Exogenous application of nitric oxide promotes hyperaccumulator Solanum nigrum L. performances, soil properties, and microbial community in cadmium contaminated soil

As an important bioactive molecule, nitric oxide (NO) can effectively alleviate the effects of drought stress on crops. However, it is still unclear whether it can increase the stress resistance of soybean. Therefore, in this study, our objective was to explore the effect of exogenous NO application on the physiological characteristics of soybean seedlings under drought stress. As test material, two soybean varieties, HN65 and HN44, were used, while sodium nitroprusside (SNP) of 100 μmol L−1, 200 μmol L−1, 500 μmol L−1, 1000 μmol L−1 served as an exogenous NO donor, and PEG-6000 as an osmotic regulator to simulate drought stress. The effects of irrigation with different SNP concentrations for different days on the physiological characteristics of the soybean seedlings under drought conditions were then investigated. The results obtained showed that the activities of antioxidant enzymes, osmotic regulator contents, as well as the abscisic acid and salicylic acid contents of the plant leaves increased with increasing SNP concentration and treatment time. However, we observed that excessively high SNP concentrations decreased the activities of key nitrogen metabolism enzymes significantly. This study provides a theoretical basis for determining a suitable exogenous NO concentration and application duration. It also highlights strategies for exploring the mechanism by which exogenous NO regulates crop drought resistance.

The elevated ultraviolet-B (UV-B) stress induces the accumulation of a variety of intracellular reactive oxygen species (ROS), which seems to cause oxidative stress for plants. To date, very little work has been done to evaluate the biological effects of a combined treatment with He-Ne laser irradiation and exogenous nitric oxide (NO) application on oxidative stress resulting from UV-B radiation. Thus, our study investigated the effects of a combination with He-Ne laser irradiation and exogenous NO treatment on oxidative damages in wheat seedlings under elevated UV-B stress. Our data showed that the reductions in ROS levels, membrane damage parameters, while the increments in antioxidant contents and antioxidant enzyme activity caused by a combination with He-Ne laser and exogenous NO treatment were greater than those of each individual treatment. Furthermore, these treatments had a similar effect on transcriptional activities of plant antioxidant enzymes. This implied that the protective effects of a combination with He-Ne laser irradiation and exogenous NO treatment on oxidative stress resulting from UV-B radiation was more efficient than each individual treatment with He-Ne laser or NO molecule. Our findings might provide beneficial theoretical references for identifying some effective new pathways for plant UV-B protection.

Salt stress negatively affects the growth, development, and yield of horticultural crops. Nitric oxide (NO) is considered a signaling molecule that plays a key role in the plant defense system under salt stress. This study investigated the impact of exogenous application of 0.2 mM of sodium nitroprusside (SNP, an NO donor) on the salt tolerance and physiological and morphological characteristics of lettuce (Lactuca sativa L.) under salt stress (25, 50, 75, and 100 mM). Salt stress caused a marked decrease in growth, yield, carotenoids and photosynthetic pigments in stressed plants as compared to control ones. Results showed that salt stress significantly affected the oxidative compounds (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX)) and non-oxidative compounds (ascorbic acid, total phenols, malondialdehyde (MDA), proline, and H2O2) in lettuce. Moreover, salt stress decreased nitrogen (N), phosphorous (P), and potassium ions (K+) while increasing Na ions (Na+) in the leaves of lettuce under salt stress. The exogenous application of NO increased ascorbic acid, total phenols, antioxidant enzymes (SOD, POD, CAT, and APX) and MDA content in the leaves of lettuce under salt stress. In addition, the exogenous application of NO decreased H2O2 content in plants under salt stress. Moreover, the exogenous application of NO increased leaf N in control, and leaf P and leaf and root K+ content in all treatments while decreasing leaf Na+ in salt-stressed lettuce plants. These results provide evidence that the exogenous application of NO on lettuce helps mitigate salt stress effects.

Excessive aluminum ions (Al3+) in acidic soil can have a toxic effect on watermelons, restricting plant growth and reducing yield and quality. In this study, we found that exogenous application of nitric oxide (NO) could increase the photochemical efficiency of watermelon leaves under aluminum stress by promoting closure of leaf stomata, reducing malondialdehyde and superoxide anion in leaves, and increasing POD and CAT activity. These findings showed that the exogenous application of NO improved the ability of watermelon to withstand aluminum stress. To further reveal the mitigation mechanism of NO on watermelons under aluminum stress, the differences following different types of treatments—normal growth, Al, and Al + NO—were shown using de novo sequencing of transcriptomes. In total, 511 differentially expressed genes (DEGs) were identified between the Al + NO and Al treatment groups. Significantly enriched biological processes included nitrogen metabolism, phenylpropane metabolism, and photosynthesis. We selected 23 genes related to antioxidant enzymes and phenylpropane metabolism for qRT-PCR validation. The results showed that after exogenous application of NO, the expression of genes encoding POD and CAT increased, consistent with the results of the physiological indicators. The expression patterns of genes involved in phenylpropanoid metabolism were consistent with the transcriptome expression abundance. These results indicate that aluminum stress was involved in the inhibition of the photosynthetic pathway, and NO could activate the antioxidant enzyme defense system and phenylpropane metabolism to protect cells and scavenge reactive oxygen species. This study improves our current understanding by comprehensively analyzing the molecular mechanisms underlying NO-induced aluminum stress alleviation in watermelons.

