Induction of salt tolerance in salicylate-deficient <Emphasis Type="Italic">NahG</Emphasis> Arabidopsis transformants using the nitric oxide donor

The effects of treatment with nitric oxide donor sodium nitroprusside (SNP, 0.5 mM) on salt tolerance of wild type (Col–0) Arabidopsis thaliana plants and Arabidopsis thaliana plants transformed with the bacterial salicylate hydroxylase gene (NahG) were compared. Basic salt tolerance level (200 mM NaCl) was higher in NahG transformants. Under salt stress conditions, these plants showed higher activity levels for antioxidant enzymes as well as higher content of sugars and anthocyanins. The treatment with NO donor induced salt tolerance in the plants of both genotypes, which could be observed as less strong growth inhibition, reduced oxidative damage, and preservation of chlorophyll pool in leaves. After the exposure to salt stress, the activity of both superoxide dismutase and guaiacol peroxidase was higher in SNP-treated wild type plants and NahG transformants than in the nontreated plants. After the imposition of salt stress, proline content in leaves of the wild type plants treated with the nitric oxide donor was lower than in the leaves of the nontreated plants. In contrast, SNP treatment of NahG transformants led to a significant increase in the proline content in leaves under the salt stress conditions. Conclusions have been made that wild type Col-0 plants and NahG transformants differ in how their systems of protection against salt stress are activated and that nitric oxideinduced mobilization of protection systems in A. thaliana may not require the presence of salicylate.     

Hydrogen peroxide-induced salt tolerance in the <Emphasis Type="Italic">Arabidopsis</Emphasis> salicylate-deficient transformants <Emphasis Type="Italic">NahG</Emphasis>

The effect of hydrogen peroxide treatment on the salt tolerance of wild-type Arabidopsis thaliana L. plants (Col-0) and plants transformed with the bacterial salicylate hydroxylase gene (NahG) was studied. The base tolerance to salt stress caused by 200 mM of NaCl in solution culture was higher in plants with the NahG genotype in comparison with the wild-type plants. Growth inhibition was observed for wild-type plants under the action of exogenous hydrogen peroxide, which was not observed for the NahG transformants; salt tolerance increased in the both types of plants after treatment, which was assessed based on the growth indicators and the ability to preserve the chlorophyll pool following NaCl treatment. The content of endogenous Н2О2 in the leaves of wild-type plants increased significantly following exogenous hydrogen peroxide treatment and salt stress, while it practically did not change in the leaves of the NahG genotype. The SOD activity increased in both genotypes after treatment with exogenous hydrogen peroxide, and remained at an elevated level after salt stress in comparison with the nontreated plants. Furthermore, the catalase activity increased in leaves of the salicylate-deficient genotype but not in the Col-0 genotype. The guaiacol peroxidase activity increased in plants of both genotypes under the action of hydrogen peroxide and salt stress, with the NahG plants demonstrating a higher degree of increase. The Н₂О₂ treatment facilitated the increase of the proline content in leaves of the plants of both genotypes under conditions of salt stress. It was concluded that there were hydrogen peroxide signal transduction pathways in Arabidopsis plants that were salicylic acid independent and that the antioxidant system functioned more effectively in salicylate-deficient Arabidopsis plants.     

Content of Osmolytes and Flavonoids under Salt Stress in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> Plants Defective in Jasmonate Signaling

The effects of the salt stress (200 mM NaCl) and exogenous jasmonic acid (JA) on levels of osmolytes and flavonoids in leaves of four-week-old Arabidopsis thaliana L. plants of the wild-type (WT) Columbia-0 (Col-0) and the mutant jin1 (jasmonate insensitive 1) with impaired jasmonate signaling were studied. The increase in proline content caused by the salt stress was higher in the Col-0 plants than in the mutant jin1. This difference was especially marked if the plants had been pretreated with exogenous 0.1 μM JA. The sugar content increased in response to the salt stress in the JA-treated WT plants but decreased in the jin1 mutant. Treatment with JA of the WT plants but not mutant defective in jasmonate signaling also enhanced the levels of anthocyanins and flavonoids absorbed in UV-B range in leaves. The presence of JA increased salinity resistance of the Col-0 plants, since the accumulation of lipid peroxidation products and growth inhibition caused by NaCl were less pronounced. Under salt stress, JA almost did not render a positive effect on the jin1 plants. It is concluded that the protein JIN1/MYC2 is involved in control of protective systems under salt stress.     

