Salicylic acid enhances nickel stress tolerance by up-regulating antioxidant defense and glyoxalase systems in mustard plants

Salicylic acid pre-treatment modulates Pb2+-induced DNA damage vis-à-vis oxidative stress in Allium cepa roots

The current study investigated the putative role of salicylic acid (SA) in modulating Pb²⁺-induced DNA and oxidative damage in Allium cepa roots. Pb²⁺ exposure enhanced free radical generation and reduced DNA integrity and antioxidant machinery after 24 h; however, SA pre-treatment (for 24 h) ameliorated Pb²⁺ toxicity. Pb²⁺ exposure led to an increase in malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) accumulation and enhanced superoxide radical and hydroxyl radical levels. SA improved the efficiency of enzymatic antioxidants (ascorbate and guaiacol peroxidases [APX, GPX], superoxide dismutases [SOD], and catalases [CAT]) at 50-μM Pb²⁺ concentration. However, SA pre-treatment could not improve the efficiency of CAT and APX at 500 μM of Pb²⁺ treatment. Elevated levels of ascorbate and glutathione were observed in A. cepa roots pre-treated with SA and exposed to 50 μM Pb²⁺ treatment, except for oxidized glutathione. Nuclear membrane integrity test demonstrated the ameliorating effect of SA by reducing the number of dark blue-stained nuclei as compared to Pb²⁺ alone treatments. SA was successful in reducing DNA damage in cell exposed to higher concentration of Pb²⁺ (500 μM) as observed through comet assay. The study concludes that SA played a major role in enhancing defense mechanism and protecting against DNA damage by acclimatizing the plant to Pb²⁺-induced toxicity.

What signals the glyoxalase pathway in plants?

Glyoxalase (GLY) system, comprising of GLYI and GLYII enzymes, has emerged as one of the primary methylglyoxal (MG) detoxification pathways with an indispensable role during abiotic and biotic stresses. MG homeostasis is indeed very closely guarded by the cell as its higher levels are cytotoxic for the organism. The dynamic responsiveness of MG-metabolizing GLY pathway to both endogenous cues such as, phytohormones, nutrient status, etc., as well as external environmental fluctuations (abiotic and biotic stresses) indicates that a tight regulation occurs in the cell to maintain physiological levels of MG in the system. Interestingly, GLY pathway is also manipulated by its substrates and reaction products. Hence, an investigation of signalling and regulatory aspects of GLY pathway would be worthwhile. Herein, we have attempted to converge all known factors acting as signals or directly regulating GLYI/II enzymes in plants. Further, we also discuss how crosstalk between these different signal molecules might facilitate the regulation of glyoxalase pathway. We believe that MG detoxification is controlled by intricate mechanisms involving a plethora of signal molecules.

Nickel stress-tolerance in plant-bacterial associations

Nickel (Ni) is an essential element for plant growth and is a constituent of several metalloenzymes, such as urease, Ni-Fe hydrogenase, Ni-superoxide dismutase. However, in high concentrations, Ni is toxic and hazardous to plants, humans and animals. High levels of Ni inhibit plant germination, reduce chlorophyll content, and cause osmotic imbalance and oxidative stress. Sustainable plant-bacterial native associations are formed under Ni-stress, such as Ni hyperaccumulator plants and rhizobacteria showed tolerance to high levels of Ni. Both partners (plants and bacteria) are capable to reduce the Ni toxicity and developed different mechanisms and strategies which they manifest in plant-bacterial associations. In addition to physical barriers, such as plants cell walls, thick cuticles and trichomes, which reduce the elevated levels of Ni entrance, plants are mitigating the Ni toxicity using their own antioxidant defense mechanisms including enzymes and other antioxidants. Bacteria in its turn effectively protect plants from Ni stress and can be used in phytoremediation. PGPR (plant growth promotion rhizobacteria) possess various mechanisms of biological protection of plants at both whole population and single cell levels. In this review, we highlighted the current understanding of the bacterial induced protective mechanisms in plant-bacterial associations under Ni stress.

