Early effects of salt stress on the physiological and oxidative status of Cakile maritima (halophyte) and Arabidopsis thaliana (glycophyte)

Early changes in physiological and oxidative status induced by salt stress were monitored in two Brassicaceae plants differing in their tolerance to salinity, Cakile maritima (halophyte) and Arabidopsis thaliana (glycophyte). Growth response and antioxidant defense of C. maritima under 400 mM NaCl were compared with those of A. thaliana exposed to 100 mM NaCl. Salinity induced early growth reduction that is less pronounced in C. maritima than in A. thaliana. Maximum hydrogen peroxide (H₂O₂) level occurred in the leaves of both species 4 h after the onset of salt treatment. A rapid decline in H₂O₂ concentration was observed thereafter in C. maritima, whereas it remained high in A. thaliana. Correlatively, superoxide dismutase, catalase and peroxidase activities increased at 4 h of treatment in C. maritima and decreased thereafter. However, the activity of these enzymes remained higher in treated plants than that in controls, regardless of the duration of treatment, in A. thaliana. The concentrations of malondialdehyde (MDA) reached maximum values at 24 h of salt stress in both species. Again, MDA levels decreased later in C. maritima, but remained high in A. thaliana. The contents of α‐tocopherol remained constant during salt stress in C. maritima and decreased during the first 24 h of salt stress and then remained low in A. thaliana. The results clearly showed that C. maritima, in contrast to A. thaliana, can rapidly evolve physiological and antioxidant mechanisms to adapt to salt and manage the oxidative stress. This may explain, at least partially, the difference in salt tolerance between halophytes and glycophytes.     

Hydrogen peroxide and nitric oxide mediate K+/Na+ homeostasis and antioxidant defense in NaCl-stressed callus cells of two contrasting poplars

Using callus cells of a salt-tolerant Populus euphratica Oliver and a salt-sensitive P. popularis 35–44 (P. popularis), the effects of NaCl stress on hydrogen peroxide (H₂O₂) and nitric oxide (NO) production and the relevance to ionic homeostasis and antioxidant defense were investigated. Results show that P. euphratica exhibited a greater capacity to tolerate NaCl stress in terms of cell viability, membrane permeability and K⁺/Na⁺ relations. NaCl salinity (150 mM) caused a rapid increase of H₂O₂ and NO in P. euphratica cells, but not in P. popularis. Moreover, salinised P. euphratica cells retained a high and stable level of H₂O₂ and NO during the period of 24-h salt stress. Noteworthy, P. eupratica cells increased activities of superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase under salinity stress, but these antioxidant enzymes were significantly inhibited by the salt treatment in P. popularis cells. Pharmacological experiments proved that the NaCl-induced H₂O₂ and NO was interdependent and contributed to the mediation of K⁺/Na⁺ homeostasis and antioxidant defense in P. euphratica cells. Given these results, we conclude that the increased H₂O₂ and NO enable P. euphratica cells to regulate ionic and ROS (reactive oxygen species) homeostasis under salinity stress in the longer term.     

Colonization with arbuscular mycorrhizal fungi improves salinity tolerance of tomato (Solanum lycopersicum L.) plants

The purpose of this study was to investigate the mechanisms underlying alleviation of salt stress by mycorrhization. Solanum lycopersicum L. cultivars Behta and Piazar with different salinity tolerance were cultivated in soil without salt (EC?=?0.63 dSm^?1), with low (EC?=?5 dSm^?1), or high (EC?=?10 dSm^?1) salinity. Plants inoculated with the arbuscular mycorrhizal fungi Glomus intraradices (+AMF) were compared to non-inoculated plants (?AMF). Under salinity, AMF-mediated growth stimulation was higher in more salt tolerant Piazar than in sensitive Behta. Mycorrhization alleviated salt-induced reduction of P, Ca, and K uptake. Ca/Na and K/Na ratios were also better in +AMF. However, growth improvement by AMF was independent from plant P nutrition under high salinity. Mycorrhization improved the net assimilation rates through both elevating stomatal conductance and protecting photochemical processes of PSII against salinity. Higher activity of ROS scavenging enzymes was concomitant with lowering of H₂O_2, less lipid peroxidation, and higher proline in +AMF. Cultivar differences in growth responses to salinity and mycorrhization could be well explained by differences in ion balance, photochemistry, and gas exchange of leaves. Function of antioxidant defenses seemed responsible for different AMF-responsiveness of cultivars under salinity. In conclusion, AMF may protect plants against salinity by alleviating the salt-induced oxidative stress.     