Salt tolerance conferred by expression of a global regulator IrrE from <Emphasis Type="Italic">Deinococcus radiodurans</Emphasis> in oilseed rape

As salinity is a major threat to sustainable agriculture worldwide, cultivation of salt-tolerant crops becomes increasingly important. IrrE acts as a global regulator and a general switch for stress resistance in Deinococcus radiodurans. In this study, to determine whether the irrE gene can improve the salt tolerance of Brassica napus, we introduced the irrE gene into B. napus by the Agrobacterium tumefaciens-mediated transformation method. Forty-two independent transgenic plants were regenerated. Polymerase chain reaction (PCR) analyses confirmed that the irrE gene had integrated into the plant genome. Northern as well as Western blot analyses revealed that the transgene was expressed at various levels in transgenic plants. Analysis for the T₁ progenies derived from four independent transformants showed that irrE had enhanced the salt tolerance of T₁ in the presence of 350 mM NaCl. Furthermore, under salt stress, transgenic plants accumulated more compatible solutes (proline) and a lower level of malondialdehyde (MDA), and they had higher activities of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD). However, agronomic traits were not affected by irrE gene overexpression in the transgenic B. napus plants. This study indicates that the irrE gene can improve the salt tolerance of B. napus and represents a promising candidate for the development of crops with enhanced salt tolerance by genetic engineering.     

<Emphasis Type="Italic">Piriformospora indica</Emphasis> Alleviates Salinity by Boosting Redox Poise and Antioxidative Potential of Tomato

More than 20% of irrigated land has been influenced by salt stress, decreasing crop production. In this research, we investigated the effect of different levels of salinity (0, 50, 100 and 150 mM NaCl) and the efficiency of Piriformospora indica on growth, biochemical traits, antioxidative defense system in tomato (Solanum lycopersicum L.). NaCl stress reduced chlorophyll content, height and biomass of plants. Higher level of salinity (150 mM) declined the plant height by 22.65%, total dry weight by 56.44% and total chlorophyll by 44.34%, however, P. indica inoculation raised plant height by 43.47%, dry weight by 69.23% and total chlorophyll content by 48.09%. Salinity stress increased H₂O₂, malondialdehyde (MDA), superoxide anion and 1,1-diphenyl-2-picrylhydrazyl (DPPH) level in leaves and roots tomato seedlings. However, P. indica inoculation reduced H₂O₂, MDA and superoxide anion and enhanced DPPH compared to non-inoculated plants at all NaCl levels. The total phenol and flavonoids increased with NaCl treatment. On the other hand, the total phenolic and flavonoid increased more in P. indica inoculated plants compared to non-inoculated ones. Moreover, inoculation of P. indica implicated noteworthy improvement of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR) activity in tomato upon salinity. Notably, colonization with P. indica significantly improved the content of reduced ascorbic acid (AsA), glutathione (GSH) and redox ratio in the tomato plants under salinity resulting in reduced redox state. Our findings confirmed that salinity had negative effect on tomato seedling; however, P. indica inoculation increased tolerance to salinity by improving the content of phenolic compounds, non-enzymatic antioxidants, and increasing the activity of antioxidant enzymes.     

