Effects of salt stress on the structure and function of the photosynthetic apparatus in Cucumis sativus and its protection by exogenous putrescine

Exogenous Tebuconazole and Trifloxystrobin Regulates Reactive Oxygen Species Metabolism Toward Mitigating Salt-Induced Damages in Cucumber Seedling

The present study investigated the role of tebuconazole (TEB) and trifloxystrobin (TRI) on cucumber plants (Cucumis sativus L. cv. Tokiwa) under salt stress (60 mM NaCl). The cucumber plants were grown semi-hydroponically in a glasshouse. Plants were exposed to two different doses of fungicides (1.375 µM TEB + 0.5 µM TRI and 2.75 µM TEB + 1.0 µM TRI) solely and in combination with NaCl (60 mM) for six days. The application of salt phenotypically deteriorated the cucumber plant growth that caused yellowing of the whole plant and significantly destructed the contents of chlorophyll and carotenoids. The oxidative damage was created under salinity by increasing the contents of malondialdehyde (MDA), hydrogen peroxide (H2O2), and electrolytic leakage (EL) resulting in the disruption of the antioxidant defense system. Furthermore, in the leaves, stems, and roots of cucumber plants increased Na+ content was observed under salt stress, whereas the K+/Na+ ratio and contents of K+, Ca2+, and Mg2+ decreased. In contrast, the exogenous application of TEB and TRI reduced the contents of MDA, H2O2, and EL by improving the activities of enzymatic and non-enzymatic antioxidants. In addition, ion homeostasis was regulated by reducing Na+ uptake and enhanced K+ accumulation and the K+/Na+ ratio after application of TEB and TRI. Therefore, this study indicates that the exogenous application of TEB and TRI enhanced salt tolerance in cucumber plants by regulating reactive oxygen speciesproduction and antioxidant defense systems.

Membrane Lipid Remodeling in Response to Salinity

Salinity is one of the most decisive environmental factors threatening the productivity of crop plants. Understanding the mechanisms of plant salt tolerance is critical to be able to maintain or improve crop yield under these adverse environmental conditions. Plant membranes act as biological barriers, protecting the contents of cells and organelles from biotic and abiotic stress, including salt stress. Alterations in membrane lipids in response to salinity have been observed in a number of plant species including both halophytes and glycophytes. Changes in membrane lipids can directly affect the properties of membrane proteins and activity of signaling molecules, adjusting the fluidity and permeability of membranes, and activating signal transduction pathways. In this review, we compile evidence on the salt stress responses of the major membrane lipids from different plant tissues, varieties, and species. The role of membrane lipids as signaling molecules in response to salinity is also discussed. Advances in mass spectrometry (MS)-based techniques have largely expanded our knowledge of salt-induced changes in lipids, however only a handful studies have investigated the underlying mechanisms of membrane lipidome regulation. This review provides a comprehensive overview of the recent works that have been carried out on lipid remodeling of plant membranes under salt treatment. Challenges and future perspectives in understanding the mechanisms of salt-induced changes to lipid metabolisms are proposed.

Stomatal and non-stomatal limitations are responsible in down-regulation of photosynthesis in melon plants grown under the saline condition: Application of carbon isotope discrimination as a reliable proxy

Salinity is one of the most severe environmental stresses limiting agricultural crop production worldwide. Photosynthesis is one of the main biochemical processes getting affected by such stress conditions. Here we investigated the stomatal and non-stomatal factors during photosynthesis in two Iranian melon genotypes "Ghobadlu" and "Suski-e-Sabz", as well as the "Galia" F1 cultivar, with an insight into better understanding the physiological mechanisms involved in the response of melon plants to increasing salinity. After plants were established in the greenhouse, they were supplied with nutrient solutions containing three salinity levels (0, 50, or 100 mM NaCl) for 15 and 30 days. With increasing salinity, almost all of the measured traits (e.g. stomatal conductance, transpiration rate, internal to ambient CO₂ concentration ratio (C_i/C_a), Rubisco and nitrate reductase activity, carbon isotope discrimination (Δ¹³C), chlorophyll content, relative water content (RWC), etc.) significantly decreased after 15 and 30 days of treatments. In contrast, the overall mean of water use efficiency (intrinsic and instantaneous WUE), leaf abscisic acid (ABA) and flavonol contents, as well as osmotic potential (Ψ_S), all increased remarkably with increasing stress, across all genotypes. In addition, notable correlations were found between Δ¹³C and leaf gas exchange parameters as well as most of the measured traits (e.g. leaf area, biomass, RWC, Ψ_S, etc.), encouraging the possibility of using Δ¹³C as an important proxy for indirect selection of melon genotypes with higher photosynthetic capacity and higher salinity tolerance. The overall results suggest that both stomatal and non-stomatal limitations play an important role in reduced photosynthesis rate in melon genotypes studied under NaCl stress. This conclusion is supported by the concurrently increased resistance to CO₂ diffusion, and lower Rubisco activity under NaCl treatments at the two sampling dates, and this was revealed by the appearance of lower C_i/C_a ratios and lower Δ¹³C in the leaves of salt-treated plants.

Photosynthetic capacity, leaf respiration and growth in two papaya (Carica papaya) genotypes with different leaf chlorophyll concentrations

Golden genotype of papaya (Carica papaya), named for its yellowish leaves, produces fruits very much appreciated by consumers worldwide. However, its growth and yield are considerably lower than those of other genotypes, such as 'Sunrise Solo', which has intensely green leaves. We undertook an investigation with the goal of evaluating key physiological traits that can affect biomass accumulation of both Golden and Sunrise Solo genotypes. Papaya seeds from two different genotypes with contrasting leaf colour 'Sunrise Solo' and Golden were grown in greenhouse conditions. Plant growth (plant height, leaf number, stem diameter, leaf area, plant dry weight), leaf gas exchanges, leaf carbon balance, RuBisCO oxygenation and carboxylation rates, nitrogen, as well as chlorophyll concentrations and fluorescence variables were assessed. Although no significant differences were observed for photosynthetic rates between genotypes, the accumulation of small differences in photosynthesis, day after day, over a long period, might contribute to some extend to a higher C-budget in Sunrise Solo, higher leaf area and, thus, to higher productivity. Additionally, we consider that physiological processes other than photosynthesis and leaf respiration can be as well involved in lower growth and yield of Golden. One of these aspects could be related to the higher rates of photorespiration observed in Sunrise Solo, which could improve the rate of N assimilation into organic compounds, such as amino acids, thus contributing to the higher biomass production in Sunrise Solo relative to Golden. Further experiments to evaluate the effects of N metabolism on physiology and growth of Golden are required as it has the potential to limit its yield.

Genetic and transcriptional variations in NRAMP-2 and OPAQUE1 genes are associated with salt stress response in wheat

SNP alleles on chromosomes 4BL and 6AL are associated with sensitivity to salt tolerance in wheat and upon validation can be exploited in the development of salt-tolerant wheat varieties. The dissection of the genetic and molecular components of salt stress response offers strong opportunities toward understanding and improving salt tolerance in crops. In this study, GWAS was employed to identify a total of 106 SNP loci (R² = 0.12-63.44%) linked to salt stress response in wheat using leaf chlorophyll fluorescence, grain quality and shoot ionic (Na⁺ and K⁺ ions) attributes. Among them, 14 SNP loci individually conferred pleiotropic effects on multiple independent salinity tolerance traits including loci at 99.04 cM (R² ≥ 14.7%) and 68.45 cM (R² ≥ 4.10%) on chromosomes 6AL and 4BL, respectively, that influenced shoot Na⁺-uptake, shoot K⁺/Na⁺ ratio, and specific energy fluxes for absorption (ABS/RC) and dissipation (DIo/RC). Analysis of the open reading frame (ORF) containing the SNP markers revealed that they are orthologous to genes involved in photosynthesis and plant stress (salt) response. Further transcript abundance and qRT-PCR analyses indicated that the genes are mostly up-regulated in salt-tolerant and down-regulated in salt-sensitive wheat genotypes including NRAMP-2 and OPAQUE1 genes on 4BL and 6AL, respectively. Both genes showed highest differential expression between contrasting genotypes when expressions of all the genes within their genetic intervals were analyzed. Possible cis-acting regulatory elements and coding sequence variation that may be involved in salt stress response were also identified in both genes. This study identified genetic and molecular components of salt stress response that are associated with Na⁺-uptake, shoot Na⁺/K⁺ ratio, ABS/RC, DIo/RC, and grain quality traits and upon functional validation would facilitate the development of gene-specific markers that could be deployed to improve salinity tolerance in wheat.