Proteomics reveals the overlapping roles of hydrogen peroxide and nitric oxide in the acclimation of citrus plants to salinity

Hydrogen peroxide (H₂O₂) and nitric oxide (˙NO) are key reactive species in signal transduction pathways leading to activation of plant defense against biotic or abiotic stress. Here, we investigated the effect of pre‐treating citrus plants (Citrus aurantium L.) with either of these two molecules on plant acclimation to salinity and show that both pre‐treatments strongly reduced the detrimental phenotypical and physiological effects accompanying this stress. A proteomic analysis disclosed 85 leaf proteins that underwent significant quantitative variations in plants directly exposed to salt stress. A large part of these changes was not observed with salt‐stressed plants pre‐treated with either H₂O₂ or sodium nitroprusside (SNP; a ˙NO‐releasing chemical). We also identified several proteins undergoing changes either in their oxidation (carbonylation; 40 proteins) and/or S‐nitrosylation (49 proteins) status in response to salinity stress. Both H₂O₂ and SNP pre‐treatments before salinity stress alleviated salinity‐induced protein carbonylation and shifted the accumulation levels of leaf S‐nitrosylated proteins to those of unstressed control plants. Altogether, the results indicate an overlap between H₂O₂‐ and ˙NO‐signaling pathways in acclimation to salinity and suggest that the oxidation and S‐nitrosylation patterns of leaf proteins are specific molecular signatures of citrus plant vigour under stressful conditions.     

Interaction between hydrogen peroxide and sodium nitroprusside following chemical priming of Ocimum basilicum L. against salt stress

Sodium nitroprusside (SNP) and hydrogen peroxide (H₂O₂)_, as priming agents, have the well-recorded property to increase plant tolerance against a range of different abiotic stresses such as salinity. In this regard, the present study was conducted to evaluate the effect of different levels of SNP (100 and 200 µM) and H₂O₂ (2.5 and 5 mM) as well as their combinations under salt stress (0 and 50 mM NaCl) on key physiological and biochemical attributes of the economically important aromatic plant basil (Ocimum basilicum L.) grown under hydroponic culture. Results revealed that morphological parameters such as plant height, root length, leaf fresh and dry weights (FW and DW) were significantly decreased by salinity stress, while SNP and H₂O₂ treatments, alone or combined, increased FW and DW thus enhancing plant tolerance to salt stress. Furthermore, 200 µM SNP + 2.5 mM H₂O₂ appeared to be the most effective treatment by causing significant increase in chlorophyll a and b, anthocyanin content and guaiacol peroxidase and ascorbate peroxidase enzymes activities under saline condition. In addition, analytical measurements showed that essential oil profile (concentration of main components) under salt stress was mostly affected by SNP and H₂O₂ treatments. The highest increase was observed for methyl chavicol (43.09–69.91%), linalool (4.8–17.9%), cadinol (1.5–3.2%) and epi-α-cadinol (0.18–10.75%) compounds. In conclusion, current findings demonstrated a positive crosstalk between SNP and H₂O₂ toward improved basil plant tolerance to salt stress, linked with regulation of essential oil composition.     

