Ozone: a tool for probing programmed cell death in plants

Activation of plant immunity by exposure to dinitrogen pentoxide gas generated from air using plasma technology

Reactive nitrogen species (RNS) play an important role in plant immunity as signaling factors. We previously developed a plasma technology to partially convert air molecules into dinitrogen pentoxide (N₂O₅), an RNS whose physiological action is poorly understood. To reveal the function of N₂O₅ gas in plant immunity, Arabidopsis thaliana was exposed to plasma-generated N₂O₅ gas once (20 s) per day for 3 days, and inoculated with Botrytis cinerea, Pseudomonas syringae pv. tomato DC3000 (Pst), or cucumber mosaic virus strain yellow (CMV(Y)) at 24 h after the final N₂O₅ gas exposure. Lesion size with B. cinerea infection was significantly (P < 0.05) reduced by exposure to N₂O₅ gas. Propagation of CMV(Y) was suppressed in plants exposed to N₂O₅ gas compared with plants exposed to the air control. However, proliferation of Pst in the N₂O₅-gas-exposed plants was almost the same as in the air control plants. These results suggested that N₂O₅ gas exposure could control plant disease depending on the type of pathogen. Furthermore, changes in gene expression at 24 h after the final N₂O₅ gas exposure were analyzed by RNA-Seq. Based on the gene ontology analysis, jasmonic acid and ethylene signaling pathways were activated by exposure of Arabidopsis plants to N₂O₅ gas. A time course experiment with qRT-PCR revealed that the mRNA expression of the transcription factor genes, WRKY25, WRKY26, WRKY33, and genes for tryptophan metabolic enzymes, CYP71A12, CYP71A13, PEN2, and PAD3, was transiently induced by exposure to N₂O₅ gas once for 20 s peaking at 1–3 h post-exposure. However, the expression of PDF1.2 was enhanced beginning from 6 h after exposure and its high expression was maintained until 24–48 h later. Thus, enhanced tryptophan metabolism leading to the synthesis of antimicrobial substances such as camalexin and antimicrobial peptides might have contributed to the N₂O₅-gas-induced disease resistance.

Antioxidants and Phytohormones Act in Coordination to Regulate Sage Response to Long Term Ozone Exposure

Antioxidants and phytohormones are hallmarks of abiotic stress responses in plants. Although it is known that they can offer cell protection or accelerate programmed cell death (PCD) depending on the level of stress, the involvement of these metabolites in stress acclimation is still not fully elucidated. Here, we showed the role of antioxidants and phytohormones in Salvia officinalis tolerance to long-term ozone (O₃) exposure (120 ppb for 36 days, 5 h day^-1). Salicylic acid (SA) content was increased under O₃ throughout the whole experiment (+150%, as average compared with control), being required to maintain the cellular redox state and potentiate defense responses. This accumulation was induced before the production of ethylene (ET), suggesting that ET was controlled by SA during O₃ exposure to modulate the magnitude of chlorosis formation and the cell redox balance (by regulating ascorbate and glutathione levels). The synthesis and/or regeneration of these antioxidants did not protect membranes from lipid peroxidation, as demonstrated by the accumulation of malondialdehyde (+23% as average). However, these processes of lipid oxidation did not include the synthesis of the membrane breakdown products, as confirmed by the unchanged values of jasmonic acid, thus indicating that this compound was not involved in the regulation of PCD strategies.

Mechanisms of ozone responses in sensitive and tolerant mungbean cultivars

Ozone (O 3) is one of the major air pollutants, with negative impacts on human health, vegetation and agricultural production. It affects plants by reducing green leaf area and leading to necrosis, lesions and chlorosis, resulting in yield loss. Four mungbean cultivars were used to study O 3 sensitivity under elevated O 3 concentrations in the range of 70-100 ppb in an O 3 open-top chamber during the growing season. Based on O 3 response mechanisms, we classified mungbean cultivars into two groups: (1) O 3 -sensitive cultivars (Chainat 3 and 4) and (2) O 3 -tolerant cultivars (Chainat 84-1-1 and Kampangsan 2). The most O 3 -sensitive cultivars (Chainat 4) had the highest visible injury symptoms and the lowest in plant biomass. This evidence was due to Chainat 4 had lower ascorbic acid, indole acetic biosynthesis protein, defence related protein related to antioxidant systems, attribute to higher H 2 O 2 accumulation and an increase in salicylic acid contents. In contrast to the most O 3 -tolerant cultivars (Chainat 84-1-1) which had higher ascorbic acid levels, an upregulation of defence related protein, especially ascorbic acid biosynthesis and regenerate, indole acetic acid and jasmonic acid biosynthesis protein resulting in balanced H 2 O 2 levels, lower salicylic acid accumulation and little visible injury under elevated O 3 concentrations. Therefore, we conclude that the increased abundance of indole acetic acid, antioxidant related proteins facilitating stomata physiology in O 3 -tolerant under O 3 stress. This is the first report of the responses of mungbean cultivars in Thailand to elevated O 3 concentrations, facilitating the selection of suitable cultivars and the biomonitoring of O 3 levels.

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).

Signalling molecules responsive to ozone-induced oxidative stress in Salvia officinalis

Tropospheric ozone (O₃) is the most important gaseous pollutant and induces a mass of negative impacts on vegetation at functional and genic levels. The aim of the present study was to investigate the role of reactive oxygen species and signalling molecules in sage plants exposed to O₃ (200 ppb, 5 h). Ozone exposure induced only a transient oxidative burst, as confirmed by the rapid peak of anion superoxide during the first hours of exposure (+16% compared to controls). The spontaneous reaction of O₃ with membrane fatty acids stimulates peroxidative processes, as demonstrated by the rise of thiobarbituric acid reactive substances concentration starting after 1 h of exposure (+25%). The formation of lipid-based signalling molecules (e.g. jasmonic acid) may be regarded as a sort of O₃-perception. The concomitant accumulation of salicylic acid suggests that sage responds early to O₃ by inducing cellular antioxidants mechanisms in order to minimize O₃-oxidative burst. The transient increase of abscisic acid (+25% at the end of the treatment) twinned with the maximal ethylene emission (about two-fold higher than controls) could be interpreted as a first attempt by plants to regulate the signalling responses induced by O₃. In order to investigate the involvement of transcription factors in managing oxidative protection, BLASTX analysis against the Salvia miltiorrhiza sequence genome was carried out using Arabidopsis thaliana WRKY sequences as queries. Six gene sequences were identified for sage WRKYs and their relative gene expression analyses were characterized. WRKY4, WRKY5, WRKY11 and WRKY46 were up-regulated by O₃ at 2 and 5 h of exposure and they showed similarity with AtWRKY48, AtWRKY22 and AtWRKY53 in A. thaliana. These results suggest that WRKYs could play a pivotal role in the signalling mechanisms during the responses of plants to O₃.

Can nutrient fertilization mitigate the effects of ozone exposure on an ozone-sensitive poplar clone?

