Polyamine regulates tolerance to water stress in leaves of white clover associated with antioxidant defense and dehydrin genes via involvement in calcium messenger system and hydrogen peroxide signaling

Different reactions of wheat, maize, and rice plants to putrescine treatment

Polyamines play an important role in growth and differentiation by regulating numerous physiological and biochemical processes at the cellular level. In addition to their roborative effect, their essential role in plant stress responses has been also reported. However, the positive effect may depend on the fine-tuning of polyamine metabolism, which influences the production of free radicals and/or signalling molecules. In the present study, 0.3 mM hydroponic putrescine treatment was tested in wheat, maize, and rice in order to reveal differences in their answers and highlight the relation of these with polyamine metabolism. In the case of wheat, the chlorophyll content and the actual quantum yield increased after putrescine treatment, and no remarkable changes were detected in the stress markers, polyamine contents, or polyamine metabolism-related gene expression. Although, in maize, the actual quantum yield decreased, and the root hydrogen peroxide content increased, no other negative effect was observed after putrescine treatment due to activation of polyamine oxidases at enzyme and gene expression levels. The results also demonstrated that after putrescine treatment, rice with a higher initial polyamine content, the balance of polyamine metabolism was disrupted and a significant amount of putrescine was accumulated, accompanied by a detrimental decrease in the level of higher polyamines. These initial differences and the putrescine-induced shift in polyamine metabolism together with the terminal catabolism or back-conversion-induced release of a substantial quantity of hydrogen peroxide could contribute to oxidative stress observed in rice.

Enriched endogenous free Spd and Spm in alfalfa (Medicago sativa L.) under drought stress enhance drought tolerance by inhibiting H2O2 production to increase antioxidant enzyme activity

Drought stress is a major factor limiting agricultural development, and exogenous polyamines (PAs) can increase plant drought resistance by enhancing antioxidant activity, but few studies have examined whether endogenous PAs enhance the plant antioxidant system. Here, to investigate the effects of endogenous PAs on the antioxidant system of alfalfa under drought stress and the underlying mechanisms, two alfalfa cultivars, Longzhong (drought resistant) and Gannong No. 3 (drought sensitive), were used as test materials, and their seedlings were treated with polyethylene glycol (PEG-6000) for 8 days at -1.2 MPa to simulate drought stress. The levels of free PAs [putrescine (Put), spermidine (Spd) and spermine (Spm)], hydrogen peroxide (H2O2), malondialdehyde (MDA), key PA metabolism enzyme [arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC), polyamine oxidase (PAO), and diamine oxidase (DAO)] activities, and antioxidant enzyme [superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)] activities were measured. These physiological indicators were used for correlation analysis to investigate the relationship between PA metabolism and the antioxidant enzyme system. The results showed that PA synthesis in alfalfa under drought stress was dominated by the ADC pathway. Spd and Spm played an important role in improving drought tolerance. The high levels of ADC and SAMDC activities were facilitated by the conversion of Put to Spd and Spm. H2O2 generation by oxidative decomposition of PAs was mainly dependent on the oxidative decomposition of DAO but not PAO. Low DAO activity favored low H2O2 production. Spd, Spm, ADC, ODC and SAMDC were positively correlated with the antioxidant enzymes SOD, CAT and POD in both cultivars under drought. Therefore, we concluded that high ADC and SAMDC activities in alfalfa promoted the conversion of Put to Spd and Spm, leading to high accumulation of Spd and Spm and low Put accumulation. Low Put levels led to low H2O2 production through low DAO activity, and low H2O2 levels induced the expression of antioxidant enzyme-encoding genes to improve antioxidant enzyme activity and reduce MDA accumulation and thereby enhanced drought resistance in alfalfa.

CsPAO2 Improves Salt Tolerance of Cucumber through the Interaction with CsPSA3 by Affecting Photosynthesis and Polyamine Conversion

Polyamine oxidases (PAOs) are key enzymes in polyamine metabolism and are related to the tolerance of plants to abiotic stresses. In this study, overexpression of cucumber (Cucumis sativus L.) PAO2 (CsPAO2) in Arabidopsis resulted in increased activity of the antioxidant enzyme and accelerated conversion from Put to Spd and Spm, while malondialdehyde content (MDA) and electrolyte leakage (EL) was decreased when compared with wild type, leading to enhanced plant growth under salt stress. Photosystem Ⅰ assembly 3 in cucumber (CsPSA3) was revealed as an interacting protein of CsPAO2 by screening yeast two-hybrid library combined with in vitro and in vivo methods. Then, CsPAO2 and CsPSA3 were silenced in cucumber via virus-mediated gene silencing (VIGS) with pV190 as the empty vector. Under salt stress, net photosynthetic rate (Pn) and transpiration rate (Tr) of CsPAO2-silencing plants were lower than pV190-silencing plants, and EL in root was higher than pV190-silencing plants, indicating that CsPAO2-silencing plants suffered more serious salt stress damage. However, photosynthetic parameters of CsPSA3-silencing plants were all higher than those of CsPAO2 and pV190-silencing plants, thereby enhancing the photosynthesis process. Moreover, CsPSA3 silencing reduced the EL in both leaves and roots when compared with CsPAO2-silencing plants, but the EL only in leaves was significantly lower than the other two gene-silencing plants, and conversion from Put to Spd and Spm in leaf was also promoted, suggesting that CsPSA3 interacts with CsPAO2 in leaves to participate in the regulation of salt tolerance through photosynthesis and polyamine conversion.