Chromotoxic effects of exogenous hydrogen peroxide (H<Subscript>2</Subscript>O<Subscript>2</Subscript>) in barley seeds exposed to salt stress

The effect of exogenous hydrogen peroxide (H₂O₂) on mitotic activity and chromosomal aberrations in root tip meristems of barley (Hordeum vulgare L. var. Tokak 157/37) germinated under salinity was analyzed. The inhibitory effect of salinity on mitotic index and the frequency of chromosomal aberrations increased with increasing salt concentration (0.00 control, 0.35, 0.40, 0.45 M, molal NaCl). The frequency of chromosomal aberrations of seeds germinated in medium with 0.40 M NaCl after pretreatment with H₂O₂ (30 μM, micromolal) was significantly higher than the control group. The highest concentration of NaCl (0.45 M) together with H₂O₂ caused total inhibition of germination. In this study, the intention was to determine the performance of H₂O₂ in alleviating detrimental effect of salt stress on mitotic activity and chromosomal aberrations. However, H₂O₂ did not reduce the detrimental effect of NaCl on these parameters. Also, it caused higher chromotoxic effect compared to those of control groups.     

Effect of salinity and waterlogging on growth,anatomical and antioxidative responses in <Emphasis Type="Italic">Mentha aquatica</Emphasis> L.

Salinity and waterlogging are two stresses which in nature often occur simultaneously. In this work, effects of combined waterlogging and salinity stresses are studied on the anatomical alteration, changes of enzymatic antioxidant system and lipid peroxidation in Mentha aquatica L. plants. Seedlings were cultured in half-strength Hoagland medium 50 days after sowing, and were treated under combination of three waterlogging levels (well drained, moderately drained and waterlogging) and NaCl (0, 50, 100, 150 mM) for 30 days. Moderately drained and waterlogging conditions induced differently aerenchyma formation in roots of M. aquatica salt-treated and untreated plants. Moreover, stele diameter and endodermis layer were also affected by salt stress and waterlogging. Salt stress significantly decreased growth, relative water content (RWC), protein level, catalase (CAT) and polyphenol oxidase (PPO) activities, and increased proline content, MDA content, H₂O₂ level and activities of superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX). Waterlogging in salt-untreated plants increased significantly growth parameters, RWC, protein content, antioxidant enzyme activity, and decreased proline content, H₂O₂ and MDA levels. In salt-treated plant, waterlogging caused strong induction of antioxidant enzymes activities especially at severe stress condition. These results suggest M. aquatica is a waterlogging tolerant plant due to significant increase of antioxidant activity, membrane stability and growth under water stress. High antioxidant capacity under waterlogging can be a protective strategy against oxidative damage, and help to salt stress alleviation.     

Salicylic Acid,as a Positive Regulator of Isochorismate Synthase,Reduces the Negative Effect of Salt Stress on Pistacia vera L. by Increasing Photosynthetic Pigments and Inducing Antioxidant Activity

Salinity, as a serious and prevalent abiotic stress, causes widespread crop losses by restricting plant growth and production throughout the world. In this study, the biochemical and molecular responses of the pistachio (Pistacia vera L.) plant were studied under NaCl and salicylic acid (SA) treatments using hydroponically grown salt tolerant (Ghazvini) and salt sensitive (Sarakhs) pistachio cultivars. NaCl treatment (250 mM) increased the production of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) and the activity of antioxidant enzymes in both cultivars. In the sensitive cultivar, the H₂O₂ content was higher than the tolerant cultivar, especially in the roots. SA application to both salt-stress-treated cultivars resulted in an increase in photosynthetic pigment contents and antioxidant enzyme activity and a decrease in the H₂O₂ and MDA contents. After NaCl treatment, the isochorismate synthase (ICS) gene was upregulated in Ghazvini which leads to an increase in the SA content of the salt tolerant pistachio cultivar. In contrast, the salt treatment downregulated the expression of the ICS gene in Sarakhs. The ICS gene expression was positively regulated by SA treatment under the salt stress condition. Our results suggest that Ghazvini has higher salinity tolerance than Sarakhs due to its higher antioxidant capacity, photosynthetic pigment content, and the cultivar-specific expression pattern of the ICS gene. In this study, the potential alleviative effects of SA on the adverse effect of salt stress in P. vera (Pistacia vera) were also identified and highlighted.

