf-level and regional-level application. In this study, we demonstrate that the photosynthetic model is superior to the multiplicative model even for leaf-level studies using measurements performed on Mangifera indica. We find that the multiplicative model overestimates the daytime stomatal conductance, when compared with measured stomatal conductance and prescribes zero conductance at night while measurements show an average conductance of 100 mmol(H₂O)m^-2s^-1 between 9 p.m. and 4 a.m. The daytime overestimation of the multiplicative model can be significantly reduced when the model is modified to include a response function for ozone-induced stomatal closure. However, nighttime pollutant uptake fluxes can only be accurately assessed with the photosynthetic model which includes the stomatal opening at night during respiration and is capable of reproducing the measured nighttime stomatal conductance. At our site, the nocturnal flux contributes 64%, 39%, 46%, and 88% of the total for NO₂ uptake in winter, summer, monsoon, and post-monsoon, respectively. For SO_2, nocturnal uptake amounts to 35%, 28%, 28%, and 44% in winter, summer, monsoon, and post-monsoon, respectively while for ozone the nighttime uptake contributes 30%, 17%, 18%, and 29% of the total stomatal uptake in winter, summer, monsoon, and post-monsoon respectively.

Simultaneous Ozone and High Light Treatments Reveal an Important Role for the Chloroplast in Co-ordination of Defense Signaling

Plants live in a world of changing environments, where they are continuously challenged by alternating biotic and abiotic stresses. To transfer information from the environment to appropriate protective responses, plants use many different signaling molecules and pathways. Reactive oxygen species (ROS) are critical signaling molecules in the regulation of plant stress responses, both inside and between cells. In natural environments, plants can experience multiple stresses simultaneously. Laboratory studies on stress interaction and crosstalk at regulation of gene expression, imply that plant responses to multiple stresses are distinctly different from single treatments. We analyzed the expression of selected marker genes and reassessed publicly available datasets to find signaling pathways regulated by ozone, which produces apoplastic ROS, and high light treatment, which produces chloroplastic ROS. Genes related to cell death regulation were differentially regulated by ozone versus high light. In a combined ozone + high light treatment, the light treatment enhanced ozone-induced cell death in leaves. The distinct responses from ozone versus high light treatments show that plants can activate stress signaling pathways in a highly precise manner.

Potential involvement of proline and flavonols in plant responses to ozone

Ozone is considered to be a major phytotoxic pollutant. It is an oxidizing molecule with harmful effects that can affect human health and vegetation. Due to its phytotoxicity, it constitutes a threat to food security in a context of climate change. Proline accumulation is induced in response to numerous stresses and is assumed to be involved in plant antioxidant defense. We therefore addressed the question of the putative involvement of proline in plant ozone responses by analyzing the responses of two Arabidopsis mutants (obtained in the Col-0 genetic background) altered in proline metabolism and different ecotypes with various degrees of ozone sensitivity, to controlled ozone treatments. Among the mutants, the p5cs1 mutant plants accumulated less proline than the double prodh1xprodh2 (p1p2) mutants. Ozone treatments did not induce accumulation of proline in Col-0 nor in the mutant plants. However, the variation of the photosynthetic parameter Fv/Fm in the p1p2 mutant suggests a positive effect of proline. Proline accumulation induced by ozone was only observed in the most ozone-sensitive ecotypes, Cvi-0 and Ler. Contrary to our expectations, proline accumulation could not be correlated with variations in protein oxidation (carbonylation). On the other hand, flavonols content, measured here, using non-destructive methods, reflected exactly the genotypes ranking according to ozone sensitivity.

Transcription factor McWRKY71 induced by ozone stress regulates anthocyanin and proanthocyanidin biosynthesis in Malus crabapple

In plants, anthocyanins and proanthocyanidins (PAs) play important roles in plant resistance to abiotic stress. In this study, ozone (O₃) treatments caused the up-regulation of Malus crabapple structural genes McANS, McCHI, McANR and McF3H, which promoted anthocyanin and PA accumulation. We identified the WRKY transcription factor (TF) McWRKY71 by screening differentially expressed genes (DEGs) that were highly expressed in response to O₃ stress from an RNA sequencing (RNA-seq) analysis. Overexpressing McWRKY71 increased the resistance of 'Orin' apple calli to O₃ stress and promoted the accumulation of anthocyanins and PAs, which facilitated reactive oxygen species scavenging to further enhance O₃ tolerance. Biochemical and molecular analyses showed that McWRKY71 interacted with McMYB12 and directly bound the McANR promoter to participate in the regulation of PA biosynthesis. These findings provide new insights into the WRKY TFs mechanisms that regulate the biosynthesis of secondary metabolites, which respond to O₃ stress, in Malus crabapple.

Individual and Interactive Effects of Elevated Ozone and Temperature on Plant Responses

From the preindustrial era to the present day, the tropospheric ozone (O3) concentration has increased dramatically in much of the industrialized world due to anthropogenic activities. O3 is the most harmful air pollutant to plants. Global surface temperatures are expected to increase with rising O3 concentration. Plants are directly affected by temperature and O3. Elevated O3 can impair physiological processes, as well as cause the accumulation of reactive oxygen species (ROS), leading to decreased plant growth. Temperature is another important factor influencing plant development. Here, we summarize how O3 and temperature elevation can affect plant physiological and biochemical characteristics, and discuss results from studies investigating plant responses to these factors. In this review, we focused on the interactions between elevated O3 and temperature on plant responses, because neither factor acts independently. Temperature has great potential to significantly influence stomatal movement and O3 uptake. For this reason, the combined influence of both factors can yield significantly different results than those of a single factor. Plant responses to the combined effects of elevated temperature and O3 are still controversial. We attribute the substantial uncertainty of these combined effects primarily to differences in methodological approaches.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks

Mesophyll conductance (gmCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O3 ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O3 and drought on gmCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O3 (ambient [35.2 nmol mol-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO2 inside a leaf. The combination of O3 and WD significantly decreased net photosynthetic rate (PN ) regardless of the species. The reduction of photosynthesis was associated with a decrease in gmCO2 and stomatal conductance (gsCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (Vcmax ) and maximum electron transport rate (Jmax ) with decreased diffusive conductance parameters. The reduction of gmCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between gmCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between gsCO2 and ABA under high O3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of PN was related to gmCO2 rather than gsCO2 . The results suggest that gmCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O3 pollution.