We tested the independent and interactive effects of nitrogen (N; 0 and 80 kg ha^-1), phosphorus (P; 0, 40 and 80 kg ha^-1), and ozone (O₃) application/exposure [ambient concentration (AA), 1.5 × AA and 2.0 × AA] for five consecutive months on biochemical traits of the O₃-sensitive Oxford poplar clone. Plants exposed to O₃ showed visible injury and an alteration of membrane integrity, as confirmed by the malondialdehyde by-product accumulation (+3 and +17% under 1.5 × AA and 2.0 × AA conditions, in comparison to AA). This was probably due to O₃-induced oxidative damage, as documented by the production of superoxide anion radical (O₂^-, +27 and +63%, respectively). Ozone per se, independently from the concentrations, induced multiple signals (e.g., alteration of cellular redox state, increase of abscisic acid/indole-3-acetic acid ratio and reduction of proline content) that might be part of premature leaf senescence processes. By contrast, nutrient fertilization (both N and P) reduced reactive oxygen species accumulation (as confirmed by the decreased O₂^- and hydrogen peroxide content), resulting in enhanced membrane stability. This was probably due to the simultaneous involvement of antioxidant compounds (e.g., carotenoids, ascorbate and glutathione) and osmoprotectants (e.g., proline) that regulate the detoxification processes of coping with oxidative stress by reducing the O₃ sensitivity of Oxford clone. These mitigation effects were effective only under AA and 1.5 × AA conditions. Nitrogen and P supply activated a free radical scavenging system that was not able to delay leaf senescence and mitigate the adverse effects of a general peroxidation due to the highest O₃ concentrations.

When "thirsty" means "less able to activate the signalling wave trigged by a pulse of ozone": A case of study in two Mediterranean deciduous oak species with different drought sensitivity

There is a lack of knowledge about the possibility that plants facing abiotic stressors, such as drought, have an altered perception of a pulse of O₃ and incur in alterations of their signalling network. This poses some concerns as to whether defensive strategy to cope episodic O₃ peaks in healthy plants may fail under stress. In this study, a set of saplings of two Mediterranean deciduous species, Quercus cerris and Q. pubescens, was subjected to water withholding (20% of daily evapotranspiration for 15 days) while another set was kept well-watered. Saplings were then subjected to a pulse of O₃ (200 nl l^-1 for 5 h) or maintained in filtered air. Q. pubescens had a more severe decline of photosynthesis and leaf PDΨ_w (about -65% and 5-fold lower than in well-watered ones) and events of cell death were observed under drought when compared to Q. cerris, which is supportive for a higher sensitivity to drought exhibited by this species. When O₃ was applied after drought, patterns of signalling compounds were altered in both species. Only in Q. pubescens, the typical O₃-induced accumulation of apoplastic reactive oxygen species, which is the first necessary step for the activation of signalling cascade, was completely lost. In Q. cerris the most frequent changes encompassed the weakening of peaks of key signalling molecules (ethylene and salicylic acid), whereas in Q. pubescens both delayed (salicylic and jasmonic acid) or weakened (ethylene and salicylic acid) peaks were observed. This is translated to a higher ability of Q. cerris to maintain a prompt activation of defensive reaction to counteract oxidative damage due to the pollutant. Our results reveal the complexity of the signalling network in plants facing multiple stresses and highlight the need to further investigate possible alteration of defensive mechanism of tree species to predict their behavior.

Foliar application of ascorbic and citric acids enhanced 'Red Spur' apple fruit quality, bioactive compounds and antioxidant activity

This study aimed to assess the effect of the foliar application of ascorbic acid (AA) and citric acid (CA) on total antioxidant activity (TAA), total phenolics, total flavonoids, total anthocyanin content, antioxidant enzymes, phenylalanine ammonialyase (PAL), and polyphenol oxidase (PPO) activities in apple 'Red Spur'. The experiment was conducted on 12-years-old trees 'Red Spur' grafted on MM₁₀₆ rootstock. The trees were sprayed with AA (0, 200 and 400 mg L^-1) and/or CA (0, 200 and 400 mg L^-1) at three different times during summer. Foliar application with AA and CA significantly (p < 0.01) enhanced all measured quality attributes and decreased the activity of PPO. Fruit from trees treated with AA at 400 mg L^-1 and CA at 200 mg L^-1 showed the highest TAA and catalase (CAT) enzyme activity. Total phenolics increased in fruits when trees were sprayed with AA and CA. Contrasting, AA treatment, CA had no significant effect on guaiacol peroxidase (G-POD). A significant decrease in PPO activity was detected in fruits when treated with both AA and CA. Both treatments significantly decreased the activity of PAL at 400 mg L^-1. Considering the results, foliar application of AA and CA, either alone or in combination improved the quality and nutraceutical properties of 'Red Spur' apple.

Transcriptomic analysis of Pak Choi under acute ozone exposure revealed regulatory mechanism against ozone stress

BACKGROUND

Ground-level ozone (O3) is one of the major air pollutants, which cause oxidative injury to plants. The physiological and biochemical mechanisms underlying the responses of plants to O3 stress have been well investigated. However, there are limited reports about the molecular basis of plant responses to O3. In this study, a comparative transcriptomic analysis of Pak Choi (Brassica campestris ssp. chinensis) exposed to different O3 concentrations was conducted for the first time.

RESULTS

Seedlings of Pak Choi with five leaves were exposed to non-filtered air (NF, 31 ppb) or elevated O3 (E-O3, 252 ppb) for 2 days (8 h per day, from 9:00-17:00). Compared with plants in the NF, a total of 675 differentially expressed genes (DEGs) were identified in plants under E-O3, including 219 DEGs with decreased expressions and 456 DEGs with increased expressions. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that O3 stress invoked multiple cellular defense pathways to mitigate the impaired cellular integrity and metabolism, including 'glutathione metabolism', 'phenylpropanoid biosynthesis', 'sulfur metabolism', 'glucosinolate biosynthesis', 'cutin, suberine and wax biosynthesis' and others. Transcription factors potentially involved in this cellular regulation were also found, such as AP2-ERF, WRKY, JAZ, MYB etc. Based on the RNA-Seq data and previous studies, a working model was proposed integrating O3 caused reactive oxygen burst, oxidation-reduction regulation, jasmonic acid and downstream functional genes for the regulation of cellular homeostasis after acute O3 stress.

CONCLUSION

The present results provide a valuable insight into the molecular responses of Pak Choi to acute O3 stress and the specific DEGs revealed in this study could be used for further functional identification of key allelic genes determining the O3 sensitivity of Pak Choi.

Ozone triggers different defence mechanisms against powdery mildew (Blumeria graminis DC. Speer f. sp. tritici) in susceptible and resistant wheat genotypes

Ozone has been proposed as a convenient elicitor against pathogens since it is known to generate different reactive oxygen species (ROS) and induce nonspecific defence by altering gene expression. The mode of action and its interaction with other defence pathways are yet to be elucidated. Besides its negative effects on plants, ozone can be used for triggering defence against environmental stresses, including pathogens, when used at appropriate concentrations. Powdery mildew, caused by the obligate biotrophic fungus Blumera graminis f.sp. tritici (Bgt), is an important plant disease that reduces crop yield and quality. We hypothesised that ozone treatment may elicit defence against Bgt by inducing ROS signalling or other routes such as the salicylic acid (SA) or jasmonic acid (JA) pathways. We conducted experiments with Bgt-susceptible (cv. Pamukova) and resistant (cv. Tahirova) wheat (Triticum aestivum L,) cultivars and treated them with different ozone concentrations before Bgt inoculation. Stress response and defence-related features such as antioxidative enzyme activity; lipid peroxidation; H2O2 and Ca+2 levels; PR1, LOX, PAL and RBOH gene expression; and disease severity were assayed. Clear discrepancies between the responses of susceptible and resistant cultivars were found, suggesting that different defence routes were activated. Here, we showed that ozone treatment was effective for diminishing Bgt invasion in the susceptible cultivar in the short term, which was probably related to defence induced via the SA pathway. Moreover, the resistant cultivar Tahirova exhibited a different mode of action against the pathogen that was triggered by ozone treatment, plausibly related to the JA pathway.