Physiological and biochemical attributes of Mentha spicata when subjected to saline conditions and cation foliar application

Marginal water, including saline water, has been proposed as an alternative source of irrigation water for partially covering plant water requirements due to scarcity of adequate water supply in hot arid and semi-arid areas, such as those usually found in the Mediterranean basin. In the present study, spearmint plants (Mentha spicata L.) were grown in a deep flow hydroponic system under saline conditions, namely 0, 25, 50, and 100 mM NaCl. Moreover, foliar application of specific cations (K, Zn, Si) was tested as a means for alleviation of salinity stress under a plant physiological and biochemical approach. The results indicated that the highest salinity level of 100 mM NaCl severely affected plant growth, photosynthetic rates, leaf stomatal conductance, content of total phenolics and antioxidant status, while low to moderate salinity levels (25-50 mM NaCl) did not significantly affect plant growth and biochemical functions. In addition, leaf potassium and calcium accumulation decreased in saline-treated plants. Cations foliar application had small to no effect on plant growth, although it increased antioxidant activity and detoxified oxidative stress products/effects, through the increased enzymatic activities and proline accumulation. The present results have demonstrated that spearmint could be considered as a salinity tolerant species which is able to grow successfully under moderate salinity levels, while cation enrichment through foliar sprays was proved as a useful means to alleviate the stress effects caused by high salinity.

Modulation of plant and bacterial polyamine metabolism during the compatible interaction between tomato and Pseudomonas syringae

The polyamines putrescine, spermidine and spermine participate in a variety of cellular processes in all organisms. Many studies have shown that these polycations are important for plant immunity, as well as for the virulence of diverse fungal phytopathogens. However, the polyamines' roles in the pathogenesis of phytopathogenic bacteria have not been thoroughly elucidated to date. To obtain more information on this topic, we assessed the changes in polyamine homeostasis during the infection of tomato plants by Pseudomonas syringae. Our results showed that polyamine biosynthesis and catabolism are activated in both tomato and bacteria during the pathogenic interaction. This activation results in the accumulation of putrescine in whole leaf tissues, as well as in the apoplastic fluids, which is explained by the induction of its synthesis in plant cells and also on the basis of its excretion by bacteria. We showed that the excretion of this polyamine by P. syringae is stimulated under virulence-inducing conditions, suggesting that it plays a role in plant colonization. However, no activation of bacterial virulence traits or induction of plant invasion was observed after the exogenous addition of putrescine. In addition, no connection was found between this polyamine and plant defence responses. Although further research is warranted to unravel the biological functions of these molecules during plant-bacterial interactions, this study contributes to a better understanding of the changes associated with the homeostasis of polyamines during plant pathogenesis.

Chlorophyll Composition, Chlorophyll Fluorescence, and Grain Yield Change in esl Mutant Rice

To evaluate the effect of changes in chlorophyll (Chl) composition and fluorescence on final yield formation, early senescence leaf (esl) mutant rice and its wild-type cultivar were employed to investigate the genotype-dependent differences in Chl composition, Chl fluorescence, and yield characteristics during the grain-filling stage. However, the temporal expression patterns of key genes involved in the photosystem II (PSII) reaction center in the leaves of two rice genotypes were analyzed by quantitative real-time polymerase chain reaction (qRT-PCR). Results showed that the seed-setting rate, 1000-grain weight, and yield per plant remarkably decreased, and the increase in the 1000-grain weight during the grain-filling stage was retarded in esl mutant rice. Chl composition, maximal fluorescence yield (Fm), variable fluorescence (Fv), a maximal quantum yield of PSII photochemistry (Fv/Fm), and net photosynthetic rate (Pn) in esl mutant rice considerably decreased, thereby indicating the weakened abilities of light energy harvesting and transferring in senescent leaves. The esl mutant rice showed an increase in the minimal fluorescence yield (F₀) and 1 - Fv/Fm and decreases in the expression levels of light-harvesting Chl a/b binding protein (Cab) and photosystem II binding protein A (PsbA), PsbB, PsbC, and PsbD encoding for the reaction center of the PSII complex during the grain-filling stage. These results indicated the PSII reaction centers were severely damaged in the mesophyll cells of senescent leaves, which resulted in the weakened harvesting quantum photon and transferring light energy to PSI and PSII for carbon dioxide assimilation, leading to enhanced heat dissipation of light energy and a decrease in Pn.

Exogenous application of calcium to 24-epibrassinosteroid pre-treated tomato seedlings mitigates NaCl toxicity by modifying ascorbate-glutathione cycle and secondary metabolites

The present study tested the efficacy of 24-epibrassinolide (EBL) and calcium (Ca) for mediating salinity tolerance in tomato. Salinity stress affected the morphological parameters of tomato as well as leaf relative water content (LRWC), photosynthetic and accessory pigments, leaf gas exchange parameters, chlorophyll fluorescence and the uptake of essential macronutrients. The salt (NaCl) treatment induced oxidative stress in the form of increased Na+ ion concentration by 146%, electrolyte leakage (EL) by 61.11%, lipid peroxidation (MDA) 167% and hydrogen peroxide (H2O2) content by 175%. Salt stress also enhanced antioxidant enzyme activities including those in the ascorbate-glutathione cycle. Plants treated with EBL or Ca after salt exposure mitigated the ill effects of salt stress, including oxidative stress, by reducing the uptake of Na+ ions by 52%. The combined dose of EBL + Ca reversed the salt-induced changes through an elevated pool of enzymes in the ascorbate-glutathione cycle, other antioxidants (superoxide dismutase, catalase), and osmoprotectants (proline, glycine betaine). Exogenously applied EBL and Ca help to optimize mineral nutrient status and enable tomato plants to tolerate salt toxicity. The ability of tomato plants to tolerate salt stress when supplemented with EBL and Ca was attributed to modifications to enzymatic and non-enzymatic antioxidants, osmolytes and metabolites.

Pumpkin CmHKT1;1 Controls Shoot Na⁺ Accumulation via Limiting Na⁺ Transport from Rootstock to Scion in Grafted Cucumber

Soil salinity adversely affects the growth and yield of crops, including cucumber, one of the most important vegetables in the world. Grafting with salt-tolerant pumpkin as the rootstock effectively improves the growth of cucumber under different salt conditions by limiting Na⁺ transport from the pumpkin rootstock to the cucumber scion. High-affinity potassium transporters (HKTs) are crucial for the long distance transport of Na⁺ in plants, but the function of pumpkin HKTs in this process of grafted cucumber plants remains unclear. In this work, we have characterized CmHKT1;1 as a member of the HKT gene family in Cucurbita moschata and observed an obvious upregulation of CmHKT1;1 in roots under NaCl stress conditions. Heterologous expression analyses in yeast mutants indicated that CmHKT1;1 is a Na⁺-selective transporter. The transient expression in tobacco epidermal cells and in situ hybridization showed CmHKT1;1 localization at plasma membrane, and preferential expression in root stele. Moreover, ectopic expression of CmHKT1;1 in cucumber decreased the Na⁺ accumulation in the plants shoots. Finally, the CmHKT1;1 transgenic line as the rootstock decreased the Na⁺ content in the wild type shoots. These findings suggest that CmHKT1;1 plays a key role in the salt tolerance of grafted cucumber by limiting Na⁺ transport from the rootstock to the scion and can further be useful for engineering salt tolerance in cucurbit crops.