Role of antioxidant systems in wild plant adaptation to salt stress

Wild plants differing in the strategies of adaptation to salinity were grown for six weeks in the phytotron and then subjected to salt stress (100 mM NaCl, 24 h). The activities of principal antioxidant enzymes and the accumulation of sodium ions and proline were studied. Independently of the level of constitutive salt tolerance, plants of all species tested accumulated sodium ions under salinity conditions but differed in their capability of stress-dependent proline accumulation and superoxide dismutase (SOD) and guaiacol-dependent peroxidase activities. Proline-accumulating species were found among both halophytes (Artemisia lerchiana and Thellungiella halophila) and glycophytes (Plantago major and Mycelis muralis). The high activities of ionically-bound and covalently bound peroxidases were characteristic of Th. halophila plants. High constitutive and stress-induced SOD activities were, as a rule, characteristic of glycophytes with the low constitutive proline level: Geum urbanum and Thalictrum aquilegifolium. Thus, a negative correlation was found between proline content and SOD activity in wild species tested; it was especially bright in the halophyte Th. halophila and glycophyte G. urbanum. An extremely high constitutive and stress-induced levels of proline and peroxidase activity in Th. halophila maybe compensate SOD low activity in this plant, and this contributed substantially into its salt resistance. Thus, monitoring of stress-dependent activities of some antioxidant enzymes and proline accumulation in wild plant species allowed a supposition of reciprocal interrelations between SOD activity and proline accumulation. It was also established that the high SOD activity is not obligatory trait of species salt tolerance. Moreover, plants with the high activity of peroxidase and active proline accumulation could acclimate to salts stress (100 mM NaCl, 24 h) independently of SOD activity.     

Growth and biochemical analysis of evergreen haloxeric tree species <Emphasis Type="Italic">Salvadora oleoides</Emphasis> and <Emphasis Type="Italic">Salvadora persica</Emphasis> under NaCl stress

In vitro growth, development, total soluble proteins and peroxidase profiles of Salvadora oleoides and Salvadora persica under NaCl stress were analysed in the present investigation. The plants are evergreen haloxeric tree species of family Salvadoraceae. Shoot apex from natural plants were initially used for screening of NaCl tolerance on MS culture medium. Shoot apex of S. oleoides and S. persica could survive optimally up to 200 and 100 mM NaCl. Axillary buds from nodal shoot segments of S. oleoides and S. persica were activated on 6 and 4 μM BAP, and were used further for extraction of total soluble proteins and peroxidases. Total soluble proteins were increased up to 150 mM NaCl in S. oleoides, but decline above 50 mM NaCl in S. persica. Peroxidase activity remained almost constant in S. oleoides at all the concentrations and duration of NaCl, but increased at 100 mM NaCl during fourth week of treatment in S. persica. Eleven peroxidase isozymes were observed in zymogram of S. oleoides. Isozymes P1, P2, P3, and P4 were slightly appeared, but P6 isozyme was lacking in S. persica. The P5 isozyme was more prominent in S. persica than S. oleoides. Isozyme P9 of S. persica was visible during the first week of NaCl treatment, but disappeared in the fourth week. Molecular biology of these plants can be useful further for the understanding of stress tolerance mechanisms for prospects.     

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.     

Effect of salt stress on the antimicrobial activity of <Emphasis Type="Italic">Ruta chalepensis</Emphasis> essential oils

In the current investigation, the biological activities of essential oils obtained from organs of Ruta chalepensis plants grown under salt stress (0, 50 and 100 mM NaCl) were analyzed. Their chemical composition was often investigated by GC/FID and GC–MS and the antimicrobial activities towards eight bacteria (Salmonella All, Salmonella K, Escherichia coli 45AG, Escherichia coli 45AI, Staphylococcus aureus 9402, Staphylococcus aureus 02B145, Listeria 477 and Pseudomonas aeruginosa ATCC 10145) and five fungi strains (Aspergillus, Saccharomycee crvisiale, Streptomyces griseus, Fusarium solani and Penicillium thomii) were studied. Results revealed that salt increased essential oil production in leaves at 50 and 100 mM NaCl. A total of 20 compounds were identified in leaves, undecan-2-one, nonan-2-one and geijerene being the dominant ones. In stems, 21 compounds were found; they were dominated by decan-2-one, geijerene, nonan-2-one and undecan-2-one. In contrast, roots exhibited a large variation with 25 volatile compounds and octyl acetate, methyl decanoate, phytyl acetate were the major ones. Salt stress induced significant antibacterial activity changes, mainly in leaves and stems. In leaves, the minimum inhibitory and bactericidal concentration decreased at 100 mM NaCl against Listeria 477, the two strains of E. coli (45AG and 45AI) and P. aeruginosa but it increased versus other bacteria. In stems, salt increased oil antibacterial activity against all strains except P. aeruginosa ATCC 10145. Root oil showed the least antibacterial activity under saline conditions versus Listeria 477 and P. aeruginosa ATCC 10145. As regards antifungal activity, NaCl reduced the antifungal activity of essential oils against the majority of fungi strains.     