Spermine-mediated metabolic homeostasis improves growth and stress tolerance in creeping bentgrass (Agrostis stolonifera) under water or high-temperature stress

Plants have developed diverse defense strategies to reduce the detrimental effects of a wide range of environmental stresses. The objectives of this study were to explore the function of spermine (Spm) on mediating growth and physiological changes in water homeostasis, photosynthetic performance, and oxidative damage and to further examine the regulatory mechanism of Spm on global metabolites reprogramming and associated metabolic pathways in horticultural creeping bentgrass (Agrostis stolonifera) under water and heat stresses. The 21-days-old plants were pretreated with or without 100 μM Spm for 3 days and then subjected to water stress (17% polyethylene glycol 6000), high-temperature stress (40/35°C, day/night), or normal condition (control without water stress and heat stress) for 18 days. Results demonstrated that exogenous application of Spm could significantly increase endogenous polyamine (PAs), putrescine (Put), spermidine (Spd), and Spm contents, followed by effective alleviation of growth retardant, water imbalance, photoinhibition, and oxidative damage induced by water and heat stress. Metabolites' profiling showed that a total of 61 metabolites were differentially or commonly regulated by Spm in leaves. Spm upregulated the accumulation of mannose, maltose, galactose, and urea in relation to enhanced osmotic adjustment (OA), antioxidant capacity, and nitrogen metabolism for growth maintenance under water and heat stress. Under water stress, Spm mainly induced the accumulation of sugars (glucose-1-phosphate, sucrose-6-phosphate, fructose, kestose, maltotriose, and xylose), amino acids (glutamic acid, methionine, serine, and threonine), and organic acids (pyruvic acid, aconitic acid, and ketoglutaric acid) involved in the respiratory pathway and myo-inositol associated with energy production, the ROS-scavenging system, and signal transduction. In response to heat stress, the accumulation of alanine, glycine, gallic acid, malic acid, or nicotinic acid was specifically enhanced by Spm contributing to improvements in antioxidant potency and metabolic homeostasis. This study provides novel evidence of Spm-induced,tolerance to water and heat stresses associated with global metabolites reprogramming in favor of growth maintenance and physiological responses in horticultural plants.

Elucidating the dialogue between arbuscular mycorrhizal fungi and polyamines in plants

The most dominant arbuscular mycorrhizal (AM) symbiont can be established on roots of most terrestrial plants by beneficial AM fungi. A type of polycationic and aliphatic compounds, polyamines (PAs), are involved in plant physiological activities including stress responses. Interestingly, small amounts of PAs such as putrescine (Put) and spermidine (Spd) were found in AM fungal spores, and they are considered to be a component involved in mycorrhizal development, including mycorrhizal colonization, appressoria formation, spore germination and mycelial growth. Thus, PAs are regulatory factors in plant-AM symbiosis. Inoculation of AM fungi also affects the metabolism of endogenous PAs in host plants, including PAs synthesis and catabolism, thus, regulating various physiological events of the host. As a result, there seems to be a dialogue between PAs and AM fungi. Existing knowledge makes us understand that endogenous or exogenous PAs are an important regulator factor in the growth of AM fungi, as well as a key substance to colonize roots, which further enhances mycorrhizal benefits in plant growth responses and root architecture. The presence of AM symbiosis in roots alters the dynamic balance of endogenous PAs, triggering osmotic adjustment and antioxidant defense systems, maintaining charge balance and acting as a stress signalling molecule, which affects various physiological activities, such as plant growth, nutrient acquisition, stress tolerance and improvement of root architecture. This review mainly elucidated (i) what is the role of fungal endogenous PAs in fungal growth and colonization of roots in host plants? (ii) how AM fungi and PAs interact with each other to alter the growth of fungi and plants and subsequent activities, providing the reference for the future combined use of AM fungi and PAs in agricultural production, although there are still many unknown events in the dialogue.