Remodeling of chloroplast proteome under salinity affects salt tolerance of Festuca arundinacea

Acclimation of photosynthetic apparatus to variable environmental conditions is an important component of tolerance to dehydration stresses, including salinity. The present study deals with the research on alterations in chloroplast proteome of the forage grasses. Based on chlorophyll fluorescence parameters, two genotypes of a model grass species-Festuca arundinacea with distinct levels of salinity tolerance: low salt tolerant (LST) and high salt tolerant (HST), were selected. Next, two-dimensional electrophoresis and mass spectrometry were applied under both control and salt stress conditions to identify proteins accumulated differentially between these two genotypes. The physiological analysis revealed that under NaCl treatment the studied plants differed in photosystem II activity, water content, and ion accumulation. The differentially accumulated proteins included ATPase B, ATP synthase, ribulose-1,5-bisphosphate carboxylase large and small subunits, cytochrome b6-f complex iron-sulfur subunit, oxygen-evolving enhancer proteins (OEE), OEE1 and OEE2, plastidic fructose-bisphosphate aldolase (pFBA), and lipocalin. A higher level of lipocalin, potentially involved in prevention of lipid peroxidation under stress, was also observed in the HST genotype. Our physiological and proteomic results performed for the first time on the species of forage grasses clearly showed that chloroplast metabolism adjustment could be a crucial factor in developing salinity tolerance.

Exogenously applied spermidine alleviates photosynthetic inhibition under drought stress in maize (Zea mays L.) seedlings associated with changes in endogenous polyamines and phytohormones

Drought stress (DS) is a major environmental factor limiting plant growth and crop productivity worldwide. It has been established that exogenous spermidine (Spd) stimulates plant tolerance to DS. The effects of exogenous Spd on plant growth, photosynthetic performance, and chloroplast ultrastructure as well as changes in endogenous polyamines (PAs) and phytohormones were investigate in DS-resistant (Xianyu 335) and DS-sensitive (Fenghe 1) maize seedlings under well-watered and DS treatments. Exogenous Spd alleviated the stress-induced reduction in growth, photosynthetic pigment content, photosynthesis rate (P_n) and photochemical quenching (q_P) parameters, including the maximum photochemistry efficiency of photosystem II (PSII) (F_v/F_m), PSII operating efficiency (ФPSII), and qP coefficient. Exogenous Spd further enhanced stress-induced elevation in non-photochemical quenching (NPQ) and the de-epoxidation state of the xanthophyll cycle (DEPS). Microscopic analysis revealed that seedlings displayed a more ordered arrangement of chloroplast ultrastructure upon Spd application during DS. Exogenous Spd increased the endogenous PA concentrations in the stressed plants. Additionally, exogenous Spd increased indoleacetic acid (IAA), zeatin riboside (ZR) and gibberellin A₃ (GA₃) and decreased salicylic acid (SA) and jasmonate (JA) concentrations under DS. These results indicate that exogenous Spd can alleviate the growth inhibition and damage to the structure and function of the photosynthetic apparatus caused by DS and that this alleviation may be associated with changes in endogenous PAs and phytohormones. This study contributes to advances in the knowledge of Spd-induced drought tolerance.

Piriformospora indica inoculation alleviates the adverse effect of NaCl stress on growth, gas exchange and chlorophyll fluorescence in tomato (Solanum lycopersicum L.)

Salinity is now an increasingly serious environmental issue that affects the growth and yield of many plants. In the present work, the influence of inoculation with the symbiotic fungus, Piriformospora indica, on gas exchange, water potential, osmolyte content, Na/K ratio and chlorophyll fluorescence of tomato plants under three salinity levels (0, 50, 100 and 150 mm NaCl) and three time periods (5, 10 and 15 days after exposure to salt) was investigated. Results indicate that P. indica inoculation improved growth parameters of tomato under salinity stress. This symbiotic fungus significantly increased photosynthetic pigment content under salinity, and more proline and glycine betaine accumulated in inoculated roots than in non-inoculated roots. P. indica further significantly improved K⁺ content and reduced Na⁺ level under salinity treatment. After inoculation with the endophytic fungus, leaf physiological parameters, such as water potential, net photosynthesis, stomatal conductance and transpiration, were all higher under the salt concentrations and durations compared with controls without P. indica. With increasing salt level and salt treatment duration, values of F₀ and qP increased but F_m , F_v /F_m , F'_v /F'_m and NPQ declined in the controls, while inoculation with P. indica improved these values. The results indicate that the negative effects of NaCl on tomato plants were alleviated after P. indica inoculation, probably by improving physiological parameters such as water status and photosynthesis.

Spermidine-mediated hydrogen peroxide signaling enhances the antioxidant capacity of salt-stressed cucumber roots

Hydrogen peroxide (H₂O₂) is a key signaling molecule that mediates a variety of physiological processes and defense responses against abiotic stress in higher plants. In this study, our aims are to clarify the role of H₂O₂ accumulation induced by the exogenous application of spermidine (Spd) to cucumber (Cucumis sativus) seedlings in regulating the antioxidant capacity of roots under salt stress. The results showed that Spd caused a significant increase in endogenous polyamines and H₂O₂ levels, and peaked at 2 h after salt stress. Spd-induced H₂O₂ accumulation was blocked under salt stress by pretreatment with a H₂O₂ scavenger and respective inhibitors of cell wall peroxidase (CWPOD; EC: 1.11.1.7), polyamine oxidase (PAO; EC: 1.5.3.11) and NADPH oxidase (NOX; EC: 1.6.3.1); among these three inhibitors, the largest decrease was found in response to the addition of the inhibitor of polyamine oxidase. In addition, we observed that exogenous Spd could increase the activities of the enzymes superoxide dismutase (SOD; EC: 1.15.1.1), peroxidase (POD; EC: 1.11.1.7) and catalase (CAT; EC: 1.11.1.6) as well as the expression of their genes in salt-stressed roots, and the effects were inhibited by H₂O₂ scavengers and polyamine oxidase inhibitors. These results suggested that, by regulating endogenous PAs-mediated H₂O₂ signaling in roots, Spd could enhance antioxidant enzyme activities and reduce oxidative damage; the main source of H₂O₂ was polyamine oxidation, which was associated with improved tolerance and root growth recovery of cucumber under salt stress.

Manipulation of glyoxalase pathway confers tolerance to multiple stresses in rice

Crop plants face a multitude of diverse abiotic and biotic stresses in the farmers' fields. Although there now exists a considerable knowledge of the underlying mechanisms of response to individual stresses, the crosstalk between response pathways to various abiotic and biotic stresses remains enigmatic. Here, we investigated if the cytotoxic metabolite methylglyoxal (MG), excess of which is generated as a common consequence of many abiotic and biotic stresses, may serve as a key molecule linking responses to diverse stresses. For this, we generated transgenic rice plants overexpressing the entire two-step glyoxalase pathway for MG detoxification. Through assessment of various morphological, physiological and agronomic parameters, we found that glyoxalase-overexpression imparts tolerance towards abiotic stresses like salinity, drought and heat and also provides resistance towards damage caused by the sheath blight fungus (Rhizoctonia solani) toxin phenylacetic acid. We show that the mechanism of observed tolerance of the glyoxalase-overexpressing plants towards these diverse abiotic and biotic stresses involves improved MG detoxification and reduced oxidative damage leading to better protection of chloroplast and mitochondrial ultrastructure and maintained photosynthetic efficiency under stress conditions. Together, our findings indicate that MG may serve as a key link between abiotic and biotic stress response in plants.

