Ozone-induced oxidative burst in the ozone biomonitor plant, tobacco Bel W3

Root-derived long-distance signals trigger ABA synthesis and enhance drought resistance in Arabidopsis

Vascular plants have evolved intricate long-distance signaling mechanisms to cope with environmental stress, with reactive oxygen species (ROS) emerging as pivotal systemic signals in plant stress responses. However, the exact role of ROS as root-to-shoot signals in the drought response has not been determined. In this study, we reveal that compared with wild-type plants, ferric reductase defective 3 (frd3) mutants exhibit enhanced drought resistance concomitant with elevated NINE-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3) transcript levels and abscisic acid (ABA) contents in leaves as well as increased hydrogen peroxide (H2O2) levels in roots and leaves. Grafting experiments distinctly illustrate that drought resistance can be conferred by the frd3 rootstock regardless of the scion genotype, indicating that long-distance signals originating from frd3 roots promote an increase in ABA levels in leaves. Intriguingly, the drought resistance conferred by the frd3 mutant rootstock is weakened by the CAT2-overexpressing scion, suggesting that H2O2 may be involved in long-distance signaling. Moreover, the results of comparative transcriptome and proteome analyses support the drought resistance phenotype of the frd3 mutant. Taken together, our findings substantiate the notion that frd3 root-derived long-distance signals trigger ABA synthesis in leaves and enhance drought resistance, providing new evidence for root-to-shoot long-distance signaling in the drought response of plants.

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.

Ante- and post-mortem cellular injury dynamics in hybrid poplar foliage as a function of phytotoxic O3 dose

After reaching phytotoxic levels during the last century, tropospheric ozone (O3) pollution is likely to remain a major concern in the coming decades. Despite similar injury processes, there is astounding interspecific-and sometimes intraspecific-foliar symptom variability, which may be related to spatial and temporal variation in injury dynamics. After characterizing the dynamics of physiological responses and O3 injury in the foliage of hybrid poplar in an earlier study, here we investigated the dynamics of changes in the cell structure occurring in the mesophyll as a function of O3 treatment, time, phytotoxic O3 dose (POD0), leaf developmental stage, and mesophyll layer. While the number of Hypersensitive Response-like (HR-like) lesions increased with higher O3 concentrations and POD0, especially in older leaves, most structural HR-like markers developed after cell death, independent of the experimental factors. The pace of degenerative Accelerated Cell Senescence (ACS) responses depended closely on the O3 concentration and POD0, in interaction with leaf age. Changes in total chlorophyll content, plastoglobuli and chloroplast shape pointed to thylakoid membranes in chloroplasts as being especially sensitive to O3 stress. Hence, our study demonstrates that early HR-like markers can provide reasonably specific, sensitive and reliable quantitative structural estimates of O3 stress for e.g. risk assessment studies, especially if they are associated with degenerative and thylakoid-related injury in chloroplasts from mesophyll.

Ozone and Bioactive Compounds in Grapes and Wine

Ozone is widely used in the agri-food and food processing industries mainly as a sanitizing agent. However, it has recently become clear that ozone exposition leads to another important benefit: in living tissues, the induced-oxidative stress triggers the antioxidant response, and, therefore, it enhances the production of antioxidant and stress-related secondary metabolites. As such, ozone can be considered an abiotic elicitor. The goal of the present review was to critically summarize knowledge about the possibility of improving bioactive compounds and, consequently, the health-related properties of grapes and wine, by using ozone. The greatest interest has been given not only to the pre- and post-harvest treatment of table and wine grapes, but also to the explanation of the mechanisms involved in the ozone-related response and the main secondary metabolites biosynthetic pathways. From the literature available, it is clear that the effect of ozone treatment on health-related properties and secondary metabolites accumulation depends on many factors, such as the cultivar, but also the form (water or gaseous), doses, and application method of ozone. Most of the published papers report an increase in antioxidant compounds (e.g., polyphenols) and stress-related volatiles, confirming the hypothesis that ozone could be used to improve berry and wine compositional and sensory quality.

Cellular autofluorescence and browning in trichomes of Chinese cabbage (Brassica campestris) in response to mechanical stimulation and senescence

There is limited information concerning the formation of dot-like browning appearing at the base of trichomes on mature leaves on the Chinese cabbage (Brassica campestris L. ssp. pekinensis). This study confirmed for the first time that enhanced autofluorescence can be induced in the base of trichomes when pressure stimuli is applied to trichomes; the enhanced autofluorescence gradually moves to the top of trichomes and the neighbouring mesophyll tissue within 15min. The excitation of autofluorescence in trichomes was found to be more effective in mature leaves compared to newly emergent leaves. Increased polyphenol oxidase (PPO) activities and reactive oxygen species (ROS) accumulation were also detected in the basal region of trichomes that were subjected to mechanical stimuli. Enhanced fluorescence was observed at the top of the trichomes in senescencing leaves. A browning in the base of the trichomes during leaf senescence was observed. In contrast, no browning occurred at the base of the trichomes in leaves that were subject to pressure stimuli. The blue fluorescence in the trichomes in senescent leaves arises mainly from the condensed cytoplasm. No direct evidence was able to prove that the enhanced autofluorescent substances in the trichomes during leaf senescence are the cause of the browning at the early growth stages.