Biostimulant Capacity of an Enzymatic Extract From Rice Bran Against Ozone-Induced Damage in Capsicum annum

Ozone is a destructive pollutant, damaging crops, and decreasing crop yield. Therefore, there is great interest in finding strategies to alleviate ozone-induced crop losses. In plants, ozone enters leaves through the stomata and is immediately degraded into reactive oxygen species (ROS), producing ROS stress in plants. ROS stress can be controlled by ROS-scavenging systems that include enzymatic or non-enzymatic mechanisms. Our research group has developed a product from rice bran, a by-product of rice milling which has bioactive molecules that act as an antioxidant compound. This product is a water-soluble rice bran enzymatic extract (RBEE) which preserves all the properties and improves the solubility of proteins and the antioxidant components of rice bran. In previous works, the beneficial properties of RBEE have been demonstrated in animals. However, to date, RBEE has not been used as a protective agent against oxidative damage in agricultural fields. The main goal of this study was to investigate the ability of RBEE to be used as a biostimulant by preventing oxidative damage in plants, after ozone exposure. To perform this investigation, pepper plants (Capsicum annuum) exposed to ozone were treated with RBEE. RBEE protected the ozone-induced damage, as revealed by net photosynthetic rate and the content of photosynthetic pigments. RBEE also decreased the induction of antioxidant enzyme activities in leaves (catalase, superoxide dismutase, and ascorbate peroxidase) due to ozone exposure. ROS generation is a common consequence of diverse cellular traumas that also activate the mitogen-activated protein kinase (MAPK) cascade. Thus, it is known that the ozone damages are triggered by the MAPK cascade. To examine the involvement of the MAPK cascade in the ozone damage CaMPK6-1, CaMPK6-2, and CaMKK5 genes were analyzed by qRT-PCR. The results showed the involvement of the MAPK pathway in both, not only in ozone damage but especially in its protection by RBEE. Taken together, these results support that RBEE protects plants against ozone exposure and its use as a new biostimulant could be proposed.

Response of rice (Oryza sativa L.) cultivars to elevated ozone stress

The plant response to elevated ozone stress reveals inter-species and intra-species disparity. Ozone-induced crop yield loss is predicted to increase in the future, posing a threat to the world economy. This study aims to evaluate the cultivar specific variation in rice exposed to elevated ozone. Fifteen short-duration rice cultivars were exposed to 50 ppb ozone for 30 days at reproductive stage. The physiological, biochemical, growth and yield traits of all test cultivars were significantly affected in response to elevated ozone. On an average, ozone stress decreased the tiller number by 22.52%, number of effective tillers by 30.43%, 1000 grain weight by 0.62% and straw weight by 23.83% over control. Spikelet sterility increased by 19.26% and linear multiregression 3D model significantly fits the spikelet sterility and photosynthetic traits with the R² of 0.74 under elevated ozone. Principal Component Analysis with total variance of 57.5% categorized 15 rice cultivars into four major groups, i.e., ozone sensitive (MDU6, TRY(R)2 and ASD16), moderately ozone sensitive (ASD18, ADT43, and MDU5), moderately ozone tolerant (ADT37, ADT(R)45, TPS5, Anna(R)4, PMK(R)3, and ADT(R)48), and ozone tolerant (CO51, CO47, and ADT36). This study indicates that the different responses of rice cultivars to elevated ozone stress through a change in plant physiology, biochemical, growth, and yield traits and the results directed to provide scientific information on plant adaptations to ozone stress and helps in efforts to search ozone tolerant gene for plant breeding.

In the tripartite combination ozone-poplar-Chrysomela populi, the pollutant alters the plant-insect interaction via primary metabolites of foliage

Ozone (O₃)-induced metabolic changes in leaves are relevant and may have several ecological significances. Here, variations in foliar chemistry of two poplar clones (Populus deltoides × maximowiczii, Eridano, and P. × euramericana, I-214) under a chronic O₃ treatment (80 ppb, 5 h d^-1 for 10 consecutive days) were investigated. The aim was to elucidate if leaf age and/or O₃-sensitivity (considering Eridano and I-214 as O₃-sensitive and O₃-resistant, respectively) can affect suitability of poplar foliage for Chrysomela populi L. (Coleoptera Chrysomelidae), in terms of palatability. Comparing controls, only low amino acid (AA) contents were reported in Eridano [about 3- and 4-fold in mature and young leaves (ML and YL, respectively)], and all the investigated primary metabolites [i.e. water soluble carbohydrates (WSC), proteins (Prot) and AA] were higher in YL than in ML of I-214 (+23, +54 and + 20%, respectively). Ozone increased WSC only in YL of Eridano (+24%, i.e. highest values among samples; O₃ effects are always reported comparing O₃-treated plants with the related controls). A concomitant decrease of Prot was observed in both ML and YL of Eridano, while only in YL of I-214 (-41, -45 and -51%, respectively). In addition, O₃ decreased AA in YL of Eridano and in ML of I-214 (-40 and -14%, respectively). Comparing plants maintained under charcoal-filtered air, total ascorbate (Asc) was lower in Eridano in both ML and YL (around -22%), and abscisic acid (ABA) was similar between clones; furthermore, higher levels of Asc were reported in YL than in ML of Eridano (+19%). Ozone increased Asc and ABA (about 2- and 3-fold, respectively) in both ML and YL of Eridano, as well as ABA in YL of I-214 (about 2-fold). Comparing leaves maintained under charcoal-filtered air, the choice feeding test showed that the 2nd instar larvae preferred YL, and the quantity of YL consumed was 9 and 4-fold higher than ML in Eridano and I-214, respectively. Comparing leaves exposed to O₃-treatment, a significant feeding preference for YL disks was also observed, regardless of the clone. The no-choice feeding test showed that larval growth was slightly higher on untreated YL than on untreated ML (+19 and + 10% in Eridano and I-214, respectively). The body mass of larvae fed with O₃-treated YL was also significantly higher than that of larvae fed with untreated YL (3- and 2-fold in Eridano and I-214). This study highlights that realistic O₃ concentrations can significantly impact the host/insect interactions, a phenomenon dependent on leaf age and O₃-sensitivity of the host.