Nitric oxide (NO) has been shown to be an important signaling molecule in mammalian and plant physiology. The notion that exogenous application of NO in the form of a solution-based NO donor, for example, sodium nitroprusside (SNP), can counteract the toxicity of heavy metals in plants has been supported experimentally in many studies in the past decade. However, some recent studies also appeared to have casted doubts about this. Moreover, there does not appear to have been any assessment of the practical or agricultural significance of applying NO exogenously for ameliorating heavy metal toxicity in plants, particularly during postgerminative seedling growth. The main features of the relevant studies were examined critically. The issues discussed in relation to the studies of applying NO and heavy metal treatment of seedlings during postgerminative growth might also be relevant to studies at other plant growth and developmental stages. It is concluded that the agricultural significance of exogenous application of NO to alleviate heavy metal toxicity in plants remains to be established.

11 - Plant metabolism adjustment in exogenously applied NO under stress

A study was conducted to assess whether foliar-applied nitric oxide (NO) could alleviate the adverse effects of salt stress on wheat (Triticum aestivum L.). Four sodium nitroprusside levels (control [water spray] and 0.05, 0.10, and 0.15 mM) were sprayed as a donor of NO on the leaves of cultivar S-24 plants grown under nonsaline and saline conditions (150 mM NaCl). Data for growth and yield, chlorophyll contents, activities of antioxidants, and concentrations of mineral nutrients were recorded. Root-medium salinity adversely affected shoot and root dry weight, shoot length, and yield attributes of the wheat plants while it enhanced the activities of antioxidants, proline accumulation, and concentrations of shoot and root Na+ and Cl-. Foliar-applied NO improved growth of only nonstressed plants. Exogenously applied NO enhanced the activities of antioxidant enzymes (superoxide dismutase [SOD], peroxidase [POD], and catalase [CAT]) and levels of soluble proteins and proline, in both stressed and nonstressed wheat plants. Overall, exogenous application of NO enhanced chlorophyll contents; activities of CAT, POD, and SOD; and levels of soluble proteins and total free proline in the salt stressed wheat plants. The exogenous application of NO had a protective role against salt-induced oxidative damage by enhancing the activities of antioxidant enzymes, thereby improving plant growth under saline stress.

This work was designed to evaluate whether external application of nitric oxide (NO) in the form of its donor S-nitroso-N-acetylpenicillamine (SNAP) could mitigate the deleterious effects of NaCl stress on chickpea (Cicer arietinum L.) plants. SNAP (50 μM) was applied to chickpea plants grown under non-saline and saline conditions (50 and 100 mM NaCl). Salt stress inhibited growth and biomass yield, leaf relative water content (LRWC) and chlorophyll content of chickpea plants. High salinity increased electrolyte leakage, carotenoid content and the levels of osmolytes (proline, glycine betaine, soluble proteins and soluble sugars), hydrogen peroxide (H2O2) and malondialdehyde (MDA), as well as the activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase in chickpea plants. Expression of the representative SOD, CAT and APX genes examined was also up-regulated in chickpea plants by salt stress. On the other hand, exogenous application of NO to salinized plants enhanced the growth parameters, LRWC, photosynthetic pigment production and levels of osmolytes, as well as the activities of examined antioxidant enzymes which is correlated with up-regulation of the examined SOD, CAT and APX genes, in comparison with plants treated with NaCl only. Furthermore, electrolyte leakage, H2O2 and MDA contents showed decline in salt-stressed plants supplemented with NO as compared with those in NaCl-treated plants alone. Thus, the exogenous application of NO protected chickpea plants against salt stress-induced oxidative damage by enhancing the biosyntheses of antioxidant enzymes, thereby improving plant growth under saline stress. Taken together, our results demonstrate that NO has capability to mitigate the adverse effects of high salinity on chickpea plants by improving LRWC, photosynthetic pigment biosyntheses, osmolyte accumulation and antioxidative defense system.