Effects of ozone water irrigation and spraying on physiological characteristics and gene expression of tomato seedlings

Tomato seedlings were used as experimental materials and treated with 1.0, 2.0, 3.0, and 4.0 mg/L ozone water irrigation and 0.2, 0.4, 0.6, and 0.8 mg/L ozone water spray treatments. Indexes including the malondialdehyde (MDA) content, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), activities, soil and plant analysis development (SPAD) value, and nitrogen content of leaves were measured. Furthermore, the expression of antioxidant enzyme, chlorophyll synthesis and nitrogen absorption genes was analyzed after optimal ozone water treatment. The results showed that the activities of antioxidant enzymes in tomato leaves were significantly increased, and the MDA content in tomato leaves was significantly reduced by ozone water irrigation and spray treatment, which indicated that ozone water treatment can significantly improve the stress tolerance of tomato seedlings. Ozone water irrigation and spraying could also significantly increase the leaf SPAD value and nitrogen content of tomato seedlings, and the optimal concentrations of ozone water irrigation and spraying were 3.0 mg/L and 0.6 mg/L, respectively. The effect of ozone water irrigation on improving the physiological characteristics of tomato seedlings was better than that of spraying. After treatment with the optimal concentration of ozone water, the relative expression of antioxidant enzyme, chlorophyll synthesis, and nitrogen absorption genes was significantly increased, and the maximum expression level was reached at 12 h. In addition, ozone water irrigation could promote the expression of genes more than ozone water spraying, which was consistent with the improvements in the physiological characteristics of the tomato seedlings.

Dynamics of Foliar Responses to O3 Stress as a Function of Phytotoxic O3 Dose in Hybrid Poplar

With background concentrations having reached phytotoxic levels during the last century, tropospheric ozone (O₃) has become a key climate change agent, counteracting carbon sequestration by forest ecosystems. One of the main knowledge gaps for implementing the recent O₃ flux-based critical levels (CLs) concerns the assessment of effective O₃ dose leading to adverse effects in plants. In this study, we investigate the dynamics of physiological, structural, and morphological responses induced by two levels of O₃ exposure (80 and 100 ppb) in the foliage of hybrid poplar, as a function of phytotoxic O₃ dose (POD₀) and foliar developmental stage. After a latency period driven by foliar ontological development, the gas exchanges and chlorophyll content decreased with higher POD₀ monotonically. Hypersensitive response-like lesions appeared early during exposure and showed sigmoidal-like dynamics, varying according to leaf age. At current POD_{1_SPEC} CL, notwithstanding the aforementioned reactions and initial visible injury to foliage, the treated poplars had still not shown any growth or biomass reduction. Hence, this study demonstrates the development of a complex syndrome of early reactions below the flux-based CL, with response dynamics closely determined by the foliar ontological stage and environmental conditions. General agreement with patterns observed in the field appears indicative of early O₃ impacts on processes relevant, e.g., biodiversity ecosystem services before those of economic significance - i.e., wood production, as targeted by flux-based CL.

Combined Acute Ozone and Water Stress Alters the Quantitative Relationships between O3 Uptake, Photosynthetic Characteristics and Volatile Emissions in Brassica nigra

Ozone (O₃) entry into plant leaves depends on atmospheric O₃ concentration, exposure time and openness of stomata. O₃ negatively impacts photosynthesis rate (A) and might induce the release of reactive volatile organic compounds (VOCs) that can quench O₃, and thereby partly ameliorate O₃ stress. Water stress reduces stomatal conductance (g_s) and O₃ uptake and can affect VOC release and O₃ quenching by VOC, but the interactive effects of O₃ exposure and water stress, as possibly mediated by VOC, are poorly understood. Well-watered (WW) and water-stressed (WS) Brassica nigra plants were exposed to 250 and 550 ppb O₃ for 1 h, and O₃ uptake rates, photosynthetic characteristics and VOC emissions were measured through 22 h recovery. The highest O₃ uptake was observed in WW plants exposed to 550 ppb O₃ with the greatest reduction and poorest recovery of g_s and A, and elicitation of lipoxygenase (LOX) pathway volatiles 10 min-1.5 h after exposure indicating cellular damage. Ozone uptake was similar in 250 ppb WW and 550 ppb WS plants and, in both treatments, O₃-dependent reduction in photosynthetic characteristics was moderate and fully reversible, and VOC emissions were little affected. Water stress alone did not affect the total amount and composition of VOC emissions. The results indicate that drought ameliorated O₃ stress by reducing O₃ uptake through stomatal closure and the two stresses operated in an antagonistic manner in B. nigra.

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.

Protecting tobacco plants from O3 injury by Bacillus velezensis with production of acetoin

Plant growth-promoting rhizobacteria (PGPRs) confer benefits to crops by producing volatile organic compounds (VOCs) to trigger induced systemic tolerance (IST). Here we show that Bacillus velezensis GJ11, a kind of PGPRs, produce VOCs such as 2,3-butanediol and acetoin to trigger IST and cause stomatal closure against O3 injury in tobacco plants. Compared to 2,3-butanediol, acetoin was more effective on triggering IST against O3 injury. The bdh-knockout strain GJ11Δbdh with a blocked metabolic pathway from acetoin to 2,3-butanediol produced more acetoin triggering stronger IST against O3 injury than GJ11. Both acetoin and GJ11Δbdh effectively enhance the antioxidant enzymes activity (e.g. superoxide dismutase and catalases) that is favorable for scavenging the reactive oxygen species like H2 O2 in leaves after exposure to O3 . Consequently, less H2 O2 accumulation was observed, and reasonably less chlorophylls and proteins were damaged by H2 O2 in the tobacco leaves treated with acetoin or GJ11Δbdh. The field experiment also showed that both acetoin and GJ11Δbdh could protect tobacco plants from O3 injury after application by root-drench. This study provides new insights into the role of rhizobacterial B. velezensis and its volatile component of acetoin in triggering defense responses against stresses such as O3 in plants.

The Water Content Drives the Susceptibility of the Lichen Evernia prunastri and the Moss Brachythecium sp. to High Ozone Concentrations

The aim of this study was to evaluate the tolerance of lichens (Evernia prunastri) and mosses (Brachythecium sp.) to short-term (1 h), acute (1 ppm) O3 fumigation under different hydration states (dry, <10% water content, metabolism almost inactive; wet, >200% water content, metabolism fully active). We hypothesized that stronger damage would occur following exposure under wet conditions. In addition, we checked for the effect of recovery (1 week) after the exposure. Ozone fumigation negatively affected the content of chlorophyll only in wet samples, but in the moss, such a difference was no longer evident after one week of recovery. Photosynthetic efficiency was always impaired by O3 exposure, irrespective of the dry or wet state, and also after one week of recovery, but the effect was much stronger in wet samples. The antioxidant power was increased in wet moss and in dry lichen, while a decrease was found for wet lichens after 1 week. Our results confirm that the tolerance to O3 of lichens and mosses may be determined by their low water content, which is the case during the peaks of O3 occurring during the Mediterranean summer. The role of antioxidant power as a mechanism of resistance to high O3 concentrations needs to be further investigated.

