Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots

The silver lining of antibiotic resistance: Bacterial-mediated reduction of tetracycline plant stress via antibiotrophy

The reuse of water using effluents containing antibiotics from anthropogenic activities has been mainly linked to the development of antibiotic resistance. However, we report that the development of bacterial tolerance promotes plant growth. In the present study, we aimed to evaluate the efficiency of inoculation of a new antibiotic-degrading bacterium, Erwinia strain S9, in augmenting the tolerance of pea (Pisum sativum L.) plants to tetracycline (TET) (10 and 20 mg/L). Physiological parameters such as tissue elongation and biomass, as well as relative water content, were remarkably lower in plants exposed to TET than in the control. The inhibitory effects of TET were associated with reduced CO2 assimilation, stomatal conductance, transpiration, dark respiration, and light saturation point (LSP). High concentrations of TET-induced oxidative stress are attested by the overproduction of superoxide radicals (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO•), resulting in increased malondialdehyde content and cell death. The high activity of antioxidant enzymes such as catalase, ascorbate peroxidase, and guaiacol peroxidase validated the proposed mechanism. Under TET stress conditions, supplementation with Erwinia strain S9 was beneficial to pea plants through osmotic adjustment, increased nutrient uptake, gas exchange optimization, and increased antioxidant activities. Its presence not only ensures plant survival and growth during antibiotic stress but also degrades TET via significant antibiotrophy. This strategy is a cost-effective environmental chemical engineering tool that can be used to depollute wastewater or to improve crop resistance in rhizofiltration treatment when treated wastewater is reused for irrigation.

Do differentially charged nanoplastics affect imidacloprid uptake, translocation, and metabolism in Chinese flowering cabbage?

Micro(nano)plastics are ubiquitous in the environment. Among the microplastics, imidacloprid (IMI) concentration has been increasing in some intensive agricultural regions, thus receiving increased attention. However, only a few studies have investigated the interaction of nanoplastics (polystyrene (PS)) and IMI in vegetable crops. We studied the effects of positively (PS-NH₂) and negatively (PS-COOH) charged nanoplastics on the uptake, translocation, and degradation of IMI in Chinese flowering cabbage grown in Hoagland solution for 28 days. PS-NH₂ co-exposure with IMI inhibited plant growth, resulting in decreased plant weight, height, and root length. Translocation of IMI from the roots to the shoots was significantly lower in the presence of PS-NH₂, whereas PS-COOH accelerated the accumulation and translocation of IMI in plants, thus potentially affecting IMI metabolism in plants. Notably, IMI-NTG and 5-OH-IMI were the two dominant metabolites. PS-NH₂ co-exposure with IMI induced significant oxidation stress and considerably affected the activities of superoxide dismutase (SOD) and peroxidase (POD), indicating that the antioxidant defense system was the main mechanism for reducing oxidative damage. Notably, both positively and negatively charged nanoplastics can accumulate in Chinese flowering cabbage. Plants in the PS-COOH alone treatment group had the highest concentration of nanoplastics in both roots and shoots. The accumulation of nanoplastics, IMI, and its metabolites in plants raises concerns about their combined potential toxicity because it compromises food safety.

Cadmium contributes to heat tolerance of a hyperaccumulator plant species Sedum alfredii

Hyperaccumulators are plant species that tolerate and accumulate very high concentrations of toxic metals, including Cd. Hyperaccumulation of heavy metals is reported to benefit plant biotic resistance; however, no prior study has examined the possible role of toxic metals on abiotic stress resistance in hyperaccumulators. A preliminary experiment found that Cd significantly improved plant growth of a hyperaccumulator, Sedum alfredii Hance, under heat stress. This study investigated the possible role of Cd in S. alfredii's heat resistance, using infrared thermography, transmission electron microscopy (TEM), real-time quantitative polymerase chain reaction (RTqPCR), and high-throughput sequencing. The results showed that high temperatures irreversibly damaged stomatal function, chloroplast structure, photosynthesis in S. alfredii, and lowered survival rates to 25%. However, Cd application significantly decreased the leaf temperature of S. alfredii and increased the survival rate to 75%. Cd penetrated the guard cells, restored stomatal function, and mitigated excessive water loss from S. alfredii under heat stress. Moreover, it activated antioxidant enzymes, promoted phytohormone biosynthesis, and upregulated a series of unigenes, thereby augmenting heat resistance in S. alfredii. These results indicate that Cd effectively improved thermotolerance in S. alfredii by regulating stomatal movement and antioxidant systems via upregulation of phytohormones and heat shock proteins.

Glucose-6-Phosphate Dehydrogenases: The Hidden Players of Plant Physiology

Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes a metabolic hub between glycolysis and the pentose phosphate pathway (PPP), which is the oxidation of glucose-6-phosphate (G6P) to 6-phosphogluconolactone concomitantly with the production of nicotinamide adenine dinucleotide phosphate (NADPH), a reducing power. It is considered to be the rate-limiting step that governs carbon flow through the oxidative pentose phosphate pathway (OPPP). The OPPP is the main supplier of reductant (NADPH) for several "reducing" biosynthetic reactions. Although it is involved in multiple physiological processes, current knowledge on its exact role and regulation is still piecemeal. The present review provides a concise and comprehensive picture of the diversity of plant G6PDHs and their role in seed germination, nitrogen assimilation, plant branching, and plant response to abiotic stress. This work will help define future research directions to improve our knowledge of G6PDHs in plant physiology and to integrate this hidden player in plant performance.

Glucose-6-phosphate dehydrogenase promotes the infection of Chilli veinal mottle virus through affecting ROS signaling in Nicotiana benthamiana

G6PDH negatively regulates viral accumulation in Nicotiana benthamiana through RBOHB-associated ROS signaling. Anti-oxidative metabolism and phytohormone-mediated immunity responses play important roles in virus infection. Glucose-6-phosphate dehydrogenase (G6PDH) is an enzyme in the pentose phosphate pathway, which plays an important role in maintaining intracellular redox homeostasis and has functions in plant growth, development and stress tolerance. However, the role of G6PDH in plants response to virus infection is poorly understood. In this study, NbG6PDH was found to be down-regulated after Chilli veinal mottle virus (ChiVMV-GFP) infection in Nicotiana benthamiana. Subcellular localization of NbG6PDH showed that it was punctate distributed in the protoplasm. Silencing of NbG6PDH reduced the sensitivity of N. benthamiana plants to ChiVMV-GFP. By contrast, transient overexpression of NbG6PDH promoted the accumulation of the virus. The results of physiological indexes showed that glutathione (GSH), catalase (CAT) and proline played an important role in maintaining plants physiological homeostasis. The results of gene expression detection showed that jasmonic acid/ethylene (JA/ET) signaling pathway was significantly correlated with the response of N. benthamiana to ChiVMV-GFP infection, and the changes of N. benthamiana respiratory burst oxidase homologues B (NbRBOHB) indicated that the NbG6PDH-dependent ROS may be regulated by NbRBOHB. Pretreatment of the inducer of reactive oxygen species (ROS) promoted virus infection, whereas inhibitor of ROS alleviated virus infection. Thus, our results indicate that the promoting effect of NbG6PDH on ChiVMV-GFP infection may be related to the NbRBOHB-regulated ROS production.

Circadian Rhythm Regulates Reactive Oxygen Species Production and Inhibits Al-Induced Programmed Cell Death in Peanut

Peanut is among the most important oil crops in the world. In the southern part of China, peanut is highly produced; however, the arable land is acidic. In acidic soils, aluminum (Al) inhibits plant growth and development by changing the properties of the cell wall and causing the disorder of the intracellular metabolic process. Circadian rhythm is an internal mechanism that occurs about every 24 h and enables plants to maintain internal biological processes with a daily cycle. To investigate the effect of photoperiod and Al stress on the Al-induced programmed cell death (PCD), two peanut varieties were treated with 100 μM AlCl₃ under three photoperiodic conditions (8/16, SD; 12/12, ND; 16/8 h, LD). The results show that Al toxicity was higher in ZH2 than in 99-1507 and higher under LD than under SD. Root length decreased by 30, 37.5, and 50% in ZH2 and decreased by 26.08, 34.78, and 47.82% in 99-1507 under SD, ND, and LD, respectively, under Al stress. Photoperiod and Al induced cell death and ROS production. MDA content, PME activity, and LOX activity increased under SD, ND, and LD, respectively, under Al stress both in ZH2 and 99-1507. APX, SOD, CAT, and POD activities were higher under SD, ND, and LD, respectively. Al stress increased the level of AhLHY expression under SD and ND but decreased it under LD in both ZH2 and 99-1507. Contrastingly, AhSTS expression levels increased exponentially and were higher under SD, LD, and ND, respectively, under Al stress. Our results will be a useful platform to research PCD induced by Al and gain new insights into the genetic manipulation of the circadian clock for plant stress response.