     

Differences in Cowpea Root Growth Triggered by Salinity and Dehydration are Associated with Oxidative Modulation Involving Types I and III Peroxidases and Apoplastic Ascorbate

The aim of this work was to investigate the balance between the activities of ascorbate peroxidase (APX) and phenol peroxidases (POD) and cowpea root growth in response to dehydration and salt stress. Root growth and indicators of oxidative response were markedly changed in response to salinity and dehydration. Salt treatment strongly inhibited root elongation, which was associated with an increase in lignin content and a significant decrease in the concentrations of apoplastic hydrogen peroxide (H₂O₂) and ascorbate. In conditions of extreme salinity, cytosol–APX activity was significantly decreased. In contrast, cell-wall POD activity was greatly increased, whereas lipid peroxidation was unchanged. These results indicate that POD could be involved in both H₂O₂ scavenging and the inhibition of root elongation under high salinity. In contrast, dehydration stimulated primary root elongation and increased lipid peroxidation and apoplastic ascorbate content, but it did not change APX and POD activities or H₂O₂ concentration. When cowpea roots were subjected to salinity followed by dehydration, the water and pressure potentials were decreased, and lipid peroxidation was markedly increased, highlighting the additive nature of the inhibitory effects caused by salt and dehydration. The proline concentration was markedly increased by dehydration alone, as well as by salt followed by dehydration, suggesting a possible role for proline in osmotic adjustment. Salinity and dehydration induce contrasting responses in the growth and morphology of cowpea roots. These effects are associated with different types of oxidative modulation involving cytosolic-APX and cell-wall POD activities and apoplast H₂O₂ and ascorbate levels.     

NH4 + induces antioxidant cellular machinery and provides resistance to salt stress in citrus plants

Key message

NH ₄ ⁺ acts as a mild oxidative stressor, which triggers antioxidant cellular machinery and provide resistance to salinity.

Abstract

NH₄ ⁺ nutrition in Carrizo citrange (Citrus sinensis L. Osbeck × Poncirus trifoliata L) plants acts as an inducer of resistance against salinity conditions. NH₄ ⁺ treatment triggers mild chronic stress that primes plant defence responses by stress imprinting and confers protection against subsequent salt stress. In this work, we studied the influence of NH₄ ⁺ nutrition on antioxidant enzymatic activities and metabolites involved in detoxification of reactive oxygen species (ROS) to clarify their involvement in NH₄ ⁺-mediated salt resistance. Our results showed that NH₄ ⁺ nutrition induces in citrus plants high levels of H₂O₂, strongly inhibits superoxide dismutase (SOD) and glutathione reductase (GR) activities, and leads to higher content of oxidised glutathione (GSSG) than in control plants in the absence of salt, thus providing evidence to confirm mild stress induced by NH₄ ⁺ nutrition. However, upon salinity, plants grown with NH₄ ⁺ (N-NH₄ ⁺ plants) showed a reduction of H₂O₂ levels in parallel to an increase of catalase (CAT), SOD, and GR activities compared with the control plants. Moreover, N-NH₄ ⁺ plants were able to keep high levels of reduced glutathione (GSH) upon salinity and were able to induce glutathione-S-transferase (GST) and phospholipid hydroperoxide glutathione peroxidise (PHGPx) mRNA accumulation. Based on this evidence, we confirm that sublethal concentrations of NH₄ ⁺ might act as a mild oxidative stressor, which triggers antioxidant cellular machinery that can provide resistance to subsequent salt stress.     

Arbuscular mycorrhizal symbiosis modulates antioxidant response in salt-stressed Trigonella foenum-graecum plants