Effects of ozone and ammonium sulfate on cauliflower: Emphasis on the interaction between plants and insect herbivores

Ammonium sulfate [(NH₄)₂SO₄] deposition and elevated ozone (O₃) concentrations may negatively affect plants and trophic interactions. This study aimed to evaluate for the first time the interactive effects of high (NH₄)₂SO₄ load and elevated O₃ levels on cauliflower (Brassica oleracea L.) under field conditions. Cauliflower seedlings were treated with 0 (AS0) or 50 (AS50) kg ha^-1 (NH₄)₂SO₄ and exposed to ambient (AOZ, ≈20 ppb) or elevated (EOZ, ≈55 ppb) O₃ for about one month, in a Free Air O₃ Concentration Enrichment (FACE) system. The oligophagous diamondback moth (Plutella xylostella Linnaeus, 1758) showed a clear preference towards the seedlings treated with AS50, which intensively grazed. Plant-herbivore interactions were driven by (NH₄)₂SO₄ availability, rather than O₃, via increased nitrogen content in the leaves. Further laboratory bioassays were followed to confirm the validity of these observations using polyphagous Eri silkmoth larvae (Samia ricini) as a biological model in a standardized experimental setup. Choice assays, where larvae could select leaves among leaf samples from the different experimental conditions, and no-choice assays, where larvae could graze leaves from just one experimental condition, were conducted. In the choice assay, the larvae preferred AS50-treated leaves, in agreement with the field observations with diamondback moth. In the no-choice assay, larval body mass growth was inhibited when fed with leaves treated with EOZ and/or AS50. Larvae fed with AS50-treated leaves displayed increased mortality. These observations coincide with higher NO₃ and Zn content in AS50-treated leaves. This study shows that plant-herbivore interactions can be driven by (NH₄)₂SO₄ availability, independently of O₃, and suggests that high N deposition may have severe health implications in animals consuming such plant tissues. Key message: Plant-herbivore interactions are driven by high (NH₄)₂SO₄ availability, independently of O₃.

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.

Genome-wide Analysis of bZIP Transcription Factors in wheat and Functional Characterization of a TabZIP under Abiotic Stress

The basic leucine zipper (bZIP) represents one of the largest as well as most diverse transcription factor (TFs) families. They are known to play role in both stress as well as in various plant developmental processes. In the present study, a total of 191 bZIP transcription factors have been identified from Triticum aestivum. Expression analysis during various stress conditions, developmental stages, different varieties and gene ontology enrichment analysis suggest their possible roles in abiotic stress as well as in developmental responses. In the current analysis, one of the members named as TabZIP (Traes_7AL_25850F96F.1) was selected for detailed analysis to understand its role under different abiotic stress conditions. Gene expression studies revealed differential expression of TabZIP in various abiotic stress conditions like heat, salinity and dehydration suggesting the possible role of bZIP in various stress mitigation mechanism. Arabidopsis transgenics overexpressing TabZIP showed enhanced tolerance to salinity, drought, heat and oxidative stress. Thus TabZIP (Traes_7AL_25850F96F.1) can serve as a candidate gene for improving heat as well as other abiotic stress tolerance and can be helpful in enhancing the crop productivity under stress conditions.

Characterization of a novel zinc finger transcription factor (TaZnF) from wheat conferring heat stress tolerance in Arabidopsis

C3HC4-type zinc finger proteins are known to play important roles in various plant processes including regulation of growth and development, signaling networks, responses to abiotic stresses etc. The current study identifies and explores the involvement of TaZnF in plant stress response, mainly heat stress. TaZnF belongs to C4HC3-type zinc finger transcription factor. Phylogenetic analysis of TaZnF revealed strong sequence similarity to Brachypodium distachyon, a model system for crop species. Gene expression studies have revealed its role under diverse stress conditions including heat and cold conditions. The transcript level of TaZnF was found to be highest in seed and starts at the post anthesis period 3-5DAA, a more sensitive stage resulting in a negative influence on the yield of crop species. TaZnF possesses transcriptional activity. Overexpression of TaZnF in Arabidopsis thaliana conferred improved tolerance to both basal and high-temperature stress as observed from various assays examining their growth and development. The transgenics were recovered and showed early flowering compared to wild-type. They had larger primary roots, more lateral branching, bigger, and more numerous leaves, resulting in heavier fresh weight. Enhanced growth and early recovery resulted in bigger plants with more yield. Additionally, the overexpression Arabidopsis transgenics also showed considerable tolerance to cold and oxidative stress. These observations suggest that TaZnF acts as a positive regulator of thermal stress and thus can be of great significance in understanding and improving temperature stress tolerance in plants.

Ozone-triggered surface uptake and stress volatile emissions in Nicotiana tabacum 'Wisconsin'

Ozone is a strong oxidant and a key stress elicitor. The immediate and longer term impacts of ozone are poorly understood in species with emission of both de novo synthesized and stored volatiles, such a tobacco (Nicotiana tabacum), which has terpene-containing glandular trichomes on the leaf surface. In this study, we exposed N. tabacum 'Wisconsin' leaves to acute ozone doses of 0 (control), 400, 600, 800, and 1000 ppb for 30 min and studied the effects of ozone exposure on ozone uptake, gas-exchange characteristics, and emissions of lipoxygenase pathway volatiles, monoterpenes, and sesquiterpenes. Foliage emissions of lipoxygenase pathway volatiles were quantitatively related to the severity of ozone exposure, but the stress dose vs. emission relationship was weaker for terpenoids. Analysis of leaf terpene content and composition indicated that several monoterpenes and sesquiterpenes were not stored in leaves and were synthesized de novo upon ozone exposure. The highest degree of elicitation for each compound was observed immediately after ozone treatment and it declined considerably during recovery. Leaf ozone uptake was dominated by non-stomatal deposition, and the emissions of total lipoxygenase pathway volatiles and mono- and sesquiterpenes were positively correlated with non-stomatal ozone deposition. Overall, this study demonstrates remarkably high ozone resistance of the studied tobacco cultivar and indicates that ozone's effects on volatile emissions primarily reflect modifications in the release of stored volatiles and reaction of ozone with the leaf surface structure.