A Novel G-Protein-Coupled Receptors Gene from Upland Cotton Enhances Salt Stress Tolerance in Transgenic Arabidopsis

Plants have developed a number of survival strategies which are significant for enhancing their adaptation to various biotic and abiotic stress factors. At the transcriptome level, G-protein-coupled receptors (GPCRs) are of great significance, enabling the plants to detect a wide range of endogenous and exogenous signals which are employed by the plants in regulating various responses in development and adaptation. In this research work, we carried out genome-wide analysis of target of Myb1 (TOM1), a member of the GPCR gene family. The functional role of TOM1 in salt stress tolerance was studied using a transgenic Arabidopsis plants over-expressing the gene. By the use of the functional domain PF06454, we obtained 16 TOM genes members in Gossypium hirsutum, 9 in Gossypium arboreum, and 11 in Gossypium raimondii. The genes had varying physiochemical properties, and it is significant to note that all the grand average of hydropathy (GRAVY) values were less than one, indicating that all are hydrophobic in nature. In all the genes analysed here, both the exonic and intronic regions were found. The expression level of Gh_A07G0747 (GhTOM) was significantly high in the transgenic lines as compared to the wild type; a similar trend in expression was observed in all the salt-related genes tested in this study. The study in epidermal cells confirmed the localization of the protein coded by the gene TOM1 in the plasma membrane. Analysis of anti-oxidant enzymes showed higher concentrations of antioxidants in transgenic lines and relatively lower levels of oxidant substances such as H₂O₂. The low malondialdehyde (MDA) level in transgenic lines indicated that the transgenic lines had relatively low level of oxidative damage compared to the wild types. The results obtained indicate that Gh_A07G0747 (GhTOM) can be a putative target gene for enhancing salt stress tolerance in plants and could be exploited in the future for the development of salt stress-tolerant cotton cultivars.

Modulation of cadmium-induced phytotoxicity in Cabomba caroliniana by urea involves photosynthetic metabolism and antioxidant status

Urea is a widespread organic pollutant, which can be a nitrogen source, playing different roles in the growth of submerged macrophytes depending on concentrations, while high cadmium (Cd) concentrations are often toxic to macrophytes. In order to evaluate the combined effect of urea and Cd on a submerged macrophyte, Cabomba caroliniana, the morphological and physiological responses of C. caroliniana in the presence of urea and Cd were studied. The results showed that high concentrations of urea (400mgL^-1) and Cd (500µmolL^-1) had negative effects on C. caroliniana. There were strong visible symptoms of toxicity after 4 days of exposure under Cd-alone, 400mgL^-1 urea, and Cd+400mgL^-1 urea treatments. In addition, 400mgL^-1 urea and Cd had adverse effects on C. caroliniana's pigment system. Significant losses in chlorophyll fluorescence and photosynthetic rates, as well as Rubisco activity were also observed under Cd-alone, 400mgL^-1 urea, and Cd+400mgL^-1 urea treatments. 400mgL^-1 urea markedly enhanced Cd toxicity in C. caroliniana, reflected by a sharp decrease in photosynthetic activity and more visible toxicity symptoms. The results of thiobarbituric acid reactive substances (TBARS) pointed to extreme oxidative stress in C. caroliniana induced under Cd or 400mgL^-1 urea exposure. Exogenous ascorbate (AsA) protected C. caroliniana from adverse damage in 400mgL^-1 urea, which further corroborated the oxidative stress claim under 400mgL^-1 urea. However, results also demonstrated that lower urea concentration (10mgL^-1) alleviated Cd-induced phytotoxicity by stimulating chlorophyll synthesis and photosynthetic activity, as well as activating the activity of catalase (CAT) and glutathione-S-transferase (GST), which may explain the alleviating effect of urea on C. caroliniana under Cd stress.

Biochemical, physiological and molecular evaluation of rice cultivars differing in salt tolerance at the seedling stage

Changes in the antioxidant enzymes, lipid peroxidation, sodium and potassium, chlorophyll, H2O2 and proline content were monitored in the leaves of 42 rice varieties which were not yet well-documented for the salinity tolerance under different salinity levels. The tolerant varieties (FL478, Hassani, Shahpasand, Gharib and Nemat) showed signs of tolerance (lower Na+/K+ ratio, high proline accumulation, less membrane damage, lower H2O2 production, and higher superoxide dismutase and catalase activity) very well. The positive relationship between the level of salt tolerance and the amount of proline accumulation in the rice varieties support the important role of proline under the salt stress. The varieties were genotyped for 12 microsatellite markers that were closely linked to SalTol QTL. The results of association analysis indicated that RM1287, RM8094, RM3412 and AP3206 markers had the high value of R2 for the regression models of the studied traits. It shows the important role of SalTol in controlling physio-biochemical traits. The results can be used in the future marker assisted selection (MAS) directly, if the results are confirmed.

Thiourea priming enhances salt tolerance through co-ordinated regulation of microRNAs and hormones in Brassica juncea

Activation of stress tolerance mechanisms demands transcriptional reprogramming. Salt stress, a major threat to plant growth, enhances ROS production and affects transcription through modulation of miRNAs and hormones. The present study delineates salt stress ameliorating action of thiourea (TU, a ROS scavenger) in Brassica juncea and provides mechanistic link between redox, microRNA and hormones. The ameliorative potential of TU towards NaCl stress was related with its ability to decrease ROS accumulation in roots and increase Na+ accumulation in shoots. Small RNA sequencing revealed enrichment of down-regulated miRNAs in NaCl + TU treated roots, indicating transcriptional activation. Ranking analysis identified three key genes including BRX4, CBL10 and PHO1, showing inverse relationship with corresponding miRNA expression, which were responsible for TU mediated stress mitigation. Additionally, ABA level was consistently higher till 24 h in NaCl, while NaCl + TU treated roots showed only transient increase at 4 h suggesting an effective stress management. Jasmonate and auxin levels were also increased, which prioritized defence and facilitated root growth, respectively. Thus, the study highlights redox as one of the "core" components regulating miRNA and hormone levels, and also strengthens the use of TU as a redox priming agent for imparting crop resilience to salt stress.

Reactive oxygen species generating system and brassinosteroids are linked to salt stress adaptation mechanisms in rice

Salinity stress is one of the major environmental challenges which adversely affects plant growth and productivity. The acquisition of salinity stress tolerance has been an interesting area of investigation for plant abiotic stress management. Recently, we investigated the interdependency of reactive oxygen species (ROS) generating and scavenging system for offering salt stress adaptation in rice. In continuation to our earlier findings, in the present study we analyzed the transcript level expression of different respiratory burst oxidase homologs (Rbohs) genes in salt sensitive and salt tolerant cultivars of rice to corroborate this result with their activities. Brassinosteroid (BR) is known to confer abiotic stress tolerance by modulating ROS machinery, and hence in the present study, the expression of key genes associated in brassinosteroid biosynthesis and signaling in salt sensitive and tolerant cultivar of rice was also conducted. In the present investigation, the other stress markers involving proline catabolism and anabolism along with chlorophyllase has been analyzed to get a better insights to our understanding of salt stress adaptation mechanisms in rice.

Exogenous γ-Aminobutyric Acid Improves the Structure and Function of Photosystem II in Muskmelon Seedlings Exposed to Salinity-Alkalinity Stress

Gamma-aminobutyric acid (GABA) is important in plant responses to environmental stresses. We wished to clarify the role of GABA in maintenance of photosynthesis in muskmelon seedlings (Cucumis melo L., cv. Yipintianxia) during saline-alkaline stress. To this end, we assessed the effect of GABA on the structure and function of the photosynthetic apparatus in muskmelon seedlings grown under saline-alkaline stress. These stresses in combination reduced net photosynthetic rate, gas-exchange, and inhibited photosystem II (PSII) electron transport as measured by the JIP-test. They also reduced the activity of chloroplast ATPases and disrupted the internal lamellar system of the thylakoids. Exogenous GABA alleviated the stress-induced reduction of net photosynthesis, the activity of chloroplast ATPases, and overcame some of the damaging effects of stress on the chloroplast structure. Based on interpretation of the JIP-test, we conclude that exogenous GABA alleviated stress-related damage on the acceptor side of PSII. It also restored energy distribution, the reaction center status, and enhanced the ability of PSII to repair reaction centers in stressed seedlings. GABA may play a crucial role in protecting the chloroplast structure and function of PSII against the deleterious effects of salinity-alkalinity stress.

Is foliar spray of proline sufficient for mitigation of salt stress in Brassica juncea cultivars?

The effects of foliar application of proline (20 mM) on growth, physio-biochemical, and yield parameters were assessed in two Brassica juncea (L.) Czern & Coss cultivars, namely, Varuna and RH-30, at different levels (2.8, 4.2, or 5.6 dsm(-1)) of NaCl in soil. At 29 days after sowing (DAS), plants were sprayed with either 20 mM proline or water in the presence or absence of NaCl stress. The NaCl negatively affected parameters related to growth, photosynthesis, and yield in both varieties but more in RH-30 than in Varuna. Exogenous application of proline counteracted the effects of salt stress in Varuna only, by increasing the antioxidative capacity of the plants. Moreover, proline was not effective in alleviating the detrimental effects of higher salt concentrations on the studied parameters. Proline application to unstressed plants increased growth, photosynthesis, and yield parameters in both varieties; however, the effects were more prominent in Varuna than in RH-30.