Salicylic acid underpins silicon in ameliorating chromium toxicity in rice by modulating antioxidant defense, ion homeostasis and cellular ultrastructure

Chromium (Cr) phytotoxicity affirmed the need of mitigation strategies to remediate polluted soils and restricts its accumulation in the food chains. Salicylic acid (SA) and silicon (Si) play pivotal roles in stimulating the plant performance and stress resilience. So far, their interactive effects against Cr-phytotoxicities are less known. Thus, we evaluated the beneficial roles of alone or/and combine applications of SA and Si in mitigating the toxic effects of Cr in the leaves and roots of rice (Oryza sativa) seedlings. Results indicated that SA (10 μM) and/or Si (5 μM) markedly retrieved the Cr (100 μM) induced toxicities by minimizing the Cr-accretion in both leaves and roots, enhancing the performance of light harvesting pigments (total chlorophylls and carotenoids), water retention and accumulation of osmolytes (water-soluble protein and total soluble sugars) and ultimately improved the growth and biomass. Additionally, SA and/or Si maintained the ionic balance by enhancing the nutrients transport, upregulated the ascorbate-glutathione (AsA-GSH) cycle enzymes, minimized the extra accumulation of reactive oxygen species (ROS) (H₂O₂ and O₂^•‒), malondialdehyde (MDA), recovered the membrane stability and damages in cellular ultrastructure in Cr-stressed rice plants. Overall findings suggested that SA underpins Si in mitigating the Cr-induced phytotoxicities on the above-reported parameters and combined applications of SA and Si were more effective than alone treatments. The uptake or cellular accumulation of Cr, osmoprotectants level and antioxidant defense system against oxidative stress can be considered as key toxicity biomarkers for the safe cultivation of rice in Cr-contaminated soils.

Salicylic acid-induced hydrogen sulphide improves lead stress tolerance in pepper plants by upraising the ascorbate-glutathione cycle

The contribution of hydrogen sulphide (H₂ S) to salicylic acid (SA) induced lead (Pb) stress tolerance modulated by the ascorbate-glutathione (AsA-GSH) cycle was examined in pepper (Capsicum annuum L.) plants. One week after germination, pepper seedlings were sprayed with 0.5 mM SA once a day for a week. Thereafter, seedlings were grown under control (no Pb) or Pb stress (Pb-S treatment consisting of 0.1 mM PbCl₂ ) for a further 2 weeks. Lead stress reduced plant growth and leaf water status as well as the activities of dehydroascorbate reductase and monodehydroascorbate reductase. However, lead stress elevated leaf Pb, the proline contents, oxidative stress, activities of glutathione reductase and ascorbate peroxidase, as well as the endogenous H₂ S content. Supplements of SA resulted in improvements in growth parameters, biomass, leaf water status and AsA-GSH cycle-related enzyme activities, as well as increasing the H₂ S content. The positive effect of SA was further enhanced when sodium hydrosulphide was added. However, 0.1 mM hypotaurine (HT) treatment reversed the beneficial effect of SA by reducing the plant H₂ S content. Application of NaHS in combination with SA + HT suppressed the adverse effect of HT mainly by restoring the plant H₂ S content, suggesting that higher H₂ S content, induced by exogenous SA supply, resulted in elevated regulation of the AsA-GSH cycle.