Biochemical indicators of root damage in rice (Oryza sativa) genotypes under zinc deficiency stress

Zn deficiency is one of the major soil constraints currently limiting rice production. Although recent studies demonstrated that higher antioxidant activity in leaf tissue effectively protects against Zn deficiency stress, little is known about whether similar tolerance mechanisms operate in root tissue. In this study we explored root-specific responses of different rice genotypes to Zn deficiency. Root solute leakage and biomass reduction, antioxidant activity, and metabolic changes were measured using plants grown in Zn-deficient soil and hydroponics. Solute leakage from roots was higher in sensitive genotypes and linked to membrane damage caused by Zn deficiency-induced oxidative stress. However, total root antioxidant activity was four-fold lower than in leaves and did not differ between sensitive and tolerant genotypes. Root metabolite analysis using gas chromatography-mass spectrometry and high performance liquid chromatography indicated that Zn deficiency triggered the accumulation of glycerol-3-phosphate and acetate in sensitive genotypes, while less or no accumulation was seen in tolerant genotypes. We suggest that these metabolites may serve as biochemical indicators of root damage under Zn deficiency.

NO-Dependent programmed cell death is involved in the formation of Zn-related lesions in tobacco leaves

A recent study indicated that the development of lesions on the leaf blades of tobacco exposed to zinc (Zn) excess can be considered a manifestation of a Zn-tolerance strategy at the organ level. Here, we investigated whether cell death leading to the formation of localized lesions is destructive in character (necrosis type) or results from programmed self-induced cell death (PCD). Selected parameters, including PCD markers, were determined in the leaves from tobacco plants grown in the presence of 200 μM Zn and compared with control conditions. TUNEL assay results showing internucleosomal DNA fragmentation in the nuclei of the cells from Zn-exposed leaves, together with an enhanced expression of three PCD marker genes (NtBI-1, Ntrboh, and NtSIPK), indicated the involvement of PCD in the formation of Zn-related lesions. It is known that NO is a key factor in the execution of PCD. Interestingly, upon exposure to high Zn, in situ localization of NO (visualized using DAF-2DA fluorescence) was restricted to groups of mesophyll cells, and was correlated with the pattern of Zn localization (determined using the fluorophore Zinpyr-1), similarly limited primarily to groups of "Zn accumulating cells". Furthermore, inhibition of the formation of lesions in the presence of l-NAME (an NO synthase inhibitor) was accompanied by the delayed appearance of Zn and by NO localization limited to these groups of cells. Altogether, we provide the first demonstration that Zn-related lesions in leaves develop from groups of mesophyll cells in which accumulation of high concentrations of Zn contributes to enhancement of the NO level and to initiation of PCD processes.

Understanding and improving global crop response to ozone pollution

Concentrations of ground-level ozone ([O3 ]) over much of the Earth's land surface have more than doubled since pre-industrial times. The air pollutant is highly variable over time and space, which makes it difficult to assess the average agronomic and economic impacts of the pollutant as well as to breed crops for O3 tolerance. Recent modeling efforts have improved quantitative understanding of the effects of current and future [O3 ] on global crop productivity, and experimental advances have improved understanding of the cellular O3 sensing, signaling and response mechanisms. This work provides the fundamental background and justification for breeding and biotechnological approaches for improving O3 tolerance in crops. There is considerable within-species variation in O3 tolerance in crops, which has been used to create mapping populations for screening. Quantitative trait loci (QTL) for O3 tolerance have been identified in model and crop species, and although none has been cloned to date, transcript profiling experiments have identified candidate genes associated with QTL. Biotechnological strategies for improving O3 tolerance are also being tested, although there is considerable research to be done before O3 -tolerant germplasm is available to growers for most crops. Strategies to improve O3 tolerance in crops have been hampered by the lack of translation of laboratory experiments to the field, and the lack of correlation between visual leaf-level O3 damage and yield loss to O3 stress. Future efforts to screen mapping populations in the field and to identify more promising phenotypes for O3 tolerance are needed.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

Biochemical and physiological processes associated with the differential ozone response in ozone-tolerant and sensitive soybean genotypes

Biochemical and physiological traits of two soybean [Glycine max (L.) Merr.] genotypes differing in sensitivity to ozone (O3 ) were investigated to determine the possible basis for the differential response. Fiskeby III (O3 -tolerant) and Mandarin (Ottawa) (O3 -sensitive) were grown in a greenhouse with charcoal-filtered air for 4 weeks, then treated with O3 for 7 h·day(-1) in greenhouse chambers. Mandarin (Ottawa) showed significantly more leaf injury and hydrogen peroxide (H2 O2 ) and superoxide (O2 (-) ) production compared with Fiskeby III. Peroxidase activity in Mandarin (Ottawa) was 31% higher with O3 but was not significantly different in Fiskeby III. Ozone did not affect superoxide dismutase or glutathione reductase activities, or leaf concentrations of glutathione or ascorbic acid. Thus, variation in O3 response between Fiskeby III and Mandarin (Ottawa) was not explained by differences in antioxidant enzymes and metabolites tested. Ethylene emission from leaves declined in Fiskeby III following O3 exposure but not in Mandarin (Ottawa). Ozone exposure reduced quantum yield (ΦPSII ), electron transport rate (ETR) and photochemical quenching (qp ) in Mandarin (Ottawa) more than in Fiskeby III, indicating that efficiency of energy conversion of PSII and photosynthetic electron transport was altered differently in the two genotypes. Short-term exposure to O3 had minimal effects on net carbon exchange rates of both soybean cultivars. A trend toward higher stomatal conductance in Mandarin (Ottawa) suggested stomatal exclusion might contribute to differential O3 sensitivity of the two genotypes. Increased sensitivity of Mandarin (Ottawa) to O3 was associated with higher H2 O2 and O2 (-) production compared with Fiskeby III, possibly associated with genotype differences in stomatal function or regulation of ethylene during the initial phases of O3 response.