Assessment of the preventive effect of vermicompost on salinity resistance in tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> cv. Ailsa Craig)

To determine the effects of vermicompost leachate (VCL) on resistance to salt stress in plants, young tomato seedlings (Solanum lycopersicum, cv. Ailsa Craig) were exposed to salinity (150 mM NaCl addition to nutrient solution) for 7 days after or during 6 mL L^??1 VCL application. Salt stress significantly decreased leaf fresh and dry weights, reduced leaf water content, significantly increased root and leaf Na⁺ concentrations, and decreased K⁺ concentrations. Salt stress decreased stomatal conductance (g_s), net photosynthesis (A), instantaneous transpiration (E), maximal efficiency of PSII photochemistry in the dark-adapted state (F_v/F_m), photochemical quenching (qP), and actual PSII photochemical efficiency (ΦPSII). VCL applied during salt stress increased leaf fresh weight and g_s, but did not reduce leaf osmotic potential, despite increased proline content in salt-treated plants. VCL reduced Na⁺ concentrations in leaves (by 21.4%), but increased them in roots (by 16.9%). VCL pre-treatment followed by salt stress was more efficient than VCL concomitant to salt stress, since VCL pre-treatment provided the greatest osmotic adjustment recorded, with maintenance of net photosynthesis and K⁺/Na⁺ ratios following salt stress. VCL pre-treatment also led to the highest proline content in leaves (50 µmol g^??1 FW) and the highest sugar content in roots (9.2 µmol g^??1 FW). Fluorescence-related parameters confirmed that VCL pre-treatment of salt-stressed plants showed higher PSII stability and efficiency compared to plants under concomitant VCL and salt stress. Therefore, VCL represents an efficient protective agent for improvement of salt-stress resistance in tomato.     

High salinity helps the halophyte <Emphasis Type="Italic">Sesuvium portulacastrum</Emphasis> in defense against Cd toxicity by maintaining redox balance and photosynthesis

Main conclusion

NaCl alleviates Cd toxicity in Sesvium portulacastrum by maintaining plant water status and redox balance, protecting chloroplasts structure and inducing some potential Cd ²⁺ chelators as GSH and proline. It has been demonstrated that NaCl alleviates Cd-induced growth inhibition in the halophyte Sesuvium portulacastrum. However, the processes that mediate this effect are still unclear. In this work we combined physiological, biochemical and ultrastructural studies to highlight the effects of salt on the redox balance and photosynthesis in Cd-stressed plants. Seedlings were exposed to different Cd concentrations (0, 25 and 50 µM Cd) combined with low (0.09 mM) (LS), or high (200 mM) NaCl (HS) in hydroponic culture. Plant–water relations, photosynthesis rate, leaf gas exchange, chlorophyll fluorescence, chloroplast ultrastructure, and proline and glutathione concentrations were analyzed after 1 month of treatment. In addition, the endogenous levels of stress-related hormones were determined in plants subjected to 25 µM Cd combined with both NaCl concentrations. In plants with low salt supply (LS), Cd reduced growth, induced plant dehydration, disrupted chloroplast structure and functioning, decreased net CO₂ assimilation rate (A) and transpiration rate (E), inhibited the maximum potential quantum efficiency (Fv/Fm) and the quantum yield efficiency (Φ _PSII) of PSII, and enhanced the non-photochemical quenching (NPQ). The addition of 200 mM NaCl (HS) to the Cd-containing medium culture significantly mitigated Cd phytotoxicity. Hence, even at similar internal Cd concentrations, HS-Cd plants were less affected by Cd than LS-Cd ones. Hence, 200 mM NaCl significantly alleviates Cd-induced toxicity symptoms, growth inhibition, and photosynthesis disturbances. The cell ultrastructure was better preserved in HS-Cd plants but affected in LS-Cd plants. The HS-Cd plants showed also higher concentrations of reduced glutathione (GSH), proline and jasmonic acid (JA) than the LS-Cd plants. However, under LS-Cd conditions, plants maintained higher concentration of salicylic acid (SA) and abscisic acid (ABA) than the HS-Cd ones. We conclude that in S. portulacastrum alleviation of Cd toxicity by NaCl is related to the modification of GSH and proline contents as well as stress hormone levels thus protecting redox balance and photosynthesis.