Alleviation of water deficit in Physalis angulata plants by nitric oxide exogenous donor

Lead (Pb) is one of the most abundant toxic heavy metals which adversely affected growth and yield of crop plants. Nitric oxide (NO), an endogenous signaling molecule, has been suggested to be involved in defense responses to biotic and abiotic stresses in plants. The present study was done to induce Pb tolerance in cowpea plants by exogenous NO application using two levels of Pb, 0 and 200 mg Pb (NO3)2 kg-1 soil and three NO levels, 0, 0.5 and 1 mM sodium nitroprusside (SNP), as NO donor. The results showed that Pb treatment caused a significant increase in Pb concentration in all plant parts. Roots had higher levels of Pb than the stems, leaves and seeds. Furthermore, lead toxicity reduced auxin (IAA), cytokinin and gibberellic acid (GA3) content but increased abscisic acid (ABA) level. Moreover Pb stress decreased stomatal conductance, leaf area and consequently seed yield of cowpea. Exogenous application of NO at 0.5 mM noticeably alleviated the lead toxicity by improving the leaf area, stomatal conductance and seed yield. NO increased Pb tolerance by lowering Pb uptake and translocation, enhancing the promoting phytohormone (IAA, cytokinin and GA3) level and reducing ABA content.

Nitric oxide (NO) is involved in numerous physiological and stress responses in higher plants. Tomato is one of the most important vegetable crops in the world and previously it has been reported that salinity induced an oxidative stress affecting its redox and NO homeostasis. Using tomato plant exposed to 120 mM NaCl, it was studied whether the exogenous application of NO could ameliorate the negative effects provoked by salinity. Thus, nitric oxide provoked a significantly increase in the main antioxidative enzymes including superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and peroxidase (POD) activities, and also a raise of some enzymes involved in nitrogen metabolism including nitrate reductase (NR) and nitrite reductase (NiR) activities. Moreover, NO-treated plants showed a higher content in both proline and ascorbate but lower content of H 2 O 2 . These data indicate that the exogenous NO application is useful to mitigate the salinity-induced oxidative stress in tomato plants.

Nitric oxide (NO) is a potent neuromodulator in the CNS and PNS. At the frog neuromuscular junction (nmj), exogenous application of NO reduces neurotransmitter release, and NO synthases (NOSs), the enzymes producing NO, are present at this synapse. This work aimed at studying the molecular mechanisms by which NO modulates synaptic efficacy at the nmj using electrophysiological recordings and Ca(2+)-imaging techniques. Bath application of the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside decreased end plate potential (EPP) amplitude as well as the frequency of miniature EPPs but not their amplitude. Ca(2+) responses elicited in presynaptic terminals by single action potentials were unaffected by NO, but responses evoked by a short train of stimuli were increased. Tonic endogenous production of NO was observed as suggested by the increase in EPP amplitude by bath application of the NO scavenger hemoglobin and the neuronal NOS inhibitor 3-bromo-7-nitroindazole sodium salt. A soluble guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY-83583), increased EPP amplitude and occluded the effects of the NO donor, suggesting that NO acts via a cGMP-dependent mechanism. High-frequency-induced depression was reduced in the presence of the NO scavenger but not by LY-83583. However, adenosine-induced depression was significantly reduced after bath perfusion of SNAP and in the presence of LY-83583. Our results indicate that NO regulates transmitter release and adenosine-induced depression via a cGMP-dependent mechanism that occurs after Ca(2+) entry and that high-frequency-induced synaptic depression is regulated by NO in a cGMP-independent manner.

Nitric oxide (NO) has received much attention since it can boost plant defense mechanisms, and plenty of studies have shown that exogenous NO improves salinity tolerance in plants. However, because of the wide range of experimental settings, it is difficult to assess the administration of optimal dosages, frequency, timing, and method of application and the overall favorable effects of NO on growth and yield improvements. Therefore, we conducted a meta-analysis to reveal the exact physiological and biochemical mechanisms and to understand the influence of plant-related or method-related factors on NO-mediated salt tolerance. Exogenous application of NO significantly influenced biomass accumulation, growth, and yield irrespective of salinity stress. According to this analysis, seed priming and foliar pre-treatment were the most effective methods of NO application to plants. Moreover, one-time and regular intervals of NO treatment were more beneficial for plant growth. The optimum concentration of NO ranges from 0.1 to 0.2 mM, and it alleviates salinity stress up to 150 mM NaCl. Furthermore, the beneficial effect of NO treatment was more pronounced as salinity stress was prolonged (>21 days). This meta-analysis showed that NO supplementation was significantly applicable at germination and seedling stages. Interestingly, exogenous NO treatment boosted plant growth most efficiently in dicots. This meta-analysis showed that exogenous NO alleviates salt-induced oxidative damage and improves plant growth and yield potential by regulating osmotic balance, mineral homeostasis, photosynthetic machinery, the metabolism of reactive oxygen species, and the antioxidant defense mechanism. Our analysis pointed out several research gaps, such as lipid metabolism regulation, reproductive stage performance, C4 plant responses, field-level yield impact, and economic profitability of farmers in response to exogenous NO, which need to be evaluated in the subsequent investigation.

Correlation of in vivo nitric oxide and cGMP with glutamate/glutamine metabolism in the rat striatum

Correlation of in vivo nitric oxide and cGMP with glutamate/glutamine metabolism in the rat striatum