Cross Talk between H2O2 and Interacting Signal Molecules under Plant Stress Response

It is well established that oxidative stress is an important cause of cellular damage. During stress conditions, plants have evolved regulatory mechanisms to adapt to various environmental stresses. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species, which is subsequently converted to H2O2. H2O2 is continuously produced as the byproduct of oxidative plant aerobic metabolism. Organelles with a high oxidizing metabolic activity or with an intense rate of electron flow, such as chloroplasts, mitochondria, or peroxisomes are major sources of H2O2 production. H2O2 acts as a versatile molecule because of its dual role in cells. Under normal conditions, H2O2 immerges as an important factor during many biological processes. It has been established that it acts as a secondary messenger in signal transduction networks. In this review, we discuss potential roles of H2O2 and other signaling molecules during various stress responses.

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.

Effects of volatile organic compound ether on cell responses and gene expressions in Arabidopsis

BACKGROUND

The volatile organic compound ether is widely used as an industrial solvent and easily released to the environment. Our previous research indicated that ether triggers reactive oxygen species (ROS) production and activates ethylene biosynthetic genes and defense gene expressions in tomato. In the present study, we investigated the effect of ether on cell responses and gene expressions in Arabidopsis and compared the ROS and phytohormones produced in Arabidopsis and tomato plants in response to different air pollutants (O₃ vs. ether).

RESULTS

Ether induced the sequential production of superoxide anion and hydrogen peroxide in Arabidopsis. Ether also triggered expressions of ethylene, salicylic acid and jasmonic acid biosynthetic genes. The temporal expression patterns of MAP kinase and protein phosphatase genes are in good accordance with those of the ethylene and salicylic acid biosynthetic genes, suggesting that induction of these phytohormone biosynthesis were through signaling pathways including both phosphorylation and/or dephosphorylation. By contrast, expression pattern of protein phosphatase PP2A3&4 coincided well with the expression of jasmonic acid biosynthetic gene LOX4, suggesting that induction of jasmonic acid biosynthesis is through PP2A3&4. However, the production of ROS and temporal expression patterns of phytohormone biosynthetic genes in Arabidopsis in response to ether were different from those to O₃ and were different from those in tomato as well.

CONCLUSIONS

Different plants have different strategies to respond to the same abiotic stress, and each plant species possesses its own unique signaling pathways that regulate the responding process.

Potato Annexin STANN1 Promotes Drought Tolerance and Mitigates Light Stress in Transgenic Solanum tuberosum L. Plants

Annexins are a family of calcium- and membrane-binding proteins that are important for plant tolerance to adverse environmental conditions. Annexins function to counteract oxidative stress, maintain cell redox homeostasis, and enhance drought tolerance. In the present study, an endogenous annexin, STANN1, was overexpressed to determine whether crop yields could be improved in potato (Solanum tuberosum L.) during drought. Nine potential potato annexins were identified and their expression characterized in response to drought treatment. STANN1 mRNA was constitutively expressed at a high level and drought treatment strongly increased transcription levels. Therefore, STANN1 was selected for overexpression analysis. Under drought conditions, transgenic potato plants ectopically expressing STANN1 were more tolerant to water deficit in the root zone, preserved more water in green tissues, maintained chloroplast functions, and had higher accumulation of chlorophyll b and xanthophylls (especially zeaxanthin) than wild type (WT). Drought-induced reductions in the maximum efficiency and the electron transport rate of photosystem II (PSII), as well as the quantum yield of photosynthesis, were less pronounced in transgenic plants overexpressing STANN1 than in the WT. This conferred more efficient non-photochemical energy dissipation in the outer antennae of PSII and probably more efficient protection of reaction centers against photooxidative damage in transgenic plants under drought conditions. Consequently, these plants were able to maintain effective photosynthesis during drought, which resulted in greater productivity than WT plants despite water scarcity. Although the mechanisms underlying this stress protection are not yet clear, annexin-mediated photoprotection is probably linked to protection against light-induced oxidative stress.

Transcript dynamics at early stages of molecular interactions of MYMIV with resistant and susceptible genotypes of the leguminous host, Vigna mungo

Initial phases of the MYMIV- Vigna mungo interaction is crucial in determining the infection phenotype upon challenging with the virus. During incompatible interaction, the plant deploys multiple stratagems that include extensive transcriptional alterations defying the virulence factors of the pathogen. Such molecular events are not frequently addressed by genomic tools. In order to obtain a critical insight to unravel how V. mungo respond to Mungbean yellow mosaic India virus (MYMIV), we have employed the PCR based suppression subtractive hybridization technique to identify genes that exhibit altered expressions. Dynamics of 345 candidate genes are illustrated that differentially expressed either in compatible or incompatible reactions and their possible biological and cellular functions are predicted. The MYMIV-induced physiological aspects of the resistant host include reactive oxygen species generation, induction of Ca²⁺ mediated signaling, enhanced expression of transcripts involved in phenylpropanoid and ubiquitin-proteasomal pathways; all these together confer resistance against the invader. Elicitation of genes implicated in salicylic acid (SA) pathway suggests that immune response is under the regulation of SA signaling. A significant fraction of modulated transcripts are of unknown function indicating participation of novel candidate genes in restricting this viral pathogen. Susceptibility on the other hand, as exhibited by V. mungo Cv. T9 is perhaps due to the poor execution of these transcript modulation exhibiting remarkable repression of photosynthesis related genes resulting in chlorosis of leaves followed by penalty in crop yield. Thus, the present findings revealed an insight on the molecular warfare during host-virus interaction suggesting plausible signaling mechanisms and key biochemical pathways overriding MYMIV invasion in resistant genotype of V. mungo. In addition to inflate the existing knowledge base, the genomic resources identified in this orphan crop would be useful for integrating MYMIV-tolerance trait in susceptible cultivars of V. mungo.