Exogenous melatonin and salicylic acid alleviates cadmium toxicity in safflower (Carthamus tinctorius L.) seedlings

The co-application of exogenous 100 µM melatonin (MT) and 100 µM salicylic acid (SA) on 21-day-old safflower seedlings grown in the presence of cadmium (Cd, 100 µM) toxicity was investigated. The application of MT, SA, or MT + SA efficiently improved toxicity symptoms and declined Cd toxicity as shown by a considerable rise in plant biomass production and chlorophyll content accompanied by decreased level of oxidative stress markers. In Cd stressed plants, the simultaneous application of MT and SA led to sharp decreases in MDA and H2O2 amounts (61.04 and 49.11%, respectively), related to plants treated with Cd alone. With respect to the control, a 41 and 48% increment in reduced glutathione (GSH) and ascorbate (ASC) content was recorded in Cd-treated seedlings. Though, with the addition of MT, SA, or MT + SA, the content of GSH and ASC increased more. The application of MT, SA, or MT + SA caused a sharp induction in phytochelatin content of the leaves of Cd-treated seedlings, while in roots, the highest PC content was recorded only in the presence of MT, which was about 1.8-fold greater than in plant treated with Cd alone. The activity of enzymes responsible for the ascorbate-glutathione cycle and glyoxalase system considerably improved by using MT, SA, or the combination of MT and SA. Our findings suggest a possible synergic interaction between MT and SA in tolerating Cd toxicity by reducing Cd uptake, improving chlorophyll biosynthesis and accelerating ascorbate-glutathione cycle as well as the modulation of glyoxalase system.

Jasmonic acid-mediated enhanced regulation of oxidative, glyoxalase defense system and reduced chromium uptake contributes to alleviation of chromium (VI) toxicity in choysum (Brassica parachinensis L.)

The cultivation of leafy vegetables on metal contaminated soil embodies a serious threat to yield and quality. In the present study, the potential role of exogenous jasmonic acid (JA; 0, 5, 10, and 20 µM) on mitigating chromium toxicity (Cr; 0, 150, and 300 µM) was investigated in choysum (Brassica parachinensis L.). With exposure to increasing Cr stress levels, a dose-dependent decline in growth, photosynthesis, and physio-biochemical attributes of choysum plants was observed. An increase in Cr levels also resulted in oxidative stress closely associated with higher lipoxygenase activity (LOX), hydrogen peroxide (H₂O₂) generation, lipid peroxidation (MDA), and methylglyoxal (MG) levels. Exogenous application of JA alleviated the Cr-induced phytotoxic effects on photosynthetic pigments, gas exchange parameters, and restored growth of choysum plants. While exposed to Cr stress, JA supplementation induced plant defense system via enhanced regulation of antioxidant enzymes, ascorbate and glutathione pool, and the glyoxalase system enzymes. The coordinated regulation of antioxidant and glyoxalase systems expressively suppressed the oxidative and carbonyl stress at both Cr stress levels. More importantly, JA restored the mineral nutrient contents, restricted Cr uptake, and accumulation in roots and shoots of choysum plants when compared to the only Cr-stressed plants. Overall, the application of JA2 treatment (10 µM JA) was more effective and counteracted the detrimental effects of 150 µM Cr stress by restoring the growth and physio-biochemical attributes to the level of control plants, while partially mitigated the detrimental effects of 300 µM Cr stress. Hence, JA application might be considered as an effective approach for minimizing Cr uptake and its detrimental effects in choysum plants grown on contaminated soils.

Salicylic acid-induced nitric oxide enhances arsenic toxicity tolerance in maize plants by upregulating the ascorbate-glutathione cycle and glyoxalase system

The role of nitric oxide (NO) in salicylic acid (SA)-induced tolerance to arsenic (As) stress in maize plants is not reported in the literature. Before starting As stress (AsS) treatments, SA (0.5 mM) was sprayed to the foliage of maize plants. Thereafter, AsV (0.1 mM as sodium hydrogen arsenate heptahydrate) stress (AsS) was initiated and during the stress period, sodium nitroprusside (SNP 0.1 mM), a NO donor, was sprayed individually or in combination with SA. Furthermore, cPTIO (0.1 mM) was also applied as a NO scavenger during the stress period. Arsenic stress led to significant reductions in plant growth, photosynthesis, water relation parameters and endogenous NO content, but it increased hydrogen peroxide, malondialdehyde, electrolyte leakage, methylglyoxal, proline, the activities of major antioxidant enzymes, and leaf and root As content. The combined treatment of SA+SNP was more effective to reverse oxidative stress related parameters and reduce the As content in both leaves and roots, with a concomitant increase in antioxidant defense system, the ascorbate-glutathione (AsA-GSH) cycle-related enzymes, glyoxalase system enzymes, plant growth, and photosynthetic traits. The beneficial effects of SA were completely abolished with cPTIO supply by blocking the NO synthesis in AsS-maize plants, indicating that NO effectively participated in SA-improved tolerance to AsS in maize plants.

