Nitric oxide affects cadmium-induced changes in the lichen Ramalina farinacea

The emerging roles of nitric oxide and its associated scavengers-phytoglobins-in plant symbiotic interactions

A key feature in the establishment of symbiosis between plants and microbes is the maintenance of the balance between the production of the small redox-related molecule, nitric oxide (NO), and its cognate scavenging pathways. During the establishment of symbiosis, a transition from a normoxic to a microoxic environment often takes place, triggering the production of NO from nitrite via a reductive production pathway. Plant hemoglobins [phytoglobins (Phytogbs)] are a central tenant of NO scavenging, with NO homeostasis maintained via the Phytogb-NO cycle. While the first plant hemoglobin (leghemoglobin), associated with the symbiotic relationship between leguminous plants and bacterial Rhizobium species, was discovered in 1939, most other plant hemoglobins, identified only in the 1990s, were considered as non-symbiotic. From recent studies, it is becoming evident that the role of Phytogbs1 in the establishment and maintenance of plant-bacterial and plant-fungal symbiosis is also essential in roots. Consequently, the division of plant hemoglobins into symbiotic and non-symbiotic groups becomes less justified. While the main function of Phytogbs1 is related to the regulation of NO levels, participation of these proteins in the establishment of symbiotic relationships between plants and microorganisms represents another important dimension among the other processes in which these key redox-regulatory proteins play a central role.

Dynamic changes in global methylation and plant cell death mechanism in response to NiO nanoparticles

This report is a first comprehensive work on the potential of engineered nickel oxide nanoparticles affecting the epigenome and modulating global methylation leading to retention of transgenerational footprints. Nickel oxide nanoparticles (NiO-NPs) are known to instigate extensive phenotypic and physiological damage to plants. In the present work, it was shown that exposure to increasing concentrations of NiO-NP-induced cell death cascades in model systems, Allium cepa and tobacco BY-2 cells. NiO-NP also generated variation in global CpG methylation; its transgenerational transmission was shown in affected cells. Plant tissues exposed to NiO-NP showed progressive replacement of essential cations, like Fe and Mg, as seen in XANES and ICP-OES data, providing earliest signs of disturbed ionic homeostasis. Fluorescent staining based confocal microscopy confirmed upsurge of H₂O₂ and nitric oxide after NiO-NP exposure. A NiO-NP concentration gradient-based switching-on of the cell death cascades was observed when autophagosomes were seen in samples exposed to lower and median concentrations of NiO-NP (10-125 mg L^-1). The apoptotic cell death marker, caspase-3 like protein, was noted in the median to higher doses (50-500 mg L^-1), and leakage of lactate dehydrogenase marking necrotic cell death was observed in samples exposed to the highest doses (125-500 mg L^-1) of NiO-NP. Concomitant increase of DNA hypermethylation (quantified by ELISA-based assay) and genomic DNA damage (evaluated through Comet-based analyses) was recorded at higher doses of NiO-NP. MSAP profiles confirmed that global methylation changes incurring in the parental generation upon NiO-NP exposure were transmitted through the two subsequent generations of BY-2 cells which was supported by data from A. cepa, too. Thus, it was evident that NiO-NP exposure incited DNA hypermethylation, as an aftermath of oxidative burst, and led to induction of autophagy, apoptotic and necrotic cell death pathways. Global methylation changes induced by NiO-NP exposure can be transmitted through subsequent cell generations.