Effects of TiO2-NPs pretreatment on UV-B stress tolerance in Arabidopsis thaliana

As the ozone hole in the North and South poles continues to increase, the entire ecosystem will face an environmental crisis caused by enhanced UV-B radiation. Considering the function of TiO₂ and the application of nanomaterials in agriculture, the effect of TiO₂-NPs on UV-B stress tolerance in Arabidopsis was investigated. The phenotype of plants was determined, and the expression patterns of antioxidant systems and related genes were analyzed. Modification of the antioxidant system and changes in the flavonoid content of plants were observed by histochemical staining. The effects of TiO₂-NPs and UV-B on mitosis were observed at the cellular level, and the degree of DNA damage was analyzed by the detection of CPDs content. The effects of TiO₂-NPs and UV-B on SOD isozymes were detected by SOD isozyme Native-PAGE electrophoresis. A laser confocal microscope was used to explore the protective mechanism of TiO₂-NPs against UV-B radiation. Results showed that pretreatment of TiO₂-NPs significantly alleviated the stress of UV-B radiation on plants. TiO₂-NPs activated the antioxidant system of plants, improved the activity of antioxidant enzymes, and promoted the synthesis of flavonoids. Moreover, TiO₂-NPs could effectively shield UV-B radiation to prevent the depolymerization of microtubules in plant cells. 10 mg/L of TiO₂-NPs is a safe and effective application dose, which has no biological toxicity to plants. Our research results reported for the first time that pretreatment of TiO₂-NPs could effectively alleviate UV-B stress to plants, providing new ideas for the application of nanomaterials in agriculture.

Effects of ethylenediurea (EDU) on apoplast and chloroplast proteome in two wheat varieties under high ambient ozone: an approach to investigate EDU's mode of action

Rising tropospheric ozone (O₃) is a serious threat to plants and animals in the present climate change scenario. High tropospheric O₃ has the capability to disrupt cellular organelles leading to impaired photosynthesis and significant yield reduction. Apoplast and chloroplast are two important cellular components in a plant system. Their proteomic response with ethylenediurea (EDU) treatment under tropospheric O₃ has not been explored till date. EDU (an organic compound) protects plants exclusively against harmful O₃ effects through activation of antioxidant defense mechanism. The present study investigated the mode of action of EDU (hereafter MAE) by identifying proteins involved in apoplast and chloroplast pathways. Two wheat varieties viz. Kundan and PBW 343 (hereafter K and P respectively) and three EDU treatments (0= control, 200, and 300 ppm) have been used for the study. In apoplast isolates, proteins such as superoxide dismutase (SOD), amino methyltransferase, catalase, and Germin-like protein have shown active role by maintaining antioxidant defense system under EDU treatment. Differential expression of these proteins leads to enhanced antioxidative defense mechanisms inside and outside the cell. Chloroplast proteins such as Rubisco, Ferredoxin NADP- reductase (FNR), fructose,1-6 bis phosphatase (FBPase), ATP synthase, vacuolar proton ATPase, and chaperonin have regulated their abundance to minimize ozone stress under EDU treatment. After analyzing apoplast and chloroplast protein abundance, we have drawn a schematic representation of the MAE working mechanism. The present study showed that plants can be capable of O₃ tolerance, which could be improved by optimizing the apoplast ROS pool under EDU treatment.

Elucidating the relevance of high temperature and elevated CO2 in plant secondary metabolites (PSMs) production

Plant secondary metabolites (PSMs) are plant products that are discontinuously distributed throughout the plant kingdom. These secondary compounds have various chemical groups and are named according to their chemical constituents. For their ability to defend biotic and abiotic stresses they are considered as plants' defensive compounds. These metabolites take part in plant protection from insects, herbivores, and extreme environmental conditions. They are indirectly involved in plants’ growth and development. Secondary metabolites are also used by people in the form of medicines, pharmaceuticals, agrochemicals, colors, fragrances, flavorings, food additives, biopesticides, and drugs development. However, the increase in atmospheric temperature by several anthropogenic activities majorly by the combustion of hydrocarbons is a great issue now. On the other hand, climate change leaves an impact on the quality and quantity of plant secondary metabolites. It is measured that several greenhouse gases (GHGs) are present in the atmosphere, like Chlorofluorocarbons (CFCs), nitrous oxides (NO_x), Carbon dioxide (CO₂), Methane (CH₄) and Ozone (O₃), etc. CO₂, the major greenhouse gas is essential for photosynthesis. On the other hand, CO₂ plays a significant role in the up-regulation of atmospheric temperature. Plants produce various types of primary metabolites such as carbohydrates, proteins, fats, membrane lipids, nucleic acids, and chlorophyll as well as a variety of secondary metabolites from photosynthesis. The high temperature in the atmosphere creates heat stress for plants. As a matter of fact many morphological, physiological and biochemical changes occur in the plant. The high temperature invariably elicits the production of several secondary metabolites within plants. Various strategies have been universally documented to improve the production of PSMs. With this objective, the focus of the current review is to further investigate and discuss futuristic scenarios the effect of elevated CO₂ and high temperature on PSMs production which may perhaps beneficial for pharmaceutical industries, biotechnology industries, and also in climate change researches.

