Aluminum Effects on Lipid Peroxidation and Antioxidative Enzyme Activities in Rice Leaves

Aluminium stress tolerance by Citrus plants: a consolidated review

Aluminium, a metallic element abundant in soils as aluminosilicates minerals, poses a toxic threat to plants, particularly in acidic soil conditions, thereby affecting their growth and development. Given their adaptability to diverse soil and climate conditions, Citrus plants have gained significant attention regarding their tolerance to Aluminium toxicity. In the North-eastern region of India, where soils are often slightly acidic with elevated aluminium levels, Citrus species are predominantly found. Understanding the tolerance mechanisms of these Citrus fruits and screening wild Citrus species for their adaptability to abiotic stresses is crucial for enhancing fruit production. Numerous investigations have demonstrated that Citrus species exhibit remarkable tolerance to aluminium contamination, surpassing the typical threshold of 30% incidence. When cultivated in acidic soils, Citrus plants encounter restricted root growth and reduced nutrient and moisture uptake, leading to various nutrient deficiency symptoms. However, promisingly, certain Citrus species such as Citrus jambhiri (Rough lemon), Poncirus trifoliata, Citrus sinensis, and Citrus grandis have shown considerable aluminium tolerance. This comprehensive review delves into the subject of aluminium toxicity and its implications, while also shedding light on the mechanisms through which Citrus plants develop tolerance to this element.

Aluminum uptake, translocation, physiological changes, and overall growth inhibition in rice genotypes (Oryza sativa) at vegetative stage

Aluminum (Al) contamination in acidic soil is a major problem in paddy field, causing grain yield loss, especially in central plains of Thailand. The objective of this study was to assess Al content in the root tissues, its translocation to the leaves, and Al toxicity in three genotypes of rice, RD35 (local acidic-tolerant), Azucena (positive-check Al-tolerant), and IR64 (high yielding) under 0 (control) or 1 mM AlCl₃ (Al toxicity) at pH 4.5. Al content in the root tissues of rice cv. RD35 under 1 mM AlCl₃ was peaked at 4.18 mg g^‒1 DW and significantly translocated to leaf tissues (0.35 mg g^‒1 DW), leading to reduced leaf greenness (SPAD) (by 44.9% over the control) and declined net photosynthetic rate (P_n) (by 54.5% over the control). In contrast, Al level in cvs. Azucena and IR64 was restricted in the roots (2.12 mg g^‒1 DW) with low amount of translocation in the leaf tissues (0.26 mg g^‒1 DW), resulting in maintained values of SPAD and P_n. In cv. RD35, root and shoot traits including root length, root fresh weight, shoot height, shoot fresh weight, and shoot dry weight in 1 mM Al treatment were significantly dropped by > 35% over the control, whereas these parameters in cvs. Azucena and IR64 were retained. Based on the results, RD35 rice genotype was identified as Al sensitive as it demonstrated Al toxicity in both aboveground and belowground parts, whereas Azucena and IR64 were found tolerant to 1 mM Al as they demonstrated storage of Al in the root tissues to reduce toxicity in the leaf tissues. The study suggests that root traits, shoot attributes, chlorophyll degradation, and photosynthetic reduction can be successfully employed for the screening of Al-tolerant genotypes in rice breeding programs.

Aluminum promotes changes in rice root structure and ascorbate and glutathione metabolism

In acidic soil, aluminum (Al) ionizes into trivalent cation and becomes highly toxic to plants. Thus, the objectives of this work were (i) to evaluate the Al concentration and identify sites of Al toxicity and its effect on the structure on rice root tips and (ii) to elucidate the adjustments involved in the activities/contents of enzymes/compounds in the roots against Al. For this, two genotypes with contrasting Al tolerance were used. Our results showed that the root length of the tolerant genotype was not affected after Al exposure. We also observed that both the genotypes used strategies to avoid Al uptake, such as the overlap of P and Al in the tolerant genotype and the presence of border cells in the sensitive genotype, which proved inefficient. In the tolerant genotype, other external Al detoxification mechanisms may have contributed to the lower Al concentration in roots and lower fluorescence of the Al-lumogallion complex. Additionally, both genotypes present the activation of key enzymes to decrease oxidative stress, such as CAT, POX, APX, and DHAR, and a more reducing redox environment, mainly due to the increase in the AA/DHA ratio. However, higher total ascorbate, AA, total glutathione, and GSH contents associated with higher SOD, GPX, and GR activities contributed to the reduction of oxidative stress in the tolerant genotype after Al exposure. Furthermore, there was a strong association between the sensitive genotype to Al concentration, O2 •- content, and MDA amount; therefore, these traits can be used as sensitivity indicators in Al studies.

Different antioxidant regulation mechanisms in response to aluminum-induced oxidative stress in Eucalyptus species

Forest ecosystems play an important role in environmental protection and maintaining ecological balance. Understanding the physiological mechanisms of tree species response to aluminum (Al) toxic is crucial to reveal the main causes of plantation decline in acid rain area. As an important afforestation tree species in tropical and subtropical areas, Eucalyptus has high economic value and plays crucial ecological roles. However, continuous fertilization and acid precipitation can exacerbate soil acidification and increase soil active Al, which has a significant negative impact on Eucalyptus growth. Hence, species and genotypes with high Al resistance are required to solve the problem of Al toxicity of acidic soils for sustainable forest production. In this study, E. urophylla was better adapted to Al stress than E. grandis or E. tereticornis; its high Al resistance was attributed to greater antioxidant enzyme activity and non-enzymatic antioxidant content, and a lower degree of membrane lipid peroxidation than E. grandis or E. tereticornis. The differences in adaptability among the three pure species were attributed to their distinct habitats. Eucalyptus urophylla × E. grandis inherited the outstanding adaptability to Al stress from its maternal species (E. urophylla), indicating that Al tolerance is highly heritable and can be selected in Eucalyptus breeding. Our results indicated that the response of Eucalyptus to Al stress may fluctuate according to the time under stress, and might be related to dynamic changes in ROS elimination and accumulation.

Metal(oid)s accumulation (Hg and As) and their biochemical effects in Halimione portulacoides (Ria de Aveiro, Portugal)

For decades, mercury (Hg) and arsenic (As) were discharged into the Aveiro Lagoon. This study was designed to assess the natural attenuation process evolution by: (1) evaluating Hg and As concentration in sediments and in Halimione portulacoides, and assess the long-term evolution of a historically contaminated salt-marsh system; (2) evaluating the stress levels imposed by Hg and As, and the mechanisms triggered to squash oxidative damage. Concentrations of Hg and As in sediments varied considerably between sampling locations. H. portulacoides did not bioconcentrate Hg and As, restricted translocation to aerial biomass and immobilized contaminants in cell walls. The ions that reached the cytosol induced oxidative stress, restrained by antioxidant mechanisms, especially SOD and CAT activity. Results show that after 25 years of natural attenuation, contamination decreased in the sediment, but remains above non-contaminated systems and continues to induce toxicity in the saltmarsh halophyte community, evidencing the persistent effect of Hg and As contamination.

Fluoride mitigates aluminum-toxicity in barley: morpho-physiological responses and biochemical mechanisms

BACKGROUND

To our knowledge, the role of exogenous fluoride (F^-) on aluminum (Al)-stress mitigation in plants has not been investigated yet. In this experiment, barley (Hordeum vulgaris) seedlings were exposed to excessive Al³⁺ concentrations (aluminum chloride, 0.5, 1.0, 2.0, 3.0, and 4.0 mM) with and without fluoride (0.025% sodium fluoride) to explore the possible roles of fluoride on the alleviation of Al-toxicity.

RESULTS

Overall, Al-stress caused inhibition of growth and the production of photosynthetic pigments. Principal component analysis showed that the growth inhibitory effects were driven by increased oxidative stress and the interruption of water balance in barley under Al-stress. Fluoride priming, on the other hand, enhanced growth traits, chlorophyll a and b content, as well as invigorated the protection against oxidative damage by enhancing overall antioxidant capacity. Fluoride also improved osmotic balance by protecting the plasma membrane. Fluoride reduced endogenous Al³⁺ content, restored Al-induced inhibition of glutathione-S-transferase, and increased  the contents of phytochelatins and metallothioneins, suggesting that fluoride reduced Al³⁺ uptake and improved chelation of Al³⁺.

