Exogenous nitric oxide donor protects Artemisia annua from oxidative stress generated by boron and aluminium toxicity

Nitric oxide induces antioxidant machinery, PSII functioning and artemisinin biosynthesis in Artemisia annua under cadmium stress

Soil contamination by heavy metals poses a significant environmental challenge, as the practical implementation of existing remediation technologies in the field has encountered numerous obstacles. This has necessitated the requirement of finding alternate solutions to reduce the harm caused to plants. In this study, nitric oxide (NO) was investigated for its potential to reduce cadmium (Cd) toxicity in A. annua plants. Although NO plays a vital role in the growth and development of plants, information on its role in reducing abiotic stress in plants is limited. A. annua plants were exposed to 20 and 40 mg/kg Cd regardless of the addition of exogenous sodium nitroprusside (SNP), a NO donor, at 200 µM concentration. Results showed that SNP treatment improved plant growth, photosynthesis, chlorophyll fluorescence, pigment content, and artemisinin production while reducing Cd accumulation and improving membrane stability in A. annua during Cd stress. The results demonstrated that NO can effectively reverse Cd-induced damage in A. annua by modulating the antioxidant system, maintaining redox homeostasis, and improving photosynthetic performance and different fluorescence parameters such as Fv/Fm, ФPSII, and ETR. The supplementation of SNP caused a substantial improvement in chloroplast ultrastructure, stomatal behavior, and different attributes relate to glandular secretory trichomes, which in turn increased artemisinin production; 14.11 % in plants exposed to Cd stress of 20 mg/kg. Our findings highlight that NO could be useful in mediating the repair of Cd-induced damage to A. annua, and suggest that it may play a critical role in plant signaling networks, improving plant adaptability to Cd stress. The results have important implications for developing new strategies to mitigate the negative impacts of environmental contaminants on plant health, and ultimately, the ecosystem.

Alleviation of cadmium toxicity in wheat by strigolactone: Regulating cadmium uptake, nitric oxide signaling, and genes encoding antioxidant defense system

Cadmium (Cd) in the food system poses a serious threat to human health. The evidence on strigolactones-mediated alleviation of abiotic stress signaling and eliciting physiological modifications in plants is scarce. Therefore, this experiment was conducted to explore the role of exogenous applied strigolactone (SL) in alleviating the toxic effects of Cd and to unravel its physiological, biochemical, and molecular mechanisms in wheat. Excessive accumulation of Cd drastically reduces growth attributes (-15%), nitric oxide signaling, and photosynthetic pigments by increasing oxidative stress biomarkers. Foliar applied SL (4 μM) decreased the Cd-induced growth inhibition (+10%), lessened plant Cd contents (-38% and -36%), shielded chlorophyll pigments (+25%), and considerably decreased Cd-induced oxidative stress in wheat. Moreover, SL applied on wheat foliage remarkably enhanced shoot and root nitric oxide content (+122% and +156%) and nitric oxide synthase activity (104% and 92%) in wheat, efficiently mitigating the Cd-induced suppression of superoxide dismutase and peroxidase, elevating the expression of genes encoding antioxidant defense system. The results of the current research exhibit that SL (GR24) could be a potential candidate for detoxification of Cd by reducing Cd contents, elevating the expression of genes encoding antioxidant defense system, and protecting wheat plants from oxidative stress by indirectly reducing oxidative stress biomarkers andsubsequently contributing to decreasing the possible risk of Cd contamination.

Artemisinin and Ambrosia trifida extract aggravate the effects of short freeze-thaw stress in winter rye (Secale cereale) seedlings

The freeze-thaw and allelopathy from alien giant ragweed (Ambrosia trifida L.) and artemisinin have led to a serious stress to plants, influencing the agricultural quality and crop yield in north-east China. Yet, little is known how allelopathy affect plants under the freeze-thaw process. In this study, the characteristics in winter rye (Secale cereale L.) seedlings were investigated by laboratory simulation. The results showed that during the freezing process, application of artemisinin and A. trifida extract significantly increased the soluble protein content and accelerated lipid peroxidation, while they significantly inhibited antioxidant enzymes, photosynthesis and respiration (P <0.05). During the thawing process, the freezing pressure decreased, and activities of antioxidant enzymes were significantly improved to mitigate artemisinin and A. trifida extract induced stress (P <0.05). In addition, the sensitivity of the investigated metabolic processes in winter rye seedlings were highest to artemisinin and A. trifida extract in the freezing process. This study suggested that the stress response induced by artemisinin and A. trifida extract on winter rye seedlings in the freezing process was greater than that in the thawing process.