Mercury Content and Amelioration of Its Toxicity by Nitric Oxide in Lichens

Mercury (Hg) content measured in five epiphytic lichen species collected in Slovakia mountain forests ranged from 30 to 100 ng/g DW and was species-specific, decreasing in the order Hypogymnia > Pseudevernia > Usnea > Xanthoria > Evernia prunastri (but polluted sites had no impact on Hg amount in Xanthoria). Evernia was therefore used to study the impact of short-term exogenous Hg (100 µM, 24 h) and possible amelioration of Hg toxicity by nitric oxide (NO) donor sodium nitroprusside (SNP). NO was efficiently released from SNP as detected by two staining reagents and fluorescence microscopy and reduced Hg-induced ROS signal and absorption of Hg by thalli of Evernia prunastri. At the same time, NO ameliorated Hg-induced depletion of metabolites such as ascorbic acid and non-protein thiols, but not of free amino acids. The amount of metabolites, including soluble phenols, was reduced by excess Hg per se. On the contrary, NO was unable to restore Hg-stimulated depletion of chlorophyll autofluorescence but mitigated the decline of some macronutrients (K and Ca). Data confirm that accumulation of Hg in the epiphytic lichens is species-specific and that NO is a vital molecule in Evernia prunastri that provides protection against Hg-induced toxicity with considerable positive impact on metabolic changes.

Calcium-enriched biochar modulates cadmium uptake depending on external cadmium dose

The impact of calcium-enriched biochar (BC, containing Ca, Al, Fe and P as dominant elements in the range of 6.9-1.3% with alkaline pH) obtained from sewage sludge (0.1 or 0.5% in the final soil) on cadmium-induced toxicity (final dose of 1.5 mg Cd/kg in control and 4.5 or 16.5 mg Cd/kg soil in low and high Cd treatment) was tested in medicinal plant Matricaria chamomilla. Low Cd dose had typically less negative impact than high Cd dose at the level of minerals and metabolites and the effect of BC doses often differed. Contrary to expectations, 0.5% BC with a high Cd dose increased Cd accumulation in plants about 2-fold. This was reflected in higher signals of reactive oxygen species, but especially the high dose of BC increased the amount of antioxidants (ascorbic acid and non-protein thiols), minerals and amino acids in shoots and/or roots and usually mitigated the negative effect of Cd. Surprisingly, the relationship between BC and soluble phenols was negative at high BC + high Cd dose, whereas the effect of Cd and BC on organic acids (mainly tartaric acid) differed in shoots and roots. Interestingly, BC alone applied to the control soil (1.5 mg total Cd/kg) reduced the amount of Cd in the plants by about 30%. PCA analyses confirmed that metabolic changes clearly distinguished the high Cd + high BC treatment from the corresponding Cd/BC treatments in both shoots and roots. Thus, it is clear that the effect of biochar depends not only on its dose but also on the amount of Cd in the soil, suggesting the use of Ca-rich biochar both for phytoremediation and safer food production.

Allantoin overaccumulation enhances production of metabolites under excess of metals but is not tightly regulated by nitric oxide

The aln-3 mutant overaccumulating allantoin and respective wild type (WT) strain of Arabidopsis thaliana were exposed to cadmium (Cd) or mercury (Hg) with or without nitric oxide (NO) donor (sodium nitroprusside, SNP) to study cross-talk, metabolic and oxidative changes between these nitrogen sources (organic vs. inorganic). The aln-3 accumulated over 10-fold more allantoin than WT with the effect of Cd and Hg differing in leaf and root tissue: aln-3 contained more ascorbic acid and phytochelatins when treated with Cd or Hg and more Cd in both organs. SNP depleted leaf Cd and root Hg accumulation in aln3 but had a positive impact on the amount of metabolites typically in WT plants, indicating potentially negative relation between allantoin and NO. In agreement, aln-3 roots showed lower NO signals in control or metal treatments, but higher ROS signal, and SNP had more pronounced impact in WT roots. Flavonol glycosides were more abundant in aln-3 and were affected more by metals than by SNP. Malate was the most affected Krebs acid with strong reaction to SNP and Hg treatment. Data indicate that allantoin overaccumulation influences the accumulation of specific metabolites but nitric oxide has a greater impact on the metabolite profile in WT.