Exogenous nitric oxide alleviates manganese toxicity in bean plants by modulating photosynthesis in relation to leaf lipid composition

Nitric oxide (NO) is a signaling molecule controlling several steps of plant development and defense process under stress conditions. NO-induced alleviation of manganese (Mn) toxicity was investigated on bean plants submitted for 28 days to 500 µM MnCl₂. Manganese excess decreased plant dry weight and elongation and increased levels of reactive oxygen species and lipid peroxidation leading to up-regulation of superoxide dismutase, catalase, and ascorbate peroxidase activities. The inhibitory effects of Mn on plant growth were associated to reduction of light-saturated carbon assimilation (A_max), stomatal conductance (g_s), and transpiration (E). By contrast, Mn induced significant increase in the apparent quantum yield (ɸ) and light compensation point (LCP). Interestingly, intracellular CO₂ (Ci) remains stable under Mn stress. Concomitantly, leaf membrane lipids have drastically reduced under high Mn concentration. After Mn exposition, leaf fatty acids exhibited a significant loss of linolenic acid, accompanied by an accumulation of palmitoleic, stearic, and linoleic acids leading to alteration of lipid desaturation. NO supply reversed Mn toxicity as evidenced by enhancement of growth biomass and recovery of A_max, E, ɸ, and LCP. Similarly, NO addition has positive effects on leaf lipid content and composition leading to restoration of lipid unsaturation. The modulation of fatty acid composition can be a way to reduce leaf membrane damages and maintain optimal photosynthesis and plant growth. Despite the absence of enough evidences in how NO is involved in lipid and photosynthesis recovery under Mn stress conditions, it is assumed that NO beneficial effects are attributable to NO/Mn cross-talk.

Biochemical Studies on the Colorectal Tissues of Wistar Rats Treated with Methanol Leaf and Stem Bark Extracts of Annona muricata (Soursop)

Annona muricata (AMC), is a tropical plant species of the Annonaceae family known for its medicinal uses in ameliorating and/or protecting several organs and tissues in the body. In this study we elucidated the influence of the methanol extracts of AMC leaf and stem bark on the biochemical and histological integrity of the colorectum. Different doses (100, 200, 400, 600, and 800mg of extract/kg body weight) of methanol extracts of the leaves and stembark were orally administered to adult male Wistar rats of average weight (100 - 150 g) for 28 days. At the end of the experimental period, the rats were sacrificed and colorectal tissues harvested for analyses. Adenosine triphosphatase (ATPases), lactate dehydrogenase (LDH), glucose-6-phosphate dehydrogenase (G6PD) activities, and tissue protein (TP) concentration, were evaluated by colorimetric method using appropriate test kits. The results in the colorectal tissue analysed showed that total ATPase, LDH and G6PD (energy markers) activities increased significantly in the groups which received methanol leaf and stem bark extracts of AMC when compared with control. However, a general decrease was observed in TP and Na+/K+-ATPase activity but there was a twist in Na+/K+-ATPase activity in stem bark doses of 200mg/kg and 400mg/kg which showed significant increases in dose dependent manner when compared with the control. Even though G6PD activity showed fluctuating patterns for both extracts, the highest dose (800mg/kg) reflected the most significant increase when compared with the control. The histology confirmed the toxicological results by the biochemical parameters. Under the condition of this study, we inferred that the methanol leaf and stem-bark extracts of AMC may significantly influence the enzymes by reducing and increasing total ATPase and Na+/K+-ATPase activities depending on the doses and administration should therefore depend on the desired outcome.

The Role of Nitric Oxide in Plant Responses to Salt Stress

The gas nitric oxide (NO) plays an important role in several biological processes in plants, including growth, development, and biotic/abiotic stress responses. Salinity has received increasing attention from scientists as an abiotic stressor that can seriously harm plant growth and crop yields. Under saline conditions, plants produce NO, which can alleviate salt-induced damage. Here, we summarize NO synthesis during salt stress and describe how NO is involved in alleviating salt stress effects through different strategies, including interactions with various other signaling molecules and plant hormones. Finally, future directions for research on the role of NO in plant salt tolerance are discussed. This summary will serve as a reference for researchers studying NO in plants.

Endogenous nitric oxide and its potential sources regulate glutathione-induced cadmium stress tolerance in maize plants

It was aimed to assess that up to what extent endogenous nitric oxide (NO) and its sources are involved in glutathione (GSH)-mediated tolerance of maize plants to cadmium (Cd) stress. The Cd-stressed maize plants were sprayed with or without GSH (1.0 mM) once every week for two weeks. Before initiating the stress treatment, the Cd-stressed plants sprayed with GSH were supplied with or without 0.1 mM, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO; a NO scavenger) for two weeks or with 0.1 mM sodium tungstate (ST; a nitrate reductase inhibitor), or 0.1 mM NG-nitro-L-arginine methyl ester hydrochloride (L-NAME). Cadmium stress suppressed the activities of dehydroascorbate reductase, monodehydroascorbate reductase, and glyoxalase II, while increased leaf NO, Cadmium content, proline, oxidative stress, the activities of glutathione reductase, ascorbate peroxidase, the key enzymes of oxidative defense system, glyoxalase I, NR and NOS. GSH reduced oxidative stress and tissue Cd²⁺ content, but it improved growth, altered water relations, and additionally increased proline levels, activities of the AsA-GSH cycle, key enzymatic antioxidants, glyoxalase I and II, NR and NOS as well as NO content. The cPTIO and ST supplementation abolished the beneficial effects of GSH by reducing the activities of NO and NR. However, L-NAME did not retreat the favorable effects of GSH, although it reduced the NOS activity without eliminating NO content, suggesting that NR might be a prospective source of NO generated by GSH in Cd-stressed plants, which in turn accelerated the activities of antioxidant enzymes.

Triacontanol attenuates drought-induced oxidative stress in Brassica juncea L. by regulating lignification genes, calcium metabolism and the antioxidant system

Effect of triacontanol on drought-induced stress was studied in Brassica juncea L. Foliage of sixteen-days-old plants was sprayed with concentrations (0, 10, 20 and 30 μM) of triacontanol (TRIA) for 7 days. Subsequently, plants were subjected to drought stress (10% polyethylene glycol, PEG6000) for 7 days. Drought stress increased oxidative stress (TBARS, O₂^●- and H₂O₂), however, their contents were reduced by TRIA. Total soluble sugars, reduced glutathione, and proline content in stressed plants were increased by TRIA. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT), and phenylalanine ammonia-lyase (PAL) activity were significantly increased in a dose-dependent manner with TRIA. Potassium (K⁺) level declined, while magnesium (Mg²⁺) and calcium (Ca²⁺) contents increased. The elevated level of lignin under drought with TRIA was significantly associated with MYB46 and PAL gene expression patterns. Altogether, our results suggest that foliar spray of 20 μM TRIA was more operative in reducing the negative impact of drought stress in B. juncea by regulating the antioxidant system, calcium, and lignification.