Insights into strigolactone (GR24) mediated regulation of cadmium-induced changes and ROS metabolism in Artemisia annua

Agricultural soil contamination and subsequently crops still require alternative solutions to reduce associated environmental risks. The effects of strigolactones (SLs) in alleviating cadmium (Cd) phytotoxicity in Artemisia annua plants were investigated during this study. Strigolactones play a vital role during plant growth and development due to their complex interplay during a plethora of biochemical processes. However, information on the potential of SLs to elicit abiotic stress signaling and trigger physiological modifications in plants is limited. In order to decipher the same, A. annua plants were exposed to different concentrations of Cd (20 and 40 mg kg^-1), with or without the supplementation of exogenous SL (GR24, a SL analogue) at 4 µM concentration. Under Cd stress, excess Cd accumulation resulted in reduced growth, physio-biochemical traits, and artemisinin content. However, the follow-up treatment of GR24 maintained a steady state equilibrium between reactive oxygen species and antioxidant enzymes, improved chlorophyll fluorescence parameters such as Fv/Fm, Ф_PSII, and ETR for improved photosynthesis, enhanced chlorophyll content, maintained chloroplast ultrastructure, improved the glandular trichome (GT) attributes and artemisinin production in A. annua. Moreover, it also resulted in improved membrane stability, reduced Cd accumulation, and regulated the behaviour of stomatal apertures for better stomatal conductance under Cd stress. The results of our study suggest that GR24 could be highly effective in alleviating Cd-induced damages in A. annua. It acts via the modulation of the antioxidant enzyme system for redox homeostasis, protection of the chloroplasts and pigments for improved photosynthetic performance, and improved GT attributes for enhanced artemisinin production in A. annua.

The impact of boron nutrient supply in mulberry (Morus alba) response to metabolomics, enzyme activities, and physiological parameters

Boron (B) is essential for normal and healthy plant growth. Therefore, Boron stress is a common abiotic stress that limits plant growth and productivity. However, how mulberry copes with boron stress remains unclear. In this study, seedlings of the Morus alba cultivar, Yu-711, were treated with five different concentrations of boric acid (H₃BO₃), including deficient (0 and 0.02 mM), sufficient (0.1 mM) and toxic (0.5 and 1 mM) levels. Physiological parameters, enzymatic activities and non-targeted liquid chromatography-mass spectrometry (LC-MS) technique were employed to evaluate the effects of boron stress on the net photosynthetic rate (Pn), chlorophyll content, stomatal conductance (Gs), transpiration rate (Tr), intercellular CO₂ concentration (Ci) and metabolome signatures. Physiological analysis revealed that Boron deficiency and toxicity induced a decline in P_n, C_i, G_s, T_r, and chlorophyll content. Also, enzymatic activities, including catalase (CAT) and superoxide dismutase (SOD), decreased, while POD activity increased in response to Boron stress. Osmotic substances such as soluble sugars, soluble proteins, and proline (PRO) presented elevated levels under all Boron concentrations. Metabolome analysis indicated that differential metabolites, including amino acids, secondary metabolites, carbohydrates, and lipids, played a key role in Yu-711's response to Boron stress. These metabolites were mainly involved in amino acid metabolism, biosynthesis of other secondary metabolites, lipid metabolism, metabolism of cofactors and vitamins, and metabolism of other amino acids pathways. Our findings reveal the various metabolites pathways in mulberry response to boron nutrient supply and may serve as fundamental knowledge in breeding resistance mulberry plants, so that it can cope with climate changes.

Involvement of abscisic acid in silicon-mediated enhancement of copper stress tolerance in Artemisia annua