CONCLUSIONS

Aluminum chloride-induced harmful effects are abridged by sodium fluoride on barely via enhancing antioxidative responses, the chelation mechanism causing reduction of Al uptake and accumulation of barely tissues. Advanced investigations are necessary to uncover the putative mechanisms underpinning fluoride-induced Al-stress tolerance in barley and other economically significant crops, where our results might serve as a solid reference.

Fluoride effect indicators in Phaseolus vulgaris seeds and seedlings

Background

Fluoride (F) is one of the main environmental pollutants, and high concentrations are commonly detected in the air and in both surface and groundwater. However, the effects of this pollutant on seed germination and on the initial growth of crop seedlings are still poorly understood. In this context, the aim of this study was to assess morphoanatomical, physiological and biochemical fluoride effect indicators in Phaseolus vulgaris L. seeds and seedlings.

Methods

P. vulgaris seeds were exposed to a liquid potassium fluoride solution (KF, pH 6.0) at concentrations of 0 (control), 10, 20, 30 mg L-1 for 7 days. A completely randomized experimental design was applied, consisting of four treatments with four replications each. During the experimental period, physiological (7 days) anatomical and histochemical (2 days), biochemical and chemical (4 days) assessments. An analysis of variance was performed followed by Dunnett's test. to determine significant differences between the KF-exposed groups and control seeds; and a multivariate analysis was performed.

Results

The germination parameters, and anatomical, morphological, physiological, biochemical and nutritional characteristics of the seedlings did not show negative effects from exposure to KF at the lowest doses evaluated. On the other hand, treatment with the highest dose of KF (30 mg L-1) resulted in a lower germination rate index and increase in abnormal seedlings, and higher electrical conductivity. A lower root length, magnesium content and photochemical efficiency were also observed. The exposure of P. vulgaris to KF, regardless the dose did not affect seeds anatomy and the accumulation of starch and proteins, in relation to the control group.

Conclusions

Our findings demonstrated that P. vulgaris seedlings were tolerant to KF solutions up to 20 mg L-1, and sensitive when exposed to 30 mg KF L-1.

Searching for biomarkers of early detection of 2,4-D effects in a native tree species from the Brazilian Cerrado biome

Biodiversity in the Brazilian Cerrado biome has been declining sharply with the continued expansion of agriculture and the excessive use of herbicides. Thus, the aim of this study was to evaluate the morphophysiological and biochemical responses in Dipteryx alata plants to various doses of the herbicide 2,4-D. Specific biomarkers that characterize the phytoindicator potential of this species were determined. Gas exchange, chlorophyll a fluorescence, photosynthetic pigments, and the activities of antioxidant enzymes and cellulase were performed after 24, 96 and/or 396 hours after 2,4-D application (HAA). The herbicide caused higher antioxidant enzymatic activity 24 HAA and damage to the photosynthetic machinery after 96 HAA. Reduction in gas exchange, chlorophyll content, and photochemical traits were observed. Increased respiratory rates, non-photochemical quenching, and carotenoid concentrations in 2,4-D-treated plants were important mechanisms in the defense against the excess energy absorbed. Furthermore, the absence of leaf symptoms suggested tolerance of D. alata to 2,4-D. Nevertheless, changes in the photosynthetic and biochemical metabolism of D. alata are useful as early indicators of herbicide contamination, especially in the absence of visual symptoms. These results are important for early monitoring of plants in conserved areas and for preventing damage to sensitive species.

Cultivar Differences in the Biochemical and Physiological Responses of Common Beans to Aluminum Stress

Soil conditions leading to high levels of available aluminum are detrimental to plant growth, but data are limited on genotypic differences in tolerance to aluminum stress in some crops. The aim of this study was to examine the morphological, biochemical, and physiological changes in roots and shoots of 25 common bean (Phaseolus vulgaris L.) cultivars (Pinto market class) under aluminum (Al) treatment. Additionally, this study aimed to assess the range of responses amongst the common bean cultivars relative to their Al toxicity tolerance and sensitivity. Plants were grown hydroponically using a simplified nutrient solution with or without 20 µM AlCl₃. Reactive oxygen species (ROS), activities of the antioxidant enzymes superoxide dismutase (SOD) and guaiacol peroxidase (POD), and malondialdehyde (MDA) concentration, an indicator of lipid peroxidation, were measured to establish the effects of Al treatment on the plants. In addition, growth parameters such as shoot and root dry weight, root-to-shoot ratio, root elongation, and root volume changes were also investigated. The cultivar effect was significant for all the measured parameters, except for shoot dry weight. Inhibition of the root and shoot dry weight for selected common bean cultivars shows that the response of common bean to Al stress is genotype-specific. Additionally, Al-induced root elongation inhibition and root volume changes varied among the cultivars. Most cultivars had significantly higher SOD activity (20 of 25 cultivars) and POD activity (12 cultivars) under AlCl₃ treatment compared to the controls. A positive significant correlation was observed between MDA and ROS, showing that Al stress induced the accumulation of ROS along with an increase in lipid peroxidation. According to the results of this study, Arapaho and AC Island cultivars could potentially be used in the future production of common beans under Al stress. Therefore, these two cultivars could also be included in Al tolerance breeding programs.

Behavior of Cucumis sativus L. in presence of aluminum stress: Germination, plant growth, and antioxidant enzymes

Aluminum (Al) is an ubiquitously present element in soil; it is considered as a major problem in crop production that affects plant growth and development on acid soils. The aim of this work was to determine the germination parameters, to quantify the water status and growth, to assess the Al accumulation, and antioxidant enzyme activities in plants to evaluate the stress exerted by aluminum in Cucumis sativus L. For germination test, increasing doses of Al were used (0, 200, 500, 1,000, and 2,000 μM). Results showed that germination was stimulated with 500 of Al. Aluminum effects on development were studied by treating the plants with different concentrations of Al (100, 200, 300, and 500 µM, Al) during 45 days. As regards to the plant's growth, water content, and dry biomass production there was a slight increase. On the other hand, the activities of the antioxidant enzymes were disturbed by aluminum stress. Data indicate that the catalase (CAT) activity showed a decrease in the different parts of the plant. However, guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) activities were significantly stimulated. Studying the effects of Al-induced stress allowed us to conclude that cucumber has a high ability to accumulate this element in the roots.

Water contamination with atrazine: is nitric oxide able to improve Pistia stratiotes phytoremediation capacity?

Atrazine is an herbicide commonly used in several countries. Due to its long half-life, associated with its use in large scales, atrazine residues remain as environmental pollutants in water bodies. Phytoremediation is often pointed out as an interesting approach to remove atrazine from the aquatic environment, but its practical application is limited by the high toxicity of this herbicide. Here, we characterize the damages triggered by atrazine in Pistia stratiotes, evaluating the role of nitric oxide (NO), a cell-signaling molecule, in increasing the tolerance to the pollutant and the phytoremediation potential of this species. Pistia stratiotes plants were exposed to four treatments: Control; Sodium nitroprusside (SNP) (0.05 mg L^-1); Atrazine (ATZ) (150 μg L^-1) and ATZ + SNP. The plants remained under those conditions for 24 h for biochemical and physiological analysis and 3 days for the evaluation of relative growth rate. The presence of atrazine in plant cells triggered a series of biochemical and physiological damages, such as the increase in the generation of reactive oxygen species, damages to cell membranes, photosynthesis impairment, and negative carbon balance. Despite this, the plants maintained greater growth rates than other aquatic macrophytes exposed to atrazine and showed high bioconcentration and translocation factors. The addition of SNP, a NO donor, decreased the herbicide toxicity, with an increase of over 60% in the IC₅₀ value (Inhibitor Concentration). Indeed, the NO signaling action was able to increase the tolerance of plants to atrazine, which resulted in increments in pollutant uptake and translocation, with the maintenance of overall cell (e.g. membranes) and organs (root system) structure, and the functioning of central physiological processes (e.g. photosynthesis). These factors allowed for more quickly and efficient removal of the pollutant from the environment, reducing costs, and increasing the viability of the phytoremediation process.