Sodium nitroprusside mediates attenuation of paraquat-mediated oxidative stress in Eruca sativa in vitro

Paraquat (PQ) causes oxidative stress, the main source of damage in plants subjected to adverse environmental factors. Sodium nitroprusside (SNP), a signaling molecule, alleviates oxidative damage. The present study was carried out to investigate the role of exogenous SNP in the amelioration of PQ-mediated oxidative stress effects on Eruca sativa plantlets cultured in MS basal media. Firstly, MS medium supplemented with 6-BA was found to be the best basal medium for seed germination. Then, a rapid micropropagation protocol was designed to produce E. sativa plantlets by using nodal segments as explants, and 0.25 mg/L 6-BA in combination with 0.1 mg/L IBA was found to be the most favorable for shoot proliferation of E. sativa. Four weeks old plants were applied with or without SNP (100 μM) and exposed to oxidative stress induced by 2.5 μM PQ. The SNP application decreased membrane damage, hydrogen peroxide, and proline contents, and increased relative water, pigments, ascorbate and total phenolic contents, and some antioxidant enzyme activities in seedlings under PQ stress compared to PQ stress alone. These results suggested that exogenous SNP could protect E. sativa plantlets propagated in vitro with PQ stress through modulation of proline and phenolics biosynthesis and antioxidant defense system.

Physiological and Molecular Alterations of Phycobionts of Genus Trebouxia and Coccomyxa Exposed to Cadmium

Several studies on aeroterrestrial microalgae are unravelling their resistance mechanisms to different abiotic stressors, including hazardous metals, pointing to their future role as bioremediation microorganisms. In the present study, physiological and molecular alterations of four phycobionts of genus Trebouxia (T. TR1 and T. TR9) and Coccomyxa (C. subellipsoidea and C. simplex) exposed to Cd were studied. Cd accumulation and subcellular distribution, cell wall structure, production of biothiols (GSH and phytochelatins), reactive oxygen species (ROS) formation, expression of key antioxidant genes and ROS-related enzymes were evaluated to determine the physiological differences among the four microalgae, with the aim to identify the most suitable microorganism for further biotechnological applications. After 7 days of Cd exposure, Coccomyxa algae showed higher capacity of Cd intake than Trebouxia species, with C. subellipsoidea being the highest Cd accumulator at both intracellular and, especially, cell wall level. Cd induced ROS formation in the four microalgae, but to a greater extent in both Coccomyxa algae. Trebouxia TR9 showed the lowest Cd-dependent oxidative stress probably due to glutathione reductase induction. All microalgae synthetized phytochelatins in response to Cd but in a species-specific and a dose-dependent manner. Results from this study agree with the notion that each microalga has evolved a distinct strategy to detoxify hazardous metals like Cd and to cope with oxidative stress associated with them. Coccomyxa subellipsoidea and Trebouxia TR9 appear as the most interesting candidates for further applications.

Calcium has protective impact on cadmium-induced toxicity in lichens

Eventual protective action of calcium (Ca, 100 or 1000 μM) against cadmium (Cd, 10 or 100 μM) toxicity in common lichen Hypogymnia physodes after 24 h of exposure was studied. Total Cd reached 482 and 2801 μg/g DW in 10 and 100 μM Cd treatments while Ca content reached over 23 mg/g DW in 1000 μM Ca treatment. Ca suppressed Cd accumulation by 23 and 38% in total fraction and completely in absorbed fraction. Fluorescence microscopy of Cd and Ca ions revealed good correlation with quantitative data. Cd stimulated increase in ROS formation and lipid peroxidation as detected using fluorescent reagents and quantification of H₂O₂ while co-application of Ca suppressed these effects. Formation of nitric oxide was mainly affected by cadmium. Cd depleted amount of amino acids but proteins or phenols remained unaffected by Cd or Ca. On the contrary, sum of thiols, reduced glutathione and ascorbic acid were depleted by Cd but reversed mainly by higher Ca dose. Among organic acids, only Cd-induced depletion of citric acid content was reversed by Ca. Data indicate that ameliorative effect of Ca under Cd excess in lichens is comparable with effect in plants and metabolic responses in various life lineages are discussed.