Heavy metal (HM) toxicity is a well-known hazard which causes deleterious impact on the growth and development of plants. The impact of abscisic acid (ABA) in presence of silicon (Si) on plant development and quality traits has largely gone unexplored. The effects of ABA and Si on the growth, yield, and quality characteristics of Artemisia annua L. plants growing under copper (Cu) stress (20 and 40 mg kg^-1) were investigated in a pot experiment. During this investigation, Cu stress caused severe damage to the plants but exogenous administration of Si and ABA ameliorated the harmful effects of Cu toxicity, and the plants displayed higher biomass and improved physio-biochemical attributes. Copper accumulated in the roots and shoots and its toxicity caused oxidative stress as demonstrated by the increased 2-thiobarbituric acid reactive substance (TBARS) content. It also resulted in the increased activity of antioxidant enzymes, however, the exogenous Si and ABA supplementation decreased the buildup of reactive oxygen species (ROS) and lipid peroxidation, alleviating the oxidative damage produced by HM stress. Copper toxicity had a considerable negative impact on glandular trichome density, ultrastructure as well as artemisinin production. However, combined Si and ABA enhanced the size and density of glandular trichomes, resulting in higher artemisinin production. Taken together, our results demonstrated that exogenous ABA and Si supplementation protect A. annua plants against Cu toxicity by improving photosynthetic characteristics, enhancing antioxidant enzyme activity, protecting leaf structure and integrity, avoiding excess Cu deposition in shoot and root tissues, and helping in enhanced artemisinin biosynthesis. Our results indicate that the combined application of Si and ABA improved the overall growth of plants and may thus be used as an effective approach for the improvement of growth and yield of A. annua in Cu-contaminated soils.

Soil Treatment with Nitric Oxide-Releasing Chitosan Nanoparticles Protects the Root System and Promotes the Growth of Soybean Plants under Copper Stress

The nanoencapsulation of nitric oxide (NO) donors is an attractive technique to protect these molecules from rapid degradation, expanding, and enabling their use in agriculture. Here, we evaluated the effect of the soil application of chitosan nanoparticles containing S-nitroso-MSA (a S-nitrosothiol) on the protection of soybeans (Glycine max cv. BRS 257) against copper (Cu) stress. Soybeans were grown in a greenhouse in soil supplemented with 164 and 244 mg kg-1 Cu and treated with a free or nanoencapsulated NO donor at 1 mM, as well as with nanoparticles without NO. There were also soybean plants treated with distilled water and maintained in soil without Cu addition (control), and with Cu addition (water). The exogenous application of the nanoencapsulated and free S-nitroso-MSA improved the growth and promoted the maintenance of the photosynthetic activity in Cu-stressed plants. However, only the nanoencapsulated S-nitroso-MSA increased the bioavailability of NO in the roots, providing a more significant induction of the antioxidant activity, the attenuation of oxidative damage, and a greater capacity to mitigate the root nutritional imbalance triggered by Cu stress. The results suggest that the nanoencapsulation of the NO donors enables a more efficient delivery of NO for the protection of soybean plants under Cu stress.

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.

Exogenous hydrogen sulphide alleviates copper stress impacts in Artemisia annua L.: Growth, antioxidant metabolism, glandular trichome development and artemisinin biosynthesis

A supply of plant micronutrients (some of which are metals) is necessary to regulate many plant processes; their excess, however, can have detrimental consequences and can hamper plant growth, physiology and metabolism. Artemisia annua is an important crop plant used in the treatment of malaria. In this investigation, the physio-biochemical mechanisms involved in exogenous hydrogen sulphide-mediated (H₂ S) alleviation of copper (Cu) stress in A. annua were assessed.. Two different levels of Cu (20, 40 mg·kg^-1 ), one H₂ S treatment (200 µm) and their combinations were introduced while one set of plants was retained as control. Results showed that the presence of excess Cu in the soil reduced growth and biomass, photosynthetic parameters, chlorophyll content and fluorescence, gas exchange parameters and induced antioxidant enzyme activity. Copper stress enhanced the production of thiobarbituric acid reactive substances (TBARS) and increased Cu content in both roots and shoots of affected plants. Exogenous application of H₂ S restored the physio-biochemical characteristics of Cu-treated A. annua plants by reducing lipid peroxidation and enhancing the activity of antioxidant enzymes in Cu-stressed plants as compared with the controls. Hydrogen sulphide also reduced the Cu content in different plant parts, increased photosynthetic efficiency, trichome density, average area of trichomes and artemisinin content. Therefore, our results provide a comprehensive assessment of the defensive role of H₂ S in Cu-stressed A. annua.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

Salicylic acid-mediated alleviation of soil boron toxicity in Mentha arvensis and Cymbopogon flexuosus: Growth, antioxidant responses, essential oil contents and components