Transcriptome analysis reveals significant difference in gene expression and pathways between two peanut cultivars under Al stress

Aluminum (Al) toxicity is an important factor in limiting peanut growth on acidic soil. The molecular mechanisms underlying peanut responses to Al stress are largely unknown. In this study, we performed transcriptome analysis of the root tips (0-1 cm) of peanut cultivar ZH2 (Al-sensitive) and 99-1507 (Al-tolerant) respectively. Root tips of peanuts that treated with 100 μM Al for 8 h and 24 h were analyzed by RNA-Seq, and a total of 8,587 differentially expressed genes (DEGs) were identified. GO and KEGG pathway analysis excavated a group of important Al-responsive genes related to organic acid transport, metal cation transport, transcription regulation and programmed cell death (PCD). These homologs were promising targets to modulate Al tolerance in peanuts. It was found that the rapid transcriptomic response to Al stress in 99-1507 helped to activate effective Al tolerance mechanisms. Protein and protein interaction analysis indicated that MAPK signal transduction played important roles in the early response to Al stress in peanuts. Moreover, weighted correlation network analysis (WGCNA) identified a predicted EIL (EIN3-like) gene with greatly increased expression as an Al-associated gene, and revealed a link between ethylene signaling transduction and Al resistance related genes in peanut, which suggested the enhanced signal transduction mediated the rapid transcriptomic responses. Our results revealed key pathways and genes associated with Al stress, and improved the understanding of Al response in peanut.

Multidimensional analysis of actin depolymerising factor family in pigeon pea under different environmental stress revealed specific response genes in each subgroup

Actin depolymerising factor (ADF) is an actin binding protein that is ubiquitous in animal and plant cells. It plays an important role in plant growth and development, as well as resistance to biotic and abiotic stress. The research of plant ADF family has been restricted to Arabidopsis thaliana (L.) Heynh. and some herb crops, but no woody cash crops have been reported to date. All members of the Cajanus cajan (L.) Millsp. ADF (CcADF) family were identified from the pigeon pea genome, and distributed among the four subfamilies by phylogenetic analysis. CcADFs were relatively conservative in gene structure evolution, protein structure and functional expression, and different CcADFs showed specific expression patterns under different treatments. The expression characteristics of several key CcADFs were revealed by analysing the stress response pattern of CcADFs and the time series RNA-seq of aluminium stress. Among them, CcADF9 in the first subgroup specifically responded to aluminium stress in the roots; CcADF3 in the second subgroup intensively responded to fungal infection in the leaves; and CcADF2 in the fourth subgroup positively responded to various stress treatments in different tissues. This study extended the relationship between plant ADF family and aluminium tolerance, as well as adding to the understanding of CcADF family in woody crops.

Evidence of drought memory in Dipteryx alata indicates differential acclimation of plants to savanna conditions

The remarkable phytogeographic characteristics of the Brazilian savanna (Cerrado) resulted in a vegetation domain composed of plants with high structural and functional diversity to tolerate climate extremes. Here we used a key Cerrado species (Dipteryx alata) to evaluate if species of this domain present a mechanism of stress memory, responding more quickly and efficiently when exposed to recurrent drought episodes. The exposure of D. alata seedlings to drought resulted in several changes, mainly in physiological and biochemical traits, and these changes differed substantially when the water deficit was imposed as an isolated event or when the plants were subjected to drought cycles, suggesting the existence of a drought memory mechanism. Plants submitted to recurrent drought events were able to maintain essential processes for plant survival when compared to those submitted to drought for the first time. This differential acclimation to drought was the result of orchestrated changes in several metabolic pathways, involving differential carbon allocation for defense responses and the reprogramming and coordination of primary, secondary and antioxidant metabolism. The stress memory in D. alata is probably linked the evolutionary history of the species and reflects the environment in which it evolved.

Tolerance of Landoltia punctata to arsenate: an evaluation of the potential use in phytoremediation programs

Plants used in phytoremediation should accumulate and tolerate a specific pollutant. Here, we aimed at evaluating a possible arsenic (As) accumulation and mechanisms of tolerance against As-induced damage in Landoltia punctata to explore this species for phytoremediation. Plants were subjected to increasing As levels. As absorption was higher with increasing As levels. The activity of superoxide dismutase and glutathione reductase as well as anthocyanin levels increased with As levels. Catalase and peroxidase activities increased in plants subjected to As levels up to 1.0 mg L^-1 and decreased at higher levels. Due to the antioxidant system, higher levels of reactive oxygen species were restrained in plants under low levels of As. However, the levels of superoxide anion, hydrogen peroxide, and lipid peroxidation increased in response to the impaired antioxidant system induced by the highest As levels. Biomass decreased in plants exposed to As and scanning electron microscopy revealed root structural damage in the root cap of plants under 3.0 mg L^-1As. This work highlights that L. punctata can be considered a hyperaccumulator species and has potential for As phytoremediation when levels are lower than 1.0 mg L^-1-a concentration 100-fold higher than that recommended for drinking water. Novelty Statement: Landoltia punctata can be considered a hyperaccumulator species and has the potential for arsenic phytoremediation when levels are lower than 1.0 mg L^-1.

Effect of glutamate on Pyricularia oryzae infection of rice monitored by changes in photosynthetic parameters and antioxidant metabolism

Considering the importance of blast caused by Pyricularia oryzae in the decrease of rice yield worldwide, this study aimed to assess the photosynthetic performance [leaf gas exchange and chlorophyll (Chl) a fluorescence parameters as well as the photosynthetic pigments concentration], the activities of antioxidant enzymes [ascorbate peroxidase, catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione-S-transferase] and concentrations of hydrogen peroxide (H₂ O₂ ) and malondialdehyde (MDA) in the leaves of rice plants non-supplied (-Glu) or supplied (+Glu) with glutamate (Glu) and non-infected or infected by P. oryzae. Blast severity was reduced in the leaves of +Glu plants. On the infected leaves of +Glu plants, the values for internal CO₂ concentration were lower while the values for net carbon assimilation rate, stomatal conductance as well as for the concentrations of Chl a, Chl b and carotenoids were higher in comparison to infected leaves of -Glu plants. The functionality of the photosynthetic apparatus was preserved in the infected leaves of +Glu plants. The activities of CAT, GPX, GR, POX and SOD increased in the infected leaves of both -Glu and +Glu plants compared to their non-inoculated counterparts, but their activities were lower for +Glu plants. The lower activity of these antioxidative enzymes was triggered by the reduced hydrogen peroxide concentration in the infected leaves of +Glu plants resulting in lower MDA concentration. It can be concluded that photosynthesis was less impaired in infected plants supplied with glutamate due to the lower biochemical constraints for CO₂ fixation. Moreover, there was a need for lower activity of reactive oxygen species scavenging enzymes in infected leaves of plants supplied with glutamate due to the lower oxidative stress imposed by P. oryzae infection.

Dipteryx alata, a tree native to the Brazilian Cerrado, is sensitive to the herbicide nicosulfuron

The expansion of land use for agricultural interests and the excessive use of herbicides are among the causes of biodiversity losses in the Brazilian Cerrado biome. Therefore, we aimed to test the hypothesis that Dipteryx alata Vogel, a common species in this biome, is sensitive to nicosulfuron because of its high phytotoxicity. We evaluated physiological, biochemical and morphological responses in D. alata plants exposed to increasing doses of the herbicide. Young plants were transplanted to 10 L pots containing substrate composed of soil and sand (2:1) after fertilization. After an acclimation period, the following doses of nicosulfuron were applied: 0 (control), 6, 12, 24, 48, and 60 g a.e. ha^-1. The experiment was conducted in a randomized block design factorial scheme with six doses of nicosulfuron, three evaluation times, and five replicates per treatment. The effects of the herbicide were assessed by measuring gas exchange, chlorophyll a fluorescence, photosynthetic pigments, membrane permeability, antioxidant enzymes and acetolactate synthase. Nicosulfuron altered the photosynthetic machinery and enzymatic metabolism of D. alata. Reductions in physiological traits, increased catalase and ascorbate peroxidase activities, enhanced malondialdehyde concentrations rate of electrolyte leakage and decreased acetolactate synthase activity in response to nicosulfuron all suggest that D. alata is sensitive to this herbicide.