Nitric oxide induced Cd tolerance and phytoremediation potential of B. juncea by the modulation of antioxidant defense system and ROS detoxification

The present study designed to illustrate correlation between cadmium induced stress and plant growth, photosynthetic pigments, morphological and physiological attributes. To study these parameters 2 weeks old seedling of B. juncea were subjected to 50 µM Cd, 100 µM Cd and 100 µM SNP separately and in combination with SNP. After 96 h, the treated plant were harvested to analyze the cellular homeostasis and metal tolerance mechanism via examining growth, stress parameters, enzymatic and non enzymatic antioxidants and expression level of NR. Higher level of Cd (100 µM) significantly increased accumulation of reactive oxygen species and malonaldehyde content in comparison to 50 µM Cd. Exogenous supplementation of SNP (100 µM) to 50 µM Cd treated plant had an additive effect on plant growth by improving the level of proline, photosynthetic pigments and activities of enzymatic antioxidants which was confirmed by histochemical staining for NADPH-d and NO fluorescence from DAF-DA staining in roots of B. juncea. Applying SNP to 50 µM Cd exposed B. juncea roots enhanced NR activity by 1.36 folds and increased NO production by 1.12 folds than individual Cd treated roots. In addition, semi quantitative RT-PCR study revealed the induction of BjNR was more pronounced in 50 µM Cd treated roots in comparison to 100 µM Cd treated roots. The present finding revealed NO confers increased B. juncea tolerance to Cd stress by stimulation of antioxidants and reestablishment of cellular redox status. Different biochemical analysis showed that plant growth, photosynthetic pigment and antioxidants were positively correlated with NO and it's negatively correlated with oxidative stress biomarkers. Therefore, NO is gaseous signalling molecule with potential role in Cd detoxification mechanism in B. juncea.

The outcomes of the functional interplay of nitric oxide and hydrogen sulfide in metal stress tolerance in plants

Phytotoxicity of metals constraints plants health, metabolism and productivity. The sustainable approaches for minimizing major metals-accrued phytotoxicity have been least explored. The gasotransmitters signaling molecules such as nitric oxide (NO) and hydrogen sulfide (H₂S) play a significant role in the mitigation of major consequences of metals stress. Versatile gaseous signaling molecules, NO and H₂S are involved in the regulation of various physiological processes in plants and their tolerance to abiotic stresses. However, literature available on NO or H₂S stand alone, and the major insights into the roles of NO and/or H₂S in plant tolerance, particularly to metals, remained unclear. Given above, this paper aimed to (a) briefly overview metals and highlight their major phytotoxicity; (b) appraises literature reporting potential mechanisms underlying the roles of NO and H₂S in plant-metal tolerance; (c) crosstalk on NO and H₂S in relation to plant metal tolerance. Additionally, major aspects so far unexplored in the current context have also been mentioned.

The opposite nitric oxide modulators do not lead to the opposite changes of metabolites under cadmium excess

Responses of common medicinal herb Matricaria chamomilla to short-term cadmium (Cd) exposure (48 h) alone or in combination with nitric oxide (NO) scavenger (PTIO) or donor (SNP) were studied. Modulators revealed expected impact on NO formation (depletion under PTIO but elevation under SNP) and confirmed anomalous impact on Cd accumulation (stimulation by both modulators). Changes of proline and free amino acids in the roots and fatty acids (mainly α-linolenic acid) in the shoots indicate rather relation to elevated Cd uptake than to altered NO formation. On the contrary, root profile of fatty acids revealed the most prominent changes in response to PTIO (elevation of monounsaturated and polyunsaturated fatty acids) which confirm that NO depletion stimulates their biosynthesis. Soluble phenols were elevated by SNP co-application in the roots while accumulation of chlorogenic acid and umbelliferone revealed clear regulation by NO (i.e. enhancement in response to SNP and depletion in response to PTIO). Data indicate that quantities of some metabolites are affected by accumulated Cd or NO formation and that the opposite NO modulators do not automatically lead to the opposite changes of plant metabolites.