Boron (B) toxicity is a notable abiotic hindrance that restricts crop productivity by disturbing several physiological and biochemical processes in plants. This study was aimed to elucidate the role of salicylic acid (SA) in conferring tolerance to B stress in Mentha arvensis and Cymbopogon flexuosus. Boron toxicity led to a considerable decrease in shoot height and root length, fresh and dry mass of shoot and root, and physiological and biochemical parameters. However, exogenously applied SA relieved the adverse effects caused by B toxicity and led to an increase in growth parameters under B stress and non-stress conditions. The treatment of B resulted in its increased accumulation in roots and shoots of both the plants which, in turn, caused oxidative damage as evident by increased content of malondialdehyde and catalase, peroxidase, superoxide dismutase and glutathione reductase enzyme activities. However, exogenous SA supply significantly affected antioxidant enzyme activities and protected the plants from excess B. Moreover, the essential oil content of two selected plants declined under B toxicity and significantly enhanced in SA-treated stressed plants. The contents of menthol and menthyl acetate in M. arvensis were lowered in B stressed plants which significantly improved in SA treated B-stressed and in their respective SA alone treatment. Similarly, citral-A and citral-B content of C. flexuosus declined under B toxicity, however, SA reversed the negative effects of B toxicity on essential oil components. This assessment stipulated the promising role of exogenously applied SA in alleviating B toxicity in M. arvensis and C. flexuosus by improving antioxidant machinery and limiting B uptake which protects the structural integrity of leaves and also helps in increasing essential oil content.

Exogenous spermidine alleviates the adverse effects of aluminum toxicity on photosystem II through improved antioxidant system and endogenous polyamine contents

Aluminum (Al) toxicity is a major yield-limiting factor for crops in acidic soils. In this work, we have investigated the potential role of spermidine (Spd) on Al toxicity in rice chloroplasts. Exogenous Spd markedly reduced Al concentration and elevated other nutrient elements such as Mn, Mg, Fe, K, Ca, and Mo in chloroplasts of Al-treated plants. Meanwhile, Spd further activated arginine decarboxylase (ADC) activity of key enzyme in polyamine (PA) synthesis, and enhanced PA contents in chloroplasts. Spd application dramatically addressed Al-induced chlorophyll (Chl) losses, inhibited thylakoid membrane protein complexes degradation, especially photosystem II (PSII), and significantly depressed the accumulations of superoxide radical (O₂·^-), hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) in chloroplasts. Spd addition activated antioxidant enzyme activities and decreased soluble sugar content in chloroplasts compared with Al treatment alone. Spd not only reversed the inhibition of photosynthesis-related gene transcript levels induced by Al toxicity, but diminished the increased expression of Chl catabolism-related genes. Furthermore, Chl fluorescence analysis showed that Spd protected PSII reaction centers and photosynthetic electron transport chain under Al stress, thus improving photosynthetic performance. These results suggest that PAs are involved in Al tolerance in rice chloroplasts and can effectively protect the integrity and function of photosynthetic apparatus, especially PSII, by mitigating oxidative damage induced by Al toxicity.

Exogenous abscisic acid mediates ROS homeostasis and maintains glandular trichome to enhance artemisinin biosynthesis in Artemisia annua under copper toxicity

One of the major abiotic stresses that cause environmental pollution is heavy metal stress. In the present investigation, copper (Cu) toxicity caused morphological and cellular damages to the Artemisia annua L. plants but supplementation of abscisic acid (ABA) ameliorated the damaging effect of Cu. Copper toxicity significantly reduced the shoot and root lengths; fresh and dry weights of shoot. However, exogenous application of ABA to Cu-treated plants significantly attenuated the damaging effects on plants caused by Cu toxicity. Copper stress also reduced the physiological and biochemical parameters, but ABA application ameliorated the negative effects of Cu in the affected plant. Accumulation of Cu in plant tissues significantly increased the membrane damage and oxidative enzyme activities such as catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD). Further, the impact of high concentration of Cu on density, area and ultrastructure of glandular trichomes and artemisinin content was studied. Moreover, the foliar application of ABA improved the area, density of glandular trichomes and secured the plant cells from Cu toxicity. Therefore, this investigation indicated that the exogenous application of ABA protects A. annua plant by increasing antioxidant enzymes activity, which helps in maintaining cell integrity of leaves and results in increased artemisinin production.