Nitric oxide and phytohormone interactions in the response of Lactuca sativa to salinity stress

MAIN CONCLUSION

Nitric oxide increased lettuce's tolerance to salinity by restoring its hormonal balance, consequently reducing Na + accumulation and activating defense mechanisms that allowed the attenuation of ionic, oxidative, and osmotic stresses. Agricultural crops are continually threatened by soil salinity. The plant's ability to tolerate soil salinity can be increased by treatment with the signaling molecule nitric oxide (NO). Involvement of NO in plant metabolism and its interactions with phytohormones have not been fully described, so knowledge about the role of this radical in signaling pathways remains fragmented. In this work, Lactuca sativa (lettuce) plants were subjected to four treatments: (1) control (nutrient solution); (2) SNP [nutrient solution containing 70 μM sodium nitroprusside (SNP), an NO donor]; (3) NaCl (nutrient solution containing 80 mM NaCl); or (4) SNP + NaCl (nutrient solution containing SNP and NaCl). The plants were exposed to these conditions for 24 h, and then, the roots and leaves were collected and used to evaluate biochemical parameters (reactive oxygen species (ROS) production, cell membrane damage, cell death, antioxidant enzymes activities, and proline concentration), physiological parameters (pigments' concentration and gas-exchange measurements), and phytohormone content. To evaluate growth, tolerance index, and nutrient concentration, the plants were exposed to the treatments for 3 days. L sativa exposure to NaCl triggered ionic, osmotic, and oxidative stress, which resulted in hormone imbalance, cell death, and decreased growth. These deleterious changes were correlated with Na+ content in the vegetative tissues. Adding NO decreased Na+ accumulation and stabilized the mineral nutrient concentration, which maintained the photosynthetic rate and re-established growth. NO-signaling action also re-established the phytohormones balance and resulted in antioxidant system activation and osmotic regulation, with consequent increase in plants tolerance to the salt.

Biochemical responses of common bean to white mold potentiated by phosphites

Considering that the mechanisms for phosphite-afforded disease control remain elusive, this study investigated whether zinc (Zn) and copper (Cu) phosphites could possible potentiate common bean resistance to white mold, caused by Sclerotinia sclerotiorum, through the stimulation of biochemical defence responses. Lesion area and disease severity were decreased by phosphites spray, but Zn phosphite outcompeted Cu phosphite. Histopathological observations revealed fewer fungal hyphae and less collapse of the mesophyll cells in the Zn and Cu phosphite-sprayed plants compared to water-sprayed ones. The S. sclerotiorum-triggered accumulation of reactive oxygen species, oxalic acid (a fungal secreted toxin) and malondialdehyde (an indicator of cellular damage) were constrained as a result of Zn and Cu phosphites spray. Activities of antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase and glutathione-S-transferase at 12 h after inoculation (hai) and catalase at 60 and 84 hai) were higher for Zn and Cu phosphites-sprayed plants than for water-sprayed ones. Activities of defence-related enzymes chitinase (CHI) at 12 hai, β-1,3-glucanase (GLU) and polyphenoloxidase (PPO) were higher at 12-84 hai for Zn, and Cu phosphites sprayed plants, phenylalanine ammonia-lyase at 36-84 hai for the Zn phosphite sprayed ones, CHI at 12-36 hai, GLU at 12-60 hai, PPO at 36 hai and PAL and lipoxygenase at 12 hai for the Cu phosphite sprayed ones upon inoculation with S. sclerotiorum relative to their water-sprayed counterparts. Concentrations of total soluble phenols and lignin-thioglycolic acid derivatives were not affected by Cu phosphite spray on infected plants but were higher and lower, respectively, for Zn phosphite sprayed plants at 60 hai compared to water-sprayed ones. Taken together, the findings from the present study shed light on the biochemical defence mechanisms involved in the Zn and Cu phosphites-mediated suppression of white mold in common bean.

Importance of Mineral Nutrition for Mitigating Aluminum Toxicity in Plants on Acidic Soils: Current Status and Opportunities

Aluminum (Al) toxicity is one of the major limitations that inhibit plant growth and development in acidic soils. In acidic soils (pH < 5.0), phototoxic-aluminum (Al3+) rapidly inhibits root growth, and subsequently affects water and nutrient uptake in plants. This review updates the existing knowledge concerning the role of mineral nutrition for alleviating Al toxicity in plants to acid soils. Here, we explored phosphorus (P) is more beneficial in plants under P-deficient, and Al toxic conditions. Exogenous P addition increased root respiration, plant growth, chlorophyll content, and dry matter yield. Calcium (Ca) amendment (liming) is effective for correcting soil acidity, and for alleviating Al toxicity. Magnesium (Mg) is able to prevent Al migration through the cytosolic plasma membrane in root tips. Sulfur (S) is recognized as a versatile element that alleviates several metals toxicity including Al. Moreover, silicon (Si), and other components such as industrial byproducts, hormones, organic acids, polyamines, biofertilizers, and biochars played promising roles for mitigating Al toxicity in plants. Furthermore, this review provides a comprehensive understanding of several new methods and low-cost effective strategies relevant to the exogenous application of mineral nutrition on Al toxicity mitigation. This information would be effective for further improvement of crop plants in acid soils.

Oxalic acid-mediated biochemical and physiological changes in the common bean-Sclerotinia sclerotiorum interaction

The success of Sclerotinia sclerotiorum infection relies mainly on the production of the non-host selective toxin named oxalic acid (OA). This toxin is known to play multiple roles in a host infected by the fungus, but its effect on photosynthesis and the antioxidant system of common bean plants remain elusive. Therefore, we performed detailed analysis of leaf gas exchange, chlorophyll a fluorescence, activities of antioxidant enzymes, concentrations of reactive oxygen species and photosynthetic pigments to investigate the OA's role during the S. sclerotiorum pathogenesis. To achieve this goal, common bean plants were sprayed with water or with oxalic acid (referred to as -OA and +OA plants, respectively) and either non-challenged or challenged with a wild-type (WT) or an OA-defective mutant (A4) of S. sclerotiorum. Irrespective of OA spray, the WT isolate was more aggressive than the A4 isolate and spraying OA increased OA concentration in the leaflets as well as the aggressiveness of both isolates. Biochemical limitations were behind S. sclerotiorum-induced photosynthetic impairments notably for the +OA plants inoculated with the WT isolate. Inoculated plants were not able to fully capture and exploit the collected energy due to the degradation of photosynthetic pigments. Photoinhibition of photosynthesis and photochemical dysfunctions were potentiated by OA. Higher activities of superoxide dismutase, peroxidase and ascorbate peroxidase besides reductions on catalase activity were noticed for plants inoculated with the WT isolate. OA was able to counteract most of the increases in the activities of antioxidant enzymes thereby increasing the generation of superoxide and hydrogen peroxide and the concurrent damage to the membranes of host cells as evidenced by the high malondialdehyde concentration. In conclusion, OA was found to enhance biochemical limitations to photosynthesis, photochemical dysfunctions and oxidative stress in the leaflets of common bean plants infected by S. sclerotiorum.

Nitric oxide mitigates the effect of water deficit in Crambe abyssinica

Crambe abyssinica is widely cultivated in the off-season in the Midwest region of Brazil with great potential for biodeisel production. Low precipitation is characteristic of this region, which can drastically affect the productivity of C. abyssinica. Signaling molecules, such as nitric oxide (NO), can potentially alleviate the effects of water stress on plants. Here we test whether nitric oxide, applied by donor sodium nitroprusside (SNP), can alleviate the occurrence of water deficit damages in Crambe plants and maintain physiological and biochemical processes. Crambe plants were sprayed with three doses of SNP (0, 75, and 150 μM) and were submitted to two water levels (100% and 50% of the maximum water holding capacity). After 32 and 136 h, leaves were analyzed to evaluate the concentration of NO, water relations, gas exchange, chlorophyll a fluorescence, chloroplastidic pigments, proline, malondialdehyde, hydrogen peroxide, superoxide anions, and the antioxidant enzymes activity. Application of SNP allowed the maintenance of gas exchange, chlorophyll fluorescence parameters, and activities of antioxidant enzymes in plants exposed to water deficit, as well as increased the concentration of NO, proline, chloroplastidic pigments and osmotic potential. The application of SNP also decreased the concentration of malondialdehyde and reactive oxygen species in plants submitted to water deficit. Thus, the application of SNP prevented the occurrence of symptoms of water deficit in Crambe plants, maintaining the physiological and biochemical responses at reference levels, even under stress conditions.