The genome sequence of Aloe vera reveals adaptive evolution of drought tolerance mechanisms

Aloe vera is a species from Asphodelaceae family having characteristics like drought resistance and numerous medicinal properties. However, the genetic basis of these phenotypes is yet unknown primarily due to unavailability of its genome sequence. Thus, we report the first Aloe vera genome sequence comprising of 12.93 Gbp and harboring 86,177 protein-coding genes. It is the first genome from Asphodelaceae family and the largest angiosperm genome sequenced and assembled till date. We also report the first genome-wide phylogeny of monocots including Aloe vera to resolve its phylogenetic position. The comprehensive comparative analysis of Aloe vera with other available high-quality monocot genomes revealed adaptive evolution in several genes of drought stress response, CAM pathway, and circadian rhythm and positive selection in DNA damage response genes in Aloe vera. This study provides clues on the genetic basis of evolution of drought stress tolerance capabilities of Aloe vera.

Nitric oxide and hydrogen sulfide modulate the NADPH-generating enzymatic system in higher plants

Nitric oxide (NO) and hydrogen sulfide (H2S) are two key molecules in plant cells that participate, directly or indirectly, as regulators of protein functions through derived post-translational modifications, mainly tyrosine nitration, S-nitrosation, and persulfidation. These post-translational modifications allow the participation of both NO and H2S signal molecules in a wide range of cellular processes either physiological or under stressful circumstances. NADPH participates in cellular redox status and it is a key cofactor necessary for cell growth and development. It is involved in significant biochemical routes such as fatty acid, carotenoid and proline biosynthesis, and the shikimate pathway, as well as in cellular detoxification processes including the ascorbate-glutathione cycle, the NADPH-dependent thioredoxin reductase (NTR), or the superoxide-generating NADPH oxidase. Plant cells have diverse mechanisms to generate NADPH by a group of NADP-dependent oxidoreductases including ferredoxin-NADP reductase (FNR), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), NADP-dependent malic enzyme (NADP-ME), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and both enzymes of the oxidative pentose phosphate pathway, designated as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). These enzymes consist of different isozymes located in diverse subcellular compartments (chloroplasts, cytosol, mitochondria, and peroxisomes) which contribute to the NAPDH cellular pool. We provide a comprehensive overview of how post-translational modifications promoted by NO (tyrosine nitration and S-nitrosation), H2S (persulfidation), and glutathione (glutathionylation), affect the cellular redox status through regulation of the NADP-dependent dehydrogenases.

Identification, Characterization, and Stress Responsiveness of Glucose-6-phosphate Dehydrogenase Genes in Highland Barley

G6PDH provides intermediate metabolites and reducing power (nicotinamide adenine dinucleotide phosphate, NADPH) for plant metabolism, and plays a pivotal role in the cellular redox homeostasis. In this study, we cloned five G6PDH genes (HvG6PDH1 to HvG6PDH5) from highland barley and characterized their encoded proteins. Functional analysis of HvG6PDHs in E. coli showed that HvG6PDH1 to HvG6PDH5 encode the functional G6PDH proteins. Subcellular localization and phylogenetic analysis indicated that HvG6PDH2 and HvG6PDH5 are localized in the cytoplasm, while HvG6PDH1, HvG6PDH3, and HvG6PDH4 are plastidic isoforms. Analysis of enzymatic activities and gene expression showed that HvG6PDH1 to HvG6PDH4 are involved in responses to salt and drought stresses. The cytosolic HvG6PDH2 is the major isoform against oxidative stress. HvG6PDH5 may be a house-keeping gene. In addition, HvG6PDH1 to HvG6PDH4 and their encoded enzymes responded to jasmonic acid (JA) and abscisic acid (ABA) treatments, implying that JA and ABA are probably critical regulators of HvG6PDHs (except for HvG6PDH5). Reactive oxygen species analysis showed that inhibition of cytosolic and plastidic G6PDH activities leads to increased H₂O₂ and O₂⁻ contents in highland barley under salt and drought stresses. These results suggest that G6PDH can maintain cellular redox homeostasis and that cytosolic HvG6PDH2 is an irreplaceable isoform against oxidative stress in highland barley.

Nitric oxide and hydrogen sulfide protect plasma membrane integrity and mitigate chromium-induced methylglyoxal toxicity in maize seedlings

The present study aims to analyse the potential crosstalk between nitric oxide (NO) and hydrogen sulfide (H₂S) in triggering resilience of maize (Zea mays L.) seedlings to hexavalent chromium (Cr VI). Exogenous application of 500 μM sodium nitroprusside (SNP, as a NO donor) or sodium hydrosulfide (NaHS, as a H₂S donor) to 9-day-old maize seedlings, countered a Cr (200 μM) -elicited reduction in embryonic axis biomass. Cr caused cellular membrane injury by enhancing the levels of superoxide and hydroxyl radicals as well as methylglyoxal, and 4-hydroxy-2-nonenal. The application of SNP or NaHS considerably improved the endogenous NO and H₂S pool, decreased oxidative stress and lipid peroxidation by suppressing lipoxygenase activity and improving some antioxidant enzymes activities in radicles and epicotyls. Radicles were more affected than epicotyls by Cr-stress with enhanced electrolyte leakage and decreased proton extrusion as indicated by lesser H⁺-ATPase activity. H₂S appeared to mitigate Cr toxicity through up-regulated H⁺-ATPase and glyoxalase pathways and by maintaining optimal GSH levels as downstream effects of ROS and MG suppression. Hence, H₂S-mediated the regeneration of GSH pool is associated with the attenuation of MG toxicity by enhancing S-lactoglutathione and D-lactate production. Taken together, our results indicate complementary roles for H₂S and GSH to strengthen membrane integrity against Cr stress in maize seedlings.

Comparative Ubiquitome Analysis under Heat Stress Reveals Diverse Functions of Ubiquitination in Saccharina japonica

Ubiquitination is a major post-translational modification involved in nearly all aspects of eukaryotic biology. Previous RNA-Seq studies showed that ubiquitination plays essential roles in the heat tolerance of Saccharina japonica, but to date, large-scale profiling of the ubiquitome in S. japonica has not been reported. To better understand the regulatory roles of ubiquitination in heat responses of S. japonica, we investigated its ubiquitome under normal and heat stress by the combination of affinity enrichment and high-resolution liquid chromatography-tandem mass spectroscopy analysis. Altogether, 3305 lysine ubiquitination sites in 1562 protein groups were identified. After normalization, 152 lysine ubiquitination sites in 106 proteins were significantly upregulated and 208 lysine ubiquitination sites in 131 proteins were significantly downregulated in response to heat stress. Protein annotation and functional analysis suggested that ubiquitination modulates a variety of essential cellular and physiological processes, including but not limited to the ubiquitin-26S proteasome system, ribosome, carbohydrate metabolism, and oxidative phosphorylation. Our results provide a global view of the heat response ubiquitome in S. japonica, and could facilitate future studies on the physiological roles of these ubiquitination-related proteins.

Label-free quantitative proteomic analysis of alfalfa in response to microRNA156 under high temperature

BACKGROUND

Abiotic stress, including heat, is one of the major factors that affect alfalfa growth and forage yield. The small RNA, microRNA156 (miR156), regulates multiple traits in alfalfa during abiotic stress. The aim of this study was to explore the role of miR156 in regulating heat response in alfalfa at the protein level.

RESULTS

In this study, we compared an empty vector control and miR156 overexpressing (miR156OE) alfalfa plants after exposing them to heat stress (40 °C) for 24 h. We measured physiological parameters of control and miR156OE plants under heat stress, and collected leaf samples for protein analysis. A higher proline and antioxidant contents were detected in miR156OE plants than in controls under heat stress. Protein samples were analyzed by label-free quantification proteomics. Across all samples, a total of 1878 protein groups were detected. Under heat stress, 45 protein groups in the empty vector plants were significantly altered (P < 0.05; |log2FC| > 2). Conversely, 105 protein groups were significantly altered when miR156OE alfalfa was subjected to heat stress, of which 91 were unique to miR156OE plants. The identified protein groups unique to miR156OE plants were related to diverse functions including metabolism, photosynthesis, stress-response and plant defenses. Furthermore, we identified transcription factors in miR156OE plants, which belonged to squamosa promoter binding-like protein, MYB, ethylene responsive factors, AP2 domain, ABA response element binding factor and bZIP families of transcription factors.

CONCLUSIONS

These results suggest a positive role for miR156 in heat stress response in alfalfa. They reveal a miR156-regulated network of mechanisms at the protein level to modulate heat responses in alfalfa.