Beneficial role of spermidine in chlorophyll metabolism and D1 protein content in tomato seedlings under salinity-alkalinity stress

Polyamines are important in protecting plants against various environmental stresses, including protection against photodamage to the photosynthetic apparatus. The molecular mechanism of this latter effect is not completely understood. Here, we have investigated the effects of salinity-alkalinity stress and spermidine (Spd) on tomato seedlings at both physiological and transcriptional levels. Salinity-alkalinity stress decreased leaf area, net photosynthetic rate, maximum net photosynthetic rate, light saturation point, apparent quantum efficiency, total chlorophyll, chlorophyll a and chlorophyll a:chlorophyll b relative to the control. The amount of D1 protein, an important component of photosystem II, was reduced compared with the control, as was the expression of psbA, which codes for D1. Expression of the chlorophyll biosynthesis gene porphobilinogen deaminase (PBGD) was reduced following salinity-alkalinity stress, whereas the expression of Chlase, which codes for chlorophyllase, was increased. These negative physiological effects of salinity-alkalinity stress were alleviated by exogenous Spd. Expression of PBGD and psbA were enhanced, whereas the expression of Chlase was reduced, when exogenous Spd was included in the stress treatment compared with when it was not. The protective effect of Spd on chlorophyll and D1 protein content during stress may maintain the photosynthetic apparatus, permitting continued photosynthesis and growth of tomato seedlings (Solanum lycopersicum cv. Jinpengchaoguan) under salinity-alkalinity stress.

Effects of salinity on the photosynthetic apparatus of two Paulownia lines

The effects of soil salinity on the functional activity of photosynthetic apparatus and pigment composition of two Paulownia lines (Paulownia tomentosa x fortunei and Paulownia elongata x elongata) were investigated. PAM chlorophyll fluorescence measurements revealed that salinity leads to: (i) an increase of the photochemical quenching coefficient (qP) and the linear electron transport rate (ETR) in both lines of Paulownia, while the maximum quantum yield of the primary photochemistry of PSII in the dark adapted state (Fv/Fm) was unaffected; (ii) improved the efficiency of the photochemical energy conversion (ФPSII); (iii) an impact on the chlorophyll fluorescence decrease ratio (RFd), which correlates to the net CO2 assimilation rate; (iv) an impact on [Formula: see text] reoxidation. The analysis of the kinetics of P700(+) reduction upon turning off far-red irradiation revealed that salinization lead to a delay of the cyclic electron transport around PSI in both studied lines as the effect on this process is more pronounced in P. tomentosa x fortunei than in (in comparison with) P. elongata x elongata. The present experimental results suggested high salt tolerance of the studied lines Paulownia, but P. tomentosa x fortunei is more tolerant to salinity than P. elongata x elongata. Molecular mechanisms involved in the Paulownia response to the soil salinity are discussed.

Mitigative effects of spermidine on photosynthesis and carbon-nitrogen balance of cucumber seedlings under Ca(NO3)2 stress

Ca(NO3)2 stress is one of the most serious constraints to plants production and limits the plants growth and development. Application of polyamines is a convenient and effective approach for enhancing plant salinity tolerance. The present investigation aimed to discover the photosynthetic carbon-nitrogen (C-N) mechanism underlying Ca(NO3)2 stress tolerance by spermidine (Spd) of cucumber (Cucumis sativus L. cv. Jinyou No. 4). Seedling growth and photosynthetic capacity [including net photosynthetic rate (P N), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr)] were significantly inhibited by Ca(NO3)2 stress (80 mM). However, a leaf-applied Spd (1 mM) treatment alleviated the reduction in growth and photosynthesis in cucumber caused by Ca(NO3)2 stress. Furthermore, the application of exogenous Spd significantly decreased the accumulation of NO3 (-) and NH4 (+) caused by Ca(NO3)2 stress and remarkably increased the activities of N metabolism enzymes simultaneously. In addition, photosynthesis N-use efficiency (PNUE) and free amino acids were significantly enhanced by exogenous Spd in response to Ca(NO3)2 stress, thus promoting the biosynthesis of N containing compounds and soluble protein. Also, the amounts of several carbohydrates (including sucrose, fructose and glucose), total C content and the C/N radio increased significantly in the presence of Spd. Based on our results, we suggest that exogenous Spd could effectively accelerate nitrate transformation into amino acids and improve cucumber plant photosynthesis and C assimilation, thereby enhancing the ability of the plants to maintain their C/N balance, and eventually promote the growth of cucumber plants under Ca(NO3)2 stress.

Exogenous Spermidine Alleviates Low Temperature Injury in Mung Bean (Vigna radiata L.) Seedlings by Modulating Ascorbate-Glutathione and Glyoxalase Pathway

The role of exogenous spermidine (Spd) in alleviating low temperature (LT) stress in mung bean (Vigna radiata L. cv. BARI Mung-3) seedlings has been investigated. Low temperature stress modulated the non-enzymatic and enzymatic components of ascorbate-glutathione (AsA-GSH) cycle, increased H₂O₂ content and lipid peroxidation, which indicate oxidative damage of seedlings. Low temperature reduced the leaf relative water content (RWC) and destroyed leaf chlorophyll, which inhibited seedlings growth. Exogenous pretreatment of Spd in LT-affected seedlings significantly increased the contents of non-enzymatic antioxidants of AsA-GSH cycle, which include AsA and GSH. Exogenous Spd decreased dehydroascorbate (DHA), increased AsA/DHA ratio, decreased glutathione disulfide (GSSG) and increased GSH/GSSG ratio under LT stress. Activities of AsA-GSH cycle enzymes such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) increased after Spd pretreatment in LT affected seedlings. Thus, the oxidative stress was reduced. Protective effects of Spd are also reflected from reduction of methylglyoxal (MG) toxicity by improving glyoxalase cycle components, and by maintaining osmoregulation, water status and improved seedlings growth. The present study reveals the vital roles of AsA-GSH and glyoxalase cycle in alleviating LT injury.

Overexpression of SpWRKY1 promotes resistance to Phytophthora nicotianae and tolerance to salt and drought stress in transgenic tobacco

WRKY transcription factors are key regulatory components of plant responses to biotic and abiotic stresses. SpWRKY1, a pathogen-induced WRKY gene, was isolated from tomato (Solanum pimpinellifolium L3708) using in silico cloning and reverse transcriptase-polymerase chain reaction (RT-PCR) methods. SpWRKY1 expression was significantly induced following oomycete pathogen infection and treatment with salt, drought, salicylic acid (SA), methyl jasmonate (MeJA) and abscisic acid (ABA). Overexpression of SpWRKY1 in tobacco conferred greater resistance to Phytophthora nicotianae infection, as evidenced by lower malondialdehyde (MDA) content; relative electrolyte leakage (REL); higher chlorophyll content; and higher peroxidase (POD, EC 1.11.1.7), superoxide dismutase (SOD, EC 1.15.1.1) and phenylalanine ammonia-lyase (PAL, EC 4.3.1.24) activities. This resistance was also coupled with enhanced expression of SA- and JA-associated genes (NtPR1, NtPR2, NtPR4, NtPR5 and NtPDF1.2), as well as of various defense-related genes (NtPOD, NtSOD and NtPAL). In addition, transgenic tobacco plants also displayed an enhanced tolerance to salt and drought stresses, mainly demonstrated by the transgenic lines exhibiting lower accumulation of MDA content and higher POD (EC 1.11.1.7), SOD (EC 1.15.1.1) activities, chlorophyll content, photosynthetic rate and stomatal conductance, accompanied by enhanced expression of defense-related genes (NtPOD, NtSOD, NtLEA5, NtP5CS and NtNCED1) under salt and drought stresses. Overall, these findings suggest that SpWRKY1 acts as a positive regulator involved in tobacco defense responses to biotic and abiotic stresses.