Hydrogen Sulfide Alleviates Aluminum Toxicity via Decreasing Apoplast and Symplast Al Contents in Rice

Hydrogen sulfide (H₂S) plays a vital role in Al³⁺ stress resistance in plants, but the underlying mechanism is unclear. In the present study, pretreatment with 2 μM of the H₂S donor NaHS significantly alleviated the inhibition of root elongation caused by Al toxicity in rice roots, which was accompanied by a decrease in Al contents in root tips under 50 μM Al³⁺ treatment. NaHS pretreatment decreased the negative charge in cell walls by reducing the activity of pectin methylesterase and decreasing the pectin and hemicellulose contents in rice roots. This treatment also masked Al-binding sites in the cell wall by upregulating the expression of OsSATR1 and OsSTAR2 in roots and reduced Al binding in the cell wall by stimulating the expression of the citrate acid exudation gene OsFRDL4 and increasing the secretion of citrate acid. In addition, NaHS pretreatment decreased the symplasmic Al content by downregulating the expression of OsNRAT1, and increasing the translocation of cytoplasmic Al to the vacuole via upregulating the expression of OsALS1. The increment of antioxidant enzyme [superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and peroxidase (POD)] activity with NaHS pretreatment significantly decreased the MDA and H₂O₂ content in rice roots, thereby reducing the damage of Al³⁺ toxicity on membrane integrity in rice. H₂S exhibits crosstalk with nitric oxide (NO) in response to Al toxicity, and through reducing NO content in root tips to alleviate Al toxicity. Together, this study establishes that H₂S alleviates Al toxicity by decreasing the Al content in the apoplast and symplast of rice roots.

Role of 24-epibrassinolide (EBL) in mediating heavy metal and pesticide induced oxidative stress in plants: A review

Industrialization and urbanization have posed serious threats to the environment. Excessive release of heavy metals from industrial effluents and overuse of pesticides in modern agriculture are limiting crop production by polluting environment and deteriorating food quality. Sustaining food quality under heavy metals and pesticide stress is crucial to meet the increasing demands for food. 24-Epibrassinolide (EBL), a ubiquitously occurring plant growth hormone shows great potential to alleviate heavy metals and pesticide stress in plants. This review sums up the potential role of EBL in ameliorating heavy metals and pesticide toxicity in plants extensively. EBL application increases plant's overall growth, biomass accumulation and photosynthetic efficiency by the modulation of numerous biochemical and physiological processes under heavy metals and pesticide stress. In addition, EBL scavenges reactive oxygen species (ROS) by triggering the production of antioxidant enzymes such as SOD, CAT, POX etc. EBL also induces the production of proline and soluble proteins that helps in maintaining osmotic potential and osmo-protection under both heavy metals and pesticide stress. At the end, future needs of research about the application of 24-epibrassinolide have also been discussed.

S-nitrosoglutathione spraying improves stomatal conductance, Rubisco activity and antioxidant defense in both leaves and roots of sugarcane plants under water deficit

Water deficit is a major environmental constraint on crop productivity and performance and nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. This study aims to test the hypothesis that leaf spraying of S-nitrosoglutathione (GSNO), an NO donor, improves the antioxidant defense in both roots and leaves of sugarcane plants under water deficit, with positive consequences for photosynthesis. In addition, the roles of key photosynthetic enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in maintaining CO₂ assimilation of GSNO-sprayed plants under water deficit were evaluated. Sugarcane plants were sprayed with water or GSNO 100 μM and subjected to water deficit, by adding polyethylene glycol (PEG-8000) to the nutrient solution. Sugarcane plants supplied with GSNO presented increases in the activity of antioxidant enzymes such as superoxide dismutase in leaves and catalase in roots, indicating higher antioxidant capacity under water deficit. Such adjustments induced by GSNO were sufficient to prevent oxidative damage in both organs and were associated with better leaf water status. As a consequence, GSNO spraying alleviated the negative impact of water deficit on stomatal conductance and photosynthetic rates, with plants also showing increases in Rubisco activity under water deficit.

Morpho-physiological analysis of tolerance to aluminum toxicity in rice varieties of North East India

Aluminum (Al) is the third most abundant metal in earth crust, whose chemical form is mainly dependent on soil pH. The most toxic form of Al with respect to plants is Al³⁺, which exists in soil pH <5. Acidic soil significantly limits crop production mainly due to Al³⁺ toxicity worldwide, impacting approximately 50% of the world’s arable land (in North-Eastern India 80% soil are acidic). Al³⁺ toxicity in plants ensues root growth inhibition leading to less nutrient and water uptake impacting crop productivity as a whole. Rice is one of the chief grains which constitutes the staple food of two-third of the world population including India and is not untouched by Al³⁺ toxicity. Al contamination is a critical constraint to plant production in agricultural soils of North East India. 24 indigenous Indica rice varieties (including Badshahbhog as tolerant check and Mashuri as sensitive check) were screened for Al stress tolerance in hydroponic plant growth system. Results show marked difference in growth parameters (relative growth rate, Root tolerance index, fresh and dry weight of root) of rice seedlings due to Al (100 μM) toxicity. Al³⁺ uptake and lipid peroxidation level also increased concomitantly under Al treatment. Histochemical assay were also performed to elucidate uptake of aluminum, loss of membrane integrity and lipid peroxidation, which were found to be more in sensitive genotypes at higher Al concentration. This study revealed that aluminum toxicity is a serious harmful problem for rice crop productivity in acid soil. Based on various parameters studied it’s concluded that Disang is a comparatively tolerant variety whereas Joymati a sensitive variety. Western blot hybridization further strengthened the claim, as it demonstrated more accumulation of Glutathione reductase (GR) protein in Disang rice variety than Joymati under stressed condition. This study also observed that the emergence of lethal toxic symptoms occurs only after 48h irrespective of the dose used in the study.

The Involvement of Nitric Oxide in Integration of Plant Physiological and Ultrastructural Adjustments in Response to Arsenic

High arsenic (As) concentrations are toxic to all the living organisms and the cellular response to this metalloid requires the involvement of cell signaling agents, such as nitric oxide (NO). The As toxicity and NO signaling were analyzed in Pistia stratiotes leaves. Plants were exposed to four treatments, for 24 h: control; SNP [sodium nitroprusside (NO donor); 0.1 mg L-1]; As (1.5 mg L-1) and As + SNP (1.5 and 0.1 mg L-1, respectively). The absorption of As increased the concentration of reactive oxygen species and triggered changes in the primary metabolism of the plants. While photosynthesis and photorespiration showed sharp decrease, the respiration process increased, probably due to chemical similarity between arsenate and phosphate, which compromised the energy status of the cell. These harmful effects were reflected in the cellular structure of P. stratiotes, leading to the disruption of the cells and a possible programmed cell death. The damages were attenuated by NO, which was able to integrate central plant physiological processes, with increases in non-photochemical quenching and respiration rates, while the photorespiration level decreased. The increase in respiratory rates was essential to achieve cellular homeostasis by the generation of carbon skeletons and metabolic energy to support processes involved in responses to stress, as well to maintaining the structure of organelles and prevent cell death. Overall, our results provide an integrated view of plant metabolism in response to As, focusing on the central role of NO as a signaling agent able to change the whole plant physiology.