Foliage visible injury in the tropical tree species, Astronium graveolens is strictly related to phytotoxic ozone dose (PODy)

The present study evaluates the development of visible injury related to phytotoxic ozone dose (PODy) in native tropical species Astronium graveolens Jacq. (Anacardiaceae) and validates the symptoms using structural markers attributed to oxidative burst and hypersensitive responses. Increasing POD₀ was associated with increasing O₃ visible injury using different metrics as the incidence (INC = number of injured plants/total number of plants × 100), severity (SF = number of injured leaves/total number of leaves on injured plant × 100), and severity leaflet (SFL = number of injured leaflets/total number leaflets injured plant × 100). The effective dose (ED), which represents the POD₀ dose responsible for inducing 20 (ED20), 50 (ED50), or 80% (ED80) of visible injury, were used to demonstrate that for this species, the response is similar even when the plants are exposed to diverse climate environments. Further investigation of the INC and SF index may help in long-term forest monitoring sites dedicated to O₃ assessment in forests, while the SFL index seems to be an excellent indicator to be used in the short term to investigate the effects of O_3.

Gaseous ozone to preserve quality and enhance microbial safety of fresh produce: Recent developments and research needs

Fresh fruits and vegetables are highly perishable and are subject to large postharvest losses due to physiological (senescence), pathologic (decay), and physical (mechanical damage) factors. In addition, contamination of fresh produce with foodborne human pathogens has become a concern. Gaseous ozone has multiple benefits including destruction of ethylene, inactivation of foodborne and spoilage microorganisms, and degradation of chemical residues. This article reviews the beneficial effects of gaseous ozone, its influence on quality and biochemical changes, foodborne human pathogens, and spoilage microorganisms, and discusses research needs with an emphasis on fruits. Ozone may induce synthesis of a number of antioxidants and bioactive compounds by activating secondary metabolisms involving a wide range of enzymes. Disparities exist in the literature regarding the impact of gaseous ozone on quality and physiological processes of fresh produce, such as weight loss, ascorbic acid, and fruit ripening. The disparities are complicated by incomplete reporting of the necessary information, such as relative humidity and temperatures at which ozone measurement and treatment were performed, which is needed for accurate comparison of results among studies. In order to fully realize the benefits of gaseous ozone, research is needed to evaluate the molecular mechanisms of gaseous ozone in inhibiting ripening, influence of relative humidity on the antimicrobial efficacy, interaction between ozone and the cuticle of fresh produce, ozone signaling pathways in the cells and tissues, and so forth. Possible adverse effects of gaseous ozone on quality of fresh produce also need to be carefully evaluated for the purpose of enhancing microbial and chemical safety of fresh produce.

Plant biochemistry influences tropospheric ozone formation, destruction, deposition, and response

Tropospheric ozone (O₃) is among the most damaging air pollutant to plants. Plants alter the atmospheric O₃ concentration in two distinct ways: (i) by the emission of volatile organic compounds (VOCs) that are precursors of O_3; and (ii) by dry deposition, which includes diffusion of O₃ into vegetation through stomata and destruction by nonstomatal pathways. Isoprene, monoterpenes, and higher terpenoids are emitted by plants in quantities that alter tropospheric O₃. Deposition of O₃ into vegetation is related to stomatal conductance, leaf structural traits, and the detoxification capacity of the apoplast. The biochemical fate of O₃ once it enters leaves and reacts with aqueous surfaces is largely unknown, but new techniques for the tracking and identification of initial products have the potential to open the black box.

Physiological Response of Maize Plants (Zea mays L.) to the Use of the Potassium Quercetin Derivative

Plant production technologies based solely on the improvement of plants themselves face obstacles resulting from the natural limitations of the biological potential of varieties. Therefore, new substances are sought that positively influence the growth and development of plants and increase resistance to various biotic and abiotic stresses, which also translates into an increase in obtained yields. The exogenous application of various phytoprotectants shows great promise in terms of cost effectiveness compared to traditional breeding methods or transgenic approaches in relation to increasing plant tolerance to abiotic stresses. Quercetin is a strong antioxidant among phenolic compounds, and it plays a physiological and biochemical role in plants. As such, the aim of this research was to assess the effect of an aqueous solution of a quercetin derivative with potassium, applied in various concentrations (0.5%, 1.0%, 3.0% and 5.0%), on the efficiency of the photosynthetic apparatus and biochemical properties of maize. Among the tested variants, compared to the control, the most stimulating effect on the course of physiological processes (PN, gs, ci, CCI, Fv/Fm, Fv/F0, PI) in maize leaves was found in 3.0 and 5.0% aqueous solutions of the quercetin derivative. The highest total antioxidant capacity and total content of polyphenolic compounds were found for plants sprayed with 5.0% quercetin derivative solution; therefore, in this study, the optimal concentration could not be clearly selected.

Ozone Induced Stomatal Regulations, MAPK and Phytohormone Signaling in Plants

Ozone (O₃) is a gaseous environmental pollutant that can enter leaves through stomatal pores and cause damage to foliage. It can induce oxidative stress through the generation of reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂) that can actively participate in stomatal closing or opening in plants. A number of phytohormones, including abscisic acid (ABA), ethylene (ET), salicylic acid (SA), and jasmonic acid (JA) are involved in stomatal regulation in plants. The effects of ozone on these phytohormones’ ability to regulate the guard cells of stomata have been little studied, however, and the goal of this paper is to explore and understand the effects of ozone on stomatal regulation through guard cell signaling by phytohormones. In this review, we updated the existing knowledge by considering several physiological mechanisms related to stomatal regulation after response to ozone. The collected information should deepen our understanding of the molecular pathways associated with response to ozone stress, in particular, how it influences stomatal regulation, mitogen-activated protein kinase (MAPK) activity, and phytohormone signaling. After summarizing the findings and noting the gaps in the literature, we present some ideas for future research on ozone stress in plants