The role of putrescine in the regulation of proteins and fatty acids of thylakoid membranes under salt stress

Polyamines can alleviate the inhibitory effects of salinity on plant growth by regulating photosynthetic efficiency. However, little information is available to explain the specific mechanisms underlying the contribution of polyamines to salt tolerance of the photosynthetic apparatus. Here, we investigated the role of putrescine (Put) on the photosynthetic apparatus of cucumber seedlings under salt stress. We found that NaCl stress resulted in severe ion toxicity and oxidative stress in cucumber chloroplasts. In addition, salinity caused a significant increase in the saturated fatty acid contents of thylakoid membranes. Put altered unsaturated fatty acid content, thereby alleviating the disintegration of thylakoid grana lamellae and reducing the number of plastoglobuli in thylakoid membranes. BN-PAGE revealed Put up-regulated the expression of ATP synthase, CP47, D1, Qb, and psbA proteins and down-regulated CP24, D2, and LHCII type III in NaCl-stressed thylakoid membranes. qRT-PCR analysis of gene expression was used to compare transcript and protein accumulation among 10 candidate proteins. For five of these proteins, induced transcript accumulation was consistent with the pattern of induced protein accumulation. Our results suggest that Put regulates protein expression at transcriptional and translational levels by increasing endogenous polyamines levels in thylakoid membranes, which may stabilise photosynthetic apparatus under salt stress.

Bottle gourd rootstock-grafting promotes photosynthesis by regulating the stomata and non-stomata performances in leaves of watermelon seedlings under NaCl stress

Previously, we found that the amelioration of photosynthetic capacity by bottle gourd (Lagenaria siceraria Standl.) rootstock in watermelon seedlings (Citrullus lanatus [Thunb.] Mansf.) with salt treatment might be closely related to the enzymes in Calvin cycle such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (Yang et al., 2012). We confirmed this and showed more details in this study that improved photosynthesis of watermelon plants by bottle gourd rootstock was associated with the decreased stomata resistance and the increased photochemical activity and photosynthetic metabolism with or without 100mM NaCl stress for 3 days. The analysis of gas exchange parameters showed that self-grafted plants suffered serious non-stomatal limitation to photosynthesis under salt stress while rootstock-grafted plants were mainly affected by stomata limitation in stress conditions. Further, results showed that NaCl stress markedly reduced the chlorophyll content, damaged the structure of photosynthetic apparatus, and inhibited photochemical activity and CO2 assimilation in self-grafted plants. In contrast, rootstock-grafting increased the chlorophyll content, especially chlorophyll b, and minimized the harmful effects on photosystem II (PSII) reaction center and the thylakoids structure induced by NaCl stress. Furthermore, rootstock-grafting enhanced the content and activity of Rubisco and thus elevated carbon fixation in the leaves of watermelon scions under salt stress. The gene expressions of enzymes related to ribulose-1,5-bisphosphate (RuBP) regeneration were also up-regulated by rootstock and this probably guaranteed the sufficient supply of RuBP for the operation of Calvin cycle in watermelon scions under salt stress. Thus, bottle gourd rootstock promoted photosynthesis by the activation of stomatal and non-stomatal abilities, especially the regulation of a variety of photosynthetic enzymes, including Rubisco in grafted watermelon plants under NaCl stress.

Disentangling the contributions of osmotic and ionic effects of salinity on stomatal, mesophyll, biochemical and light limitations to photosynthesis

There are conflicting opinions on the relative importance of photosynthetic limitations under salinity. Quantitative limitation analysis of photosynthesis provides insight into the contributions of different photosynthetic limitations, but it has only been applied under saturating light conditions. Using experimental data and modelling approaches, we examined the influence of light intensity on photosynthetic limitations and quantified the osmotic and ionic effects of salinity on stomatal (LS ), mesophyll (LM ), biochemical (LB ) and light (LL ) limitations in cucumber (Cucumis sativus L.) under different light intensities. Non-linear dependencies of LS , LM and LL to light intensity were found. Osmotic effects on LS and LM increased with the salt concentration in the nutrient solution (Ss ) and the magnitude of LM depended on light intensity. LS increased with the Na(+) concentration in the leaf water (Sl ) and its magnitude depended on Ss . Biochemical capacity declined linearly with Sl but, surprisingly, the relationship between LB and Sl was influenced by Ss . Our results suggest that (1) improvement of stomatal regulation under ionic stress would be the most effective way to alleviate salinity stress in cucumber and (2) osmotic stress may alleviate the ionic effects on LB but aggravate the ionic effects on LS .

Molecular and physiological changes in response to salt stress in Citrus macrophylla W plants overexpressing Arabidopsis CBF3/DREB1A

Plant stress induced by high salinity has leading to an important reduction in crop yields. Due to their tropical origin, citrus fruits are highly sensitive to salts. Rootstocks are the root system of fruit trees, regulating ion uptake and transport to the canopy. Therefore, increasing their salt tolerance could improve the salt tolerance of the fruit tree. For this, we genetically-transformed an important rootstock for lemon, Citrus macrophylla W, to constitutively express the CBF3/DREB1A gene from Arabidopsis, a well-studied salinity tolerance transcription factor. Transgenic lines showed normal size, with no dwarfism. Under salt stress, some transgenic lines showed greater growth, similar accumulation of chloride and sodium in the leaves and better stomatal conductance, in comparison to wild-type plants. Quantitative real-time analyses showed a similar expression of several CBF3/DREB1A target genes, such as COR15A, LEA 4/5, INV, SIP1, P5CS, GOLS, ADC2 and LKR/SDH, in transgenic lines and wild type plants, with the exception of INV that shows increased expression in line 4C15. Under salt stress, all measured transcript increased in both wild type and transgenics lines, with the exception of INV. Altogether, these results suggest a higher salt tolerance of transgenic C. macrophylla plants induced by the overexpression of AtCBF3/DREB1A.

Lesion simulating disease 1 and enhanced disease susceptibility 1 differentially regulate UV-C-induced photooxidative stress signalling and programmed cell death in Arabidopsis thaliana

As obligate photoautotrophs, plants are inevitably exposed to ultraviolet (UV) radiation. Because of stratospheric ozone depletion, UV has become more and more dangerous to the biosphere. Therefore, it is important to understand UV perception and signal transduction in plants. In the present study, we show that lesion simulating disease 1 (LSD1) and enhanced disease susceptibility 1 (EDS1) are antagonistic regulators of UV-C-induced programmed cell death (PCD) in Arabidopsis thaliana. This regulatory dependence is manifested by a complex deregulation of photosynthesis, reactive oxygen species homeostasis, antioxidative enzyme activity and UV-responsive genes expression. We also prove that a UV-C radiation episode triggers apoptotic-like morphological changes within the mesophyll cells. Interestingly, chloroplasts are the first organelles that show features of UV-C-induced damage, which may indicate their primary role in PCD development. Moreover, we show that Arabidopsis Bax inhibitor 1 (AtBI1), which has been described as a negative regulator of plant PCD, is involved in LSD1-dependent cell death in response to UV-C. Our results imply that LSD1 and EDS1 regulate processes extinguishing excessive energy, reactive oxygen species formation and subsequent PCD in response to different stresses related to impaired electron transport.