Salicylic acid restrains arsenic induced oxidative burst in two varieties of Artemisia annua L. by modulating antioxidant defence system and artemisinin production

Arsenic is a harmful and toxic substance to the growth and development of plants. Salicylic acid (SA) acts as a signaling molecule, plays pivotal roles in the overall growth and development of plants under various environmental stresses. Artemisinin extracted from the leaves of A. annua helps in malarial treatment. The present investigation is aimed to find out the possible ameliorative role of exogenously-applied salicylic acid (SA) on two varieties of Artemisia annua L., namely 'CIM-Arogya' and 'Jeevan Raksha' under arsenic (As) stress conditions. For this, growth, physiological and biochemical characterization, and artemisinin production was assessed. The various treatments applied on the plants were Control, 10^-6 M SA, 10^-5 M SA, 45 mg kg^-1As, 45 mg kg^-1 As + 10^-6 M SA, and 45 mg kg^-1 As + 10^-5 M SA. Arsenic at 45 mg kg^-1 of soil, reducing the overall performance of both varieties at 90 and 120 DAP. However, the levels of antioxidants were enhanced in As-stressed plants, and the supplementation of SA further increased these antioxidants in SA-treated plants. It has been observed that minimum reduction in growth and yield occurs with enhanced production of artemisinin in the case of 'CIM-Arogya' compared to 'Jeevan Raksha' under As stress (45 mg kg^-1 of soil). Leaf-applied SA significantly increased the content (49.0% & 43.4%) and yield (53.3% & 46.3%) of artemisinin in both tolerant and sensitive varieties as compared to their respective controls. Thus, the variety 'CIM-Arogya' showed tolerant behavior over 'Jeevan Raksha' and is much adapted to higher As stress.

Salicylic Acid Stimulates Antioxidant Defense and Osmolyte Metabolism to Alleviate Oxidative Stress in Watermelons under Excess Boron

Boron (B) is a microelement required in vascular plants at a high concentration that produces excess boron and toxicity in many crops. B stress occurs widely and limits plant growth and crop productivity worldwide. Salicylic acid (SA) is an essential hormone in plants and is a phenolic compound. The goal of this work is to explore the role of SA in the alleviation of excess B (10 mg L⁻¹) in watermelon plants at a morphological and biochemical level. Excess boron altered the nutrient concentrations and caused a significant reduction in morphological criteria; chlorophyll a, b, and carotenoids; net photosynthetic rate; and the stomatal conductance and transpiration rate of watermelon seedlings, while intercellular carbon dioxide (CO₂) was significantly increased compared to the control plants (0.5 mg L⁻¹ B). Furthermore, excess boron accelerated the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and induced cellular oxidative injury. The application of exogenous SA significantly increased chlorophyll and carotenoid contents in plants exposed to excess B (10 mg L⁻¹), in line with the role of SA in alleviating chlorosis caused by B stress. Exogenously applied SA promoted photosynthesis and, consequently, biomass production in watermelon seedlings treated with a high level of B (10 mg L⁻¹) by reducing B accumulation, lipid peroxidation, and the generation of H₂O₂, while significantly increasing levels of the most reactive ROS, OH⁻. SA also activated antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) and protected the seedlings from an ROS induced cellular burst. In conclusion, SA can be used to alleviate the adverse effects of excess boron.

Impact of Long-Term Copper Exposure on Growth, Photosynthesis, Antioxidant Defence System and Artemisinin Biosynthesis in Soil-Grown Artemisia annua Genotypes

The effects of copper (Cu) exposure on growth and physiological characteristics of three genotypes (CN-12, Cim-Sanjeevani and Cim-Arogya) of Artemisia annua L. were elucidated. The plants were grown under naturally illuminated greenhouse conditions and were harvested after physiological maturity (120 days after sowing). Results suggest that 10 mg kg^- 1 Cu significantly enhanced the growth and physiological parameters like enzyme activities, photosynthesis. At higher concentrations, Cu inhibited the growth, biomass, photosynthetic parameters; while increased lipid peroxidation in all the genotypes. The activities of antioxidant enzymes viz. catalase, peroxidase and superoxide dismutase were upregulated by the Cu stress. The highest applied concentration of Cu (60 mg kg^- 1) proved most toxic for plants. Moreover, artemisinin content was increased upto 10 mg kg^- 1 of Cu treatment, compared with control, however, the artemisinin accumulation decreased at higher doses of Cu in all the genotypes. On the basis of studied parameters, Cim-Arogya was found to be most tolerant among all for Cu toxicity.

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are considered as new 'gasotransmitter' molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂ S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂ S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂ S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.