Differential effects of ozone on photosynthesis of winter wheat among cultivars depend on antioxidative enzymes rather than stomatal conductance

Five modern cultivars of winter wheat (Triticum aestivum L.): Yangmai16 (Y16), Yangmai 15 (Y15), Yangfumai 2 (Y2), Yannong 19 (Y19) and Jiaxing 002 (J2) were investigated to determine the impacts of elevated ozone concentration (E-O₃) on photosynthesis-related parameters and the antioxidant system under fully open-air field conditions in China. The plants were exposed to E-O₃ at 1.5 times the ambient ozone concentration (A-O₃) from the initiation of tillering to final harvest. Pigments, gas exchange rates, chlorophyll a fluorescence, antioxidants contents, antioxidative enzyme activity and lipid oxidation were measured in three replicated plots throughout flag leaf development. Results showed that significant O₃ effects on most variables were only found during the mid-grain filling stage. Across five cultivars, E-O₃ significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to non-stomatal factors, e.g. lower maximum carboxylation capacity and electron transport rates. There were strong interactions between O₃ and cultivar in photosynthetic pigments, light-saturated photosynthesis rate and chlorophyll a fluorescence with O₃-sensitive (Y19, Y2 and Y15) and O₃-tolerant (J2, Y16) cultivars being clearly differentiated in their responses to E-O₃. E-O₃ significantly influenced the antioxidative enzymes but not antioxidant contents. Significant interactions between O₃ and cultivar were found in antioxidative enzymes, such as SOD and CAT, but not in stomatal conductance (g_s). Therefore, it can be concluded that antioxidative enzymes rather than g_s or antioxidants are responsible for the differential responses to E-O₃ among cultivars. These findings provide important information for the development of accurate modeling O₃ effects on crops, especially with respect to the developmental stage when O₃ damage to photosynthesis becomes manifest.

A comparative analysis of transcriptomic, biochemical, and physiological responses to elevated ozone identifies species-specific mechanisms of resilience in legume crops

Current concentrations of tropospheric ozone ([O3]) pollution negatively impact plant metabolism, which can result in decreased crop yields. Interspecific variation in the physiological response of plants to elevated [O3] exists; however, the underlying cellular responses explaining species-specific differences are largely unknown. Here, a physiological screen has been performed on multiple varieties of legume species. Three varieties of garden pea (Pisum sativum L.) were resilient to elevated [O3]. Garden pea showed no change in photosynthetic capacity or leaf longevity when exposed to elevated [O3], in contrast to varieties of soybean (Glycine max (L.) Merr.) and common bean (Phaseolus vulgaris L.). Global transcriptomic and targeted biochemical analyses were then done to examine the mechanistic differences in legume responses to elevated [O3]. In all three species, there was an O3-mediated reduction in specific leaf weight and total non-structural carbohydrate content, as well as increased abundance of respiration-related transcripts. Differences specific to garden pea included a pronounced increase in the abundance of GLUTATHIONE REDUCTASE transcript, as well as greater contents of foliar glutathione, apoplastic ascorbate, and sucrose in elevated [O3]. These results suggest that garden pea may have had greater capacity for detoxification, which prevented net losses in CO2 fixation in an elevated [O3] environment.

Plant signalling in acute ozone exposure

Exposure of plants to high ozone concentrations causes lesion formation in sensitive plants. Plant responses to ozone involve fast and massive changes in protein activities, gene expression and metabolism even before any tissue damage can be detected. Degradation of ozone and subsequent accumulation of reactive oxygen species (ROS) in the extracellular space activates several signalling cascades, which are integrated inside the cell into a fine-balanced network of ROS signalling. Reversible protein phosphorylation and degradation plays an important role in the regulation of signalling mechanisms in a complex crosstalk with plant hormones and calcium, an essential second messenger. In this review, we discuss the recent advances in understanding the molecular mechanisms of ozone uptake, perception and signalling pathways activated during the early steps of ozone response, and discuss the use of ozone as a tool to study the function of apoplastic ROS in signalling.

Genetic dissection of ozone tolerance in rice (Oryza sativa L.) by a genome-wide association study

Tropospheric ozone causes various negative effects on plants and affects the yield and quality of agricultural crops. Here, we report a genome-wide association study (GWAS) in rice (Oryza sativa L.) to determine candidate loci associated with ozone tolerance. A diversity panel consisting of 328 accessions representing all subgroups of O. sativa was exposed to ozone stress at 60 nl l(-1) for 7h every day throughout the growth season, or to control conditions. Averaged over all genotypes, ozone significantly affected biomass-related traits (plant height -1.0%, shoot dry weight -15.9%, tiller number -8.3%, grain weight -9.3%, total panicle weight -19.7%, single panicle weight -5.5%) and biochemical/physiological traits (symptom formation, SPAD value -4.4%, foliar lignin content +3.4%). A wide range of genotypic variance in response to ozone stress were observed in all phenotypes. Association mapping based on more than 30 000 single-nucleotide polymorphism (SNP) markers yielded 16 significant markers throughout the genome by applying a significance threshold of P<0.0001. Furthermore, by determining linkage disequilibrium blocks associated with significant SNPs, we gained a total of 195 candidate genes for these traits. The following sequence analysis revealed a number of novel polymorphisms in two candidate genes for the formation of visible leaf symptoms, a RING and an EREBP gene, both of which are involved in cell death and stress defence reactions. This study demonstrated substantial natural variation of responses to ozone in rice and the possibility of using GWAS in elucidating the genetic factors underlying ozone tolerance.

Climatic factors influence leaf structure and thereby affect the ozone sensitivity of Ipomoea nil 'Scarlet O'Hara'

Phenotypic plasticity of the leaves can interfere with the plant sensitivity to ozone (O3) toxic effect. This study aimed to assess whether the leaf structure of Ipomoea nil changes due to climatic variations and whether these changes affect the species' sensitivity. Field exposures, in different seasons (winter and spring) were made. The leaves that developed during the winter were thinner, with a lower proportion of photosynthetic tissues, higher proportion of intercellular spaces and lower density and stomatal index compared to those developed during the spring. The temperature and relative humidity positively influenced the leaf thickness and stomatal index. The visible injuries during winter were positively correlated with the palisade parenchyma thickness and negatively correlated with the percentage of spongy parenchyma; during the spring, the symptoms were positively correlated with the stomatal density. In conclusion, the leaf structure of I. nil varied among the seasons, interfering in its sensitivity to O3.

Towards Understanding Extracellular ROS Sensory and Signaling Systems in Plants

Reactive Oxygen Species (ROS) are ubiquitous metabolites in all aerobic organisms. Traditionally ROS have been considered as harmful, accidental byproducts of cellular functions involving electron transport chains or electron transfer. However, it is now recognized that controlled production of ROS has significant signaling functions, for example, in pathogen defense, in the regulation of stomatal closure, or in cell-to-cell signaling. ROS formation in subcellular compartments is critical to act as “alarm” signal in the response to stress, and the concept of ROS as primarily signaling substances has emerged. The involvement of ROS in several developmental and inducible processes implies that there must be coordinated function of signaling network(s) that govern ROS responses and subsequent processes. The air pollutant ozone can be used as a useful tool to elucidate the function of apoplastic ROS: O3 degrades in cell wall into various ROS which are interpreted as ROS with signaling function inducing downstream responses. We have used ozone as a tool in mutant screens and transcript profiling-reverse genetics to identify genes involved in processes related to the signaling function of ROS. We review here our recent findings in the elucidation of apoplastic ROS sensing, signaling, and interaction with various symplastic components.