Melatonin alleviates nickel phytotoxicity by improving photosynthesis, secondary metabolism and oxidative stress tolerance in tomato seedlings

Arable land contamination with nickel (Ni) has become a major threat to worldwide crop production. Recently, melatonin has appeared as a promising stress-relief substance that can alleviate heavy metal-induced phytotoxicity in plants. However, the plausible underlying mechanism of melatonin function under Ni stress has not been fully substantiated in plants. Herein, we conducted an experiment that unveiled critical mechanisms in favor of melatonin-mediated Ni-stress tolerance in tomato. Ni stress markedly inhibited growth and biomass by impairing the photosynthesis, photosystem function, mineral homeostasis, root activity, and osmotic balance. In contrast, melatonin application notably reinforced the plant growth traits, increased photosynthesis efficiency in terms of chlorophyll content, upregulation of chlorophyll synthesis genes, i.e. POR, CAO, CHL G, gas exchange parameters, and PSII maximum efficiency (Fv/Fm), decreased Ni accumulation and increased mineral nutrient homeostasis. Moreover, melatonin efficiently restricted the hydrogen peroxide (H₂O₂) and superoxide radical production and increased RBOH expression and restored cellular integrity (less malondialdehyde and electrolyte leakage) through triggering the antioxidant enzyme activities and modulating AsA-GSH pools. Notably, oxidative stress was effectively mitigated by upregulation of several defense genes (SOD, CAT, APX, GR, GST, MDHAR, DHAR) and melatonin biosynthesis-related genes (TDC, T5S, SNAT, ASMT). Besides, melatonin treatment enhanced secondary metabolites (phenols, flavonoids, and anthocyanin) contents along with their encoding genes (PAL, CHS) expression, and these metabolites potentially restricted excess H₂O₂ accumulation. In conclusion, our findings deciphered the potential functions of melatonin in alleviating Ni-induced phytotoxicity in tomato through boosting the biomass production, photosynthesis, nutrient uptake, redox balance, and secondary metabolism.

The role of Fe-nano particles in scarlet sage responses to heavy metals stress

Despite the stabilized ornamental markets for scarlet sage (Salvia splendens), little is known about the stress resistance of heavy metals (HMs). Therefore, a hydroponic study was conducted to determine whether the addition of Fe nanoparticles (Fe NPs) at 0, 5, 10, 20 and 30 µM in Hoagland's nutrient solution reduce the toxicity caused by 100 μM of HMs (Cd, Cu, Ni, Cr and Pb). Exposure to HMs significantly reduced relative growth rate (RGR), chlorophyll content, chlorophyll fluorescence (Fv/Fm), total antioxidant activity (TAA), total phenol content (TPC) and antioxidant power assay (FRAP), while the malondialdehyde (MDA) accumulation, H₂O₂ generation and electrolyte leakage (EL) significantly increased. Fe NPs improved HMs toxicity by significant reduction in MDA content, H₂O₂ generation and EL while increase in the PGR, chlorophyll content, Fv/Fm, the TAA, TPC and FRAP. Exposure to HMs caused Fe deficiency-induced chlorosis while Fe NPs reduced the negative effects of HM by preventing further reduction of leaf Fe. The results highlighted that although using Fe NPs significantly improved plant growth and motivated the plant defense mechanisms in response to HMs toxicity, it had a negative effect on the phytoremediation properties of salvia splendens by reducing the accumulation of HMs in plant organs.