Alternative Pathway is Involved in Nitric Oxide-Enhanced Tolerance to Cadmium Stress in Barley Roots

Alternative pathway (AP) has been widely accepted to be involved in enhancing tolerance to various environmental stresses. In this study, the role of AP in response to cadmium (Cd) stress in two barley varieties, highland barley (Kunlun14) and barley (Ganpi6), was investigated. Results showed that the malondialdehyde (MDA) content and electrolyte leakage (EL) level under Cd stress increased in two barley varieties. The expressions of alternative oxidase (AOX) genes (mainly AOX1a), AP capacity (V_alt), and AOX protein amount were clearly induced more in Kunlun14 under Cd stress, and these parameters were further enhanced by applying sodium nitroprussid (SNP, a NO donor). Moreover, H₂O₂ and O₂⁻ contents were raised in the Cd-treated roots of two barley varieties, but they were markedly relieved by exogenous SNP. However, this mitigating effect was aggravated by salicylhydroxamic acid (SHAM, an AOX inhibitor), suggesting that AP contributes to NO-enhanced Cd stress tolerance. Further study demonstrated that the effect of SHAM application on reactive oxygen species (ROS)-related scavenging enzymes and antioxidants was minimal. These observations showed that AP exerts an indispensable function in NO-enhanced Cd stress tolerance in two barley varieties. AP was mainly responsible for regulating the ROS accumulation to maintain the homeostasis of redox state.

Tolerance of Eugenia dysenterica to Aluminum: Germination and Plant Growth

Native Cerrado plants are exposed to soils with low pH and high availability of Al. In this study, we measured the Al content in adult plants, and investigated the effects of various Al doses on germination and early development of Eugenia dysenterica plants. For germination tests, the seeds were soaked in Al solution and evaluated for twenty days in growth chambers. In a second experiment, young plants were cultivated in hydroponic systems with various Al concentrations to evaluate the morphological, anatomical and physiological characteristics of E. dysenterica. Anatomical changes and low germinative vigor were observed in seeds germinated in 600 and 800 μmol Al3+ L-1. In the hydroponic system, 200 μmol Al3+ L-1 stimulated root growth in young plants. The activity of antioxidant enzymes and the accumulation of phenolic compounds were greatest at the highest Al doses, preventing changes in gas exchange and chlorophyll a fluorescence. Starch grain accumulation was noted in plant cells exposed to 200 and 400 μmol Al3+ L-1. Adult E. dysenterica trees also accumulated Al in leaves, bark and seeds. These data suggest that E. dysenterica is tolerant to Al.

Overcoming limitation of "recalcitrant areas" to phytoextraction process: The synergistic effects of exogenous cytokinins and nitrogen treatments

The aim of the present work was to test the efficiency of the phytoextraction process involving the use of exogenous phytohormone (cytokinins, CKs) and fertilizer (nitrogen, N) treatments in phytotechnologies to address risk management in "recalcitrant areas". The CKs and N treatments, alone or combined (CKs + N) in a Modulated Application (MA), were tested on the crop plant Helianthus annuus, common to Mediterranean area, fast growing and with high biomass production. Plants were grown on boron (B) contaminated sediments (collected from a geothermal area located in Tuscany (Italy). Plant growth, B uptake, together with plant stress parameters were investigated. Boron is easily taken up and translocated by some crop plants, but the high phytotoxicity can dramatically impact the plant growth and consequently the applicability and efficiency of the phytoextraction process. As indicators of plant stress, oxidative balance and photosynthetic parameters were investigated to give a deeper insight of phytotoxic mechanisms. Results showed that while each treatment (CKs and N alone) had significantly positive effects on plant health, the MA treatment provided a synergistic effect on morphological parameters and biomass production as a whole. After MA treatment, plants showed antioxidant activity comparable to that of the control (unpolluted sediments) and showed an increase of net photosynthesis. Moreover, our data showed very high values of B uptake and translocation (about 800 mg kg^-1 in shoots), without any alteration triggered by the treatments (CKs and N alone or combined in MA). B phytoextraction resulted increased about fivefold with the MA treatments, while each treatment alone increased only two or three folds when treated with either CKs or N. The MA treatment is not "contaminant specific", so it could be applied in other "recalcitrant areas" where different types of contaminations occur, in order to overcome limitations of plant growth.