Scanning ion-selective electrode technique and X-ray microanalysis provide direct evidence of contrasting Na+ transport ability from root to shoot in salt-sensitive cucumber and salt-tolerant pumpkin under NaCl stress

Grafting onto salt-tolerant pumpkin rootstock can increase cucumber salt tolerance. Previous studies have suggested that this can be attributed to pumpkin roots with higher capacity to limit the transport of Na(+) to the shoot than cucumber roots. However, the mechanism remains unclear. This study investigated the transport of Na(+) in salt-tolerant pumpkin and salt-sensitive cucumber plants under high (200 mM) or moderate (90 mM) NaCl stress. Scanning ion-selective electrode technique showed that pumpkin roots exhibited a higher capacity to extrude Na(+), and a correspondingly increased H(+) influx under 200 or 90 mM NaCl stress. The 200 mM NaCl induced Na(+)/H(+) exchange in the root was inhibited by amiloride (a Na(+)/H(+) antiporter inhibitor) or vanadate [a plasma membrane (PM) H(+) -ATPase inhibitor], indicating that Na(+) exclusion in salt stressed pumpkin and cucumber roots was the result of an active Na(+)/H(+) antiporter across the PM, and the Na(+)/H(+) antiporter system in salt stressed pumpkin roots was sufficient to exclude Na(+) X-ray microanalysis showed higher Na(+) in the cortex, but lower Na(+) in the stele of pumpkin roots than that in cucumber roots under 90 mM NaCl stress, suggesting that the highly vacuolated root cortical cells of pumpkin roots could sequester more Na(+), limit the radial transport of Na(+) to the stele and thus restrict the transport of Na(+) to the shoot. These results provide direct evidence for pumpkin roots with higher capacity to limit the transport of Na(+) to the shoot than cucumber roots.

Effects of exogenous spermidine on photosynthetic capacity and expression of Calvin cycle genes in salt-stressed cucumber seedlings

We investigated the effects of exogenous spermidine (Spd) on growth, photosynthesis and expression of the Calvin cycle-related genes in cucumber seedlings (Cucumis sativus L.) exposed to NaCl stress. Salt stress reduced net photosynthetic rates (PN), actual photochemical efficiency of PSII (ΦPSII) and inhibited plant growth. Application of exogenous Spd to salinized nutrient solution alleviated salinity-induced the inhibition of plant growth, together with an increase in PN and ΦPSII. Salinity markedly reduced the maximum carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Vcmax), the maximal velocity of RuBP regeneration (Jmax), triose-phosphate utilization capacity (TPU) and carboxylation efficiency (CE). Spd alleviated the negative effects on CO2 assimilation induced by salt stress. Moreover, Spd significantly increased the activities and contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose-1,6-biphosphate aldolase (ALD; aldolase) in the salt-stressed cucumber leaves. On the other hand, salinity up-regulated the transcriptional levels of ribulose-1,5-bisphosphate (RCA), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribrokinase (PRK) and down-regulated the transcriptional levels of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RbcL), ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (RbcS), ALD, triose-3-phosphate isomerase (TPI), fructose-1,6-bisphosphate phosphatase (FBPase) and 3-phosphoglyceric acid kinase (PGK). However, Spd application to salt-stressed plant roots counteracted salinity-induced mRNA expression changes in most of the above-mentioned genes. These results suggest that Spd could improve photosynthetic capacity through regulating gene expression and activity of key enzymes for CO2 fixation, thus confers tolerance to salinity on cucumber plants.

The photoprotective role of spermidine in tomato seedlings under salinity-alkalinity stress

Polyamines are small, ubiquitous, nitrogenous compounds that scavenge reactive oxygen species and stabilize the structure and function of the photosynthetic apparatus in response to abiotic stresses. Molecular details underlying polyamine-mediated photoprotective mechanisms are not completely resolved. This study investigated the role of spermidine (Spd) in the structure and function of the photosynthetic apparatus. Tomato seedlings were subjected to salinity-alkalinity stress with and without foliar application of Spd, and photosynthetic and morphological parameters were analyzed. Leaf dry weight and net photosynthetic rate were reduced by salinity-alkalinity stress. Salinity-alkalinity stress reduced photochemical quenching parameters, including maximum photochemistry efficiency of photosystem II, quantum yield of linear electron flux, and coefficient of photochemical quenching (qP). Salinity-alkalinity stress elevated nonphotochemical quenching parameters, including the de-epoxidation state of the xanthophyll cycle and nonphotochemical quenching (NPQ). Microscopic analysis revealed that salinity-alkalinity stress disrupted the internal lamellar system of granal and stromal thylakoids. Exogenous Spd alleviated the stress-induced reduction of leaf dry weight, net photosynthetic rate, and qP parameters. The NPQ parameters increased by salinity-alkalinity stress were also alleviated by Spd. Seedlings treated with exogenous Spd had higher zeaxanthin (Z) contents than those without Spd under salinity-alkalinity stress. The chloroplast ultrastructure had a more ordered arrangement in seedlings treated with exogenous Spd than in those without Spd under salinity-alkalinity stress. These results indicate that exogenous Spd can alleviate the growth inhibition and thylakoid membrane photodamage caused by salinity-alkalinity stress. The Spd-induced accumulation of Z also may have an important role in stabilizing the photosynthetic apparatus.

Drought-inhibited ribulose-1,5-bisphosphate carboxylase activity is mediated through increased release of ethylene and changes in the ratio of polyamines in pakchoi

To study the mechanisms of drought inhibiting photosynthesis and the role of PAs and ethylene, the photosynthetic rate (Pn), the maximal photochemical efficiency of PSII (Fv/Fm), the intercellular CO2 concentration (Ci), photorespiratory rate (Pr), the amount of chlorophyll (chl), antioxidant enzyme activity, ethylene levels, RuBPC (ribulose-1,5-bisphosphate carboxylase) activity and endogenous polyamine levels of pakchoi were examined, and an inhibitor of S-adenosylmethionine decarboxylase (SAMDC) and an inhibitor of ethylene synthesis and spermidine (Spd) were used to induce the change of endogenous polyamine levels. The results show that drought induced a decrease in Pn and RuBPC activity, an increase in the intercellular CO2 concentration (Ci), but no change in the actual photochemical efficiency of PSII (ΦPSII), and chlorophyll content. In addition, drought caused an increase in the free putrescine (fPut), the ethylene levels, a decrease in the Spd and spermine (Spm) levels, and the PAs/fPut ratio in the leaves. The exogenous application of Spd and amino oxiacetic acid (AOAA, an inhibitor of ethylene synthesis) markedly reversed these drought-induced effects on polyamine, ethylene, Pn, the PAs/fPut ratio and RuBPCase activity in leaves. Methylglyoxal-bis(guanylhydrazone) (MGBG), an inhibitor of SAMDC resulting in the inability of activated cells to synthesize Spd and Spm, exacerbates the negative effects induced by drought. These results suggest that the decrease in Pn is at least partially attributed to the decrease of RuBPC activity under drought stress and that drought inhibits RuBPC activity by decreasing the ratio of PAs/fPut and increasing the release of ethylene.

Maintenance of Chloroplast Structure and Function by Overexpression of the Rice MONOGALACTOSYLDIACYLGLYCEROL SYNTHASE Gene Leads to Enhanced Salt Tolerance in Tobacco

In plants, the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactodiacylglycerol (DGDG) are major constituents of photosynthetic membranes in chloroplasts. One of the key enzymes for the biosynthesis of these galactolipids is MGDG synthase (MGD). To investigate the role of MGD in the plant's response to salt stress, we cloned an MGD gene from rice (Oryza sativa) and generated tobacco (Nicotiana tabacum) plants overexpressing OsMGD. The MGD activity in OsMGD transgenic plants was confirmed to be higher than that in the wild-type tobacco cultivar SR1. Immunoblot analysis indicated that OsMGD was enriched in the outer envelope membrane of the tobacco chloroplast. Under salt stress, the transgenic plants exhibited rapid shoot growth and high photosynthetic rate as compared with the wild type. Transmission electron microscopy observation showed that the chloroplasts from salt-stressed transgenic plants had well-developed thylakoid membranes and properly stacked grana lamellae, whereas the chloroplasts from salt-stressed wild-type plants were fairly disorganized and had large membrane-free areas. Under salt stress, the transgenic plants also maintained higher chlorophyll levels. Lipid composition analysis showed that leaves of transgenic plants consistently contained significantly higher MGDG (including 18:3-16:3 and 18:3-18:3 species) and DGDG (including 18:3-16:3, 18:3-16:0, and 18:3-18:3 species) contents and higher DGDG-MGDG ratios than the wild type did under both control and salt stress conditions. These results show that overexpression of OsMGD improves salt tolerance in tobacco and that the galactolipids MGDG and DGDG play an important role in the regulation of chloroplast structure and function in the plant salt stress response.