Ozone responses in Arabidopsis: beyond stomatal conductance

Tropospheric ozone (O3) is a major air pollutant that decreases yield of important crops worldwide. Despite long-lasting research of its negative effects on plants, there are many gaps in our knowledge on how plants respond to O3. In this study, we used natural variation in the model plant Arabidopsis (Arabidopsis thaliana) to characterize molecular and physiological mechanisms underlying O3 sensitivity. A key parameter in models for O3 damage is stomatal uptake. Here we show that the extent of O3 damage in the sensitive Arabidopsis accession Shahdara (Sha) does not correspond with O3 uptake, pointing toward stomata-independent mechanisms for the development of O3 damage. We compared tolerant (Col-0) versus sensitive accessions (Sha, Cvi-0) in assays related to photosynthesis, cell death, antioxidants, and transcriptional regulation. Acute O3 exposure increased cell death, development of lesions in the leaves, and decreased photosynthesis in sensitive accessions. In both Sha and Cvi-0, O3-induced lesions were associated with decreased maximal chlorophyll fluorescence and low quantum yield of electron transfer from Photosystem II to plastoquinone. However, O3-induced repression of photosynthesis in these two O3-sensitive accessions developed in different ways. We demonstrate that O3 sensitivity in Arabidopsis is influenced by genetic diversity given that Sha and Cvi-0 developed accession-specific transcriptional responses to O3. Our findings advance the understanding of plant responses to O3 and set a framework for future studies to characterize molecular and physiological mechanisms allowing plants to respond to high O3 levels in the atmosphere as a result of high air pollution and climate change.

Tropospheric ozone rapidly decreases root growth by altering carbon metabolism and detoxification capability in growing soybean roots

High tropospheric ozone (O3) concentrations lead to significant global soybean (Glycine max) yield reductions. Research concerning O3 impacts on soybean has focused on the contributions of above-ground tissues. In this study, Mandarin (Ottawa) (O3-sensitive) and Fiskeby III (O3-tolerant) soybean genotypes provide contrasting materials to investigate O3 effects on root growth. We compared root morphological and proteomic changes when 16-day-old plants were treated with charcoal-filtered (CF) air or elevated O3 (80 ppb O3 for 7 h/day) in continuously stirred-tank reactors (CSTR) for 7 days. Our results showed that in Mandarin (Ottawa), decreased expression of enzymes involved in the tricarboxylic acid (TCA) cycle contributes to reduction of root biomass and diameter under elevated O3. In contrast, O3 tolerance in Fiskeby III roots was associated with O3-dependent induction of enzymes involved in glycolysis and O3-independent expression of enzymes involved in the ascorbate-glutathione cycle. We conclude that a decreased abundance of key redox enzymes in roots due to limited carbon availability rapidly alters root growth under O3 stress. However, maintaining a high abundance of enzymes associated with redox status and detoxification capability contributes to overall O3 tolerance in roots.

cROStalk for Life: Uncovering ROS Signaling in Plants and Animal Systems, from Gametogenesis to Early Embryonic Development

This review explores the role of reactive oxygen species (ROS)/Ca2+ in communication within reproductive structures in plants and animals. Many concepts have been described during the last years regarding how biosynthesis, generation products, antioxidant systems, and signal transduction involve ROS signaling, as well as its possible link with developmental processes and response to biotic and abiotic stresses. In this review, we first addressed classic key concepts in ROS and Ca2+ signaling in plants, both at the subcellular, cellular, and organ level. In the plant science field, during the last decades, new techniques have facilitated the in vivo monitoring of ROS signaling cascades. We will describe these powerful techniques in plants and compare them to those existing in animals. Development of new analytical techniques will facilitate the understanding of ROS signaling and their signal transduction pathways in plants and mammals. Many among those signaling pathways already have been studied in animals; therefore, a specific effort should be made to integrate this knowledge into plant biology. We here discuss examples of how changes in the ROS and Ca2+ signaling pathways can affect differentiation processes in plants, focusing specifically on reproductive processes where the ROS and Ca2+ signaling pathways influence the gametophyte functioning, sexual reproduction, and embryo formation in plants and animals. The study field regarding the role of ROS and Ca2+ in signal transduction is evolving continuously, which is why we reviewed the recent literature and propose here the potential targets affecting ROS in reproductive processes. We discuss the opportunities to integrate comparative developmental studies and experimental approaches into studies on the role of ROS/ Ca2+ in both plant and animal developmental biology studies, to further elucidate these crucial signaling pathways.

Ozone Response of Leaf Physiological and Stomatal Characteristics in Brassica juncea L. at Supraoptimal Temperatures

Plants are affected by the features of their surrounding environment, such as climate change and air pollution caused by anthropogenic activities. In particular, agricultural production is highly sensitive to environmental characteristics. Since no environmental factor is independent, the interactive effects of these factors on plants are essential for agricultural production. In this context, the interactive effects of ozone (O3) and supraoptimal temperatures remain unclear. Here, we investigated the physiological and stomatal characteristics of leaf mustard (Brassica juncea L.) in the presence of charcoal-filtered (target concentration, 10 ppb) and elevated (target concentration, 120 ppb) O3 concentrations and/or optimal (22/20 °C day/night) and supraoptimal temperatures (27/25 °C). Regarding physiological characteristics, the maximum rate of electron transport and triose phosphate use significantly decreased in the presence of elevated O3 at a supraoptimal temperature (OT conditions) compared with those in the presence of elevated O3 at an optimal temperature (O conditions). Total chlorophyll content was also significantly affected by supraoptimal temperature and elevated O3. The chlorophyll a/b ratio significantly reduced under OT conditions compared to C condition at 7 days after the beginning of exposure (DAE). Regarding stomatal characteristics, there was no significant difference in stomatal pore area between O and OT conditions, but stomatal density under OT conditions was significantly increased compared with that under O conditions. At 14 DAE, the levels of superoxide (O2-), which is a reactive oxygen species, were significantly increased under OT conditions compared with those under O conditions. Furthermore, leaf weight was significantly reduced under OT conditions compared with that under O conditions. Collectively, these results indicate that temperature is a key driver of the O3 response of B. juncea via changes in leaf physiological and stomatal characteristics.