Modulation of Cadmium Tolerance in Rice: Insight into Vanillic Acid-Induced Upregulation of Antioxidant Defense and Glyoxalase Systems

Cadmium (Cd) is a toxic heavy metal that enters the human food chain from the soil via plants. Increased Cd uptake and translocation in plants alters metabolism andreduces crop production. Maintaining crop yield therefore requires both soil remediation andenhanced plant tolerance to Cd. In this study, we investigated the effects of vanillic acid (VA) on Cd accumulation and Cd stress tolerance in rice (Oryza sativa L. cv. BRRI dhan54). Thirteen-day-old rice seedlings treated with CdCl₂ (1.0 and 2.0 mM) for 72 h showed reduced growth, biomass accumulation, and water and photosynthetic pigment contents, as well as increased signs of oxidative stress (elevated levels of malondialdehyde, hydrogen peroxide, methylglyoxal, and lipoxygenase) and downregulated antioxidant and glyoxalase systems. Cadmium-induced changes in leaf relative turgidity, photosynthetic pigment content, ascorbate pool size, and glutathione content were suppressed by VA under both mild and severe Cd toxicity stress. The supplementation of VA under Cd stress conditions also increased antioxidant and glyoxylase enzyme activity. Vanillic acid also increased phytochelatin content and the biological accumulation factor, biological accumulation co-efficient, and Cd translocation factor. Vanillic acid, therefore appears to enhance Cd stress tolerance by increasing metal chelation and sequestration, by upregulating antioxidant defense and glyoxalase systems, and by facilitating nutrient homeostasis.

The role of endogenous nitric oxide in salicylic acid-induced up-regulation of ascorbate-glutathione cycle involved in salinity tolerance of pepper (Capsicum annuum L.) plants

An experimentation was carried out to appraise whether or not nitric oxide (NO) contributes to salicylic acid (SA)-induced salinity tolerance particularly by regulating ascorbate-glutathione (AsA-GSH) cycle. Before starting salinity stress (SS), SA (0.5 mM) was sprayed to the foliage of plants once every other day for a week and then seedlings were grown under control or SS (100 mM NaCl), for five weeks. Salinity stress enhanced the AsA-GSH cycle-related enzymes, glutathione reductase (GR), ascorbate peroxidase (APX), and dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR). Furthermore, SS caused substantial decreases in plant physiological-related traits such as leaf potassium (K) contents, K⁺/Na⁺ ratio, the ratios of reduced ascorbate/dehydroascorbic acid (AsA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG), but in contrast, significant increases occurred in leaf hydrogen peroxide, malondialdehyde, electron leakage, proline, the premier antioxidant enzymes' activities, Na⁺ and NO. SA reduced leaf Na⁺ content and oxidative stress-related traits, but improved all earlier-mentioned traits compared with those in plants treated with SS alone. All positive effects of SA were eliminated by NO scavenger, 0.1 mM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1- oxyl-3-oxide (c-PTIO) by reducing NO, suggesting that NO produced by SA up-regulated the activities of AsA-GSH cycle and antioxidant enzymes, so it could play a central function as a signal molecule in salt tolerance of pepper plants.

Modulation of growth performance and coordinated induction of ascorbate-glutathione and methylglyoxal detoxification systems by salicylic acid mitigates salt toxicity in choysum (Brassica parachinensis L.)