An experiment was conducted to evaluate the influence of Glomus intraradices colonization on the activity of antioxidant enzymes [superoxide dismutase (SOD), catalase (CAT), peroxidase (PX), ascorbate peroxidase (APX), and glutathione reductase (GR)] and the accumulation of nonenzymatic antioxidants (ascorbic acid, α-tocopherol, glutathione, and carotenoids) in roots and leaves of fenugreek plants subjected to varying degrees of salinity (0, 50, 100, and 200 mM NaCl) at two time intervals (1 and 14 days after saline treatment, DAT). The antioxidative capacity was correlated with oxidative damage in the same tissue. Under salt stress, lipid peroxidation and H₂O₂ concentration increased with increasing severity and duration of salt stress (DoS). However, the extent of oxidative damage in mycorrhizal plants was less compared to nonmycorrhizal plants. The study reveals that mycorrhiza-mediated attenuation of oxidative stress in fenugreek plants is due to enhanced activity of antioxidant enzymes and higher concentrations of antioxidant molecules. However, the significant effect of G. intraradices colonization on individual antioxidant molecules and enzymes varied with plant tissue, salinity level, and DoS. The significant effect of G. intraradices colonization on antioxidative enzymes was more evident at 1DAT in both leaves and roots, while the concentrations of antioxidant molecules were significantly influenced at 14DAT. It is proposed that AM symbiosis can improve antioxidative defense systems of plants through higher SOD activity in M plants, facilitating rapid dismutation of O₂ ^- to H₂O₂, and subsequent prevention of H₂O₂ build-up by higher activities of CAT, APX, and PX. The potential of G. intraradices to ameliorate oxidative stress generated in fenugreek plants by salinity was more evident at higher intensities of salt stress.     

Changes in the antioxidative systems of Ocimum basilicum L. (cv. Fine) under different sodium salts

The effects of different sodium salts on some physiological parameters and antioxidant responses were investigated in a medicinal and aromatic plant, Ocimum basilicum L. (cultivar Fine). Plants were subjected to an equimolar concentration of Na₂SO₄ (25?mM) and NaCl (50?mM) for 15 and 30?days. Growth, oxidative stress parameters [electrolyte leakage, peroxidation, and hydrogen peroxide (H₂O₂) concentration], antioxidant enzyme activities [ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), and peroxidases (POD, EC 1.11.1.7)], as well as antioxidant molecules [ascorbate and glutathione] were determined. The two salts affected leaf growth rates to the same extent, after 15 or 30?days of treatment, indicating a similar effect of Na₂SO₄ and NaCl salinity on growth, even if different (enzymatic and non-enzymatic) antioxidant mechanisms were involved in H₂O₂ detoxification. However, under both salts, the efficiency of the antioxidant metabolism seemed to be sufficient to avoid the deleterious effects of reactive oxygen species (ROS). Indeed, both ion leakage and peroxidation did not change under either Na₂SO₄ or NaCl salinity. As a whole, these data suggest that a cooperative process between the antioxidant systems is important for the tolerance of Ocimum basilicum L., cv. Fine to Na₂SO₄ and NaCl salinity.     

Rapid breakdown of exogenous extracellular hydrogen peroxide by lichens

All organisms, even highly stress‐tolerant lichens, produce a variety of reactive oxygen species (ROS) during and after stress. Furthermore, the cell walls of some lichens in Suborder Peltigerineae contain laccases, and therefore can produce quinone radicals that can break down to yield ROS. While the extracellular ROS produced by these enzymes probably play important roles in the biology of these lichens, they may also be potentially harmful and need to be rapidly broken down. To test this, rates of breakdown of exogenously supplied H₂O₂ were measured in a range of lichen species. Considerable diversity existed in rates of H₂O₂ breakdown but rates were on average almost double in members of Suborder Peltigerineae. While all lichens tested appeared to lack extracellular peroxidases and catalases, enzymes normally involved in breaking down H₂O₂, extracellular tyrosinase activity could be readily detected in the Peltigerineae. A role for tyrosinases in H₂O₂ breakdown was supported by the results from experiments involving inhibitors, and demonstration of the simultaneous release into an incubation solution of tyrosinase activity and the ability to breakdown H₂O₂. Rates of breakdown were very high, and tyrosinase appeared to break down H₂O₂ by a catalase‐like mechanism. However, significant rates of breakdown of H₂O₂ also occurred in species that did not possess cell wall redox enzymes. These species probably took up the exogenously supplied H₂O₂ intracellularly and then broke it down by the usual catalases and peroxidases. The importance of H₂O₂ degradation is discussed in terms of its possible role in defence against the harmful effects of ROS.     