Role of antioxidant pool in management of ozone stress through soil nitrogen amendments in two cultivars of a tropical legume

The present experiment was done on two different cultivars of a tropical legume, Cymopsis tetragonoloba L. Taub. (cluster bean) cvv. Pusa-Naubahar (PUSA-N) and Selection-151 (S-151). The experiment was conducted under ambient ozone (O3) conditions with inputs of three different doses of inorganic nitrogen (N1, recommended; N2, 1.5-times recommended and N3, 2-times recommended) as well as control plants. The objective of this study was to evaluate the effectiveness of soil nitrogen amendments in management of ambient ozone stress in the two cultivars of C. tetragonoloba. Our experiment showed that nitrogen amendments can be an efficient measure to manage O3 injury in plants. Stimulation of antioxidant enzyme activities under nitrogen amendments is an important feature of plants that help plants cope with ambient O3 stress. Nitrogen amendments strengthened the antioxidant machinery in a more effective way in the tolerant cultivar PUSA-N, while in the sensitive cultivar S-151, avoidance strategy marked by more reduction in stomatal conductance was more prominent. Enzymes of the Halliwell-Asada pathway, especially ascorbate peroxidase and glutathione reductase, were more responsive and synchronised in PUSA-N than S-151, under similar nitrogen amendment regimes and were responsible for the differential sensitivities of the two cultivars of C. tetragonoloba. The present study shows that 1.5-times recommended dose of soil nitrogen amendments was sufficient in partial mitigation of O3 injury and the higher nitrogen dose (2-times recommended, in our case), did not provide any extra advantage to the plant's metabolism compared with plants treated with the lower nitrogen dose (1.5-times recommended).

Ozone uptake at night is more damaging to plants than equivalent day-time flux

MAIN CONCLUSION

Plants exposed to equivalent ozone fluxes administered during day-time versus night-time exhibited greater losses in biomass at night and this finding is attributed to night-time depletion of cell wall-localised ascorbate. The present study employed Lactuca sativa and its closest wild relative, L. serriola, to explore the relative sensitivity of plants to ozone-induced oxidative stress during day-time versus night-time. By controlling atmospheric ozone concentration and measuring stomatal conductance, equivalent ozone uptake into leaves was engineered during day and night, and consequences on productivity and net CO2 assimilation rate were determined. Biomass losses attributable to ozone were significantly greater when an equivalent dose of ozone was taken-up by foliage at night compared to the day. Linkages between ozone impacts and ascorbic acid (AA) content, redox status and cellular compartmentation were probed in both species. Leaf AA pools were depleted by exposure of plants to darkness, and then AA levels in the apoplast and symplast were monitored on subsequent transfer of plants to the light. Apoplast AA appeared to be more affected by light-dark transition than the symplast pool. Moreover, equivalent ozone fluxes administered to leaves with contrasting AA levels resulted in contrasting effects on the light-saturated rate of CO2 assimilation (Asat) in both species. Once apoplast AA content recovered to pre-treatment levels, the same ozone flux resulted in no impacts on Asat. The results of the present investigation reveal that plants are significantly more sensitive to equivalent ozone fluxes taken-up at night compared with those during the day and were consistent with diel shifts in apoplast AA content and/or redox status. Furthermore, findings suggest that some thought should be given to weighing regional models of ozone impacts for extraordinary night-time ozone impacts.

Root-type ferredoxin-NADP+ oxidoreductase isoforms in Arabidopsis thaliana: Expression patterns, location and stress responses

In Arabidopsis, two leaf-type ferredoxin-NADP⁺ oxidoreductase (LFNR) isoforms function in photosynthetic electron flow in reduction of NADP⁺ , while two root-type FNR (RFNR) isoforms catalyse reduction of ferredoxin in non-photosynthetic plastids. As the key to understanding, the function of RFNRs might lie in their spatial and temporal distribution in different plant tissues and cell types, we examined expression of RFNR1 and RFNR2 genes using β-glucuronidase (GUS) reporter lines and investigated accumulation of distinct RFNR isoforms using a GFP approach and Western blotting upon various stresses. We show that while RFNR1 promoter is active in leaf veins, root tips and in the stele of roots, RFNR2 promoter activity is present in leaf tips and root stele, epidermis and cortex. RFNR1 protein accumulates as a soluble protein within the plastids of root stele cells, while RFNR2 is mainly present in the outer root layers. Ozone treatment of plants enhanced accumulation of RFNR1, whereas low temperature treatment specifically affected RFNR2 accumulation in roots. We further discuss the physiological roles of RFNR1 and RFNR2 based on characterization of rfnr1 and rfnr2 knock-out plants and show that although the function of these proteins is partly redundant, the RFNR proteins are essential for plant development and survival.

Age-dependent increase in α-tocopherol and phytosterols in maize leaves exposed to elevated ozone pollution

Tropospheric ozone is a major air pollutant that significantly damages crop production. Crop metabolic responses to rising chronic ozone stress have not been well studied in the field, especially in C4 crops. In this study, we investigated the metabolomic profile of leaves from two diverse maize (Zea mays) inbred lines and the hybrid cross during exposure to season-long elevated ozone (~100 nl L-1) in the field using free air concentration enrichment (FACE) to identify key biochemical responses of maize to elevated ozone. Senescence, measured by loss of chlorophyll content, was accelerated in the hybrid line, B73 × Mo17, but not in either inbred line (B73 or Mo17). Untargeted metabolomic profiling further revealed that inbred and hybrid lines of maize differed in metabolic responses to ozone. A significant difference in the metabolite profile of hybrid leaves exposed to elevated ozone occurred as leaves aged, but no age-dependent difference in leaf metabolite profiles between ozone conditions was measured in the inbred lines. Phytosterols and α-tocopherol levels increased in B73 × Mo17 leaves as they aged, and to a significantly greater degree in elevated ozone stress. These metabolites are involved in membrane stabilization and chloroplast reactive oxygen species (ROS) quenching. The hybrid line also showed significant yield loss at elevated ozone, which the inbred lines did not. This suggests that the hybrid maize line was more sensitive to ozone exposure than the inbred lines, and up-regulated metabolic pathways to stabilize membranes and quench ROS in response to chronic ozone stress.