Long-term impact of cadmium in protonema cultures of Physcomitrella patens

Antioxidative responses of axenic protonema cultures of the moss Physcomitrella patens exposed to 10 μM Cd over 40 d were studied. Cd treatment suppressed growth by ca. 75% with concomitant browning of some filaments and suppression of chlorophyll autofluorescence but had no impact on tissue water content. Despite this negative growth responses which could be related to enhanced ROS formation (as detected using fluorescence staining reagents for total ROS, hydroperoxides and lipid peroxidation), some metabolites revealed strong elevation by Cd which could contribute to attenuation of long-term Cd stress (elevation of ascorbic, malic and citric acids). Molar ratio of malate to Cd was 12.7 and citrate to Cd 2.5, thus potentially contributing to Cd chelation. Interestingly, GSH/GSSG pool and nitric oxide formation remained unaltered by Cd. Accumulation of Cd reached 82 μg/g DW with bioaccumulation factor of 73. Data indicate that Cd induces elevation of potentially protective metabolites even after prolonged exposure though they do not prevent oxidative stress sufficiently.

Endogenous NO Is Involved in Dissimilar Responses to Rehydration and Pb(NO3)2 in Ramalina farinacea Thalli and Its Isolated Phycobionts

Lichens undergo desiccation/rehydration cycles and are permeable to heavy metals, which induce free radicals. Nitrogen monoxide (NO) regulates important cellular functions, but the research on lichen NO is still very scarce. In Ramalina farinacea thalli, NO seems to be involved in the peroxidative damage caused by air pollution, antioxidant defence and regulation of lipid peroxidation and photosynthesis. Our hypothesis is that NO also has a critical role during the rehydration and in the responses to lead of its isolated phycobionts (Trebouxia sp. TR9 and Trebouxia jamesii). Therefore, we studied the intracellular reactive oxygen species (ROS) production, lipid peroxidation and chlorophyll autofluorescence during rehydration of thalli and isolated microalgae in the presence of a NO scavenger and Pb(NO₃)₂. During rehydration, NO scavenging modulates free radical release and chlorophyll autofluorescence but not lipid peroxidation in both thalli and phycobionts. Pb(NO₃)₂ reduced free radical release (hormetic effect) both in the whole thallus and in microalgae. However, only in TR9, the ROS production, chlorophyll autofluorescence and lipid peroxidation were dependent on NO. In conclusion, Pb hormetic effect seems to depend on NO solely in TR9, while is doubtful for T. jamesii and the whole thalli.

Roles of nitric oxide in heavy metal stress in plants: Cross-talk with phytohormones and protein S-nitrosylation

Heavy metal (HM) stress is a major hazard, which significantly affects plant growth and development. In order to confront HM stress, plants directly or indirectly regulate the levels of endogenous nitric oxide (NO), a redox-related signaling molecule involved in wide range of plant growth and development as well as in response to HM stress. In addition, there is now compelling experimental evidence that NO usually mediates signaling processes through interactions with different biomolecules like phytohormones to regulate HM tolerance. Apart from phytohormones, NO partly operates through posttranslational modification of proteins, notably via S-nitrosylation in response to HM stress. Recently, the roles of S-nitrosylation as a regulator of plant responses to HM stress and S-nitrosylated candidates have also been established and detected. Here, we describe the roles of NO in confronting HM phytotoxicity in plants with a particular focus on the presentation and discussion of recent data obtained in this field, which involves in the function of various phytohormones and S-nitrosylation during plant responses to HM stress. Additionally, both importance and challenges of future work are outlined in order to further elucidate the specific mechanisms underlying the roles of NO in plant responses to HM stress.