Investigating the effect of cadmium and aluminium on growth and stress-induced responses in the micropropagated medicinal plant Hypoxis hemerocallidea

Hypoxis hemerocallidea is a highly utilized medicinal plant in South Africa. Its cultivation has received considerable attention in order to meet the high demand. High levels of cadmium (Cd) and aluminum (Al) in H. hemerocallidea plants sold in traditional medicinal markets was previously reported. The present study used an in vitro propagation model to investigate the uptake of Cd and Al by H. hemerocallidea and their effect on plant growth, elemental uptake and some stress-induced responses such as pigment, malondialdehyde (MDA), proline content and ultrastructural changes. Shoot and root growth of plantlets exposed to Cd, Cd:Al and high concentrations of Al was significantly reduced. Highest concentrations of Cd accumulated in the corms of Cd-treated plantlets while highest Al concentrations occurred in the leaves and roots. There was higher accumulation of Cd and Al when applied singularly compared to the Cd:Al combination treatments. Cd and Al also reduced accumulation of trace elements in micropropagted H. hemerocallidea with lowest concentrations in the Cd:Al combination treatments. Exposure to Cd, Al and Cd:Al significantly reduced the level of chlorophyll but increased the levels of carotenoids, MDA and proline. Ultrastructural changes were also observed in H. hemerocallidea exposed to Cd and Al. All these factors contributed to the inhibition of plant growth and could potentially affect the ability of this important medicinal plant to synthesize bioactive compounds. It is thus necessary to understand heavy metal stress-induced responses in this highly valued medicinal plant to ensure a high quality product for the consumer.

Changes in the Antioxidant System in Soybean Leaves Infected by Corynespora cassiicola

Considering the importance of target spot, caused by the fungus Corynespora cassiicola, to reduce soybean yield in Brazil and that more basic information regarding the soybean-C. cassiicola interaction is needed, the present study aimed to investigate whether the cellular damage caused by C. cassiicola infection could activate the antioxidant system and whether a more efficient antioxidant system could be associated with an increase in soybean resistance to target spot. The activities of the antioxidant enzymes superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase, glutathione S-transferase as well as the concentrations of ascorbate (AsA), hydrogen peroxide (H2O2), superoxide (O2•-), and malondialdehyde (MDA) were measured in soybean plants from two cultivars differing in resistance to the pathogen. The number of lesions per square centimeter was significantly reduced by 14% in plants from cultivar Fundacep 59 compared with plants from cultivar TMG 132. The area under the disease progress curve was significantly lower, by 15%, in plants from Fundacep 59 than in plants from TMG 132. Generally, antioxidant enzyme activities and AsA concentration significantly increased in response to C. cassiicola infection in plants from both cultivars, however more prominent increases were recorded for plants from Fundacep 59. The concentrations of MDA, H2O2, and O2•- also increased, particularly for plants from TMG 132. The results from this study highlight the importance of a more efficient antioxidative system in the removal of reactive oxygen species generated in soybean plants during C. cassiicola infection, contributing to the resistance to target spot.

Alterations in Gas Exchange and Oxidative Metabolism in Rice Leaves Infected by Pyricularia oryzae are Attenuated by Silicon

Rice blast, caused by Pyricularia oryzae, is the most important disease in rice worldwide. This study investigated the effects of silicon (Si) on the photosynthetic gas exchange parameters (net CO2 assimilation rate [A], stomatal conductance to water vapor [gs], internal-to-ambient CO2 concentration ratio [Ci/Ca], and transpiration rate [E]); chlorophyll fluorescence a (Chla) parameters (maximum photochemical efficiency of photosystem II [Fv/Fm], photochemical [qP] and nonphotochemical [NPQ] quenching coefficients, and electron transport rate [ETR]); concentrations of pigments, malondialdehyde (MDA), and hydrogen peroxide (H2O2); and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and lypoxigenase (LOX) in rice leaves. Rice plants were grown in a nutrient solution containing 0 or 2 mM Si (-Si or +Si, respectively) with and without P. oryzae inoculation. Blast severity decreased with higher foliar Si concentration. The values of A, gs and E were generally higher for the +Si plants in comparison with the -Si plants upon P. oryzae infection. The Fv/Fm, qp, NPQ, and ETR were greater for the +Si plants relative to the -Si plants at 108 and 132 h after inoculation (hai). The values for qp and ETR were significantly higher for the -Si plants in comparison with the +Si plants at 36 hai, and the NPQ was significantly higher for the -Si plants in comparison with the +Si plants at 0 and 36 hai. The concentrations of Chla, Chlb, Chla+b, and carotenoids were significantly greater in the +Si plants relative to the -Si plants. For the -Si plants, the MDA and H2O2 concentrations were significantly higher than those in the +Si plants. The LOX activity was significantly higher in the +Si plants than in the -Si plants. The SOD and GR activities were significantly higher for the -Si plants than in the +Si plants. The CAT and APX activities were significantly higher in the +Si plants than in the -Si plants. The supply of Si contributed to a decrease in blast severity, improved the gas exchange performance, and caused less dysfunction at the photochemical level.

Sulfur metabolism: different tolerances of two aquatic macrophytes exposed to arsenic

The toxicity of arsenic (As) and the mechanisms of response to this pollutant were analyzed in two aquatic plant species, one sensitive and one tolerant to the pollutant, Salvinia minima and Lemna gibba, respectively. The plants, grown in nutrient solution at pH 6.5, were exposed to As concentrations of 0.0 and 1.0mgL(-1) for 3 days. Both species accumulated As in their tissues, which resulted in increases in H2O2 production. L. gibba accumulated eleven times more As than S. minima. However, L. gibba was more tolerant, as shown by the absence of cell membrane damage and, despite greater accumulation, smaller growth reduction than S. minima. Indeed, the index of tolerance to As was twenty percent higher in L. gibba than in S. minima, which most likely results from the presence of a more efficient defense system. This defense system in L. gibba is most likely based on sulfate absorption, assimilation and metabolism. L. gibba showed an increase in sulfate absorption and adenosine-5'-triphosphate (ATP) sulfurylase activity (the first enzyme of the inorganic sulfate assimilation pathway) following exposure to As. Consequently, the plant produced greater concentrations of sulfur-containing compounds that are involved in cellular detoxification, such as glutathione and non-protein thiols, and demonstrated greater enzymatic activity of γ-glutamylcysteine synthetase, glutathione S-transferase and glutathione reductase. Therefore, the plant׳s ability to increase absorption, assimilation and metabolism of sulfur are key steps for tolerance to oxidative stress triggered by metals.

Effects of adding nitroprusside on arsenic stressed response of Pistia stratiotes L. under hydroponic conditions

Effect of nitric oxide (NO) in mitigating stress induced by arsenic (As) was assessed in Pistia stratiotes, with NO supplied as sodium nitroprusside (SNP). Plants were exposed to four treatments: control, SNP (0.1 mg L(-1)), As (1.5 mg L(-1)), As + SNP (1.5 and 0.1 mg L(-1)), for seven days (analyses of growth, absorption of As and mineral nutrients) and for 24 h (analyses of concentration of reactive oxygen intermediates (ROIs), antioxidant capacity and photosynthesis). P. stratiotes accumulated high concentrations of As and this accumulation wasn't affected by the addition of SNP, but the tolerance index of the plant to As increased. SNP attenuated effects of As on the absorption of mineral nutrients (Ca, Fe, Mn, and Mg), but not for phosphorus, and maintained concentrations of ROIs to normal levels, probably due to the increase in antioxidant capacity. The As damaged the photosynthesis by the decrease in pigment contents and by disturbance the photochemical (loss of PSII efficiency and increases in non-photochemical quenching coefficient) and biochemical (reductions in carbon assimilation, increase in the C(i)/C(a) and phi(PSII)/phi(CO2) ratios) steps. The addition of SNP restored these parameters to normal levels. Thus, NO was able to increasing the resistance of P. stratiotes to As.