Valine-Glutamine Proteins in Plant Responses to Oxygen and Nitric Oxide

Multigene families coding for valine-glutamine (VQ) proteins have been identified in all kind of plants but chlorophytes. VQ proteins are transcriptional regulators, which often interact with WRKY transcription factors to regulate gene expression sometimes modulated by reversible phosphorylation. Different VQ-WRKY complexes regulate defense against varied pathogens as well as responses to osmotic stress and extreme temperatures. However, despite these well-known functions, new regulatory activities for VQ proteins are still to be explored. Searching public Arabidopsis thaliana transcriptome data for new potential targets of VQ-WRKY regulation allowed us identifying several VQ protein and WRKY factor encoding genes that were differentially expressed in oxygen-related processes such as responses to hypoxia or ozone-triggered oxidative stress. Moreover, some of those were also differentially regulated upon nitric oxide (NO) treatment. These subsets of VQ and WRKY proteins might combine into different VQ-WRKY complexes, thus representing a potential regulatory core of NO-modulated and O₂-modulated responses. Given the increasing relevance that gasotransmitters are gaining as plant physiology regulators, and particularly considering the key roles exerted by O₂ and NO in regulating the N-degron pathway-controlled stability of transcription factors, VQ and WRKY proteins could be instrumental in regulating manifold processes in plants.

Assessing the effects of elevated ozone on physiology, growth, yield and quality of soybean in the past 40 years: A meta-analysis

Soybean (Glycine max) production is seriously threatened by ground-level ozone (O3) pollution. The goal of our study is to summarize the impacts of O3 on physiology, growth, yield, and quality of soybean, as well as root parameters. We performed meta-analysis on the collated 48 peer-reviewed papers published between 1980 and 2019 to quantitatively summarize the response of soybean to elevated O3 concentrations ([O3]). Relative to charcoal-filtered air (CF), elevated [O3] significantly accelerated chlorophyll degradation, enhanced foliar injury, and inhibited growth of soybean, evidenced by great reductions in leaf area (-20.8%), biomass of leaves (-13.8%), shoot (-22.8%), and root (-16.9%). Shoot of soybean was more sensitive to O3 than root in case of biomass. Chronic ozone exposure of about 75.5 ppb posed pronounced decrease in seed yield of soybean (-28.3%). In addition, root environment in pot contributes to higher reduction in shoot biomass and yield of soybean. Negative linear relationships were observed between yield loss and intensity of O3 treatment, AOT40. The larger loss in seed yield was significantly associated with higher reduction in shoot biomass and other yield component. This meta-analysis demonstrates the effects of elevated O3 on soybean were pronounced, suggesting that O3 pollution is still a soaring threat to the productivity of soybean in regions with high ozone levels.

The Effects of Ozone on Herbivore-Induced Volatile Emissions of Cultivated and Wild Brassica Rapa

Since preindustrial times, concentrations of tropospheric ozone, a phytotoxic pollutant, have risen in the Northern Hemisphere. Selective breeding has intentionally modified crop plant traits to improve yield but may have altered plant defenses against abiotic and biotic stresses. This study aims to determine if cultivated and wild plants respond differently to herbivory under elevated ozone. We studied the volatile emissions of four cultivated Brassica rapa ssp. oleifera varieties and one wild population after exposure to ozone or Plutella xylostella larval feeding either individually or together. Ozone modulated the volatiles emitted in response to herbivory by all plant varieties to different extents. We did not observe a clear difference in the effects of ozone on wild and cultivated plants, but cultivated plants had higher volatile emission rates in response to herbivory and ozone had either no effect or increased the herbivore-induced response. Larvae tended to feed more on elevated ozone-treated plants; however, we could not link the increase of feeding to the change in volatile emissions. Our study complements recent studies reporting that selective breeding might not have weakened chemical defenses to biotic and abiotic stresses of cultivated plants.

SlTLFP8 reduces water loss to improve water-use efficiency by modulating cell size and stomatal density via endoreduplication

Improving plant water-use efficiency (WUE) is important to plant survival and crop yield in the context of water limitation. In this study, SlTLFP8 (Tubby-like F-box protein 8) was identified as an osmotic-induced gene in tomato. Transgenic tomato with up-regulated expression of SlTLFP8 showed enhanced water-deficient resistance, whereas knockout mutants generated by CRISPR/Cas9 were more sensitive to water deficit. SlTLFP8 overexpression significantly enhanced WUE by suppressing transpiration under both water-sufficient and water-deficient conditions. Further study showed that overexpressing SlTLFP8 significantly increased leaf epidermal cell size and thereby decreased stomatal density 10-20%, conversely SlTLFP8 knockout resulted in decreased cell size and thereby increased stomatal density 20-50%. SlTLFP8 overexpression and knockout modulated ploidy levels in leaf cells. Changes in expression of cell cycle related genes also indicated that SlTLFP8 affected cell size and stomatal density through endocycle transition. Despite changes in stomata density and transpiration, altering the expression of SlTLFP8 did not change photosynthesis. Additionally, biomass was not altered and there was little difference in fruit yield for transgenic and wild type lines under water-sufficient and water-deficient conditions. Our results demonstrate the effect of SlTLFP8 on endoreduplication and the potential of SlTLFP8 for improvement of WUE. BRIEF SUMMERY: This work found a new mechanism of TLP (Tubby like protein) response to water-deficient stress. SlTLFP8, a member of TLP family, regulates water-deficient resistance by modulating water loss via affecting stomatal density. Expression of SlTLFP8 was induced by osmotic stress. Transgenic tomato lines with SlTLFP8 overexpression or SlTLFP8 knockout showed significantly differences in water-use efficiency (WUE) and water-deficient resistance. The difference of leaf water loss caused by transpiration is the main explanation of the difference in WUE and water-deficient resistance. Additionally, overexpressing SlTLFP8 significantly decreased stomatal density, while SlTLFP8 knockout resulted in increased stomatal density, and SlTLFP8 affected stomatal density through endoreduplication and altered epidermal cell size. Despite changes in stomata density, altering the expression of SlTLFP8 did not result in distinct changes in photosynthesis, biomass and yield of tomato.