Effects of nitric oxide on nitrogen metabolism and the salt resistance of rice (Oryza sativa L.) seedlings with different salt tolerances

Salt stress inhibits rice productivity seriously. Nitric oxide (NO) is an endogenous signaling molecule in plants that can improve the resistance of rice to abiotic stresses. Previous studies also showed that nitrogen metabolism is essential for rice stress-tolerance. However, the physiological and molecular mechanisms by how NO affects the nitrogen metabolisms of rice seedlings remain unclear. A hydroponic experiment with two rice varieties, Jinyuan85 (salt tolerant) and Liaojing763 (salt sensitive), was carried out to explore whether NO could alleviate the negative effects of salt stress on nitrogen metabolism and increase salt resistance of rice seedlings. The results showed that (1) the application of NO alleviated the inhibitory effects of salt stress on plant height and biomass accumulation, and increased the nitrogen content of rice leaf. (2) the accumulation of the sucrose and proline was markedly increased in salt stress after application of NO, and peroxidase activities was increased by 107% and 67.7% for Jinyuan85 and Liaojing763, respectively. (3) NO significantly increased the activities of glutamate dehydrogenase, sucrose synthase and sucrose phosphate synthase in both rice varieties under salt stress. (4) Additionally, NO regulated the expression levels of AMT, NIA and SUT genes, but these regulation effects are different with rice varieties and treatments. The results suggested that NO mainly increased the glutamate dehydrogenase and peroxidase activities and sucrose accumulation to enhance the nitrogen metabolism and antioxidative capacity, and alleviated the negative effects of salt stress on rice performance.

Nitric oxide production is involved in maintaining energy state in Alfalfa (Medicago sativa L.) nodulated roots under both salinity and flooding

In Medicago sativa nodulated roots, NR-dependent NO production is involved in maintaining energy state, presumably through phytoglobin NO respiration, under both salinity and hypoxia stress. The response to low and average salinity stress and to a 5 day-long flooding period was analyzed in M. sativa nodulated roots. The two treatments result in a decrease in the biological nitrogen fixation capacity and the energy state (evaluated by the ATP/ADP ratio), and conversely in an increase nitric oxide (NO) production. Under salinity and hypoxia treatments, the use of either sodium tungstate, an inhibitor of nitrate reductase (NR), or carboxy-PTIO, a NO scavenger, results in a decrease in NO production and ATP/ADP ratio, meaning that NR-dependent NO production participates to the maintenance of the nodulated roots energy state.

Evaluation of seed nitrate assimilation and stimulation of phenolic-linked antioxidant on pentose phosphate pathway and nitrate reduction in three feed-plant species

BACKGROUND

Soil and water pollution due to nitrate are becoming increasingly serious worldwide. The government also put forward relevant governance policies, and a large number of scholars studied chemical physics and other methods to remove nitrate in water, but the cost was substantial. Studies have found that planting systems including grasses have the potential to remove nitrates. However, there are few studies on nitrate linked pathway and nitrate assimilation during its early growth.

RESULTS

We have evaluated three different feed-plant species with three levels of overnight seed nitrate treatments along with a control. The activity of different enzymes from 2 weeks old shoots was measured to get a comprehension of proline-associated pentose phosphate pathway coupled with nitrate assimilation and phenolic-linked antioxidant response system in these species under nitrate treatments. All three feed-plant species showed high nitrate tolerance during germination and early growth stages. It is perceived that the accumulation of total soluble phenolics and total antioxidant activity was high in all three feed-plant species under high nitrate treatments. In terms of high G6PDH activity along with low SDH activity in alfalfa, there may be a shift of carbon flux in this species under high nitrate treatments. Higher activity of these enzymes along with higher SOD and GPX activity was observed in alfalfa. The efficient mechanism of nitrate stress tolerance of alfalfa also correlated with higher photochemical efficiency. Perennial ryegrass also showed excellent potential under high nitrate treatments by adopting an efficient mechanism to counter nitrate-induced oxidative stress.

CONCLUSIONS

Under the condition of nitrate treatment, the germination rates of the three feed-plant species are still ideal, and they have good enzyme activity and have the potential to remove nitrate.

Enhancement of polyphenolic metabolism as an adaptive response of lettuce (Lactuca sativa) roots to aluminum stress

Polyphenols, pivotal secondary metabolites, are involved in plant adaption to abiotic stresses. Here, we investigated the role and metabolism profile of polyphenols under aluminum (Al) stress in different lettuce genotypes grown in 0.5 mM CaCl2 solution with AlCl3 (pH = 4.5). The complementary use of high-resolution mass spectrometry and quantitative biochemical approaches allowed the characterization of total and unique phenols, as well as their roles in Al tolerance. By comparing the most tolerant and sensitive genotype, 8 polyphenols, including 4 phenolic acids, 2 flavonoids, 1 xanthone and 1 unknown compound, were identified in the roots of the tolerant genotype. The total phenolic and flavonoid contents significantly increased in the tolerant genotype under Al stress. Seedlings with more phenolic accumulation usually performed greater Al tolerance. Meanwhile, principal enzymes related to phenolic biosynthesis significantly increased in roots of the tolerance genotype after Al treatment, with phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase, and 4-coumarate coenzyme A ligase increased by 16, 18 and 30%, respectively. The elevated total phenolics were significantly suppressed by AIP, a highly specific PAL inhibitor. Consequently, the antioxidant capacity was inhibited, leading to lettuce sensitivity to Al stress. These results clearly suggested the enhancement of unique polyphenolic biosynthesis as an adaptive strategy of lettuce to Al stress by protecting plants from oxidative stress.

Involvement of active MKK9-MAPK3/MAPK6 in increasing respiration in salt-treated Arabidopsis callus

Mitogen-activated protein kinase kinase 9 (MKK9) is an upstream activator of mitogen-activated protein kinase 3 (MAPK3) and MAPK6 in planta. To investigate MKK9 roles in mitochondrial respiration in Arabidopsis, MKK9^DD, the active allele with mutations of Thr-201 and Ser-205 to Asp, and MKK9^KR, the allele lacking MKK9 activity with a mutation of Lys-76 to Arg, were used. Results showed that the total respiratory rate (V_t), alternative pathway capacity (V_alt) and cytochrome pathway capacity (V_cyt) increased under 0-100 mM NaCl treatments but decreased under 150-300 mM NaCl treatments in Col-0 callus. However, the activation of MKK9 by dexamethasone (DEX) increased V_t, V_alt and V_cyt under 200 mM NaCl treatment; moreover, V_alt showed more increase than V_cyt. The activation of MKK9 in MKK9^DD callus sharply increased AOX protein expression under normal and NaCl conditions, but the increase was not observed in MKK9^KR callus. Further results indicated that MAPK3 and MAPK6 were involved in the MKK9-induced increase of AOX protein levels. qRT-PCR results showed that MKK9-MAPK3/MAPK6 was involved in the NaCl-induced AOX1b and AOX1d expression, but only MKK9-MAPK3 was necessary for AOX2 expression; in addition, MAPK3 regulated the AOX1a transcription in an MKK9-independent manner. MKK9 positively regulated SOD and CAT activities by affecting MAPK3 and MAPK6 and negatively regulated APX and POD activities by affecting MAPK3. Moreover, MKK9 functions as a positive factor in H₂O₂ accumulation under salt stress. The regulation of ethylene on alternative respiration was also associated with MKK9 under salt stress. Taken together, the MKK9-MAPK3/MAPK6 pathway plays a pivotal role in increasing alternative respiration in the salt-treated Arabidopsis callus.