     

Salinity and drought stresses improve antioxidant potential of <Emphasis Type="Italic">Allium roseum</Emphasis> L., an edible medicinal plant

Allium roseum L., a spontaneous comestible species in North Africa, presented a nutraceutical potential in terms of essential nutrients and phytonutrients responsible for health benefits. In the present study, growth parameters, soluble sugars, principal secondary metabolites contents (total polyphenols, vitamin C) and their antioxidant properties were determined in A. roseum subjected to salt or drought stresses during 30 days. Plants were subjected to drought stress by suspending watering, while other plants were submitted to salt stress through irrigation with a nutrient solution containing 50, 100, 150 and 200 mM NaCl. The obtained results reveal that salt and drought stress did not significantly affect the growth parameters (p?≥?0.05), while these stressful conditions increased the bulbs dry matter. Moreover, the applied salt and drought stresses increased the soluble sugars, total polyphenols and vitamin C contents of the plant. Cultivating A. roseum under drought or salt stresses could be a good alternative to valorize antioxidant molecules from this species in industrial applications.     

Foliar-application of α-tocopherol enhanced salt tolerance of <Emphasis Type="Italic">Carex leucochlora</Emphasis>

Several different concentrations of α-tocopherol were applied to Carex leucochlora after plants had been treated with high salinity (0.8 % NaCl) in a greenhouse for one month. The results revealed that 0.8 mM α-tocopherol treatment showed the greatest alleviation of growth inhibition and cell membrane damage induced by salt stress. In comparison with NaCl alone, the 0.8 mM α-tocopherol application significantly decreased the content of hydrogen peroxide and the rate of superoxide radical generation, and increased the content of chlorophyll b, carotenoids, free proline, and soluble protein, but had no effect on the content of chlorophyll a and soluble sugar. These results suggest that α-tocopherol could effectively protect C. leucochlora plants from salt stress damage presumably by quenching the excessive reactive oxygen species to protect the photosynthetic pigments and by enhancing the osmotic adjustment.     

Salicylic acid promotes plant growth and salt-related gene expression in <Emphasis Type="Italic">Dianthus superbus</Emphasis> L. (Caryophyllaceae) grown under different salt stress conditions

Salt stress is a critical factor that affects the growth and development of plants. Salicylic acid (SA) is an important signal molecule that mitigates the negative effects of salt stress on plants. To elucidate salt tolerance in large pink Dianthus superbus L. (Caryophyllaceae) and the regulatory mechanism of exogenous SA on D. superbus under different salt stresses, we conducted a pot experiment to evaluate leaf biomass, leaf anatomy, soluble protein and sugar content, and the relative expression of salt-induced genes in D. superbus under 0.3, 0.6, and 0.9% NaCl conditions with and without 0.5 mM SA. The result showed that exposure of D. superbus to salt stress lead to a decrease in leaf growth, soluble protein and sugar content, and mesophyll thickness, together with an increase in the expression of MYB and P5CS genes. Foliar application of SA effectively increased leaf biomass, soluble protein and sugar content, and upregulated the expression of MYB and P5CS in the D. superbus, which facilitated in the acclimation of D. superbus to moderate salt stress. However, when the plants were grown under severe salt stress (0.9% NaCl), no significant difference in plant physiological responses and relevant gene expression between plants with and without SA was observed. The findings of this study suggest that exogenous SA can effectively counteract the adverse effects of moderate salt stress on D. superbus growth and development.