Signaling molecules and cell death in Melissa officinalis plants exposed to ozone

The study focuses on the interaction between reactive oxygen species and hormones that regulate the programmed cell death in plants of Melissa officinalis exposed to ozone. Interaction between hormone and redox signaling pathways has been investigated in ozone-stressed (200 ppb, 5 h) lemon balm to verify if the response resembles the biotic defense reactions. In comparison to controls, plants exhibited foliar injury and the cell death was induced by (1) biphasic production of hydrogen peroxide and superoxide radical; (2) hormonal regulation of ozone-induced lesion formation with a significant production of ethylene, salicylic, jasmonic and abscisic acid; (3) ozone degradation to reactive oxygen species and their detoxification by some enzymatic (such as superoxide dismutase) and non-enzymatic antioxidant systems (such as ascorbic acid, glutathione and carotenoids), that worked in cooperation without providing a defense against free radicals (such as confirmed by the modification of the antioxidant properties of leaf tissue). This integrated view showed that reactive oxygen species interact with hormonal signaling pathway regulating cell death and the sensitivity of lemon balm to ozone.

Impacts of acute ozone stress on superoxide dismutase (SOD) expression and reactive oxygen species (ROS) formation in rice leaves

Tropospheric ozone is an air pollutant harmful to plants and animals. Its rapid increase at the ground surface has raised serious concern over damage to the quality and yield of agricultural crops. Reactive oxygen species (ROS) are formed in plant cells when that are exposed to a high concentration of ozone, and the ROS are thought to alter gene expression and result in cellular death. Clarification of the ROS formation will provide us with a better understanding of the physiological responses to and signaling pathways of ozone stress in plants. In this study, we investigated the mechanisms of the ROS formation in rice (Oryza sativa L.), the premier crop in Asia. To determine ROS distribution in rice leaves under acute ozone stress, we analyzed superoxide dismutase (SOD) expression, lipid peroxidation, NADPH oxidase activity, and in vivo H2O2 formation. Interestingly, chloroplastic, peroxisomal and mitochondrial SODs down-regulated their expression levels under ozone stress, whereas cytosolic SODs maintained their expression level. Higher lipid peroxidation occurred after the end of ozone exposure, which suggests lipid peroxidation may not be due to ozone directly, but rather to metabolic changes caused by the ozone exposure. Activity of NADPH oxidase did not show significant change. The in vivo analysis indicated accelerated formation of H2O2 about 24 h after the onset of exposure, which suggests that cellular death occurred around this time.

Increasing vitamin C content in plant foods to improve their nutritional value-successes and challenges

Vitamin C serves as a cofactor in the synthesis of collagen needed to support cardiovascular function, maintenance of cartilage, bones, and teeth, as well as being required in wound healing. Although vitamin C is essential, humans are one of the few mammalian species unable to synthesize the vitamin and must obtain it through dietary sources. Only low levels of the vitamin are required to prevent scurvy but subclinical vitamin C deficiency can cause less obvious symptoms such as cardiovascular impairment. Up to a third of the adult population in the U.S. obtains less than the recommended amount of vitamin C from dietary sources of which plant-based foods constitute the major source. Consequently, strategies to increase vitamin C content in plants have been developed over the last decade and include increasing its synthesis as well as its recycling, i.e., the reduction of the oxidized form of ascorbic acid that is produced in reactions back into its reduced form. Increasing vitamin C levels in plants, however, is not without consequences. This review provides an overview of the approaches used to increase vitamin C content in plants and the successes achieved. Also discussed are some of the potential limitations of increasing vitamin C and how these may be overcome.

Post-transcriptional regulation of GORK channels by superoxide anion contributes to increases in outward-rectifying K⁺ currents

· Ion fluxes are ubiquitous processes in the plant and animal kingdoms, controlled by fine-tuned regulations of ion channel activity. Yet the mechanism that cells employ to achieve the modification of ion homeostasis at the molecular level still remains unclear. This is especially true when it comes to the mechanisms that lead to cell death. · In this study, Arabidopsis thaliana cells were exposed to ozone (O₃). Ion flux variations were analyzed by electrophysiological measurements and their transcriptional regulation by RT-PCR. Reactive oxygen species (ROS) generation was quantified by luminescence techniques and caspase-like activities were investigated by laser confocal microscopy. · We highlighted the delayed activation of K(+) outward-rectifying currents after an O₃ -induced oxidative stress leading to programmed cell death (PCD). Caspase-like activities are detected under O₃ exposure and could be decreased by K(+) channel blocker. Molecular experiments revealed that the sustained activation of K(+) outward current could be the result of an unexpected O₂ ·⁻ post-transcriptional regulation of the guard cell outward-rectifying K(+) (GORK) channels. · This consists of a likely new mode of regulating the processing of the GORK mRNA, in a ROS-dependent manner, to allow sustained K(+) effluxes during PCD. These data provide new mechanistic insights into K(+) channel regulation during an oxidative stress response.

Physiological, biochemical and molecular responses to a combination of drought and ozone in Medicago truncatula

Drought and tropospheric ozone are escalating climate change problems that can co-occur. In this study, we observed Medicago truncatula cultivar Jemalong that is sensitive to ozone and drought stress when applied singly, showed tolerance when subjected to a combined application of these stresses. Lowered stomatal conductance may be a vital tolerance mechanism to overcome combined ozone and drought. Sustained increases in both reduced ascorbate and glutathione in response to combined stress may play a role in lowering reactive oxygen species and nitric oxide toxicity. Transcriptome analysis indicated that genes associated with glucan metabolism, responses to temperature and light signalling may play a role in dampening ozone responses due to drought-induced stomatal closure during combined occurrence of these two stresses. Gene ontologies for jasmonic acid signalling and innate immunity were enriched among the 300 differentially expressed genes unique to combined stress. Differential expression of transcription factors associated with redox, defence signalling, jasmonate responses and chromatin modifications may be important for evoking novel gene networks during combined occurrence of drought and ozone. The alterations in redox milieu and distinct transcriptome changes in response to combined stress could aid in tweaking the metabolome and proteome to annul the detrimental effects of ozone and drought in Jemalong.

A role for oxalic acid generation in ozone-induced signallization in Arabidopis cells

Ozone (O(3) ) is an air pollutant with an impact increasingly important in our industrialized world. It affects human health and productivity in various crops. We provide the evidences that treatment of Arabidopsis thaliana with O(3) results in ascorbate-derived oxalic acid production. Using cultured cells of A. thaliana as a model, here we further showed that oxalic acid induces activation of anion channels that trigger depolarization of the cell, increase in cytosolic Ca(2+) concentration, generation of reactive oxygen species and cell death. We confirmed that O(3) reacts with ascorbate in the culture, thus resulting in production of oxalic acid and this could be part of the O(3) -induced signalling pathways that trigger programmed cell death.

L-ascorbic Acid: a multifunctional molecule supporting plant growth and development

L-Ascorbic acid (vitamin C) is as essential to plants as it is to animals. Ascorbic acid functions as a major redox buffer and as a cofactor for enzymes involved in regulating photosynthesis, hormone biosynthesis, and regenerating other antioxidants. Ascorbic acid regulates cell division and growth and is involved in signal transduction. In contrast to the single pathway responsible for ascorbic acid biosynthesis in animals, plants use multiple pathways to synthesize ascorbic acid, perhaps reflecting the importance of this molecule to plant health. Given the importance of ascorbic acid to human nutrition, several technologies have been developed to increase the ascorbic acid content of plants through the manipulation of biosynthetic or recycling pathways. This paper provides an overview of these approaches as well as the consequences that changes in ascorbic acid content have on plant growth and function. Discussed is the capacity of plants to tolerate changes in ascorbic acid content. The many functions that ascorbic acid serves in plants, however, will require highly targeted approaches to improve their nutritional quality without compromising their health.