Chryseobacterium salivictor sp. nov., a plant-growth-promoting bacterium isolated from freshwater

A Gram-stain-negative, non-motile, rod-shaped bacterial strain, designated NBC 122^T, was isolated from freshwater of the Nakdong River Republic of Korea. Growth occurred at pH 5.0-8.0 (optimum, pH 7.0), at 4-37 °C (optimum, 25 °C), and with 0-3.5% (w/v) NaCl (optimum, 1%). Phylogenetic analyses based on 16S rRNA gene sequences showed that strain NBC 122^T belongs to the genus Chryseobacterium and is most closely related to Chryseobacterium antarcticum AT1013^T (97.85%). The average nucleotide identity and in silico DNA-DNA hybridization (DDH) values between strain NBC 122^T and related Chryseobacterium species were 77.77-80.28 and 20.9-23.2%, respectively. The strain NBC 122^T contained MK-6 as the major respiratory quinone. The major fatty acids included anteiso C_15:0, iso C_15:0, summed feature 9 (C_17:1 iso ω9c, C_16:0 10-methyl) and iso C_17:0 3-OH, and the polar lipids were phosphatidylethanolamine, five unidentified aminolipids and six unidentified lipids. The DNA G + C content was 37.5 mol%. On the basis of a 16S rRNA gene phylogeny and comparative phenotypic analyses, strain NBC 122^T represents a novel species in the genus Chryseobacterium, for which the name Chryseobacterium salivictor sp. nov. is proposed. The type strain is NBC122^T (= KCTC 72248^T = JCM 33980^T).

Sulphur potentiates selenium to alleviate arsenic-induced stress by modulating oxidative stress, accumulation and thiol-ascorbate metabolism in Brassica juncea L

The present study was designed to see the influence of selenium (Se) and sulphur (S) in the alleviation of arsenic (As)-induced stress in Brassica juncea plant. Se-induced alterations in physiological and biochemical responses due to deficient S (DS), normal S (NS) and additional S (AS) conditions were evaluated in 14-day-old seedlings of B. juncea variety Varuna. During the last 7 days of the 14-day-old seedlings, supplementation with arsenite (As^III, 300 μM) alone and its combination with selenite (Se^IV, 50 μM) along with different S treatments was done which are as follows: (i) control; (ii) As; (iii) As+Se+DS; (iv) As+Se + NS; (v) As+Se + AS. Experimental results showed that the application of AS in spite of NS supplied with Se influenced plant growth, oxidative stress and thiol-ascorbate-related parameters more prominently under As stress. The plants with As+Se+AS treatment exhibited lower ROS (superoxide and hydrogen peroxide ion), malondialdehyde (MDA) accumulation and lipoxygenase activity with increased activities of superoxide dismutase, catalase and ascorbate peroxidase compared with As+Se+NS condition. These plants also exhibited an increase in cysteine, non-protein thiols and phytochelatins, along with reduced, oxidised and redox content of glutathione and ascorbate. Furthermore, the application of S along with Se increased the activities of glutathione reductase, glutathione S-transferase, glutathione peroxidase, monodehydroascorbate and dehydroascorbate to minimise As stress. However, we observed that these responses were reversed under As+Se+DS condition and induced oxidative stress, which was almost similar to As only treatment. It indicated that AS nutrition potentiated Se to alleviate As-inhibited plant growth by modulating antioxidants including thiol-ascorbate-based mechanism and reducing As accumulation in B. juncea plants.

Genome-Wide Analysis of the Glucose-6-Phosphate Dehydrogenase Family in Soybean and Functional Identification of GmG6PDH2 Involvement in Salt Stress

Glucose-6-phosphate dehydrogenase (G6PDH) is known as a critical enzyme responsible for nicotinamide adenine dinucleotide phosphate (NADPH) generation in the pentose phosphate pathway (PPP), and has an essential function in modulating redox homeostasis and stress responsiveness. In the present work, we characterized the nine members of the G6PDH gene family in soybean. Phylogenic analysis and transit peptide prediction showed that these soybean G6PDHs are divided into plastidic (P) and cytosolic (Cy) isoforms. The subcellular locations of five GmG6PDHs were further verified by confocal microscopy in Arabidopsis mesophyll protoplasts. The respective GmG6PDH genes had distinct expression patterns in various soybean tissues and at different times during seed development. Among them, the Cy-G6PDHs were strongly expressed in roots, developing seeds and nodules, while the transcripts of P-G6PDHs were mainly detected in green tissues. In addition, the activities and transcripts of GmG6PDHs were dramatically stimulated by different stress treatments, including salt, osmotic and alkali. Notably, the expression levels of a cytosolic isoform (GmG6PDH2) were extraordinarily high under salt stress and correlated well with the G6PDH enzyme activities, possibly implying a crucial factor for soybean responses to salinity. Enzymatic assay of recombinant GmG6PDH2 proteins expressed in Escherichia coli showed that the enzyme encoded by GmG6PDH2 had functional NADP⁺-dependent G6PDH activity. Further analysis indicated overexpression of GmG6PDH2 gene could significantly enhance the resistance of transgenic soybean to salt stress by coordinating with the redox states of ascorbic acid and glutathione pool to suppress reactive oxygen species generation. Together, these results indicate that GmG6PDH2 might be the major isoform for NADPH production in PPP, which is involved in the modulation of cellular AsA-GSH cycle to prevent the oxidative damage induced by high salinity.

Interactive role of exogenous 24 Epibrassinolide and endogenous NO in Brassica juncea L. under salinity stress: Evidence for NR-dependent NO biosynthesis

The present study unravels origin of nitric oxide (NO) and the interaction between 24-Epibrassinolide (EBL) and nitrate reductase (NR) for NO production in Indian mustard (Brassica juncea L.) under salinity stress. Two independent experiments were performed to check whether (i) Nitrate reductase or Nitric oxide synthase takes part in the biosynthesis of endogenous NO and (ii) EBL has any regulatory effect on NR-dependent NO biosynthesis in the alleviation of salinity stress. Results revealed that NR-inhibitor tungstate significantly (P ≤ 0.05) decreased the NR activity and endogenous NO content, while NOS inhibitor l-NAME did not influence NO biosynthesis and plant growth. Under salinity stress, inhibition in NR activity decreased the activities of antioxidant enzymes, increased H₂O₂, MDA, protein carbonyl content and caused DNA damage, implying that antioxidant defense might be related to NO signal. EBL supplementation enhanced the NR activity but did not influence NOS activity, suggesting that NR was involved in endogenous NO production. EBL supplementation alleviated the inhibitory effects of salinity stress and improved the plant growth by enhancing nutrients, photosynthetic pigments, compatible osmolytes, and performance of AsA-GSH cycle. It also decreased the superoxide ion accumulation, leaf epidermal damages, cell death, DNA damage, and ABA content. Comet assay revealed significant (P ≤ 0.05) enhancement in tail length and olive tail moment, while flow cytometry did not showed any significant (P ≤ 0.05) changes in genome size and ploidy level under salinity stress. Moreover, EBL supplementation increased the G6PDH activity and S-nitrosothiol content which further boosted the antioxidant responses under salinity stress. Taken together, these results suggested that NO production in mustard occurred in NR-dependent manner and EBL in association with endogenous NO activates the antioxidant system to counter salinity stress.

Nitric oxide modulating ion balance in Hylotelephium erythrostictum roots subjected to NaCl stress based on the analysis of transcriptome, fluorescence, and ion fluxes

Soil salinization is one of the main stress factors that affect both growth and development of plants. Hylotelephium erythrostictum exhibits strong resistance to salt, but the underlying genetic mechanisms remain unclear. In this study, hydroponically cultured seedlings of H. erythrostictum were exposed to 200 mM NaCl. RNA-Seq was used to determine root transcriptomes at 0, 5, and 10 days, and potential candidate genes with differential expression were analyzed. Transcriptome sequencing generated 89.413 Gb of raw data, which were assembled into 111,341 unigenes, 82,081 of which were annotated. Differentially expressed genes associated to Na+ and K+ transport, Ca2+ channel, calcium binding protein, and nitric oxide (NO) biosynthesis had high expression levels in response to salt stress. An increased fluorescence intensity of NO indicated that it played an important role in the regulation of the cytosolic K+/Na+ balance in response to salt stress. Exogenous NO donor and NO biosynthesis inhibitors significantly increased and decreased the Na+ efflux, respectively, thus causing the opposite effect for K+ efflux. Moreover, under salt stress, exogenous NO donors and NO biosynthesis inhibitors enhanced and reduced Ca2+ influx, respectively. Combined with Ca2+ reagent regulation of Na+ and K+ fluxes, this study identifies how NaCl-induced NO may function as a signaling messenger that modulates the K+/Na+ balance in the cytoplasm via the Ca2+ signaling pathway. This enhances the salt resistance in H. erythrostictum roots.