The role of elevated ozone on growth, yield and seed quality amongst six cultivars of mung bean

Tropospheric ozone (O3) can be deleterious to plants by decreasing crop yield and quality. Present study was conducted on six cultivars of mung bean (HUM-1, HUM-2, HUM-6, HUM-23, HUM-24 and HUM-26) grown under ambient O3 (NFC) and elevated O3 levels (ambient+10 ppb; NFC+) in open top chambers (OTCs) for two consecutive years. Ozone monitoring data showed high mean ambient concentration of O3 at the experimental site, which was above the threshold value of 40 ppb. Ozone exposure induced symptoms of foliar injury and also depicted accumulation of reactive oxygen species (ROS) which led to increased membrane damage vis-a-vis solute leakage. Root/shoot allometric coefficient (k), yield and seed quality showed negative response to O3. Differential response of mung bean cultivars against elevated O3 was assessed by comparing the levels of antioxidants, metabolites, growth, total biomass and yield. Cultivar HUM-1 showed maximum sensitivity towards O3 as compared to other cultivars. Findings of present study emphasized the possibility of selection of suitable O3 resistant cultivars for the areas experiencing high concentrations of O3.

Fusaric acid induced cell death and changes in oxidative metabolism of Solanum lycopersicum L

BACKGROUND

Fusaric acid (FA) has been shown to stimulate the rapid development of disease symptoms, such as necrosis and foliar desiccation. In this study, we have evaluated the phytotoxicity of FA on tomato plants (Solanum lycopersicum L.). FA induced necrotic lesions in detached leaves, which are reminiscent of hypersensitive response (HR) lesions induced by plant-pathogen interactions and other abiotic stress factors.

RESULTS

FA-treated tomato leaves exhibited visible necrotic lesion as a result of cell death which was evident by Evans blue staining, enhanced reactive oxygen species (ROS) levels and DNA degradation. Changes in the generation of O2.- and H2O2 as well as the activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) were examined in FA-treated tomato leaves. It was observed that FA exposure stimulated oxidative burst in the leaves, resulting in a lasting activation of O2.- and H2O2 production. After first day of FA application, the H2O2 scavenging enzymes CAT and APX showed a strong activity decrease followed by gradual recovery to the control level after 2 and 3 days.

CONCLUSION

A concomitant increase in ROS production, the down regulation of antioxidative enzymes activities and upregulation of lipid peroxidation were crucial for the onset of cell death. These results suggested that FA-induced damage might result from ROS pathways. Thus, our experiments provide a useful model plant system for research on FA-induced plant cell death.

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.

Apoplastic antioxidant enzyme responses to chronic free-air ozone exposure in two different ozone-sensitive wheat cultivars

The effects of elevated ozone concentrations [O3] on two different ozone-sensitive wheat (Triticum aestivum L.) cultivars [Yangmai16 (Y16) and Yannong19 (Y19)] were investigated to determine the different apoplastic antioxidant mechanisms under O3-FACE (free-air controlled enrichment) condition. The results indicated that elevated [O3] (1.5 × ambient [O3]) induced increases in the production of superoxide anion (O2(-)), hydroxyl radical (HO), hydrogen peroxide (H2O2) and lipid peroxidation, and these results were more pronounced in the apoplasts of Y19 than in those of Y16. Apoplastic antioxidant enzymes were developmentally regulated and the effect of elevated [O3] depended on the developmental stage of wheat for both cultivars. In cultivar Y19, continuous O3 stress induced a decrease in the activity of apoplastic superoxide dismutase (SOD; EC 1.15.1.1), peroxidase (POD; EC 1.11.1.7) and ascorbate peroxidase (APX; EC 1.11.1.11) in the later growing stages, indicating Y19 appears to be the more sensitive cultivar and is prone to oxidative stress. The strategic response of antioxidant enzymes activities by Y16 in four different plant development stages (booting, flowering, filling and ripening) resulted in O3 stress-induced antioxidant defense responses, which indicated its higher tolerance to O3 stress. The same patterns of activity of apoplastic SOD and APX isozymes were observed in both Y16 and Y19 cultivars, while POD isozymes differed by cultivar in terms of the pattern of bands. The results of the present study show that O3 tolerance can be improved by regulating apoplastic ROS metabolism through the responses of apoplastic antioxidant enzymes to O3 stress in different plant development stages.

Priming-mediated systemic resistance in cucumber induced by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22 against Colletotrichum orbiculare

Induced systemic resistance (ISR) can be activated by biotic agents, including root-associated beneficial bacteria to inhibit pathogen infection. We investigated priming-mediated ISR in cucumber induced by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22 against Colletotrichum orbiculare (causal fungus of anthracnose). In addition, we examined whether this ISR expression was bacterial density-dependent by assessing peroxidase activity in the presence and absence of the pathogen. As a result, root treatment with the ISR-eliciting strains GC-B19 and MM-B22 or the chemical inducer DL-β-amino-n-butyric acid (positive control) significantly inhibited fungal infection process (conidial germination and appressorium formation) and disease severity compared with the non-ISR-eliciting strain, Pseudomonas aeruginosa PK-B09 (negative control), and MgSO4 solution (untreated control). These treatments effectively induced rapid elicitation of hypersensitive reaction-like cell death with H2O2 generations, and accumulation of defense-related enzymes (β-1,3-glucanase, chitinase, and peroxidase) in cucumber leaves in the "primed" state against C. orbiculare. In addition, ISR expression was dependent on the bacterial cell density in the rhizosphere. This ISR expression was derived from the presence of sustained bacterial populations ranging from 10(4) to 10(6) cells/g of potting mix over a period of time after introduction of bacteria (10(6) to 10(10) cells/g of potting mix) into the rhizosphere. Taken together, these results suggest that priming-mediated ISR against C. orbiculare in cucumber can be induced in a bacterial density-dependent manner by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22.