Volatiles emission patterns in poplar clones varying in response to ozone

The volatiles emitted from young and old leaves of two poplar clones (Populus deltoides x maximowiczii, Eridano, and P. x euramericana, I-214) were sampled after exposure to ozone (80 ppb, 5 h d(-1), for 10 consecutive days) by solid phase microextraction and characterized by GC-MS. Only mature leaves of the ozone-sensitive Eridano clone developed necrosis in response to ozone exposure, and their membrane integrity was significantly affected by ozone (+86 and +18 % of levels of thiobarbituric acid reactive substances in mature and young leaves). The headspace of the poplar clones studied here contained mono- and sesquiterpenes, both hydrocarbons and oxygenated ones in Eridano, and only hydrocarbons in the clone I-214. Furthermore, some non-terpenes, such as C(9)-C(15) straight-chain aldehydes and C(12)-C(16) saturated and unsaturated aliphatic hydrocarbons, were detected. Other common non-terpene volatiles were oxygenated aliphatic compounds, mainly C(6)-alcohols and their acetates. Ozone exposure induced a strong change in volatile profiles, depending on clones and leaf age. Regardless of leaf age, in clone I-214, quantities of oxygenated monoterpenes tended to increase after ozone exposure, however, "O(3) x leaf age" was not significant. In clone Eridano, increases were observed in emissions of hydrocarbons and oxygenated sesquiterpenes in response to ozone treatment. (Z)-3-Hexen-1-ol and (Z)-3-hexenol acetate were present in traces in the headspace of untreated Eridano mature leaves, but quantities slightly increased after ozone treatment. Quantities of non-terpene oxygenated compounds dropped in the headspace of young leaves of both clones (-24 and -44 % in Eridano and I-214) and also in mature ones of I-214 (-50 %) after ozone exposure. Similarly, quantities of non-terpene hydrocarbons in the emissions from mature leaves of both clones (-58 and -49 %, respectively) decreased, while these compounds increased in young leaves of Eridano (+83 %). We suggest that the resistance of the poplar clone I-214 to O(3) is achieved by: i) monoterpenes constitutively present in young leaves and ii) increase of monoterpene content induced by O(3) in mature leaves.

Root and shoot gas exchange respond additively to moderate ozone and methyl jasmonate without induction of ethylene: ethylene is induced at higher O3 concentrations

The available literature is conflicting on the potential protection of plants against ozone (O(3)) injury by exogenous jasmonates, including methyl jasmonate (MeJA). Protective antagonistic interactions of O(3) and MeJA have been observed in some systems and purely additive effects in others. Here it is shown that chronic exposure to low to moderate O(3) concentrations (4-114 ppb; 12 h mean) and to MeJA induced additive reductions in carbon assimilation (A (n)) and root respiration (R (r)), and in calculated whole plant carbon balance. Neither this chronic O(3) regime nor MeJA induced emission of ethylene (ET) from the youngest fully expanded leaves. ET emission was induced by acute 3 h pulse exposure to much higher O(3) concentrations (685 ppb). ET emission was further enhanced in plants treated with MeJA. Responses of growth, allocation, photosynthesis, and respiration to moderate O(3) concentrations and to MeJA appear to be independent and additive, and not associated with emission of ET. These results suggest that responses of Pima cotton to environmentally relevant O(3) are not mediated by signalling pathways associated with ET and MeJA, though these pathways are inducible in this species and exhibit a synergistic O(3)×MeJA interaction at very high O(3) concentrations.

NaJAZh regulates a subset of defense responses against herbivores and spontaneous leaf necrosis in Nicotiana attenuata plants

The JASMONATE ZIM DOMAIN (JAZ) proteins function as negative regulators of jasmonic acid signaling in plants. We cloned 12 JAZ genes from native tobacco (Nicotiana attenuata), including nine novel JAZs in tobacco, and examined their expression in plants that had leaves elicited by wounding or simulated herbivory. Most JAZ genes showed strong expression in the elicited leaves, but NaJAZg was mainly expressed in roots. Another novel herbivory-elicited gene, NaJAZh, was analyzed in detail. RNA interference suppression of this gene in inverted-repeat (ir)JAZh plants deregulated a specific branch of jasmonic acid-dependent direct and indirect defenses: irJAZh plants showed greater trypsin protease inhibitor activity, 17-hydroxygeranyllinalool diterpene glycosides accumulation, and emission of volatile organic compounds from leaves. Silencing of NaJAZh also revealed a novel cross talk in JAZ-regulated secondary metabolism, as irJAZh plants had significantly reduced nicotine levels. In addition, irJAZh spontaneously developed leaf necrosis during the transition to flowering. Because the lesions closely correlated with the elevated expression of programmed cell death genes and the accumulations of salicylic acid and hydrogen peroxide in the leaves, we propose a novel role of the NaJAZh protein as a repressor of necrosis and/or programmed cell death during plant development.

Ozone and aging up-regulate type II metacaspase gene expression and global metacaspase activity in the leaves of field-grown maize (Zea mays L.) plants

Maize plants (Zea mays L. cv. NK Perform) were exposed to O(3)-enriched air, using a new field fumigation system. Transcriptional changes for three type II-metacaspase genes were studied in the leaves (ranks 10 and 12), using quantitative real-time PCR. Global metacaspase activity was measured using metacaspase-specific synthetic tripeptide Boc-GRR-AMC. Aging had little effect on mRNA accumulation whereas four to six-fold increases were observed for the most O(3)-responsive type II metacaspase genes, in the older leaves 10. Global metacaspase activity increased by 257% and 333% in leaves 12 and 10, respectively, in response to the highest cumulated concentration. In non-fumigated plants, metacaspase activity progressively increased over the course of the experiment and always was higher in the older leaves 10. Together, these results suggest that metacaspase-mediated proteolysis is a crucial step in leaf responses to both O(3) and age-mediated senescence.

Plant volatile organic compounds (VOCs) in ozone (O3) polluted atmospheres: the ecological effects

Tropospheric ozone (O3) is an important secondary air pollutant formed as a result of photochemical reactions between primary pollutants, such as nitrogen oxides (NOx), and volatile organic compounds (VOCs). O3 concentrations in the lower atmosphere (troposphere) are predicted to continue increasing as a result of anthropogenic activity, which will impact strongly on wild and cultivated plants. O3 affects photosynthesis and induces the development of visible foliar injuries, which are the result of genetically controlled programmed cell death. It also activates many plant defense responses, including the emission of phytogenic VOCs. Plant emitted VOCs play a role in many eco-physiological functions. Besides protecting the plant from abiotic stresses (high temperatures and oxidative stress) and biotic stressors (competing plants, micro- and macroorganisms), they drive multitrophic interactions between plants, herbivores and their natural enemies e.g., predators and parasitoids as well as interactions between plants (plant-to-plant communication). In addition, VOCs have an important role in atmospheric chemistry. They are O3 precursors, but at the same time are readily oxidized by O3, thus resulting in a series of new compounds that include secondary organic aerosols (SOAs). Here, we review the effects of O3 on plants and their VOC emissions. We also review the state of current knowledge on the effects of ozone on ecological interactions based on VOC signaling, and propose further research directions.