Increased suicidal erythrocyte death in patients with hepatitis B-related acute-on-chronic liver failure

Anemia is a common complication of hepatitis B-related acute-on-chronic liver failure (HB-ACLF). Eryptosis, a suicidal erythrocyte death characterized by phosphatidylserine (PS) externalization and red blood cell-derived microparticle (RMP) generation, decreases erythrocyte lifespan. Herein, we investigated whether enhanced eryptosis is involved in the anemia pathophysiology associated with HB-ACLF. PS exposure, cell volume, cytosolic Ca²⁺, and reactive oxygen species (ROS) production were determined using flow cytometry. RMPs were extracted using a polyethylene glycol (PEG)-based method. We found that hemoglobin (Hb) and hematocrit (Hct) were significantly lower in patients with HB-ACLF than in healthy controls (HC), patients with chronic hepatitis B (CHB), and patients with cirrhosis. The direct antiglobulin test positive rate was 75.9% in patients with HB-ACLF while its intensity was associated with anemia. The ratio of abnormal erythrocytes was higher in patients with HB-ACLF than in HC, CHB, and cirrhosis. The percentage of PS-exposed erythrocytes was higher in patients with HB-ACLF (2.07 ± 0.11%) compared with HC (0.37 ± 0.05%), CHB (0.38 ± 0.03%), and cirrhosis (0.38 ± 0.04%). The cytosolic Ca²⁺ and ROS abundance were also higher in patients with HB-ACLF compared with HC, patients with CHB, and patients with cirrhosis, and were inversely correlated with the anemia in patients with HB-ACLF. PS exposure of erythrocytes collected from HC was significantly pronounced following incubation in plasma from patients with HB-ACLF compared with incubation in plasma from HC. The protein concentration and RMPs size significantly increased in patients with HB-ACLF compared with HC. Thus, the anemia in patients with HB-ACLF is associated with increased eryptosis, which is partially triggered by increased cytosolic Ca²⁺ and oxidative stress.NEW & NOTEWORTHY Acute chronic liver failure (ACLF) is a critical syndrome characterized by multiple organ failures and high short-term mortality. A common complication of HB-ACLF is anemia, however, the mechanism of anemia in HB-ACLF remains to be elucidated. We confirm that the accelerated eryptosis is involved in the pathophysiology of anemia associated with HB-ACLF, which progressively aggravates the clinical outcome. Our study illustrates the mechanism regarding the anemia pathogenesis of HB-ACLF, which may be utilized further toward therapeutic ends.

Polyamines Metabolism Interacts with γ-Aminobutyric Acid, Proline and Nitrogen Metabolisms to Affect Drought Tolerance of Creeping Bentgrass

Due to increased global warming and climate change, drought has become a serious threat to horticultural crop cultivation and management. The purpose of this study was to investigate the effect of spermine (Spm) pretreatment on metabolic alterations of polyamine (PAs), γ-aminobutyric acid (GABA), proline (Pro), and nitrogen associated with drought tolerance in creeping bentgrass (Agrostis stolonifera). The results showed that drought tolerance of creeping bentgrass could be significantly improved by the Spm pretreatment, as demonstrated by the maintenance of less chlorophyll loss and higher photosynthesis, gas exchange, water use efficiency, and cell membrane stability. The Spm pretreatment further increased drought-induced accumulation of endogenous PAs, putrescine, spermidine, and Spm, and also enhanced PAs metabolism through improving arginine decarboxylases, ornithine decarboxylase, S-adenosylmethionine decarboxylase, and polyamine oxidase activities during drought stress. In addition, the Spm application not only significantly improved endogenous GABA content, glutamate content, activities of glutamate decarboxylase and α-ketoglutarase, but also alleviated decline in nitrite nitrogen content, nitrate reductase, glutamine synthetase, glutamate synthetase, and GABA aminotransferase activities under drought stress. The Spm-pretreated creeping bentgrass exhibited significantly lower ammonia nitrogen content and nitrite reductase activity as well as higher glutamate dehydrogenase activity than non-pretreated plants in response to drought stress. These results indicated beneficial roles of the Spm on regulating GABA and nitrogen metabolism contributing towards better maintenance of Tricarboxylic acid (TCA) cycle in creeping bentgrass. Interestingly, the Spm-enhanced Pro metabolism rather than more Pro accumulation could be the key regulatory mechanism for drought tolerance in creeping bentgrass. Current findings provide a comprehensive understanding of PAs interaction with other metabolic pathways to regulate drought tolerance in grass species.

Exogenous application of spermidine mitigates the adverse effects of drought stress in faba bean (Vicia faba L.)