Nitric oxide protects carbon assimilation process of watermelon from boron-induced oxidative injury

Nitric oxide (NO) mediates plant response to a variety of abiotic stresses; however, limited information is available on its effect on boron (B)-stressed watermelon plants. The present study investigates the mechanism through which NO protects watermelon seedlings from B deficiency and toxicity stresses. Five days old watermelon seedlings were exposed to B (0, 0.5 and 10 mg L^-1) alone or with 75 μmole of NO donor sodium nitroprusside (SNP) for 30 days. Both low and high B concentrations in the media altered nutrient accumulation and impaired various physiological processes of watermelon seedlings, leading to a significant reduction in biomass production. The plants exposed to B deficient or toxic concentrations had 66 and 69% lower shoot dry weight, respectively compared with optimum B levels. B toxicity-induced growth inhibition of watermelon seedlings was associated with high B translocation to shoot tissues, which caused lipid membrane peroxidation (12% increase) and chlorophyll destruction (25% reduction). In contrast, B deficiency accelerated generation of reactive oxygen species (ROS), specifically OH^-1 and induced cellular oxidative injury. Exogenously applied SNP promoted leaf chlorophyll, photosynthesis and consequently biomass production in B-stressed watermelon seedlings by reducing B accumulation, lipid membrane peroxidation and ROS generation. It also activated antioxidant enzymes such as SOD, POD and APX, and protected the seedlings from ROS-induced cellular burst.

Exogenous nitric oxide improves sugarcane growth and photosynthesis under water deficit

Nitric oxide (NO)-mediated redox signaling plays a role in alleviating the negative impact of water stress in sugarcane plants by improving root growth and photosynthesis. Drought is an environmental limitation affecting sugarcane growth and yield. The redox-active molecule nitric oxide (NO) is known to modulate plant responses to stressful conditions. NO may react with glutathione (GSH) to form S-nitrosoglutathione (GSNO), which is considered the main reservoir of NO in cells. Here, we investigate the role of NO in alleviating the effects of water deficit on growth and photosynthesis of sugarcane plants. Well-hydrated plants were compared to plants under drought and sprayed with mock (water) or GSNO at concentrations ranging from 10 to 1000 μM. Leaf GSNO sprayed plants showed significant improvement of relative water content and leaf and root dry matter under drought compared to mock-sprayed plants. Additionally, plants sprayed with GSNO (≥ 100 μM) showed higher leaf gas exchange and photochemical activity as compared to mock-sprayed plants under water deficit and after rehydration. Surprisingly, a raise in the total S-nitrosothiols content was observed in leaves sprayed with GSH or GSNO, suggesting a long-term role of NO-mediated responses to water deficit. Experiments with leaf discs fumigated with NO gas also suggested a role of NO in drought tolerance of sugarcane plants. Overall, our data indicate that the NO-mediated redox signaling plays a role in alleviating the negative effects of water stress in sugarcane plants by protecting the photosynthetic apparatus and improving shoot and root growth.

Alternative oxidase pathway is involved in the exogenous SNP-elevated tolerance of Medicago truncatula to salt stress

Exogenous application of sodium nitroprusside (SNP) would enhance the tolerance of plants to stress conditions. Some evidences suggested that nitric oxide (NO) could induce the expression of alternative oxidase (AOX). In this study, Medicago truncatula (Medicago) was chosen to study the role of AOX in the SNP-elevated resistance to salt stress. Our results showed that the expression of AOX genes (especially AOX1 and AOX2b1) and cyanide-resistant respiration rate (Valt) could be significantly induced by salt stress. Exogenous application of SNP could further enhance the expression of AOX genes and Valt. Exogenous application of SNP could alleviate the oxidative damage and photosynthetic damage caused by salt stress. However, the stress resistance was significantly decreased in the plants which were pretreated with n-propyl gallate (nPG). More importantly, the damage in nPG-pretreated plants could not be alleviated by application of SNP. Further study showed that effects of nPG on the activities of antioxidant enzymes were minor. These results showed that AOX pathway played an important role in the SNP-elevated resistance of Medicago to salt stress. AOX could contribute to regulating the accumulation of reactive oxygen (ROS) and protect of photosystem, and we proposed that all these were depend on the ability of maintaining the homeostasis of redox state.