Production of reactive oxygen species and induction of signaling pathways for the ACO gene expressions in tomato plants triggered by the volatile organic compound ether

Diethyl ether (ether), a volatile organic compound, is widely used as an industrial solvent and easily released to the environment. Acute exposure of tomato plants to high concentrations of ether caused young leaves to curl. Histochemical analyses revealed that superoxide anion (•O(2-) and hydrogen peroxide were formed sequentially by ether, and that (•O(2-) was the major ROS produced in response to ether exposure. We observed cell death by microscopic inspection of Evans blue-stained samples, following fumigation with ether for 6 h. The ethylene biosynthetic gene, 1-aminocyclopropane-1-carboxylic acid oxidase (ACO), was induced as early as 15-30 min after ether fumigation and could be activated at ether concentration as low as 1 μL/L. Induction of ACO gene expression occurred simultaneously with ROS accumulation and coincided with the occurrence of cell death. Simultaneous treatment of tomato plants with mechanical wounding and ether induced differential expression of the ACO gene family. Ether strongly induced ACO4 and moderately induced ACO1, whereas mechanical wounding strongly induced ACO1 and slightly induced ACO4. Induction of the ACO gene family by ether occurred via different signaling pathways. While the ACO1 gene was induced via protein phosphorylation, the ACO4 gene was induced through protein dephosphorylation. Induction of ACO1 and ACO4 might be through MPK1, MPK2, MPK3, and PP2Ac1. These results suggest that the cellular responses of tomato plants to ether are different from the plant responses to ozone, and that tomato plants respond to different air pollutants through different perceptions and downstream signaling pathways.

Investigations on N-rich protein (NRP) of Arabidopsis thaliana under different stress conditions

Stress response and adaptation are important physiological mechanisms in plants. As plants are not able to avoid stressful environments by moving away, as animals, they have developed diverse mechanisms to respond to stressful situations. One of the genes involved in these mechanisms is NRP (Asparagine-rich protein or N-rich protein). In this study, NRP expression, protein localization and nrp knockout plants were investigated for further understanding of NRP function. NaCl-induced salt stress, oxidative stress (ozone exposure) and mechanical perturbation (touch treatment) were used to induce abiotic stress. NRP expression was up-regulated in the early phase of stress response to all three elicitors. Stressed nrp knockout seedlings revealed a more pronounced growth inhibition compared to wildtype (salt and osmotic stress). Seedlings showed NRP-GFP expression in the apical meristem, leaf veins, central cylinder, root hair zone and root tip. Analyses of NRP-GFP localization in root cells and protoplasts revealed cytosolic distribution under non-stress conditions and translocation of NRP-GFP to mitochondria due to stress response. Summarizing, our findings point to a contribution of NRP in signal transduction of the initial phase of general stress response in Arabidopsis thaliana.

Tropospheric ozone and plants: absorption, responses, and consequences

Ozone is now considered to be the second most important gaseous pollutant in our environment. The phytotoxic potential of O₃ was first observed on grape foliage by B.L. Richards and coworkers in 1958 (Richards et al. 1958). To date, unsustainable resource utilization has turned this secondary pollutant into a major component of global climate change and a prime threat to agricultural production. The projected levels to which O₃ will increase are critically alarming and have become a major issue of concern for agriculturalists, biologists, environmentalists and others plants are soft targets for O₃. Ozone enters plants through stomata, where it disolves in the apoplastic fluid. O₃ has several potential effects on plants: direct reaction with cell membranes; conversion into ROS and H₂O₂ (which alters cellular function by causing cell death); induction of premature senescence; and induction of and up- or down-regulation of responsive components such as genes , proteins and metabolites. In this review we attempt to present an overview picture of plant O₃ interactions. We summarize the vast number of available reports on plant responses to O₃ at the morphological, physiological, cellular, biochemical levels, and address effects on crop yield, and on genes, proteins and metabolites. it is now clear that the machinery of photosynthesis, thereby decreasing the economic yield of most plants and inducing a common morphological symptom, called the "foliar injury". The "foliar injury" symptoms can be authentically utilized for biomonitoring of O₃ under natural conditions. Elevated O₃ stress has been convincingly demonstrated to trigger an antioxidative defense system in plants. The past several years have seen the development and application of high-throughput omics technologies (transcriptomics, proteomics, and metabolomics) that are capable of identifying and prolifiling the O₃-responsive components in model and nonmodel plants. Such studies have been carried out ans have generated an inventory of O₃-Responsive components--a great resource to the scientific community. Recently, it has been shown that certain organic chemicals ans elevated CO₂ levels are effective in ameliorating O₃-generated stress. Both targeted and highthroughput approaches have advanced our knowledge concerning what O₃-triggerred signaling and metabolic pathways exist in plants. Moreover, recently generated information, and several biomarkers for O₃, may, in the future, be exploited to better screen and develop O₃-tolerant plants.

Differential impact of chronic ozone exposure on expanding and fully expanded poplar leaves

Populus tremula L. × Populus alba L. (Populus ×c anescens (Aiton) Smith) - clone INRA 717-1-B4 saplings (50 cm apex to base and carrying 19 leaves on average) - were followed for 28 days. Half of the trees were grown in charcoal-filtered air while the other half were exposed to 120 ppb ozone for 11 h a day during the light period. The expanding leaf number 4 was tagged at the beginning of the experiment and finished expansion between 7 and 14 days. These leaves were harvested weekly for biochemical and proteome analyses using quantitative bidimensional electrophoresis (DiGE). Independent of the ozone treatment, all the analyses allowed a distinction between expanding and adult leaves. The results indicate that during the expansion phase (Days 0-7) the enzymatic machinery of the leaves is set up, and remains dynamically stable in the adult leaves (Days 14-28). Although ozone had no apparent effect on expanding leaves, the metabolic stability in fully expanded leaves observed in ozone-free plants was disturbed after 2 weeks of exposure and a stress-induced response became apparent.

Increased anion channel activity is an unavoidable event in ozone-induced programmed cell death

BACKGROUND

Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant.

PRINCIPAL FINDINGS

By exposing cells to a strong pulse of ozonized air, an acute cell death was observed in suspension cells of Arabidopsis thaliana used as a model. We demonstrated that O(3) treatment induced the activation of a plasma membrane anion channel that is an early prerequisite of O(3)-induced cell death in A. thaliana. Our data further suggest interplay of anion channel activation with well known plant responses to O(3), Ca(2+) influx and NADPH-oxidase generated reactive oxygen species (ROS) in mediating the oxidative cell death. This interplay might be fuelled by several mechanisms in addition to the direct ROS generation by O(3); namely, H(2)O(2) generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death.

SIGNIFICANCE

Collectively, our data indicate that anion efflux is an early key component of morphological and biochemical events leading to O(3)-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation.

Modulation of transcription factor and metabolic pathway genes in response to water-deficit stress in rice

Water-deficit stress is detrimental for rice growth, development, and yield. Transcriptome analysis of 1-week-old rice (Oryza sativa L. var. IR64) seedling under water-deficit stress condition using Affymetrix 57 K GeneChip® has revealed 1,563 and 1,746 genes to be up- and downregulated, respectively. In an effort to amalgamate data across laboratories, we identified 5,611 differentially expressing genes under varying extrinsic water-deficit stress conditions in six vegetative and one reproductive stage of development in rice. Transcription factors (TFs) involved in ABA-dependent and ABA-independent pathways have been found to be upregulated during water-deficit stress. Members of zinc-finger TFs namely, C₂H₂, C₂C₂, C₃H, LIM, PHD, WRKY, ZF-HD, and ZIM, along with TF families like GeBP, jumonji, MBF1 and ULT express differentially under water-deficit conditions. NAC (NAM, ATAF and CUC) TF family emerges to be a potential key regulator of multiple abiotic stresses. Among the 12 TF genes that are co-upregulated under water-deficit, salt and cold stress conditions, five belong to the NAC TF family. We identified water-deficit stress-responsive genes encoding key enzymes involved in biosynthesis of osmoprotectants like polyols and sugars; amino acid and quaternary ammonium compounds; cell wall loosening and structural components; cholesterol and very long chain fatty acid; cytokinin and secondary metabolites. Comparison of genes responsive to water-deficit stress conditions with genes preferentially expressed during panicle and seed development revealed a significant overlap of transcriptome alteration and pathways.