Interactive effect of leaf age and ozone on mesophyll conductance in Siebold's beech

Mesophyll conductance (G_m ) is one of the most important factors determining photosynthesis. Tropospheric ozone (O₃ ) is known to accelerate leaf senescence and causes a decline of photosynthetic activity in leaves. However, the effects of age-related variation of O₃ on G_m have not been well investigated, and we, therefore, analysed leaf gas exchange data in a free-air O₃ exposure experiment on Siebold's beech with two levels (ambient and elevated O₃ : 28 and 62 nmol mol^-1 as daylight average, respectively). In addition, we examined whether O₃ -induced changes on leaf morphology (leaf mass per area, leaf density and leaf thickness) may affect CO₂ diffusion inside leaves. We found that O₃ damaged the photosynthetic biochemistry progressively during the growing season. The G_m was associated with a reduced photosynthesis in O₃ -fumigated Siebold's beech in August. The O₃ -induced reduction of G_m was negatively correlated with leaf density, which was increased by elevated O₃ , suggesting that the reduction of G_m was accompanied by changes in the physical structure of mesophyll cells. On the other hand, in October, the O₃ -induced decrease of G_m was diminished because G_m decreased due to leaf senescence regardless of O₃ treatment. The reduction of photosynthesis in senescent leaves after O₃ exposure was mainly due to a decrease of maximum carboxylation rate (V_cmax ) and/or maximum electron transport rate (J_max ) rather than diffusive limitations to CO₂ transport such as G_m . A leaf age×O₃ interaction of photosynthetic response will be a key for modelling photosynthesis in O₃ -polluted environments.

The phytotoxic air-pollutant O3 enhances the emission of herbivore-induced volatile organic compounds (VOCs) and affects the susceptibility of black mustard plants to pest attack

Stress-induced changes to plant biochemistry and physiology can influence plant nutritional quality and subsequent interactions with herbivorous pests. However, the effects of stress combinations are unpredictable and differ to the effects of individual stressors. Here we studied the effects of exposure to the phytotoxic air-pollutant ozone (O₃), feeding by larvae of the large cabbage white butterfly (Pieris brassicae), and a combination of the two stresses, on the emission of volatile organic compounds (VOCs) by black mustard plants (Brassica nigra) under field and laboratory conditions. Field-grown B. nigra plants were also measured for carbon-nitrogen (C-N) content, net photosynthetic activity (Pn), stomatal conductance (gs) and biomass. The effects of O₃ on interactions between plants and a herbivorous pest were addressed by monitoring the abundance of wild diamondback moth larvae (Plutella xylostella) and feeding-damage to B. nigra plants in an O₃-free air concentration enrichment (O₃-FACE) field site. Herbivore-feeding induced the emission of VOCs that were not emitted by undamaged plants, both under field and laboratory conditions. The combination of O₃ and herbivore-feeding stresses resulted in enhanced emission rates of several VOCs from field-grown plants. Short-term O₃ exposure (of 10 days) and P. brassicae-feeding did not affect C-N content, but chronic O₃ exposure (of 34 and 47 days) and P. brassicae-feeding exacerbated suppression of Pn. Ozone exposure also caused visible injury and decreased the plant biomass. Field-grown B. nigra under elevated O₃ were infested with fewer P. xylostella larvae and received significantly less feeding damage. Our results suggest that plants growing in a moderately polluted environment may be of reduced quality and less attractive to foraging herbivores.

How does Malus crabapple resist ozone? Transcriptomics and metabolomics analyses

Ozone (O₃), an oxidizing toxic air pollutant, is ubiquitous in industrialized and developing countries. To understand the effects of O₃ exposure on apple (Malus) and to explore its defense mechanisms, we exposed 'Hongjiu' crabapple to O₃ and monitored its responses using physiological, transcriptomics, and metabolomics analyses. Exposure to 300 nL L^-1 O₃ for 3 h caused obvious damage to the leaves of Malus crabapple, affected chlorophyll and anthocyanin contents, and activated antioxidant enzymes. The gene encoding phospholipase A was highly responsive to O₃ in Malus crabapple. McWRKY75 is a key transcription factor in the response to O₃ stress, and its transcript levels were positively correlated with those of flavonoid-related structural genes (McC4H, McDFR, and McANR). The ethylene response factors McERF019 and McERF109-like were also up-regulated by O₃. Exogenous methyl jasmonate (MeJA) decreased the damaging effects of O₃ on crabapple and was most effective at 200 μmol L ^-1. Treatments with MeJA altered the metabolic pathways of crabapple under O₃ stress. In particular, MeJA activated the flavonoid metabolic pathway in Malus, which improved its resistance to O₃ stress.

Does ozone exposure affect herbivore-induced plant volatile emissions differently in wild and cultivated plants?

Concentrations of tropospheric ozone have more than doubled in the Northern Hemisphere since pre-industrial times. Plant responses to single abiotic or biotic stresses, such as ozone exposure and herbivore-feeding, have received substantial attention, especially for cultivated plants. Modern cultivated plants have been subjected to selective breeding that has altered plant chemical defences. To understand how ozone might affect plant responses to herbivore-feeding in wild and cultivated plants, we studied the volatile emissions of brassicaceous plants after exposure to ambient (~ 15 ppb) or elevated ozone (80 ppb), with and without Plutella xylostella larvae-feeding. Results indicated that most of the wild and cultivated plants increased volatile emissions in response to herbivore-feeding. Ozone alone had a weaker and less consistent effect on volatile emissions, but appeared to have a greater effect on wild plants than cultivated plants. This study highlights that closely related species of the Brassicaceae have variable responses to ozone and herbivore-feeding stresses and indicates that the effect of ozone may be stronger in wild than cultivated plants. Further studies should investigate the mechanisms by which elevated ozone modulates plant volatile emissions in conjunction with biotic stressors.