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.

The forgotten role of toxicodynamics: How habitat quality alters the mite, Oppia nitens, susceptibility to zinc, independent of toxicokinetics

Soil habitat quality is thought to influence metal toxicity via changes in speciation and thereby toxicokinetics. Here, we assessed the toxicokinetic and toxicodynamic effects of habitat quality on mite, Oppia nitens when exposed to zinc (Zn) contaminated soils. Forty-seven soils were ranked into three habitat qualities; high, medium, and low based on biological reproduction of Folsomia candida, Enchytraeus crypticus, and Elymus lanceolatus. From the 47 soils, eighteen soils (comprising of six soils from each habitat quality) were randomly selected and dosed with field relevant concentrations of Zn. Mite survival and reproduction were assessed after 28 days. Total Zn, bioaccessible Zn, Zn bioavailability, Zn body burden, lactate dehydrogenase activity (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) activities of the mites were determined. Zinc toxicity and potency were much less in the high compared to low quality soils and the mites in the high habitat quality soils tolerated higher zinc body burdens (2040 ± 130 μg/g b.w) than the lower habitat quality (1180 ± 310 μg/g b.w). Lower LDH activity (20 ± 2 μU mg^-1) in the high quality soils compared to lower quality soils (50 ± 8 μU mg^-1) suggested that there was less stress in the high habitat quality mites. Despite changes in speciation across habitat qualities, bioavailability of zinc was similar (∼20%) irrespective of habitat quality. Our results suggest that the influence of soil properties on survival is modulated by toxicodynamics rather than toxicokinetics. Restoring habitat quality may be more important for soil invertebrate protection than metal concentration at contaminated sites.

Nitric oxide and abscisic acid protects against PEG-induced drought stress differentially in Brassica genotypes by combining the role of stress modulators, markers and antioxidants

The present study was designed to see the effect of exogenous nitric oxide (NO) and abscisic acid (ABA) and their interaction on physiological and biochemical activities in leaves and roots of two Indian mustard (Brassica juncea) cultivars [cv. Pusa Jagannath (PJN) and Varuna (VAR)] exposed to polyethylene glycol (PEG)-induced drought stress. Seven days old hydroponically grown seedlings were treated with PEG (10%), sodium nitroprusside, a NO donor [NO (100 μM)] and abscisic acid [ABA (10 μM)], using different combinations as: Control, ABA, NO, PEG, PEG + ABA, PEG + NO, and PEG + NO + ABA. Results revealed that in response to PEG-induced drought stress leaf relative water content, chlorophyll, carotenoid and protein content decreased with increased production of O₂^-●, MDA, H₂O₂, cysteine content and non-enzymatic antioxidants (including proline, flavonoid, phenolic, anthocyanin, and ascorbic acid), whereas, the enzymatic antioxidants (including SOD, CAT, APX, GR) showed the response range from no effect to increase or decrease in certain enzymes in both Brassica cultivars. The application of NO or/and ABA in PEG-stressed cultivars showed that both enzymatic and non-enzymatic antioxidants responded differently to attenuate oxidative stress in leaves and roots of both cultivars. Overall, PJN had the antioxidant protection mainly through the accumulation of non-enzymatic antioxidants, whereas VAR showed tolerance by the enhancement of both enzymatic and non-enzymatic antioxidant activities. Altogether, the study concluded that the independent NO and its interaction with ABA (PEG + NO and PEG + NO + ABA) were much effective than independent ABA (PEG + ABA) in lowering PEG-drought stress in Brassica cultivars.

Nitric oxide synthase-mediated early nitric oxide burst alleviates water stress-induced oxidative damage in ammonium-supplied rice roots

BACKGROUND

Nutrition with ammonium (NH4+) can enhance the drought tolerance of rice seedlings in comparison to nutrition with nitrate (NO3-). However, there are still no detailed studies investigating the response of nitric oxide (NO) to the different nitrogen nutrition and water regimes. To study the intrinsic mechanism underpinning this relationship, the time-dependent production of NO and its protective role in the antioxidant defense system of NH4+- or NO3--supplied rice seedlings were studied under water stress.

RESULTS

An early NO burst was induced by 3 h of water stress in the roots of seedlings subjected to NH4+ treatment, but this phenomenon was not observed under NO3- treatment. Root oxidative damage induced by water stress was significantly higher for treatment with NO3- than with NH4+ due to reactive oxygen species (ROS) accumulation in the former. Inducing NO production by applying the NO donor 3 h after NO3- treatment alleviated the oxidative damage, while inhibiting the early NO burst by applying the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) increased root oxidative damage in NH4+ treatment. Application of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester(L-NAME) completely suppressed NO synthesis in roots 3 h after NH4+ treatment and aggravated water stress-induced oxidative damage. Therefore, the aggravation of oxidative damage by L-NAME might have resulted from changes in the NOS-mediated early NO burst. Water stress also increased the activity of root antioxidant enzymes (catalase, superoxide dismutase, and ascorbate peroxidase). These were further induced by the NO donor but repressed by the NO scavenger and NOS inhibitor in NH4+-treated roots.

CONCLUSION

These findings demonstrate that the NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by water stress by enhancing the antioxidant defenses in roots supplemented with NH4+.

Cytosolic Glucose-6-Phosphate Dehydrogenase Is Involved in Seed Germination and Root Growth Under Salinity in Arabidopsis

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) is the key regulatory enzyme in the oxidative pentose phosphate pathway (OPPP). The cytosolic isoforms including G6PD5 and G6PD6 account for the major part of the G6PD total activity in plant cells. Here, we characterized the Arabidopsis single null mutant g6pd5 and g6pd6 and double mutant g6pd5/6. Compared to wild type, the mutant seeds showed a reduced germination rate and root elongation under salt stress. The seeds and seedlings lacking G6PD5 and G6PD6 accumulate more reactive oxygen species (ROS) than the wild type under salt stress. Cytosolic G6PD (cy-G6PD) affected the expression of NADPH oxidases and the G6PD enzymatic activities in the mutant atrbohD/F, in which the NADPH oxidases genes are disrupted by T-DNA insertion and generation of ROS is inhibited, were lower than that in the wild type. The NADPH level in mutants was decreased under salt stress. In addition, we found that G6PD5 and G6PD6 affected the activities and transcript levels of various antioxidant enzymes in response to salt stress, especially the ascorbate peroxidase and glutathione reductase. Exogenous application of ascorbate acid and glutathione rescued the seed and root phenotype of g6pd5/6 under salt stress. Interestingly, the cytosolic G6PD negatively modulated the NaCl-blocked primary root growth under salt stress in the root meristem and elongation zone.

Involvement of G6PD5 in ABA response during seed germination and root growth in Arabidopsis

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) functions in supply of NADPH, which is required for plant defense responses to stresses. However, whether G6PD functions in the abscisic acid (ABA) signaling pathway remains to be elucidated. In this study, we investigated the involvement of the cytosolic G6PD5 in the ABA signaling pathway in Arabidopsis.

RESULTS

We characterized the Arabidopsis single null mutant g6pd5. Phenotypic analysis showed that the mutant is more sensitive to ABA during seed germination and root growth, whereas G6PD5-overexpressing plants are less sensitive to ABA compared to wild type (WT). Furthermore, ABA induces excessive accumulation of reactive oxygen species (ROS) in mutant seeds and seedlings. G6PD5 participates in the reduction of H₂O₂ to H₂O in the ascorbate-glutathione cycle. In addition, we found that G6PD5 suppressed the expression of Abscisic Acid Insensitive 5 (ABI5), the major ABA signaling component in dormancy control. When G6PD5 was overexpressed, the ABA signaling pathway was inactivated. Consistently, G6PD5 negatively modulates ABA-blocked primary root growth in the meristem and elongation zones. Of note, the suppression of root elongation by ABA is triggered by the cell cycle B-type cyclin CYCB1.

CONCLUSIONS

This study showed that G6PD5 is involved in the ABA-mediated seed germination and root growth by suppressing ABI5.