The early history of polyamine research

In 1678 Antonie van Leeuwenhoek identified crystalline substances in human semen. The structure of these crystals, named "spermine", was not elucidated by Rosenheim until 250 years later. Subsequently a triamine (spermidine) and a diamine (putrescine; 1,4-diaminobutane) were isolated from prokaryotic and eukaryotic systems. Soon it became apparent that polyamines can promote the growth of fastidious bacteria. Subsequently a group in Helsinki studied the accumulation of polyamines in regenerating rat liver, while Caldarera and his group studied polyamine synthesis in the developing chick embryo. These investigations led to metabolic studies. Ornithine decarboxylase was identified as a key enzyme in polyamine biosynthesis, while polyamine and diamine oxidations were studied by Mondovì. alpha-Diflouromethylornithine (DFMO) was synthesized by Merrell-Dow and became a potent inhibitor of ornithine decarboxylase. The findings of Russell that polyamines are excreted in the urine of cancer patients drew the attention of oncologists, who attempted the use new technologies for the detection of cancer and improving therapy. With the advance of molecular biology the structure of polyamine-biosynthetic enzymes was elaborated. Plants served as another important tool to study the physiological functions of polyamines. Bagni and his group at Bologna were pioneers in that field and for more than forty-six years set the foundation of a most interesting discipline.

Effectiveness of different EDU concentrations in ameliorating ozone stress in carrot plants

Ethylenediurea (EDU) is suggested for use to evaluate plant response under ambient ozone (O(3)) concentrations. Four EDU treatments, viz. 0 (non-EDU), 150, 300 and 450 mg L(-1), applied as soil drench at 10 days interval to carrot (Daucus carota L. var. Pusa Kesar), grown at a tropical suburban site of Varanasi experiencing mean O(3) concentration of 36.1 ppb during the experimental period. EDU treated plants showed significantly higher antioxidative defense, assimilation capability and reduced membrane lipid peroxidation, which led to better growth and significant yield increments compared to non-EDU treated ones. The magnitude of positive responses was highest at 150 mg L(-1) EDU treatment at 60 DAG, representing the metabolically most active phase of root filling in carrot. This study suggests that the lowest EDU concentration was sufficient to provide protection against negative effects of O(3).

No interaction between methyl jasmonate and ozone in Pima cotton: growth and allocation respond independently to both

Ozone (O3) is damaging to plants, inducing signalling pathways involving antagonism between jasmonates and ethylene. These pathways mediate O3 responses, particularly to acute exposure, and their manipulation protected several species against acute and chronic O3. We use chronic daily exposure of up to 163 ppb O3, and twice weekly application of up to 320 microg plant(-1) methyl jasmonate (MeJA) to test two hypothesizes: 1) a low rate of MeJA does not affect growth but increases O3 sensitivity; 2) a high rate inhibits growth but reduces O3 sensitivity. Both hypotheses were rejected. Growth declined with increases in both MeJA and O3. MeJA at 40 microg plant(-1) caused no direct effect, and at 160 microg plant(-1) reduced growth similarly at all O3. Neither rate altered O3 sensitivity. These additive responses are not consistent with protection by MeJA in this system. They may reflect inter-specific differences in signalling, since O3 concentrations used here exceeded some reported acute exposures. Alternatively, parallel responses to O3 and MeJA may suggest that O3-induced jasmonates play a developmental role in chronic response but no protective role in the absence of lesions characteristic of acute exposure. MeJA appears useful as a probe of these mechanisms.

Ethylene and salicylic acid control glutathione biosynthesis in ozone-exposed Arabidopsis thaliana

Ozone produces reactive oxygen species and induces the synthesis of phytohormones, including ethylene and salicylic acid. These phytohormones act as signal molecules that enhance cell death in response to ozone exposure. However, some studies have shown that ethylene and salicylic acid can instead decrease the magnitude of ozone-induced cell death. Therefore, we studied the defensive roles of ethylene and salicylic acid against ozone. Unlike the wild-type, Col-0, Arabidopsis mutants deficient in ethylene signaling (ein2) or salicylic acid biosynthesis (sid2) generated high levels of superoxide and exhibited visible leaf injury, indicating that ethylene and salicylic acid can reduce ozone damage. Macroarray analysis suggested that the ethylene and salicylic acid defects influenced glutathione (GSH) metabolism. Increases in the reduced form of GSH occurred in Col-0 6 h after ozone exposure, but little GSH was detected in ein2 and sid2 mutants, suggesting that GSH levels were affected by ethylene or salicylic acid signaling. We performed gene expression analysis by real-time polymerase chain reaction using genes involved in GSH metabolism. Induction of gamma-glutamylcysteine synthetase (GSH1), glutathione synthetase (GSH2), and glutathione reductase 1 (GR1) expression occurred normally in Col-0, but at much lower levels in ein2 and sid2. Enzymatic activities of GSH1 and GSH2 in ein2 and sid2 were significantly lower than in Col-0. Moreover, ozone-induced leaf damage observed in ein2 and sid2 was mitigated by artificial elevation of GSH content. Our results suggest that ethylene and salicylic acid protect against ozone-induced leaf injury by increasing de novo biosynthesis of GSH.

Protection of palak (Beta vulgaris L. var Allgreen) plants from ozone injury by ethylenediurea (EDU): roles of biochemical and physiological variations in alleviating the adverse impacts

Ameliorative effects of ethylenediurea (N-[2-(2-oxo-1-imidazolinidyl) ethyl]-N' phenylurea, abbreviated as EDU) against ozone stress were studied on selected growth, biochemical, physiological and yield characteristics of palak (Beta vulgaris L. var Allgreen) plants grown in field at a suburban site of Varanasi, India. Mean eight hourly ozone concentration varied from 52 to 73 ppb which was found to produce adverse impacts on plant functioning and growth characteristics. The palak plants were treated with 300 ppm EDU at 10 days after germination at 10 days interval up to the plant maturity. Lipid peroxidation in EDU treated plants declined significantly as compared to non-EDU treated ones. Significant increment in F(v)/F(m) ratio in EDU treated plants as compared to non-EDU treated ones was recorded. EDU treated plants showed significant increment in ascorbic acid contents and reduction in peroxidase activity as compared to non-EDU treated ones. As a result of the protection provided by EDU against ozone induced stress on biochemical and physiological characteristics of palak, the morphological parameters also responded positively. Significant increments were recorded in shoot length, number of leaves plant(-1), leaf area and root and shoot biomass of EDU treated plants as compared to non-EDU treated ones. Contents of Na, K, Ca, Mg and Fe were higher in EDU treated plants as compared to non-EDU treated ones. The present investigation proves the usefulness of EDU in partially ameliorating ozone injury in ambient conditions.

Detoxification and repair process of ozone injury: from O3 uptake to gene expression adjustment

Plants react to O(3) threat by setting up a variety of defensive strategies involving the co-ordinated modulation of stress perception, signalling and metabolic responses. Although stomata largely controls O(3) uptake, differences in O(3) tolerance cannot always be ascribed to changes in stomatal conductance but cell protective and repair processes should be taken into account. O(3)-driven ROS production in the apoplast induces a secondary, active, self-propagating generation of ROS, whose levels must be finely tuned, by many enzymatic and non-enzymatic antioxidant systems, to induce gene activation without determining uncontrolled cell death. Additional signalling molecules, as ethylene, jasmonic and salicylic acid are also crucial to determine the spreading and the containment of leaf lesions. The main recent results obtained on O(3) sensing, signal transduction, ROS formation and detoxification mechanisms are here discussed.