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.

Mutation of a Short Variable Region in HCpro Protein of Potato virus A Affects Interactions with a Microtubule-Associated Protein and Induces Necrotic Responses in Tobacco

Helper component proteinase (HCpro) is a multifunctional protein of potyviruses (genus Potyvirus). HCpro of Potato virus A (PVA) interacts with the microtubule-associated protein HIP2 in host cells, and depletion of HIP2 reduces virus accumulation. This study shows that HCpro of Potato virus Y and Tobacco etch virus also interact with HIP2. The C-proximal portion of PVA HCpro determines the interaction with HIP2 and was found to contain a stretch of six residues comprising a highly variable region (HVR) in potyviruses. Mutations in HVR reduced PVA accumulation in tobacco plants and induced necrotic symptoms novel to PVA. Microarray and quantitative reverse transcription polymerase chain reaction analyses revealed induction of many defense-related genes including ethylene- and jasmonic acid-inducible pathways in systemically infected leaves at necrosis onset. Salicylic acid-mediated signaling was dispensable for the response. Genes related to microtubule functions were down-regulated. Structural modeling of HCpro suggested that all mutations in HVR caused conformational changes in adjacent regions containing functionally important motifs conserved in potyviruses. Those mutations, which also caused conformational changes in HVR, led to the greatest reduction of fitness. Our results implicate HVR in the regulation of HCpro conformation and virus-host interactions and suggest that mutation of HVR induces host defense.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

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.

Ethylene-induced overproduction of reactive oxygen species is responsible for the development of watersoaking in immature cucumber fruit

Watersoaking is an ethylene-induced disorder observed in some members of the Cucurbitaceae including cucumber (Cucumis sativus L.), watermelon (Citrullus lanatus Thunb. Matsum and Nakai), and tropical pumpkin (Cucurbita moschata Duch.). Previous studies have found that immature beit-alpha cucumber (cv. Manar) exhibit watersoaking after 6d of continuous exposure to 10 μLL(-1) ethylene in air (21 kPa O(2)). The present study was designed to investigate the early dynamics of ethylene responses in immature cucumber fruit in order to provide insight into the watersoaking triggering mechanism. Changes in respiration, epidermal color, firmness, reactive oxygen species (ROS) production and electrolyte leakage were evaluated as a function of time under different ethylene concentrations and exposure duration. Ethylene concentrations exceeding 10 μLL(-1) did not accelerate changes in any of the evaluated responses. The first detectable change was a significant rise in respiration on day 2, followed by a significant rise in ROS on day 4, and significant degreening, mesocap softening, and increased electrolyte leakage on day 6; the latter responses coincident with incipient watersoaking. Varying the duration of exposure to ethylene indicated that the critical exposure time is between 2 and 4d. Notably, all deleterious responses to ethylene were suppressed under a hypoxic atmosphere. A model is proposed in which ethylene induces a sharp increase in respiration with a concomitant sharp rise in ROS, which the immature fruit is incapable of quenching. The resulting production of excess ROS leads to discoloration and membrane deterioration, leading to the release of cytoplasmic content, rapid softening, and the visual symptom of watersoaking.

Digital gene expression analysis of early root infection resistance to Sporisorium reilianum f. sp. zeae in maize

The maize smut fungus, Sporisorium reilianum f. sp. zeae, which is an important biotrophic pathogen responsible for extensive crop losses, can infect maize by invading the root during the early seedling stage. In order to investigate disease-resistance mechanisms at this early seedling stage, digital gene expression analysis, which applies a dual-enzyme approach, was used to identify the transcriptional changes in the roots of Huangzao4 (susceptible) and Mo17 (resistant) after root inoculation with S. reilianum. During the infection in the roots, the expression pattern of pathogenesis-related genes in Huangzao4 and Mo17 were significantly differentially regulated at different infection stages. The glutathione S-transferase enzyme activity and reactive oxygen species levels also showed changes before and after inoculation. The total lignin contents and the pattern of lignin depositions in the roots differed during root colonization of Huangzao4 and Mo17. These results suggest that the interplay between S. reilianum and maize during the early infection stage involves many important transcriptional and physiological changes, which offer several novel insights to understanding the mechanisms of resistance to the infection of biotrophic fungal pathogens.

Calcium supplementation modulates arsenic-induced alterations and augments arsenic accumulation in callus cultures of Indian mustard (Brassica juncea (L.) Czern.)

In the present study, the effect of arsenate (AsV) exposure either alone or in combination with calcium (Ca) was investigated in callus cultures of Brassica juncea (L.) Czern. cv. Pusa Bold grown for a period up to 24 h. The AsV (250 μM) + Ca (10 mM) treatment resulted in a significantly higher level of As (464 μg g(-1) dry weight (DW)) than AsV without Ca (167 μg g(-1) DW) treatment at 24 h. Furthermore, AsV + Ca-treated calli had a higher percent of AsIII (24-47%) than calli subjected to AsV treatment (12-14%). Despite this, AsV + Ca-treated calli did not show any signs of hydrogen peroxide (H(2)O(2)) accumulation or cell death upon in vivo staining, while AsV-exposed calli had increased H(2)O(2), shrinkage of cytoplasmic contents, and cell death. Thus, AsV treatment induced oxidative stress, which in turn elicited a response of antioxidant enzymes and metabolites as compared with control and AsV + Ca treatment. The positive effects of Ca supplementation were also correlated to an increase in thiolic constituents', viz., cysteine, reduced glutathione, and glutathione reductase in AsV + Ca than in AsV treatment. An analysis of selected signaling related genes, e.g., mitogen-activated protein kinases (MAPK3 and MAPK6) and jasmonate ZIM-domain (JAZ3) suggested that AsV and AsV + Ca followed variable pathways to sense and signal the As stress. In AsV-alone treatment, jasmonate signaling was seemingly activated, while MAPK3 was not involved. In contrast, AsV + Ca treatment appeared to specifically inhibit jasmonate signaling and activate MAPK3. In conclusion, Ca supplementation may hold promise for achieving increased As accumulation in plants without compromising their tolerance.