Lipids and proteins--major targets of oxidative modifications in abiotic stressed plants

Stress factors provoke enhanced production of reactive oxygen species (ROS) in plants. ROS that escape antioxidant-mediated scavenging/detoxification react with biomolecules such as cellular lipids and proteins and cause irreversible damage to the structure of these molecules, initiate their oxidation, and subsequently inactivate key cellular functions. The lipid- and protein-oxidation products are considered as the significant oxidative stress biomarkers in stressed plants. Also, there exists an abundance of information on the abiotic stress-mediated elevations in the generation of ROS, and the modulation of lipid and protein oxidation in abiotic stressed plants. However, the available literature reflects a wide information gap on the mechanisms underlying lipid- and protein-oxidation processes, major techniques for the determination of lipid- and protein-oxidation products, and on critical cross-talks among these aspects. Based on recent reports, this article (a) introduces ROS and highlights their relationship with abiotic stress-caused consequences in crop plants, (b) examines critically the various physiological/biochemical aspects of oxidative damage to lipids (membrane lipids) and proteins in stressed crop plants, (c) summarizes the principles of current technologies used to evaluate the extent of lipid and protein oxidation, (d) synthesizes major outcomes of studies on lipid and protein oxidation in plants under abiotic stress, and finally, (e) considers a brief cross-talk on the ROS-accrued lipid and protein oxidation, pointing to the aspects unexplored so far.

The endogenous nitric oxide mediates selenium-induced phytotoxicity by promoting ROS generation in Brassica rapa

Selenium (Se) is suggested as an emerging pollutant in agricultural environment because of the increasing anthropogenic release of Se, which in turn results in phytotoxicity. The most common consequence of Se-induced toxicity in plants is oxidative injury, but how Se induces reactive oxygen species (ROS) burst remains unclear. In this work, histofluorescent staining was applied to monitor the dynamics of ROS and nitric oxide (NO) in the root of Brassica rapa under Se(IV) stress. Se(IV)-induced faster accumulation of NO than ROS. Both NO and ROS accumulation were positively correlated with Se(IV)-induced inhibition of root growth. The NO accumulation was nitrate reductase (NR)- and nitric oxide synthase (NOS)-dependent while ROS accumulation was NADPH oxidase-dependent. The removal of NO by NR inhibitor, NOS inhibitor, and NO scavenger could alleviate Se(IV)-induced expression of Br_Rbohs coding for NADPH oxidase and the following ROS accumulation in roots, which further resulted in the amelioration of Se(IV)-induced oxidative injury and growth inhibition. Thus, we proposed that the endogenous NO played a toxic role in B. rapa under Se(IV) stress by triggering ROS burst. Such findings can be used to evaluate the toxic effects of Se contamination on crop plants.

ROS generation and proline metabolism in calli of halophyte Nitraria tangutorum Bobr. to sodium nitroprusside treatment

Nitric oxide (NO) is a stress factor or a signal molecule involved in various plant physiological and developmental processes. In the present study, the generation of reactive oxygen species and the metabolism of proline due to different sodium nitroprusside (SNP, an NO donor) concentrations were investigated in callus from halophyte Nitraria tangutorum Bobr. Treatment with SNP led to significant increases of hydrogen peroxide (H2O2) content and cell viability but notable reductions in hydrogen radical level and lipid peroxidation degree, and superoxide onion (O2 (-)) content also enhanced in 100 μM SNP-treated calli. Using a chemical inhibitor for plasma membrane (PM) NADPH oxidase diphenylene iodonium (DPI), we found low O2 (-) generation in untreated and 25 μM SNP-treated calli, whereas in those treated with 100 μM SNP O2 (-) level exhibited a very little alteration, comparable to the absence of DPI. These suggest a high activity of PM NADPH oxidase in untreated calli. H2O2 scavenging enzymes (catalase, peroxidase [POD] and ascorbate peroxidase) and H2O2 forming enzymes (superoxide dismutase [SOD], cell wall-POD and diamine oxidase [DAO]) stimulated significantly in calli treated with different SNP concentrations while glutathione reductase activity decreased. In addition, a reduction in proline content was observed in SNP-treated calli. Moreover, different SNP concentrations stimulated proline dehydrogenase (PDH) and ornithine δ-aminotransferase but inhibited r-glutamyl kinase (GK). In conclusion, our results suggest that the increasing H2O2 generation was associated with the stimulation of SOD, cell wall-POD and DAO, and that the reduction of proline content might be the consequence of increased PDH activity and decreased GK activity in N. tangutorum Bobr. calli under SNP treatment.