Plant cell death and cellular alterations induced by ozone: key studies in Mediterranean conditions

An account of histo-cytological and ultrastructural studies on ozone effect on crop and forest species in Italy is given, with emphasis on induced cell death and the underlying mechanisms. Cell death phenomena possibly due to ambient O(3) were recorded in crop and forest species. In contrast, visible O(3) effects on Mediterranean vegetation are often unclear. Microscopy is thus suggested as an effective tool to validate and evaluate O(3) injury to Mediterranean vegetation. A DAB-Evans blue staining was proposed to validate O(3) symptoms at the microscopic level and for a pre-visual diagnosis of O(3) injury. The method has been positively tested in some of the most important crop species, such as wheat, tomato, bean and onion and, with some restriction, in forest species, and it also allows one to gain some very useful insights into the mechanisms at the base of O(3) sensitivity or tolerance.

RNAi-mediated suppression of isoprene biosynthesis in hybrid poplar impacts ozone tolerance

Isoprene is the most abundant volatile compound emitted by vegetation. It influences air chemistry and is thought to take part in plant defense reactions against abiotic stress such as high temperature or ozone. However, whether or not isoprene emission impacts ozone tolerance of plants is still in discussion. In this study, we exploited the transgenic non-isoprene emitting grey poplar (Populus x canescens (Aiton) Sm.) in a biochemical and physiological model study to investigate the effect of acute ozone stress on the elicitation of defense-related emissions of plant volatiles, on photosynthesis and on the antioxidative system. We recorded that non-isoprene emitting poplars were more resistant to ozone as indicated by less damaged leaf area and higher assimilation rates compared to ozone-exposed wild-type (WT) plants. The integral of green leaf volatile emissions was different between the two poplar phenotypes and was a reliable early marker for subsequent leaf damage. For other stress-induced volatiles, such as mono-, homo- and sesquiterpenes and methyl salicylate, similar time profiles, pattern and emission intensities were observed in both transgenic and WT plants. However, unstressed non-isoprene emitting poplars are characterized by elevated levels of ascorbate and alpha-tocopherol as well as by a more effective de-epoxidation ratio of xanthophylls than the WT. Since ozone quenching properties of ascorbate are much higher than those of isoprene and furthermore alpha-tocopherol is also an essential antioxidant, non-isoprene emitting poplars might benefit from changes within the antioxidative system by providing them with enhanced ozone tolerance.

Gene networks in plant ozone stress response and tolerance

For many plant species ozone stress has become much more severe in the last decade. The accumulating evidence for the significant effects of ozone pollutant on crop and forest yield situate ozone as one of the most important environmental stress factors that limits plant productivity worldwide. Today, transcriptomic approaches seem to give the best coverage of genome level responses. Therefore, microarray serves as an invaluable tool for global gene expression analyses, unravelling new information about gene pathways, in-species and cross-species gene expression comparison, and for the characterization of unknown relationships between genes. In this review we summarize the recent progress in the transcriptomics of ozone to demonstrate the benefits that can be harvested from the application of integrative and systematic analytical approaches to study ozone stress response. We focused our consideration on microarray analyses identifying gene networks responsible for response and tolerance to elevated ozone concentration. From these analyses it is now possible to notice how plant ozone defense responses depend on the interplay between many complex signaling pathways and metabolite signals.

Elevated ozone alters soybean-virus interaction

Increasing concentrations of ozone (O(3)) in the troposphere affect many organisms and their interactions with each other. To analyze the changes in a plant-pathogen interaction, soybean plants were infected with Soybean mosaic virus (SMV) while they were fumigated with O(3). In otherwise natural field conditions, elevated O(3) treatment slowed systemic infection and disease development by inducing a nonspecific resistance against SMV for a period of 3 weeks. During this period, the negative effect of virus infection on light-saturated carbon assimilation rate was prevented by elevated O(3) exposure. To identify the molecular basis of a soybean nonspecific defense response, high-throughput gene expression analysis was performed in a controlled environment. Transcripts of fungal, bacterial, and viral defense-related genes, including PR-1, PR-5, PR-10, and EDS1, as well as genes of the flavonoid biosynthesis pathways (and concentrations of their end products, quercetin and kaempherol derivatives) increased in response to elevated O(3). The drastic changes in soybean basal defense response under altered atmospheric conditions suggest that one of the elements of global change may alter the ecological consequences and, eventually, coevolutionary relationship of plant-pathogen interactions in the future.

Protection of ash (Fraxinus excelsior) trees from ozone injury by ethylenediurea (EDU): roles of biochemical changes and decreased stomatal conductance in enhancement of growth

Treatments with ethylenediurea (EDU) protect plants from ozone foliar injury, but the processes underlying this protection are poorly understood. Adult ash trees (Fraxinus excelsior), with or without foliar ozone symptoms in previous years, were treated with EDU at 450 ppm by gravitational trunk infusion in May-September 2005 (32.5 ppm h AOT40). At 30-day intervals, shoot growth, gas exchange, chlorophyll a fluorescence, and water potential were determined. In September, several biochemical parameters were measured. The protective influence of EDU was supported by enhancement in the number of leaflets. EDU did not contribute its nitrogen to leaf tissue as a fertiliser, as determined from lack of difference in foliar N between treatments. Both biochemical (increase in ascorbate-peroxidase and ascorbic acid, and decrease in apoplastic hydrogen peroxide) and biophysical (decrease in stomatal conductance) processes regulated EDU action. As total ascorbic acid increased only in the asymptomatic trees, its role in alleviating O(3) effects on leaf growth and visible injury is controversial.

Transcriptomic changes induced by acute ozone in resistant and sensitive Medicago truncatula accessions

BACKGROUND

Tropospheric ozone, the most abundant air pollutant is detrimental to plant and animal health including humans. In sensitive plant species even a few hours of exposure to this potent oxidant (200-300 nL. L-1) leads to severe oxidative stress that manifests as visible cell death. In resistant plants usually no visible symptoms are observed on exposure to similar ozone concentrations. Naturally occurring variability to acute ozone in plants provides a valuable resource for examining molecular basis of the differences in responses to ozone. From our earlier study in Medicago truncatula, we have identified cultivar Jemalong is ozone sensitive and PI 464815 (JE154) is an ozone-resistant accession. Analyses of transcriptome changes in ozone-sensitive and resistant accession will provide important clues for understanding the molecular changes governing the plant responses to ozone.

RESULTS

Acute ozone treatment (300 nL L-1 for six hours) led to a reactive oxygen species (ROS) burst in sensitive Jemalong six hours post-fumigation. In resistant JE154 increase in ROS levels was much reduced compared to Jemalong. Based on the results of ROS profiling, time points for microarray analysis were one hour into the ozone treatment, end of treatment and onset of an ozone-induced ROS burst at 12 hours. Replicated temporal transcriptome analysis in these two accessions using 17 K oligonucleotide arrays revealed more than 2000 genes were differentially expressed. Significantly enriched gene ontologies (GOs) were identified using the Cluster Enrichment analysis program. A striking finding was the alacrity of JE154 in altering its gene expression patterns in response to ozone, in stark contrast to delayed transcriptional response of Jemalong. GOs involved in signaling, hormonal pathways, antioxidants and secondary metabolism were altered in both accessions. However, the repertoire of genes responding in each of these categories was different between the two accessions. Real-time PCR analysis confirmed the differential expression patterns of a subset of these genes.

CONCLUSION

This study provided a cogent view of the unique and shared transcriptional responses in an ozone-resistant and sensitive accession that exemplifies the complexity of oxidative signaling in plants. Based on this study, and supporting literature in Arabidopsis we speculate that plants sensitive to acute ozone are impaired in perception of the initial signals generated by the action of this oxidant. This in turn leads to a delayed transcriptional response in the ozone sensitive plants. In resistant plants rapid and sustained activation of several signaling pathways enables the deployment of multiple mechanisms for minimizing the toxicity effect of this reactive molecule.