A novel thylakoid ascorbate peroxidase from Jatrophacurcas enhances salt tolerance in transgenic tobacco

Ascorbate peroxidase (APX) plays an important role in the metabolism of hydrogen peroxide in higher plants. In the present study, a novel APX gene (JctAPX) was cloned from Jatropha curcas L. The deduced amino acid sequence was similar to that of APX of some other plant species. JctAPX has a chloroplast transit peptide and was localized to the chloroplasts by analysis with a JctAPX-green fluorescent protein (GFP) fusion protein. Quantitative polymerase chain reaction (qPCR) analysis showed that JctAPX was constitutively expressed in different tissues from J. curcas and was upregulated by NaCl stress. To characterize its function in salt tolerance, the construct p35S: JctAPX was created and successfully introduced into tobacco by Agrobacterium-mediated transformation. Compared with wild type (WT), the transgenic plants exhibited no morphological abnormalities in the no-stress condition. However, under 200 mM NaCl treatment, JctAPX over-expressing plants showed increased tolerance to salt during seedling establishment and growth. In addition, the transgenic lines showed higher chlorophyll content and APX activity, which resulted in lower H2O2 content than WT when subjected to 400 mM NaCl stress. These results suggest that the increased APX activity in the chloroplasts from transformed plants increased salt tolerance by enhancing reactive oxygen species (ROS)-scavenging capacity under short-term NaCl stress conditions.

Biochemical changes in the leaves of wheat plants infected by Pyricularia oryzae

Blast, caused by the fungus Pyricularia oryzae, is a major disease of the wheat crop in the Brazilian Cerrado and represents a potential threat to world wheat production. However, information about the wheat-P. oryzae interaction is still limited. In this work, the activities of the enzymes superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione-S-transferase (GST), ascorbate peroxidase (APX), glutathione reductase (GR), and glutathione peroxidase (GPX) and the concentrations of superoxide (O₂(-)), hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) as well as the electrolyte leakage (EL) were studied in wheat plants 'BR 18' and 'BRS 229', which are susceptible and partially resistant, respectively, to leaf blast at the vegetative growth stage, during the infection process of P. oryzae. The blast severity in BRS 229 was 50% lower than in BR 18 at 96 h after inoculation (hai). The activities of SOD, POX, APX, and GST increased for both cultivars in the inoculated plants compared with noninoculated plants and the increases were more pronounced for BRS 229 than for BR 18 at 96 hai. The GR and CAT activities only increased in inoculated plants from BRS 229 at 96 hai. For BR 18, the GR activity was not influenced by plant inoculation, and the CAT activity was lower in inoculated plants. The GPX activity only increased in inoculated plants from BR 18 at 48 and 72 hai. The P. oryzae infection increased the O₂(-), H₂O₂, and MDA concentrations and EL. However, the greater increases of the SOD, POX, APX, GST, GR, and CAT activities for BRS 229 compared with BR 18 contributed to the lower O₂(-), H₂O₂, and MDA concentrations and EL verified in the former. These results show that a more efficient antioxidative system in the removal of excess of reactive oxygen species generated during the infection process of P. oryzae limits the cellular damage caused by the fungus, thus contributing to greater wheat resistance to blast.

Aluminum stress response in rice: effects on membrane lipid composition and expression of lipid biosynthesis genes

The presence of aluminum (Al) in acidic soils is a major abiotic stress limiting the production of cultivated plants. Cell membranes are the main targets of environmental stresses and there is growing evidence for the involvement of membrane lipids in plant adaptation. The aim of this study was to evaluate the mid-long effects of Al on membrane lipid content and composition in the roots and shoots of rice plants grown under hydroponic conditions. Four rice cultivars were compared: two acknowledged as Al-resistant (Koshihikari) and Al-sensitive (Kasalath), respectively, and two Vietnamese cultivars, OM6073 and OM1490. Al treatment inhibited root and shoot growth in the sensitive cultivars and the observed changes in root and shoot lipid and fatty acid composition revealed patterns associated with Al sensitivity: larger decreases in lipid content and decreases in fatty acid unsaturation. In the roots, phospholipids, and particularly phosphatidylcholine (PC), decreased dramatically in the susceptible cultivars whereas the amount of lipid classes remained unchanged in the tolerant ones. In the shoots, the glycolipids monogalactosyldiacylglycerol and digalactosyldiacylglycerol as well as PC were mostly affected by Al treatment in the susceptible varieties. mRNA accumulation corresponding to genes coding for galactolipid synthases, enzymes of the PC and phosphatidylethanolamine biosynthetic pathways and fatty acid desaturases correlated well with changes in lipid contents in roots and partly explained lipid changes in leaves. The results suggested that the capacity to maintain the proper functioning of some lipid biosynthetic activities and hence the stability of lipid composition may help the rice plant to withstand Al stress.

Physiological and biochemical aspects of the resistance of banana plants to Fusarium wilt potentiated by silicon

Silicon amendments to soil have resulted in a decrease of diseases caused by several soilborne pathogens affecting a wide number of crops. This study evaluated the physiological and biochemical mechanisms that may have increased resistance of banana to Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense, after treatment with silicon (Si) amendment. Plants from the Grand Nain (resistant to F. oxysporum f. sp. cubense) and "Maçã" (susceptible to F. oxysporum f. sp. cubense) were grown in plastic pots amended with Si at 0 or 0.39 g/kg of soil (-Si or +Si, respectively) and inoculated with race 1 of F. oxysporum f. sp. cubense. Relative lesion length (RLL) and asymptomatic fungal colonization in tissue (AFCT) were evaluated at 40 days after inoculation. Root samples were collected at different times after inoculation with F. oxysporum f. sp. cubense to determine the level of lipid peroxidation, expressed as equivalents of malondialdehyde (MDA), hydrogen peroxide (H(2)O(2)), pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids), total soluble phenolics (TSP), and lignin-thioglycolic acid (LTGA) derivatives; the activities of the enzymes phenylalanine ammonia-lyases glucanases (PALs), peroxidases (POXs), polyphenoloxidases (PPOs), β-1,3-glucanases (GLUs), and chitinases (CHIs); and Si concentration in roots. Root Si concentration was significantly increased by 35.3% for the +Si treatment compared with the -Si treatment. For Grand Nain, the root Si concentration was significantly increased by 12.8% compared with "Maçã." Plants from Grand Nain and "Maçã" in the +Si treatment showed significant reductions of 40.0 and 57.2%, respectively, for RLL compared with the -Si treatment. For the AFCT, there was a significant reduction of 18.5% in the +Si treatment compared with the -Si treatment. The concentration of MDA significantly decreased for plants from Grand Nain and "Maçã" supplied with Si compared with the -Si treatment while the concentrations of H(2)O(2) on roots and pigments on leaves significantly increased. The concentrations of TSP and LTGA derivatives as well as the PALs, PPOs, POXs, GLUs, and CHIs activities significantly increased on roots of plants from Grand Nain and "Maçã" from the +Si treatment compared with the -Si treatment. Results of this study suggest that the symptoms of Fusarium wilt on roots of banana plants supplied with Si decreased due to an increase in the concentrations of H(2)O(2), TSP, and LTGA derivatives and greater activities of PALs, PPOs, POXs, GLUs, and CHIs.

Leaf gas exchange and oxidative stress in sorghum plants supplied with silicon and infected by Colletotrichum sublineolum

Considering the economic importance of anthracnose, caused by Colletotrichum sublineolum, and silicon (Si) to enhance sorghum resistance against this disease, this study aimed to investigate the effect of this element on leaf gas exchange and also the antioxidative system when infected by C. sublineolum. Plants from sorghum line CMSXS142 (BR 009 [Tx623] - Texas), growing in hydroponic culture with (+Si, 2 mM) or without (-Si) Si, were inoculated with C. sublineolum. Disease severity was assessed at 2, 4, 6, 8, and 10 days after inoculation (dai) and data were used to calculate the area under anthracnose progress curve (AUAPC). Further, the net carbon assimilation rate (A), stomatal conductance to water vapor (g(s)), internal-to-ambient CO₂ concentration ratio (C(i)/C(a)), and transpiration rate (E); the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR); the electrolyte leakage (EL), and the concentrations of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) were determined. The AUAPC was reduced by 86% for the +Si plants compared with the -Si plants. The values of A, g(s), and E were lower upon inoculation of -Si plants in contrast to inoculated +Si plants with decreases of 31 and 60% for A, 34 and 61% for g(s), and 27 and 57% for E, respectively, at 4 and 8 dai. For the noninoculated plants, there was no significant difference between the -Si and +Si treatments for the values of A, g(s), and E. The C(i)/C(a) ratio was similar between the -Si and +Si treatments, regardless of the pathogen inoculation. The activities of SOD, CAT, APX, and GR tended to be higher in the +Si plants compared with the -Si plants upon inoculation with C. sublineolum. The EL significantly increased for -Si plants compared with +Si plants. The MDA concentration significantly increased by 31 and 38% at 4 and 8 dai, respectively, for the -Si plants compared with the +Si plants. Based on these results, Si may have a positive effect on sorghum physiology when infected by C. sublineolum through the maintenance of carbon fixation and also by enhancing the antioxidant system, which resulted in an increase in reactive oxygen species scavenging and, ultimately, reduced damage to the cell membranes.