In Tunisia, drought stress is a major environmental factor limiting crop production and causing relatively low and unstable faba bean yields. In the present study, we explored the putative role of spermidine (0.5, 1, 1.5 and 2mM) in ameliorating the effects of drought stress induced by polyethylene glycol (PEG-6000, -0.58MPa) in faba bean seedlings. Drought stress reduced photosynthetic performance, chlorophyll and relative water content in leaves of faba bean variety Badii. Moreover, drought increased proline, electrolyte leakage and malondialdehyde content by inducing reactive oxygen species (hydrogen peroxide) generation in leaves. However, applying spermidine increased the activities of catalase, superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase. The results show that the application of spermidine especially at a rate of 1.5mM effectively reduces oxidative damage and alleviates negative effects caused by drought stress. In addition, exogenous spermidine increased the expression of polyamine biosynthetic enzymes' genes (VfADC , VfSAMDC and VfSPDS ), and reduced the expression of VfSPMS suggesting that exogenous spermidine can regulate polyamines' metabolic status under drought challenge, and consequently may enhance drought stress tolerance in faba bean. Real-time quantitative polymerase chain reaction analysis revealed that some drought responsive genes (VfNAC , VfHSP , VfNCED , VfLEA , VfCAT , VfAPX , VfRD22 , VfMYB , VfDHN , VfERF , VfSOD and VfWRKY ) from various metabolic pathways were differentially expressed under drought stress. Overall, these genes were more abundantly transcribed in the spermidine-treated plants compared to untreated suggesting an important role of spermidine in modulating faba bean drought stress response and tolerance.

Biocontrol Potential of Endophytic Streptomyces malaysiensis 8ZJF-21 From Medicinal Plant Against Banana Fusarium Wilt Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Banana (Musa spp.) is an important fruit crop cultivated in most tropical countries. Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is the most destructive fungal disease. Biocontrol using endophytic microorganisms is considered as a safety and sustainable strategy. Actinomycetes have a potential for the production of diverse metabolites. Isolation of endophytic actinomycetes with high efficiency and broad-spectrum antagonism is key for exploring biocontrol agents. Our previous study showed that a total of 144 endophytic actinomycetes were isolated from different tissues of medicinal plants in Hainan, China. Especially, strain 8ZJF-21 exhibited a broad-spectrum antifungal activity. Its morphological, physiological, and biochemical characteristics were consistent with the genus Streptomyces. The phylogenetic tree demonstrated that strain 8ZJF-21 formed a distinct clade with Streptomyces malaysiensis. Average nucleotide identity (ANI) was 98.49% above the threshold of novel species. The pot experiment revealed that endophytic Streptomyces malaysiensis 8ZJF-21 could improve the plant resistance to Foc TR4 by enhancing the expression levels of defense-related and antioxidant enzyme genes. It also promoted the plant growth by producing several extracellular enzymes and metabolites. Antifungal mechanism assays showed that S. malaysiensis 8ZJF-21 extract inhibited mycelial growth and spore germination of Foc TR4 in vitro. Pathogenic cells occurred cytoplasmic heterogeneity, disappeared organelles, and ruptured ultrastructure. Sequencing and annotation of genome suggested that S. malaysiensis 8ZJF-21 had a potential of producing novel metabolites. Nineteen volatile organic compounds were obtained from the extract by Gas Chromatography-Mass Spectrometry (GC-MS). Hence, endophytic Streptomyces strains will become essential biocontrol agents of modern agricultural practice.

Effect of heat stress on oxidative damage and antioxidant defense system in white clover (Trifolium repens L.)

This study leads to advances in the field of heat tolerance among different plant species. We concluded that a coordinated, increased antioxidant defense system enabled white clover to reduce heat-induced oxidative damage. The rise in global ambient temperature has a wide range of effects on plant growth, and, therefore, on the activation of various molecular defenses before the appearance of heat damage. Elevated temperatures result in accelerated generation of reactive oxygen species (ROS), causing an imbalance between ROS production and the ability of scavenging systems to detoxify and remove the reactive intermediates. The aim of this study was to determine the role of antioxidant defense systems in the alleviation of heat stress (HS) consequences in white clover (Trifolium repens L.), which is cultivated worldwide. We evaluated how temperature and time parameters contribute to the thermotolerance of white clover at different growth stages. We revealed HS protection in white clover from 37 to 40 °C, with 40 °C providing the greatest protection of 3-day-old seedlings and 28-day-old adult plants. Heat-provoked oxidative stress in white clover was confirmed by substantial changes in electrolyte leakage, malondialdehyde (MDA), and chlorophyll content, as well as superoxide anion (O₂^·-) and hydrogen peroxide (H₂O₂) production. Furthermore, superoxide dismutase (SOD) and ascorbate peroxidase (APX) as well as a high level of GSH non-enzymatic antioxidant were the most responsive, and were associated with acquired thermotolerance through the regulation of ROS generation. We demonstrated, by studying protoplast transient gene expression, direct genetic evidence of endogenous antioxidant-related genes that confer HS tolerance in white clover. Our present study clearly establishes that oxidative stress ensues from HS, which triggers the induction of antioxidant defense systems for ROS scavenging in white clover.