Role of Nitric Oxide and Hydrogen Peroxide During the Salt Resistance Response

Ion homeostasis is essential for plant cell resistance to salt stress. Under salt stress, to avoid cellular damage and nutrient deficiency, plant cells need to maintain adequate K nutrition and a favorable K to Na ratio in the cytosol. Recent observations revealed that both nitric oxide (NO) and hydrogen peroxide (H₂O₂) act as signaling molecules to regulate K to Na ratio in calluses from Populus euphratica under salt stress. Evidence indicated that NO mediating H₂O₂ causes salt resistance via the action of plasma membrane H⁺-ATPase but that activity of plasma membrane NADPH oxidase is dependent on NO. Our study demonstrated the signaling transduction pathway. In this addendum, we proposed a testable hypothesis for NO function in regulation of H₂O₂ mediating salt resistance.Key Words: hydrogen peroxide, nitric oxide, signaling molecule, salt resistanceUnder salinity conditions, tolerant plant cells achieve ion homeostasis by extruding Na to the external medium and/or compartmentalizing into vacuoles, maintaining K uptake and high K and low Na in the cytosol.^1,2 Control of Na movement across the plasma membrane (PM) and tonoplast in order to maintain a low Na concentration in the cytoplasm is a key factor of cellular adaptation to salt stress.^3,4 Na transport across the PM is dependent on the electrochemical gradient created by the PM H⁺-ATPase.^5,6 It has been proven that the activity of the PM H⁺-ATPase is a key index of plant adaptation to salt stress.⁷ Therefore, the regulation of expression of the PM H⁺-ATPase may represent an important cellular mechanism for salt resistance. In contrast to our understanding of the regulation of PM H⁺-ATPase by other factors, the roles of NO and H₂O₂ act as signals under salt stress have been less known.Previous studies have shown that both NO and H₂O₂ function as stress signals in plants, mediating a range of resistance mechanisms in plants under stress conditions.^8–10 We have previously shown that NO serves as a signal in inducing salt resistance by increasing the K to Na ratio, which is dependent on the increased PM H⁺-ATPase activity in calluses from reed.¹¹ Although NO acts as a signal molecule under salt stress and induces salt resistance by increasing PM H⁺-ATPase activity, our research results also indicated NO can not activate purified PM H⁺-ATPase activity, at least in vitro. Subsequently, we set out to find the other signal molecules and factors between NO and PM H⁺-ATPase activity. Since our studies have indicated that NO can not induce salt resistance directly, what roles dose it play in salt resistance in tolerant cells under salt stress? We initially hypothesized ABA or H₂O₂ might be downstream signal molecules to regulate the activity of PM H⁺-ATPase. Further results indicated H₂O₂ content increased greatly under salt stress. Since H₂O₂ might be the candidate downstream signal molecule, we tested PM H⁺-ATPase activity and K to Na ratio in calluses by adding H₂O₂. The results suggested that H₂O₂ inducing an increased PM H⁺-ATPase activity resulted in an increased K to Na ratio. Summing up this new assay that allows us to speculate NO maybe regulate the H₂O₂ generation.Since H₂O₂ is involved in downstream signal molecule of NO, PM NADPH oxidase, the main source of H₂O₂ production, might be the regulated target of NO. We took a pharmacological approach to examine the speculation. The results indicated that PM NADPH oxidase is required for H₂O₂ accumulation and PM NADPH oxidase activity could attribute to NO in calluses under salt stress. These results also raised another question regarding what concentrations of NO can induce such effects. In our experiments, NO content was induced 1.6 times higher than the control values under salt treatment. We speculated there exists an effective balance point in NO signal system similar to previous reports by Delledonne et al.¹² in disease resistance.Further research work is required to decipher the mechanism through which NO and H₂O₂ acts and how K and Na elements uptake might be connected with salt resistance. We would like to propose a simple testable model that accounts for the results reported in this paper (Fig. 1). According to our model, H₂O₂ rather than NO is the major signaling molecular that mediated directly PM H⁺-ATPase under salt stress. Normally, NO generated from nitric oxide synthase (NOS) acts as a signal molecule to regulate other mechanisms. Under salt stress, accumulated NO activates PM NADPH oxidase activity. Then, a number of H₂O₂ is produced from PM NADPH oxidase. The PM H⁺-ATPase is activated greatly by the accumulated H₂O₂. Eventually, the transmembrane electrochemical gradient is created and K to Na ratio increases. The model we have proposed here is testable and should provide further insights into salt resistance mechanism regulated by NO and H₂O₂ signal molecules.Open in a separate windowFigure 1Hypothetical model for the potential function of NO and H₂O₂ as signaling molecules in inducing salt resistance. Salt stress activates a signal transduction cascade that leads to the increased activity of PM H⁺-ATPase, whose expression produces salt resistance. NO is generated by NOS, and H₂O₂ is produced by NADPH oxidase attributed to NO. The activity of PM H⁺-ATPase is regulated by H₂O₂ directly under salt stress. The model is based on the recent results in calluses from P. euphratica¹² and those previously reported on the NO function in reed.¹¹Research on roles of NO and H₂O₂ under stress conditions in plant is advancing rapidly. Further analysis of salt resistance mechanism with novel technology will certainly increase our knowledge in this field.     