Aluminium Toxicity Targets in Plants

Aluminium (Al) is the third most abundant metallic element in soil but becomes available to plants only when the soil pH drops below 5.5. At those conditions, plants present several signals of Al toxicity. As reported by literature, major consequences of Al exposure are the decrease of plant production and the inhibition of root growth. The root growth inhibition may be directly/indirectly responsible for the loss of plant production. In this paper the most remarkable symptoms of Al toxicity in plants and the latest findings in this area are addressed. Root growth inhibition, ROS production, alterations on root cell wall and plasma membrane, nutrient unbalances, callose accumulation, and disturbance of cytoplasmic Ca2+ homeostasis, among other signals of Al toxicity are discussed, and, when possible, the behavior of Al-tolerant versus Al-sensitive genotypes under Al is compared.

Nitric oxide enhances aluminum tolerance by affecting cell wall polysaccharides in rice roots

Nitric oxide (NO) is a key signal molecule involved in many physiological processes in plants. To study the mechanisms of exogenous NO contribution to alleviate the aluminum (Al) toxicity, roots of rice (Oryza sativa) seedlings pre-treated with sodium nitroprusside (SNP, a NO donor) were used to investigate the effect of Al in this study. Results indicated that NO alleviated the lipid peroxidation induced by Al and promoted the root elongation, whereas butylated hydroxyanisole (BHA), an efficient lipophilic antioxidant, alleviated the lipid peroxidation only. Rice seedling roots pre-treated with SNP followed by Al treatment had lower contents of pectin and hemicellulose, lower Al accumulation in root tips and cell walls, higher degree of methylation of pectin and lower wall Al-binding capacity than the roots with Al treatment only. Therefore, the decreased Al accumulation in the cell walls of rice roots is likely to be the reason for the NO-induced increase of Al tolerance in rice, and it seems that exogenous NO enhanced Al tolerance in rice roots by decreasing the contents of pectin and hemicellulose, increasing the degree of methylation of pectin, and decreasing Al accumulation in root cell walls.

Proteomic identification of OsCYP2, a rice cyclophilin that confers salt tolerance in rice (Oryza sativa L.) seedlings when overexpressed

BACKGROUND

High Salinity is a major environmental stress influencing growth and development of rice. Comparative proteomic analysis of hybrid rice shoot proteins from Shanyou 10 seedlings, a salt-tolerant hybrid variety, and Liangyoupeijiu seedlings, a salt-sensitive hybrid variety, was performed to identify new components involved in salt-stress signaling.

RESULTS

Phenotypic analysis of one protein that was upregulated during salt-induced stress, cyclophilin 2 (OsCYP2), indicated that OsCYP2 transgenic rice seedlings had better tolerance to salt stress than did wild-type seedlings. Interestingly, wild-type seedlings exhibited a marked reduction in maximal photochemical efficiency under salt stress, whereas no such change was observed for OsCYP2-transgenic seedlings. OsCYP2-transgenic seedlings had lower levels of lipid peroxidation products and higher activities of antioxidant enzymes than wild-type seedlings. Spatiotemporal expression analysis of OsCYP2 showed that it could be induced by salt stress in both Shanyou 10 and Liangyoupeijiu seedlings, but Shanyou 10 seedlings showed higher OsCYP2 expression levels. Moreover, circadian rhythm expression of OsCYP2 in Shanyou 10 seedlings occurred earlier than in Liangyoupeijiu seedlings. Treatment with PEG, heat, or ABA induced OsCYP2 expression in Shanyou 10 seedlings but inhibited its expression in Liangyoupeijiu seedlings. Cold stress inhibited OsCYP2 expression in Shanyou 10 and Liangyoupeijiu seedlings. In addition, OsCYP2 was strongly expressed in shoots but rarely in roots in two rice hybrid varieties.

CONCLUSIONS

Together, these data suggest that OsCYP2 may act as a key regulator that controls ROS level by modulating activities of antioxidant enzymes at translation level. OsCYP2 expression is not only induced by salt stress, but also regulated by circadian rhythm. Moreover, OsCYP2 is also likely to act as a key component that is involved in signal pathways of other types of stresses-PEG, heat, cold, or ABA.

Hydrogen sulfide alleviates aluminum toxicity in germinating wheat seedlings

Protective role of hydrogen sulfide (H(2)S) on seed germination and seedling growth was studied in wheat (Triticum) seeds subjected to aluminum (Al(3+)) stress. We show that germination and seedling growth of wheat is inhibited by high concentrations of AlCl(3). At 30 mmol/L AlCl(3) germination is reduced by about 50% and seedling growth is more dramatically inhibited by this treatment. Pre-incubation of wheat seeds in the H(2)S donor NaHS alleviates AlCl(3)-induced stress in a dose-dependant manner at an optimal concentration of 0.3 mmol/L. We verified that the role of NaHS in alleviating Al(3+) stress could be attributed to H(2)S/HS(-) by showing that the level of endogenous H(2)S increased following NaHS treatment. Furthermore, other sodium salts containing sulfur were ineffective in alleviating Al(3+) stress. NaHS pretreatment significantly increased the activities of amylases and esterases and sustained much lower levels of MDA and H(2)O(2) in germinating seeds under Al(3+) stress. Moreover, NaHS pretreatment increased the activities of guaiacol peroxidase, ascorbate peroxidase, superoxide dismutase and catalase and decreased that of lipoxygenase. NaHS pretreatment also decreased the uptake of Al(3+) in AlCl(3)-treated seed. Taken together these results suggest that H(2)S could increase antioxidant capability in wheat seeds leading to the alleviation of Al(3+) stress.

Nitric oxide reduces aluminum toxicity by preventing oxidative stress in the roots of Cassia tora L

Nitric oxide (NO) as a key signaling molecule has been involved in mediation of various biotic and abiotic stress-induced physiological responses in plants. In the present study, we investigated the effect of NO on Cassia tora L. plants exposed to aluminum (Al). Plants pre-treated for 12 h with 0.4 mM sodium nitroprusside (SNP), an NO donor, and subsequently exposed to 10 microM Al treatment for 24 h exhibited significantly greater root elongation as compared with the plants without SNP treatment. The NO-promoted root elongation was correlated with a decrease in Al accumulation in root apexes. Furthermore, oxidative stress associated with Al treatment increased lipid peroxidation and reactive oxygen species, and the activation of lipoxygenase and antioxidant enzymes was reduced by NO. Such effects were confirmed by the histochemical staining for the detection of peroxidation of lipids and loss of membrane integrity in roots. The ameliorating effect of NO was specific, because the NO scavenger cPTIO [2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide] completely reversed the effect of NO on root growth in the presence of Al. These results indicate that NO plays an important role in protecting the plant against Al-induced oxidative stress.

Making the life of heavy metal-stressed plants a little easier

The contamination of soils and water with metals has created a major environmental problem, leading to considerable losses in plant productivity and hazardous health effects. Exposure to toxic metals can intensify the production of reactive oxygen species (ROS), which are continuously produced in both unstressed and stressed plants cells. Some of the ROS species are highly toxic and must be detoxified by cellular stress responses, if the plant is to survive and grow. The aim of this review is to assess the mode of action and role of antioxidants in protecting plants from stress caused by the presence of heavy metals in the environment.