Evaluation of the Relevance of Myriophyllum alterniflorum (Haloragaceae) Cadmium-Sensitive Biomarkers for Ecotoxicological Surveys

Toxicity caused by trace metal elements in water is a major concern, leading to environmental disturbances and public health problems. The effect of cadmium on clonal macrophyte populations is poorly documented despite its high level of toxicity among aquatic organisms. Our aim here is to highlight the strong relationship existing between the physiological responses of Myriophyllum alterniflorum and the cadmium level over a long exposure period. Nine potential biomarkers of cadmium stress are tested, with three of them appearing to be highly sensitive: free proline, Hsp70, and malondialdehyde. Long-term follow-up analysis after metal exposure (27 days) also proves to be quite beneficial by providing a detailed overview of ecotoxicological events that is more complete and extensive than data recordings conducted over a few days. Taken together, these results support our initial hypothesis that leads to recommending biomarker analyses over at least 2 weeks of metal exposure.

Nitric oxide-mediated integrative alterations in plant metabolism to confer abiotic stress tolerance, NO crosstalk with phytohormones and NO-mediated post translational modifications in modulating diverse plant stress

Nitric oxide (NO) is a major signaling biomolecule associated with signal transduction in plants. The beneficial role of NO in plants, exposed to several abiotic stresses shifted our understanding as it being not only free radical, released from the toxic byproducts of oxidative metabolism but also helps in plant sustenance. An explosion of research in plant NO biology during the last two decades has revealed that NO is a key signal associated with plant growth, germination, photosynthesis, leaf senescence, pollen growth and reorientation. NO is beneficial as well as harmful to plants in a dose-dependent manner. Exogenous application of NO at lower concentrations promotes seed germination, hypocotyl elongation, pollen development, flowering and delays senescence but at higher concentrations it causes nitrosative damage to plants. However, this review concentrates on the beneficial impact of NO in lower concentrations in the plants and also highlights the NO crosstalk of NO with other plant hormones, such as auxins, gibberellins, abscisic acid, cytokinins, ethylene, salicylic acid and jasmonic acid, under diverse stresses. While concentrating on the multidimensional role of NO, an attempt has been made to cover the role of NO-mediated genes associated with plant developmental processes, metal uptake, and plant defense responses as well as stress-related genes. More recently, several NO-mediated post translational modifications, such as S-nitrosylation, N-end rule pathway operates under hypoxia and tyrosine nitration also occurs to modulate plant physiology.

Salt-Sensitive Signaling Networks in the Mediation of K+/Na+ Homeostasis Gene Expression in Glycyrrhiza uralensis Roots

We investigated the effects of salt-sensitive signaling molecules on ionic fluxes and gene expression related to K+/Na+ homeostasis in a perennial herb, Glycyrrhiza uralensis, during short-term NaCl stress (100 mM, 24 h). Salt treatment caused more pronounced Na+ accumulation in root cells than in leaf cells. Na+ ions were mostly compartmentalized in vacuoles. Roots exposed to NaCl showed increased levels of extracellular ATP (eATP), cytosolic Ca2+, H2O2, and NO. Steady-state flux recordings revealed that these salt-sensitive signaling molecules enhanced NaCl-responsive Na+ efflux, due to the activated Na+/H+ antiport system in the plasma membrane (PM). Moreover, salt-elicited K+ efflux, which was mediated by depolarization-activated cation channels, was reduced with the addition of Ca2+, H2O2, NO, and eATP. The salt-adaptive effects of these molecules (Na+ extrusion and K+ maintenance) were reduced by pharmacological agents, including LaCl3 (a PM Ca2+ channel inhibitor), DMTU (a reactive oxygen species scavenger), cPTIO (an NO scavenger), or PPADS (an antagonist of animal PM purine P2 receptors). RT-qPCR data showed that the activation of the PM Na+/H+ antiport system in salinized roots most likely resulted from the upregulation of two genes, GuSOS1 and GuAHA, which encoded the PM Na+/H+ antiporter, salt overly sensitive 1 (SOS1), and H+-ATPase, respectively. Clear interactions occurred between these salt-sensitive agonists to accelerate transcription of salt-responsive signaling pathway genes in G. uralensis roots. For example, Ca2+, H2O2, NO, and eATP promoted transcription of GuSOS3 (salt overly sensitive 3) and/or GuCIPK (CBL-interacting protein kinase) to activate the predominant Ca2+-SOS signaling pathway in salinized liquorice roots. eATP, a novel player in the salt response of G. uralensis, increased the transcription of GuSOS3, GuCIPK, GuRbohD (respiratory burst oxidase homolog protein D), GuNIR (nitrate reductase), GuMAPK3, and GuMAPK6 (the mitogen-activated protein kinases 3 and 6). Moreover, GuMAPK3 and GuMAPK6 expression levels were enhanced by H2O2 in NaCl-stressed G. uralensis roots. Our results indicated that eATP triggered downstream components and interacted with Ca2+, H2O2, and NO signaling to maintain K+/Na+ homeostasis. We propose that a multiple signaling network regulated K+/Na+ homeostasis in NaCl-stressed G. uralensis roots.

Atmospheric emission of nitric oxide and processes involved in its biogeochemical transformation in terrestrial environment

Nitric oxide (NO) is an intra- and intercellular gaseous signaling molecule with a broad spectrum of regulatory functions in biological system. Its emissions are produced by both natural and anthropogenic sources; however, soils are among the most important sources of NO. Nitric oxide plays a decisive role in environmental-atmospheric chemistry by controlling the tropospheric photochemical production of ozone and regulates formation of various oxidizing agents such as hydroxyl radical (OH), which contributes to the formation of acid of precipitates. Consequently, for developing strategies to overcome the deleterious impact of NO on terrestrial ecosystem, it is mandatory to have reliable information about the exact emission mechanism and processes involved in its transformation in soil-atmospheric system. Although the formation process of NO is a complex phenomenon and depends on many physicochemical characteristics, such as organic matter, soil pH, soil moisture, soil temperature, etc., this review provides comprehensive updates about the emission characteristics and biogeochemical transformation mechanism of NO. Moreover, this article will also be helpful to understand the processes involved in the consumption of NO in soils. Further studies describing the functions of NO in biological system are also discussed.

Involvement of ABA- and H2O2-dependent cytosolic glucose-6-phosphate dehydrogenase in maintaining redox homeostasis in soybean roots under drought stress

The roles of abscisic acid (ABA) and hydrogen peroxide (H2O2) in inducing glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and the possible roles of G6PDH in regulating ascorbate-glutathione (AsA-GSH) cycle were investigated in soybean (Glycine max L.) roots under drought stress. Drought caused a marked increase of the total and cytosolic G6PDH activities and triggered a rapid ABA and H2O2 accumulation in soybean roots. Exogenous ABA or H2O2 treatment elevated the total and cytosolic G6PDH activities, whereas suppressing ABA or H2O2 production inhibited the drought-induced increase in total and cytosolic G6PDH activities, suggesting that ABA and H2O2 are required for drought-induced increase of total G6PDH activity, namely cytosolic G6PDH activity. Furthermore, ABA induced H2O2 production by stimulating NADPH oxidase activity under drought stress. Moreover, drought significantly increased the contents of AsA and GSH and the activities of key enzymes in AsA-GSH cycle, while application of G6PDH inhibitor to seedlings significantly reduced the above effect induced by drought. Taken together, these results indicate that H2O2 acting as a downstream signaling molecule of ABA mediates drought-induced increase in cytosolic G6PDH activity, and that enhanced cytosolic G6PDH activity maintains cellular redox homeostasis by regulating AsA-GSH cycle in soybean roots.

Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants

Glucose 6 phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is well-known as the main regulatory enzyme of the oxidative pentose phosphate pathway (OPPP) in living organisms. Namely, in Planta, different G6PDH isoforms may occur, generally localized in cytosol and plastids/chloroplasts. These enzymes are differently regulated by distinct mechanisms, still far from being defined in detail. In the last decades, a pivotal function for plant G6PDHs during the assimilation of nitrogen, providing reductants for enzymes involved in nitrate reduction and ammonium assimilation, has been described. More recently, several studies have suggested a main role of G6PDH to counteract different stress conditions, among these salinity and drought, with the involvement of an ABA depending signal. In the last few years, this recognized vision has been greatly widened, due to studies clearly showing the non-conventional subcellular localization of the different G6PDHs, and the peculiar regulation of the different isoforms. The whole body of these considerations suggests a central question: how do the plant cells distribute the reductants coming from G6PDH and balance their equilibrium? This review explores the present knowledge about these mechanisms, in order to propose a scheme of distribution of reductants produced by G6PDH during nitrogen assimilation and stress.

Nitrate reductase-mediated NO production enhances Cd accumulation in Panax notoginseng roots by affecting root cell wall properties

Panax notoginseng (Burk) F. H. Chen is a traditional medicinal herb in China. However, the high capacity of its roots to accumulate cadmium (Cd) poses a potential risk to human health. Although there is some evidence for the involvement of nitric oxide (NO) in mediating Cd toxicity, the origin of Cd-induced NO and its function in plant responses to Cd remain unknown. In this study, we examined NO synthesis and its role in Cd accumulation in P. notoginseng roots. Cd-induced NO production was significantly decreased by application of the nitrate reductase inhibitor tungstate but not the nitric oxide synthase inhibitor L-NAME (N(G)-methyl-l-arginine acetate), indicating that nitrate reductase is the major contributor to Cd-induced NO production in P. notoginseng roots. Under conditions of Cd stress, sodium nitroprusside (SNP, an NO donor) increased Cd accumulation in root cell walls but decreased Cd translocation to the shoot. In contrast, the NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and tungstate both significantly decreased NO-increased Cd retention in root cell walls. The amounts of hemicellulose 1 and pectin, together with pectin methylesterase activity, were increased with the addition of SNP but were decreased by cPTIO and tungstate. Furthermore, increases or decreases in hemicellulose 1 and pectin contents as well as pectin methylesterase activity fit well with the increased or decreased retention of Cd in the cell walls of P. notoginseng roots. The results suggest that nitrate reductase-mediated NO production enhances Cd retention in P. notoginseng roots by modulating the properties of the cell wall.

G6PDH activity highlights the operation of the cyclic electron flow around PSI in Physcomitrella patens during salt stress

Photosynthetic performances and glucose-6-phosphate dehydrogenase (G6PDH) activity in Physcomitrella patens changed greatly during salt stress and recovery. In P. patens, the cyclic electron flow around photosystem (PS) I was much more tolerant to high salt stress than PSII. After high salt stress, the PSII activity recovered much more slowly than that of PSI, which was rapidly restored to pretreatment levels even as PSII was almost inactivate. This result suggested that after salt stress the recovery of the cyclic electron flow around PSI was independent of PSII activity. In addition, G6PDH activity and NADPH content increased under high salt stress. When G6PDH activity was inhibited by glucosamine (Glucm, a G6PDH inhibitor), the cyclic electron flow around PSI and the NADPH content decreased significantly. Additionally, after recovery in liquid medium containing Glucm, the PSI activity was much lower than in liquid medium without Glucm. These results suggested the PSI activity was affected significantly by G6PDH activity and the NADPH content. Based on the above results, we propose that G6PDH in P. patens has a close relationship with the photosynthetic process, possibly providing NADPH for the operation of the cyclic electron flow around PSI during salt stress and promoting the restoration of PSI.

Identification of Novel and Conserved miRNAs from Extreme Halophyte, Oryza coarctata, a Wild Relative of Rice

Oryza coarctata, a halophyte and wild relative of rice, is grown normally in saline water. MicroRNAs (miRNAs) are non-coding RNAs that play pivotal roles in every domain of life including stress response. There are very few reports on the discovery of salt-responsive miRNAs from halophytes. In this study, two small RNA libraries, one each from the control and salt-treated (450 mM NaCl for 24 h) leaves of O. coarctata were sequenced, which yielded 338 known and 95 novel miRNAs. Additionally, we used publicly available transcriptomics data of O. coarctata which led to the discovery of additional 48 conserved miRNAs along with their pre-miRNA sequences through in silico analysis. In total, 36 known and 7 novel miRNAs were up-regulated whereas, 12 known and 7 novel miRNAs were down-regulated under salinity stress. Further, 233 and 154 target genes were predicted for 48 known and 14 novel differentially regulated miRNAs respectively. These targets with the help of gene ontology analysis were found to be involved in several important biological processes that could be involved in salinity tolerance. Relative expression trends of majority of the miRNAs as detected by real time-PCR as well as predicted by Illumina sequencing were found to be coherent. Additionally, expression of most of the target genes was negatively correlated with their corresponding miRNAs. Thus, the present study provides an account of miRNA-target networking that is involved in salinity adaption of O. coarctata.

Plasticity in ploidy: a generalized response to stress

Endoreduplication, the replication of the genome without mitosis, leads to an increase in the cellular ploidy of an organism over its lifetime, a condition termed 'endopolyploidy'. Endopolyploidy is thought to play significant roles in physiology and development through cellular, metabolic, and genetic effects. While the occurrence of endopolyploidy has been observed widely across taxa, studies have only recently begun to characterize and manipulate endopolyploidy with a focus on its ecological and evolutionary importance. No compilation of these examples implicating endoreduplication as a generalized response to stress has thus far been made, despite the growing evidence supporting this notion. We review here the recent literature of stress-induced endopolyploidy and suggest that plants employ endoreduplication as an adaptive, plastic response to mitigate the effects of stress.

Silicon does not mitigate cell death in cultured tobacco BY-2 cells subjected to salinity without ethylene emission

KEY MESSAGE

Silicon induces cell death when ethylene is suppressed in cultured tobacco BY-2 cells. There is a crosstalk between Si and ethylene signaling. Silicon (Si) is beneficial for plant growth. It alleviates both biotic and abiotic stresses in plants. How Si works in plants is still mysterious. This study investigates the mechanism of Si-induced cell death in tobacco BY-2 cell cultures when ethylene is suppressed. Results showed that K2SiO3 alleviated the damage of NaCl stress. Si treatment rapidly increased ethylene emission and the expression of ethylene biosynthesis genes. Treatments with Si + Ag and Si + aminooxyacetic acid (AOA, ethylene biosynthesis inhibitor) reduced the cell growth and increased cell damage. The treatment with Si + Ag induced hydrogen peroxide (H2O2) generation and ultimately cell death. Some nucleus of BY-2 cells treated with Si + Ag appeared TUNEL positive. The inhibition of H2O2 and nitric oxide (NO) production reduced the cell death rate induced by Si + Ag treatment. Si eliminated the up-regulation of alternative pathway by Ag. These data suggest that ethylene plays an important role in Si function in plants. Without ethylene, Si not only failed to enhance plant resistance, but also elevated H2O2 generation and further induced cell death in tobacco BY-2 cells.

Positive correlation between PSI response and oxidative pentose phosphate pathway activity during salt stress in an intertidal macroalga

Studies have demonstrated that photosynthetic limitations and starch degradation are responses to stress; however, the relationship between the two is seldom described in detail. In this article, the effects of salt stress on photosynthesis, the levels of NADPH and total RNA, the starch content and the activities of glucose-6-phosphate dehydrogenase (G6PDH) and ribulose-5-phosphate kinase (RPK) were evaluated. In thalli that underwent salt treatments, the cyclic electron flow through PSI showed greater stress tolerance than the flow through PSII. Even though the linear electron flow was suppressed by DCMU, the cyclic electron flow still operated. The electron transport rate I (ETRI) increased as the salinity increased when the thalli recovered in seawater containing DCMU. These results suggested that PSI receives electrons from a source other than PSII. Furthermore, the starch content and RPK activity decreased, while the content of NADPH and total RNA, and the activity of G6PDH increased under salt stress. Soluble sugar from starch degradation may enter the oxidative pentose phosphate pathway (OPPP) to produce NADPH and ribose 5-phosphate. Data analysis suggests that NADPH provides electrons for PSI in Ulva prolifera during salt stress, the OPPP participates in the stress response and total RNA is synthesized in excess to assist recovery.