Overexpression of the white clover TrSAMDC1 gene enhanced salt and drought resistance in Arabidopsis thaliana

S-adenosylmethionine decarboxylase (SAMDC) mediates the biosynthesis of polyamines (PAs) and plays a positive role in plants' response to adversity stress tolerance. In this study, we isolated a SAMDC gene from white clover, which is located in mitochondria. It was strongly induced when white clover exposed to drought (15% PEG6000), salinity (200 mM NaCl), 20 μM spermidine, 100 μM abscisic acid, and 10 mM H₂O₂, especially in leaves. The INVSc1 yeast introduced with TrSAMDC1 had tolerance to drought, salt, and oxidative stress. Overexpression of TrSAMDC1 in Arabidopsis showed higher fresh weight and dry weight under drought and salt treatment and without growth inhibition under normal conditions. Leaf senescence induced by drought and saline was further delayed in transgenic plants, regardless of cultivation in 1/2 MS medium and soil. During drought and salt stress, transgenic plants exhibited a significant increase in relative water content, maximum photosynthesis efficiency (Fv/Fm), performance index on the absorption basis (PI_ABS), activities of antioxidant protective enzymes such as SOD, POD, CAT, and APX, and a significant decrease in accumulation of MDA and H₂O₂ as compared to the WT. The concentrations of total PAs, putrescine, spermidine, and spermidine in transgenic lines were higher in transgenic plants than in WT under normal and drought conditions. These results suggested that TrSAMDC1 could effectively mitigate abiotic stresses without the expense of production and be a potential candidate gene for improving the drought and salt resistance of crops.

Exogenous Spermidine Priming Mitigates the Osmotic Damage in Germinating Seeds of Leymus chinensis Under Salt-Alkali Stress

Spermidine (Spd) is known to protect macromolecules involved in physiological and biochemical processes in plants. However, it is possible that Spd also plays an osmotic regulatory role in promoting the seed germination of Leymus chinensis (L. chinensis) under salt-alkali stress. To investigate this further, seeds of L. chinensis were soaked in Spd solution or distilled water, and a culture experiment was performed by sowing the soaked seeds in saline-alkaline soils. The data showed that the Spd priming resulted in an increase of more than 50% in soluble sugar content and an increase of more than 30% in proline content in the germinating seeds. In addition, the Spd priming resulted in an increase of more than 30% in catalase activity and an increase of more than 25% in peroxidase activity in the germinating seeds and effectively mitigated the oxidative damage to the plasma membrane in the germinating seeds under salt-alkali stress. Moreover, the Spd priming of seeds affected the accumulation of polyamine (PA) and maintained the activities of macromolecules involved in physiological metabolism in germinating seeds exposed to salt-alkali stress. Furthermore, the Spd priming treatment increased the hydrogen peroxide (H₂O₂) level to more than 30% and the Ca²⁺ concentration to more than 20% in the germinating seeds, thus breaking the dormancy induction pathways in L. chinensis seeds through beneficial hormone enrichment. This study provides an insight into the Spd-mediated regulation pathway during exogenous Spd priming of L. chinensis seeds, which mitigates osmotic and oxidative damage and maintains the integrality of the cell lipid membrane. Thus, exogenous Spd priming increases PA oxidase activity and maintains the accumulation of H₂O₂. We found that the H₂O₂ beneficially affected the balance of Ca²⁺ and hormones, promoting the vigor and germination of L. chinensis in response to salt-alkali stress.

Adaptability to abiotic stress regulated by γ-aminobutyric acid in relation to alterations of endogenous polyamines and organic metabolites in creeping bentgrass

The frequency and severity of global abiotic stresses such as heat, drought, and salt stress are increasing due to climate changes. Objectives of this study were to investigate effects of γ-aminobutyric acid (GABA) priming on inducing plants' acclimation to abiotic stress associated with alterations of endogenous polyamines (PAs), amino acids, and sugars in creeping bentgrass (Agrostis stolonifera). The pretreatment with GABA fertigation significantly alleviated heat-, drought-, and salt-induced declines in leaf relative water content, chlorophyll content, cell membrane stability, photochemical efficiency (Fv/Fm), and performance index on absorption basis (PIABS), and also further decreased stress-caused decline in osmotic potential in leaves. The GABA priming uniformly increased total PAs, spermidine, amino acids involved in GABA shunt (GABA, glutamic acid, and alanine), and other amino acids (phenylalanine, aspartic acid, and glycine) accumulation under heat, drought, and salt stress. The GABA priming also significantly improved methionine content under heat and drought stress, maltose, galactose, and talose content under heat and salt stress, or cysteine, serine, and threonine content under drought and salt stress. Interestingly, the GABA priming uniquely led to significant accumulation of spermine, fructose, and glucose under heat stress, putrescine, proline, and mannose under drought stress, or arginine, trehalose and xylose under salt stress, respectively. These particular PAs, sugars, and amino acids differentially or commonly regulated by GABA could play critical roles in osmotic adjustment, osmoprotection, antioxidant, energy source, and signal molecular for creeping bentgrass to acclimate diverse abiotic stresses.