Sodic alkaline stress mitigation by interaction of nitric oxide and polyamines involves antioxidants and physiological strategies in Solanum lycopersicum

Nitric oxide (NO) and polyamines (PAs) are two kinds of important signal in mediating plant tolerance to abiotic stress. In this study, we observed that both NO and PAs decreased alkaline stress in tomato plants, which may be a result of their role in regulating nutrient balance and reactive oxygen species (ROS), thereby protecting the photosynthetic system from damage. Further investigation indicated that NO and PAs induced accumulation of each other. Furthermore, the function of PAs could be removed by a NO scavenger, cPTIO. On the other hand, application of MGBG, a PA synthesis inhibitor, did little to abolish the function of NO. To further elucidate the mechanism by which NO and PAs alleviate alkaline stress, the expression of several genes associated with abiotic stress was analyzed by qRT-PCR. NO and PAs significantly upregulated ion transporters such as the plasma membrane Na(+)/H(+) antiporter (SlSOS1), vacuolar Na(+)/H(+) exchanger (SlNHX1 and SlNHX2), and Na(+) transporter and signal components including ROS, MAPK, and Ca(2+) signal pathways, as well as several transcription factors. All of these play important roles in plant adaptation to stress conditions.

Polyamines in response to abiotic stress tolerance through transgenic approaches

The distribution, growth, development and productivity of crop plants are greatly affected by various abiotic stresses. Worldwide, sustainable crop productivity is facing major challenges caused by abiotic stresses by reducing the potential yield in crop plants by as much as 70%. Plants can generally adapt to one or more environmental stresses to some extent. Physiological and molecular studies at transcriptional, translational, and transgenic plant levels have shown the pronounced involvement of naturally occurring plant polyamines (PAs), in controlling, conferring, and modulating abiotic stress tolerance in plants. PAs are small, low molecular weight, non-protein polycations at physiological pH, that are present in all living organisms, and that have strong binding capacity to negatively charged DNA, RNA, and different protein molecules. They play an important role in plant growth and development by controlling the cell cycle, acting as cell signaling molecules in modulating plant tolerance to a variety of abiotic stresses. The commonly known PAs, putrescine, spermidine, and spermine tend to accumulate together accompanied by an increase in the activities of their biosynthetic enzymes under a range of environmental stresses. PAs help plants to combat stresses either directly or by mediating a signal transduction pathway, as shown by molecular cloning and expression studies of PA biosynthesis-related genes, knowledge of the functions of PAs, as demonstrated by developmental studies, and through the analysis of transgenic plants carrying PA genes. This review highlights how PAs in higher plants act during environmental stress and how transgenic strategies have improved our understanding of the molecular mechanisms at play.

Plastid-associated polyamines: their role in differentiation, structure, functioning, stress response and senescence

Polyamines are low-molecular weight biogenic amines. They are a specific group of cell growth and development regulators. In the past decade biochemical, molecular and genetic studies have contributed much to a better understanding of the biological role of polyamines in the plant cell. Substantial evidence has also been added to our understanding of the role of polyamines in plastid development. In developing chloroplasts, polyamines serve as a nitrogen source for protein and chlorophyll synthesis. In chloroplast structure, thylakoid proteins linked to polyamines belong mainly to antenna proteins of light-harvesting chlorophyll a/b-protein complexes. The fact that LHCII oligomeric forms are much more intensely labelled by polyamines, in comparison to monomeric forms, suggests that polyamines participate in oligomer stabilisation. In plastid metabolism, polyamines modulate effectiveness of photosynthesis. The role of polyamines in mature chloroplasts is also related to the photo-adaptation of the photosynthetic apparatus to low and high light intensity and its response to environmental stress. The occurrence of polyamines and enzymes participating in their metabolism at every stage of plastid development indicates that polyamines play a role in plastid differentiation, structure, functioning and senescence.

Comparative proteomic and physiological analyses reveal the protective effect of exogenous polyamines in the bermudagrass (Cynodon dactylon) response to salt and drought stresses

Polyamines conferred enhanced abiotic stress tolerance in multiple plant species. However, the effect of polyamines on abiotic stress and physiological change in bermudagrass, the most widely used warm-season turfgrasses, are unknown. In this study, pretreatment of exogenous polyamine conferred increased salt and drought tolerances in bermudagrass. Comparative proteomic analysis was performed to further investigate polyamines mediated responses, and 36 commonly regulated proteins by at least two types of polyamines in bermudagrass were successfully identified, including 12 proteins with increased level, 20 proteins with decreased level and other 4 specifically expressed proteins. Among them, proteins involved in electron transport and energy pathways were largely enriched, and nucleoside diphosphate kinase (NDPK) and three antioxidant enzymes were extensively regulated by polyamines. Dissection of reactive oxygen species (ROS) levels indicated that polyamine-derived H2O2 production might play dual roles under abiotic stress conditions. Moreover, accumulation of osmolytes was also observed after application of exogenous polyamines, which is consistent with proteomics results that several proteins involved in carbon fixation pathway were mediated commonly by polyamines pretreatment. Taken together, we proposed that polyamines could activate multiple pathways that enhance bermudagrass adaption to salt and drought stresses. These findings might be applicable for genetically engineering of grasses and crops to improve stress tolerance.

Salt-induced modulation in growth, photosynthesis and antioxidant system in two varieties of Brassica juncea

The present study was carried out to examine salt-induced modulation in growth, photosynthetic characteristics and antioxidant system in two cultivars of Brassica juncea Czern and Coss varieties (Varuna and RH-30). The surface sterilized seeds of these varieties were sown in the soil amended with different levels (2.8, 4.2 or 5.6 dsm(-1)) of sodium chloride under a simple randomized block design. The salt treatment significantly decreased growth, net photosynthetic rate and its related attributes, chlorophyll fluorescence, SPAD value of chlorophyll, leaf carbonic anhydrase activity and leaf water potential, whereas electrolyte leakage, proline content, and activity of catalase, peroxidase and superoxide dismutase enzymes increased in both the varieties at 30 d stage of growth. The variety Varuna was found more resistant than RH-30 to the salt stress and possessed higher values for growth, photosynthetic attributes and antioxidant enzymes. Out of the graded concentrations (2.8, 4.2 or 5.6 dsm(-1)) of sodium chloride, 2.8 sm(-1) was least toxic and 5.6 dsm(-1) was most harmful. The variation in the responses of these two varieties to salt stress is attributed to their differential photosynthetic traits, SPAD chlorophyll value and antioxidant capacity, which can be used as potential markers for screening mustard plants for salt tolerance.

Effects of exogenous spermine on chlorophyll fluorescence, antioxidant system and ultrastructure of chloroplasts in Cucumis sativus L. under salt stress

The effects of exogenous spermine (Spm) on plant growth, chlorophyll fluorescence, ultrastructure and anti-oxidative metabolism of chloroplasts were investigated in Cucumis sativus L. under NaCl stress. Salt stress significantly reduced plant growth, chlorophylls content and F(v)/F(m). These changes could be alleviated by foliar spraying with Spm. Salt stress caused an increase in malondialdehyde (MDA) content and superoxide anion [Formula: see text] generation rate in chloroplasts. Application of Spm significantly increased activities of superoxidase dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), and ascorbate peroxidase (APX, EC 1.11.1.11) which decreased the levels of [Formula: see text] and MDA in the salt-stressed chloroplasts. Salt stress decreased the activities of dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) in the chloroplasts and reduced the contents of dehydroascorbate (DAsA) and glutathione (GSH), but increased monodehydroascorbate reductase (MDAR, EC 1.6.5.4) activity. On the other hand, Spm significantly increased the activities of antioxidant enzymes and levels of antioxidants in the salt-stressed chloroplasts. Further analysis of the ultrastructure of chloroplasts indicated that salinity induced destruction of the chloroplast envelope and increased the number of plastoglobuli with aberrations in thylakoid membranes. However, Spm application to salt-stressed plant leaves counteracted the adverse effects of salinity on the structure of the photosynthetic apparatus. These results suggest that Spm alleviates salt-induced oxidative stress through regulating antioxidant systems in chloroplasts of cucumber seedlings, which is associated with an improvement of the photochemical efficiency of PSII.