Gene Knockout Study Reveals That Cytosolic Ascorbate Peroxidase 2(OsAPX2) Plays a Critical Role in Growth and Reproduction in Rice under Drought,Salt and Cold Stresses

Plant ascorbate peroxidases (APXs), enzymes catalyzing the dismutation of H₂O₂ into H₂O and O₂, play an important role in reactive oxygen species homeostasis in plants. The rice genome has eight OsAPXs, but their physiological functions remain to be determined. In this report, we studied the function of OsAPX2 gene using a T-DNA knockout mutant under the treatment of drought, salt and cold stresses. The Osapx2 knockout mutant was isolated by a genetic screening of a rice T-DNA insertion library under 20% PEG-2000 treatment. Loss of function in OsAPX2 affected the growth and development of rice seedlings, resulting in semi-dwarf seedlings, yellow-green leaves, leaf lesion mimic and seed sterility. OsAPX2 expression was developmental- and spatial-regulated, and was induced by drought, salt, and cold stresses. Osapx2 mutants had lower APX activity and were sensitive to abiotic stresses; overexpression of OsAPX2 increased APX activity and enhanced stress tolerance. H₂O₂ and MDA levels were high in Osapx2 mutants but low in OsAPX2-OX transgenic lines relative to wild-type plants after stress treatments. Taken together, the cytosolic ascorbate peroxidase OsAPX2 plays an important role in rice growth and development by protecting the seedlings from abiotic stresses through scavenging reactive oxygen species.     

Oxygen dynamics in a salt-marsh soil and in Suaeda maritima during tidal submergence

Habitats occupied by many halophytes are not only saline, but are also prone to flooding and yet surprisingly few studies have evaluated submergence tolerance in halophytes. Sediment, floodwater, and intra-plant O₂ dynamics were evaluated during tidal submergence for the leaf-succulent halophyte Suaeda maritima (L.) Dum. For S. maritima growing in soil just above the mud flat in a UK salt marsh, the soil was only moderately hypoxic just prior to tidal inundation, presumably owing to drainage and O₂ entry facilitated by frequent, large cracks. O₂ declined to very low levels following high tide. By contrast, mud flat sediment remained waterlogged, lacked cracks, and was anoxic. Plant O₂ dynamics were investigated using field-collected plants in sediment blocks transported to a controlled-submergence system in a glasshouse. Submergence during night-time resulted in anoxia within leaves, whereas during day-time O₂ was produced by underwater photosynthesis. The thin lateral roots of S. maritima presumably access some O₂ from hypoxic sediments, but could also experience transient episodes of severe hypoxia/anoxia, especially as any internal O₂ movement from shoots would be small owing to the low gas-filled porosity in roots. Fermentative metabolism to lactate, producing some ATP in the absence of O₂, might contribute to tolerance of transient O₂ deficits. Lactate was high in root tissues, whereas ethanol production (tissue and incubation medium contents) was low, both in comparison with values reported for other species. Our findings demonstrate the importance of tolerance to transient waterlogging and submergence for the halophyte S. maritima growing in a tidal salt marsh.