Salicylic acid - a potential biomarker of tobacco Bel-W3 cell death developed as a response to ground level ozone under ambient conditions

Salicylic acid content and benzoic acid 2-hydroxylase (BA2H) activity were investigated in tobacco Bel-W3 and Bel-B leaves after exposure to tropospheric ozone in the conditions of ambient air. Plants were exposed in accordance with a standard methodology for ozone biomonitoring, in a three-year experiment. Free salicylic acid (SA), conjugated with glucose (SAG), and as a product of the BA2H activity was quantified with HPLC. In order to evaluate ozone injuries of leaves, an open source image analysis software was employed. Plants exposure to ambient ozone resulted in enhanced BA2H activity and intensified salicylic acid biosynthesis in leaves of Bel-W3 cultivar showing visible ozone injuries. The BA2H activity significantly correlated with SAG for ozone-exposed Bel-W3 plants. Both injuries and salicylic acid biosynthesis rate depended on the growth phase of leaves and nearly linear correlation between SA content and injuries was found for particular leaves of Bel-W3.

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.

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.

Intracellular chromium localization and cell physiological response in the unicellular alga Micrasterias

Various contaminants like metals and heavy metals are constantly released into the environment by anthropogenic activities. The heavy metal chromium has a wide industrial use and exists in two stable oxidation states: trivalent and hexavalent. Chromium can cause harm to cell metabolism and development, when it is taken up by plants instead of necessary micronutrients such as for example iron. The uptake of Cr VI into plant cells has been reported to be an active process via carriers of essential anions, while the cation Cr III seems to be taken up inactively. Micrasterias denticulata, an unicellular green alga of the family Desmidiaceae is a well-studied cell biological model organism. Cr III and VI had inhibiting effects on its cell development, while cell division rates were only impaired by Cr VI. Transmission electron microscopy (TEM) revealed ultrastructural changes such as increased vacuolization, condensed cytoplasm and dark precipitations in the cell wall after 3 weeks of Cr VI treatment. Electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) were applied to measure intracellular chromium distribution. Chromium was only detected after 3 weeks of 10 μM Cr VI treatment in electron dense precipitations found in bag-like structures along the inner side of the cell walls together with iron and elevated levels of oxygen, pointing toward an accumulation respectively extrusion of chromium in form of an iron-oxygen compound. Atomic emission spectroscopy (EMS) revealed that Micrasterias cells are able to accumulate considerable amounts of chromium and iron. During chromium treatment the Cr:Fe ratio shifted in favor of chromium, which implied that chromium may be taken up instead of iron. Significant and rapid increase of ROS production within the first 5 min of treatment confirms an active Cr VI uptake. SOD and CAT activity after Cr VI treatment did not show a response, while the glutathione pool determined by immuno-TEM decreased significantly in chromium treated cells, showing that glutathione is playing a major role in intracellular ROS and chromium detoxification.

Enhanced sensitivity to higher ozone in a pathogen-resistant tobacco cultivar

Investigations of the effects of elevated ozone (O(3)) on the virus-plant system were conducted to inform virus pathogen management strategies better. One susceptible cultivar of tobacco (Nicotiana tabacum L. cv. Yongding) and a resistant cultivar (Nicotiana tabacum L. cv. Vam) to Potato virus Y petiole necrosis strain (PVY(N)) infection were grown in open-top chambers under ambient and elevated O(3) concentrations. Above-ground biomass, foliage chlorophyll, nitrogen and total non-structural carbohydrate (TNCs), soluble protein, total amino acid (TAA) and nicotine content, and peroxidase (POD) activity were measured to estimate the effects of elevated O(3) on the impact of PVY(N) in the two cultivars. Results showed that under ambient O(3), the resistant cultivar possessed greater biomass and a lower C/N ratio after infection than the susceptible cultivar; however, under elevated O(3), the resistant cultivar lost its biomass advantage but maintained a lower C/N ratio. Variation of foliar POD activity could be explained as a resistance cost which was significantly correlated with biomass and C/N ratio of the tobacco cultivar. Chlorophyll content remained steady in the resistant cultivar but decreased significantly in the susceptible cultivar when stressors were applied. Foliar soluble protein and free amino acid content, which were related to resistance cost changes, are also discussed. This study indicated that a virus-resistant tobacco cultivar showed increased sensitivity to elevated O(3) compared to a virus-sensitive cultivar.

Redox state and energetic equilibrium determine the magnitude of stress in Hydrilla verticillata upon exposure to arsenate

Arsenic (As) is a potential hazard to plants' health, however the mechanisms of its toxicity are yet to be properly understood. To determine the impact of redox state and energetic in stress imposition, plants of Hydrilla verticillata (L.f.) Royle, which are known to be potential accumulator of As, were exposed to 100 and 500 μM arsenate (AsV) for 4 to 96 h. Plants demonstrated significant As accumulation with the maximum being at 500 μM after 96 h (568 μg g(-1) dry weight, dw). The accumulation of As led to a significant increase in the level of reactive oxygen species, nitric oxide, carbonyl, malondialdehyde, and percentage of DNA degradation. In addition, the activity of pro-oxidant enzymes like NADPH oxidase and ascorbate oxidase also showed significant increases. These parameters collectively indicated oxidative stress, which in turn caused an increase in percentage of cell death. These negative effects were seemingly linked to an altered energetic and redox equilibrium [analyzed in terms of ATP/ADP, NADH/NAD, NADPH/NADP, reduced glutathione/oxidized glutathione, and ascorbate/dehydroascobate ratios]. Although there was significant increase in the levels of phytochelatins, the As chelating ligands, a large amount of As was presumably present as free ion particularly at 500 μM AsV, which supposedly produced toxic responses. In conclusion, the study demonstrated that the magnitude of disturbance to redox and energetic equilibrium of plants upon AsV exposure determines the extent of toxicity to plants.

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.