Spermine Regulates Water Balance Associated with Ca2+-Dependent Aquaporin (TrTIP2-1, TrTIP2-2 and TrPIP2-7) Expression in Plants under Water Stress

Spermine (Spm) regulates water balance involved in water channel proteins, aquaporins (AQPs), in plants. An increase in endogenous Spm content via exogenous Spm application significantly improved cell membrane stability, photosynthesis, osmotic adjustment (OA) and water use efficiency (WUE) contributing to enhanced tolerance to water stress in white clover. Spm upregulated TrTIP2-1, TrTIP2-2 and TrPIP2-7 expressions and also increased the abundance of TIP2 and PIP2-7 proteins in white clover under water stress. Spm quickly activated intracellular Ca2+ signaling and Spm-induced TrTIP2-2 and TrPIP2-7 expressions could be blocked by Ca2+ channel blockers and the inhibitor of Ca2+-dependent protein kinase in leaves of white clover. TrSAMS in relation to Spm biosynthesis was first cloned from white clover and the TrSAMS was located in the nucleus. Transgenic Arabidopsis overexpressing the TrSAMS had significantly higher endogenous Spm content and improved cell membrane stability, photosynthesis, OA, WUE and transcript levels of AtSIP1-1, AtSIP1-2, AtTIP2-1, AtTIP2-2, AtPIP1-2, AtPIP2-1 and AtNIP2-1 than wild type in response to water stress. Current findings indicate that Spm regulates water balance via an enhancement in OA, WUE and water transport related to Ca2+-dependent AQP expression in plants under water stress.

Bitter Melon (Momordica charantia L.) Rootstock Improves the Heat Tolerance of Cucumber by Regulating Photosynthetic and Antioxidant Defense Pathways

High temperature is considered a critical abiotic stressor that is increasing continuously, which is severely affecting plant growth and development. The use of heat-resistant rootstock grafting is a viable technique that is practiced globally to improve plant resistance towards abiotic stresses. In this experiment, we explored the efficacy of bitter melon rootstock and how it regulates photosynthesis and the antioxidant defense system to alleviate heat stress (42 °C/32 °C) in cucumber. Our results revealed that bitter-melon-grafted seedlings significantly relieved heat-induced growth inhibition and photoinhibition, maintained better photosynthesis activity, and accumulated a greater biomass than self-grafted seedlings. We measured the endogenous polyamine and hydrogen peroxide (H2O2) contents to determine the inherent mechanism responsible for these effects, and the results showed that heat stress induced a transient increase in polyamines and H2O2 in the inner courtyard of grafted seedlings. This increment was greater and more robust in bitter-melon-grafted seedlings. In addition, the use of polyamine synthesis inhibitors MGBG (methylglyoxal bis-guanylhydrazone) and D-Arg (D-arginine), further confirmed that the production of H2O2 under heat stress is mediated by the accumulation of endogenous polyamines. Moreover, compared with other treatments, the bitter-melon-grafted seedlings maintained high levels of antioxidant enzyme activity under high temperature conditions. However, these activities were significantly inhibited by polyamine synthesis inhibitors and H2O2 scavengers (dimethylthiourea, DMTU), indicating that bitter melon rootstock not only maintained better photosynthetic activity under conditions of high temperature stress but also mediated the production of H2O2 through the regulation of the high level of endogenous polyamines, thereby boosting the antioxidant defense system and comprehensively improving the heat tolerance of cucumber seedlings. Taken together, these results indicate that grafting with a resistant cultivar is a promising alternative tool for reducing stress-induced damage.

Tinkering Cis Motifs Jigsaw Puzzle Led to Root-Specific Drought-Inducible Novel Synthetic Promoters

Following an in-depth transcriptomics-based approach, we first screened out and analyzed (in silico) cis motifs in a group of 63 drought-inducible genes (in soybean). Six novel synthetic promoters (SynP14-SynP19) were designed by concatenating 11 cis motifs, ABF, ABRE, ABRE-Like, CBF, E2F-VARIANT, G-box, GCC-Box, MYB1, MYB4, RAV1-A, and RAV1-B (in multiple copies and various combination) with a minimal 35s core promoter and a 222 bp synthetic intron sequence. In order to validate their drought-inducibility and root-specificity, the designed synthetic assemblies were transformed in soybean hairy roots to drive GUS gene using pCAMBIA3301. Through GUS histochemical assay (after a 72 h 6% PEG6000 treatment), we noticed higher glucuronidase activity in transgenic hairy roots harboring SynP15, SynP16, and SynP18. Further screening through GUS fluorometric assay flaunted SynP16 as the most appropriate combination of efficient drought-responsive cis motifs. Afterwards, we stably transformed SynP15, SynP16, and SynP18 in Arabidopsis and carried out GUS staining as well as fluorometric assays of the transgenic plants treated with simulated drought stress. Consistently, SynP16 retained higher transcriptional activity in Arabidopsis roots in response to drought. Thus the root-specific drought-inducible synthetic promoters designed using stimulus-specific cis motifs in a definite fashion could be exploited in developing drought tolerance in soybean and other crops as well. Moreover, the rationale of design extends our knowledge of trial-and-error based cis engineering to construct synthetic promoters for transcriptional upgradation against other stresses.