Salinity represents a serious environmental threat to crop production and by extension, to world food supply, social and economic prosperity of the developing world. Salicylic acid (SA) is an endogenous plant signal molecule involved in regulating various plant responses to stress. In the present study, we characterized the regulatory role of exogenous SA for their ability to ameliorate deleterious effects of salt stress (0, 100, 150, 200 mM NaCl) in choysum plants through coordinated induction of antioxidants, ascorbate glutathione (AsA-GSH) cycle, and the glyoxalase enzymes. An increase in salt stress dramatically declined root and shoot growth, leaf chlorophyll and relative water content (RWC), subsequently increased electrolyte leakage (EL) and osmolytes accumulation in choysum plants. Salt stress disrupted the antioxidant and glyoxalase defense systems which persuaded oxidative damages and carbonyl toxicity, indicated by increased H₂O₂ generation, lipid peroxidation, and methylglyoxal (MG) content. However, application of SA had an additive effect on the growth of salt-affected choysum plants, which enhanced root length, plant biomass, chlorophyll contents, leaf area, and RWC. Moreover, SA application effectively eliminated the oxidative and carbonyl stress by improving AsA and GSH pool, upregulating the activities of antioxidant enzymes and the enzymes associated with AsA-GSH cycle and glyoxalase system. Overall, SA application completely counteracted the salinity-induced deleterious effects of 100 and 150 mM NaCl and partially mediated that of 200 mM NaCl stress. Therefore, we concluded that SA application induced tolerance to salinity stress in choysum plants due to the synchronized increase in activities of enzymatic and non-enzymatic antioxidants, enhanced efficiency of AsA-GSH cycle and the MG detoxification systems.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C₇H₆O₃ compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C7H6O3 compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

Plant growth regulators improve growth, photosynthesis, mineral nutrient and antioxidant system under cadmium stress in menthol mint (Mentha arvensis L.)

Menthol mint (Mentha arvensis L.) cultivation is significantly affected by the heavy metals like cadmium (Cd) which also imposes severe health hazards. Two menthol mint cultivars namely Kosi and Kushal were evaluated under Cd stress conditions. Impact of plant growth regulators (PGRs) like salicylic acid (SA), gibberellic acid (GA₃) and triacontanol (Tria) on Cd stress tolerance was assessed. Reduced growth, photosynthetic parameters, mineral nutrient concentration, and increased oxidative stress biomarkers like electrolyte leakage, malondialdehyde, and hydrogen peroxide contents were observed under Cd stress. Differential upregulation of proline content and antioxidant activities under Cd stress was observed in both the cultivars. Interestingly, low electrolyte leakage, lipid peroxidation, hydrogen peroxide and Cd concentration in leaves were observed in Kushal compared to Kosi. Among all the PGRs tested, SA proved to be the best in improving Cd-stress tolerance in both the cultivars but Kushal responded better than Kosi.

A multivariate analysis of comparative effects of heavy metals on cellular biomarkers of phytoremediation using Brassica oleracea

The biochemical/physiological variations in plant responses to heavy metals stress govern plant's ability to phytoremediate/tolerate metals. So, the comparative effects of different types of heavy metals on various plant responses can better elucidate the mechanisms of metal toxicity and detoxification. This study compared the physiological modifications, photosynthetic performance and detoxification potential of Brassica oleracea under different levels of chromium (Cr), nickel (Ni) and selenium (Se). All the heavy metals induced a severe phytotoxicity to B. oleracea in terms of chlorophyll contents, Ni being the most toxic (76% decrease). Brassica oleracea showed high lipid oxidation: 87% and 273%, respectively in leaves and roots. Furthermore, all the metals increased the activities of catalase and peroxidase, while decreased superoxide dismutase and ascorbate peroxidase. Interestingly, heavy metals decreased hydrogen peroxide contents perhaps due to their possible transformation to another form of reactive oxygen species such as hydroxyl radical. Among the three metals, Ni was more phytotoxic than Cr and Se. Moreover, the phytoremediation/tolerance potential of B. oleracea to Ni, Cr and Se stress varied with the type of metal, their applied levels, response variables and plant organ type (root/shoot). The multivariate analysis separated different plant response variables and heavy metal treatments into different groups based on their correlations.