Nitrogen nutrition modulates oxidative stress and metabolite production in Hypericum perforatum

Impact of various nitrate concentrations (14.12 mM, 3.53 mM, no nitrate) or ammonium presence (14.12 mM) on physiological and metabolic changes in Hypericum perforatum after 14 days of cultivation was monitored. Nitrate deficiency suppressed growth of shoots but stimulated root growth while ammonium suppressed root growth: concomitant changes of ascorbic acid and glutathione supported these growth changes, e.g., unaltered level in roots under nitrate deficiency but depleted in ammonium treatment. Soluble proteins and water content were more suppressed by nitrate deficiency but total ROS, nitric oxide formation, and antioxidative enzyme activities (APX and SOD) indicate higher sensitivity of plants to ammonium. Though both extreme treatments (NO₃^- deficiency or ammonium) stimulated accumulation of total soluble phenols and affected PAL activity (in comparison with full or 1/4× nitrate dose), major phenols (chlorogenic acid and three flavonoids) were elevated mainly by NO₃^- deficiency. At the level of specific metabolites, NO₃^- deficiency had stimulatory impact on pseudohypericin (but not hypericin) content while hyperforin decreased. Expression of earlier putative gene of hypericin biosynthesis (hyp-1) showed rather partial correlation with pseudohypericin amount. Data indicate that depletion of NO₃^- is useful to obtain Hypericum plants with higher amount of health-positive secondary metabolites.

Research Progress on the Functions of Gasotransmitters in Plant Responses to Abiotic Stresses

Abiotic stress is one of the major threats affecting plant growth and production. The harm of abiotic stresses includes the disruption of cellular redox homeostasis, reactive oxygen species (ROS) production, and oxidative stress in the plant. Plants have different mechanisms to fight stress, and these mechanisms are responsible for maintaining the required homeostasis in plants. Recently, the study of gasotransmitters in plants has attracted much attention, especially for abiotic stress. In the present review, abiotic stressors were mostly found to induce gasotransmitter production in plants. Meanwhile, these gasotransmitters can enhance the activity of several antioxidant enzymes, alleviate the harmfulness of ROS, and enhance plant tolerance under various stress conditions. In addition, we introduced the interaction of gasotransmitters in plants under abiotic stress. With their promising applications in agriculture, gasotransmitters will be adopted in the near future.

Possible use of a Nicotiana tabacum 'Bright Yellow 2' cell suspension as a model to assess phytotoxicity of pharmaceuticals (diclofenac)

Growth and developmental changes in plants induced by pharmaceuticals reflect changes in processes at the cellular and subcellular levels. Due to their growth and cellular characteristics, plant cell suspension cultures can be a suitable model for assessing toxicity. In this study, 10-1000 μg/L of the non-steroidal anti-inflammatory drug diclofenac (DCF) decreased the viability of Nicotiana tabacum BY-2 cells after 24 h of treatment. Further, 0.1-10 mg/L DCF diminished the density of the cell suspension by 9-46% after 96 h of treatment, but at 1 and 10 μg/L, DCF increased the density by 13% and 5%, respectively, after 120 h. These changes were accompanied by increased production of total reactive oxygen species (ROS) and mitochondrial superoxide (up to 17-fold and 5-fold, respectively), and a decrease in the mitochondrial membrane potential (by ∼64%) especially at 1000 μg/L DCF. The increased ROS production was accompanied by decrease in level of reactive nitrogen species (RNS; by 36%) and total thiols (by 61%). Damage to BY-2 cells was evidenced by accumulation of neutral red in acidic compartments (up to 10-fold at 1000 μg/L DCF), and increase of autophagic vacuole formation (up to 8-fold at 1000 μg/L DCF). Furthermore, irregular or stretched nuclei were observed in nearly 27% and 50% of cells at 100 and 1000 μg/L DCF, respectively. Highest levels of chromatin condensation (11% of cells) and apoptotic DNA fragmentation (7%) were found at 10 μg/L DCF. The results revealed a significant effect of DCF on BY-2 cells after 24 h of exposure. Changes in the growth and viability parameters were indisputably related to ROS and RNS production, changes in mitochondrial function, and possible activation of processes leading to cell death.