Polyamines and Legumes: Joint Stories of Stress, Nitrogen Fixation and Environment

Polyamines (PAs) are natural aliphatic amines involved in many physiological processes in almost all living organisms, including responses to abiotic stresses and microbial interactions. On other hand, the family Leguminosae constitutes an economically and ecologically key botanical group for humans, being also regarded as the most important protein source for livestock. This review presents the profuse evidence that relates changes in PAs levels during responses to biotic and abiotic stresses in model and cultivable species within Leguminosae and examines the unreviewed information regarding their potential roles in the functioning of symbiotic interactions with nitrogen-fixing bacteria and arbuscular mycorrhizae in this family. As linking plant physiological behavior with "big data" available in "omics" is an essential step to improve our understanding of legumes responses to global change, we also examined integrative MultiOmics approaches available to decrypt the interface legumes-PAs-abiotic and biotic stress interactions. These approaches are expected to accelerate the identification of stress tolerant phenotypes and the design of new biotechnological strategies to increase their yield and adaptation to marginal environments, making better use of available plant genetic resources.

Using intervarietal substitution lines for the identification of wheat chromosomes involved in early responses to water-deficit stress

Water deficit induces reactive oxygen species (ROS) overproduction, which in turn inhibits plant growth and development. High concentrations of ROS disrupt the osmotic balance in plant cells and alter membrane integrity. Chromosomes carrying structural or regulatory genes must be detected to better understand plant response mechanisms to stress. The aim of our study was to identify Triticum aestivum L. chromosomes involved in early responses to short-term water-deficit stress (1, 3 and 6 h). In the present study, intervarietal substitution lines of drought-tolerant 'Saratovskaya 29' and sensitive 'Janetzkis Probat' wheat cultivars were examined. We studied the biochemical plant response system and conducted an analysis of catalase, ascorbate peroxidase and guaiacol peroxidase activities, levels of lipid peroxidation and changes in relative water content. Our results determined that the first reaction was a significant increase in guaiacol peroxidase (GPX) activity. However, the strongest impact on plant responses was found for catalase (CAT), which caused a significant decrease in lipid peroxidation (LPO) levels. Our findings indicate that chromosomes 5A, 4B, 6B and 7D are associated with early responses to short-term osmotic stress in wheat.

Indole-3-acetic acid modulates phytohormones and polyamines metabolism associated with the tolerance to water stress in white clover

Endogenous hormones and polyamines (PAs) could interact to regulate growth and tolerance to water stress in white clover. The objective of this study was to investigate whether the alteration of endogenous indole-3-acetic acid (IAA) level affected other hormones level and PAs metabolism contributing to the regulation of tolerance to water stress in white clover. Plants were pretreated with IAA or L-2-aminooxy-3-phenylpropionic acid (L-AOPP, the inhibitor of IAA biosynthesis) for 3 days and then subjected to water-sufficient condition and water stress induced by 15% polyethylene glycol 6000 for 8 days in growth chambers. Exogenous application of IAA significantly increased endogenous IAA, gibberellin (GA), abscisic acid (ABA), and polyamine (PAs) levels, but had no effect on cytokinin content under water stress. The increase in endogenous IAA level enhanced PAs anabolism via the improvement of enzyme activities and transcript level of genes including arginine decarboxylase, ornithine decarboxylase, and S-adenosylmethionine decarboxylase. Exogenous application of IAA also affected PAs catabolism, as manifested by an increase in diamine oxidase and a decrease in polyamine oxidase activities and genes expression. More importantly, the IAA deficiency in white clover decreased endogenous hormone levels (GA, ABA, and PAs) and PAs anabolism along with decline in antioxidant defense and osmotic adjustment (OA). On the contrary, exogenous IAA effectively alleviated stress-induced oxidative damage, growth inhibition, water deficit, and leaf senescence through the maintenance of higher chlorophyll content, OA, and antioxidant defense as well as lower transcript levels of senescence marker genes SAG101 and SAG102 in leaves under water stress. These results indicate that IAA-induced the crosstalk between endogenous hormones and PAs could be involved in the improvement of antioxidant defense and OA conferring tolerance to water stress in white clover.

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.

Alteration of Transcripts of Stress-Protective Genes and Transcriptional Factors by γ-Aminobutyric Acid (GABA) Associated with Improved Heat and Drought Tolerance in Creeping Bentgrass (Agrostis stolonifera)

Gamma-aminobutyric acid (GABA) may play a positive role in regulating plant tolerance to drought or heat stress. The objectives of this study were to investigate the physiological effects of GABA on tolerance of creeping bentgrass (Agrostis stolonifera) to heat and drought stress and to determine whether enhanced heat and drought tolerance due to GABA treatment was associated with the up-regulation of selected genes and transcriptional factors involved in stress protection. Creeping bentgrass (cultivar “Penncross”) plants were treated with 0.5 mM GABA or water (untreated control) as a foliar spray and were subsequently exposed to heat stress (35/30 °C, day/night), drought stress by withholding irrigation, or non-stress conditions in controlled-environment growth chambers. Exogenous application of GABA significantly improved plant tolerance to heat and drought stress, as reflected by increased leaf water content, cell membrane stability, and chlorophyll content. The analysis of gene transcript level revealed that exogenous GABA up-regulated the expression of ABF3, POD, APX, HSP90, DHN3, and MT1 during heat stress and the expression of CDPK26, MAPK1, ABF3, WRKY75, MYB13, HSP70, MT1, 14-3-3, and genes (SOD, CAT, POD, APX, MDHAR, DHAR, and GR) encoding antioxidant enzymes during drought stress. The up-regulation of the aforementioned stress-protective genes and transcriptional factors could contribute to improved heat and drought tolerance in creeping bentgrass.

The inhibition of polyamine biosynthesis weakens the drought tolerance in white clover (Trifolium repens) associated with the alteration of extensive proteins

Changes of endogenous polyamine (PA) levels could be a key adaptive response to drought in plants. White clover pretreated with or without dicyclohexylamine (DCHA), an inhibitor of PA biosynthesis, was subjected to drought stress induced by 18% polyethylene glycol 6000 for 8 days in controlled growth chambers. Results showed that drought stress significantly increased endogenous PA content, whereas DCHA significantly decreased PA accumulation under drought stress. The attenuate PA biosynthesis was unfavorable for plant growth and drought tolerance, as reflected by significantly lower relative water content, relative growth rate, instantaneous water use efficiency, and cell membrane stability in leaves in response to drought. On the basis of proteomic analysis, the inhibition of PA synthesis decreased the accumulation of many key differentially expressed proteins including (1) ribosomal structure and biogenesis: elongation factor, ribosomal protein S10E, and 30S ribosomal protein; (2) amino acid transport and metabolism: cysteine synthase, delta-1-pyrroline-5-carboxylate synthetase, and glutamate decarboxylase; (3) carbohydrate metabolism and energy production: photosystem apoprotein, sucrose-phosphate synthase, phosphogluconate dehydrogenase, sucrose-phosphatase, NADH oxidoreductase, and ATP synthase; (4) antioxidant metabolism: catalase, peroxidase I, ascorbate peroxidase, and glutathione S-transferase; and (5) other biological processes: heat shock protein 70, heat shock protein 90, and calcium-dependent protein kinase associated with the decreased drought tolerance in white clover. These findings indicate that PAs play a critical role in the regulation of growth, ribosome, amino acid and energy metabolism, and antioxidant reactions in white clover under drought stress. Drought-induced increases in endogenous PAs could be one of key adaptive responses against drought stress in white clover.

Transgenic Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) Overexpressing S-Adenosylmethionine Decarboxylase (SAMDC) Gene for Improved Cold Tolerance Through Involvement of H2O2 and NO Signaling

Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) is an important warm-season turfgrass species. Transgenic centipedgrass plants overexpressing S-adenosylmethionine decarboxylase from bermudagrass (CdSAMDC1) that was induced in response to cold were generated in this study. Higher levels of CdSAMDC1 transcript and sperimidine (Spd) and spermin (Spm) concentrations and enhanced freezing and chilling tolerance were observed in transgenic plants as compared with the wild type (WT). Transgenic plants had higher levels of polyamine oxidase (PAO) activity and H₂O₂ than WT, which were blocked by pretreatment with methylglyoxal bis (guanylhydrazone) or MGBG, inhibitor of SAMDC, indicating that the increased PAO and H₂O₂ were a result of expression of CdSAMDC1. In addition, transgenic plants had higher levels of nitrate reductase (NR) activity and nitric oxide (NO) concentration. The increased NR activity were blocked by pretreatment with MGBG and ascorbic acid (AsA), scavenger of H₂O₂, while the increased NO level was blocked by MGBG, AsA, and inhibitors of NR, indicating that the enhanced NR-derived NO was dependent upon H₂O₂, as a result of expression CdSAMDC1. Elevated superoxide dismutase (SOD) and catalase (CAT) activities were observed in transgenic plants than in WT, which were blocked by pretreatment with MGBG, AsA, inhibitors of NR and scavenger of NO, indicating that the increased activities of SOD and CAT depends on expression of CdSAMDC1, H₂O₂, and NR-derived NO. Our results suggest that the elevated cold tolerance was associated with PAO catalyzed production of H₂O₂, which in turn led to NR-derived NO production and induced antioxidant enzyme activities in transgenic plants.

Metabolic Pathways Regulated by Chitosan Contributing to Drought Resistance in White Clover

Increased endogenous chitosan (CTS) could be associated with improved drought resistance in white clover (Trifolium repens). Plants were pretreated with or without 1 mg/mL CTS and then were subjected to optimal or water-limited condition in controlled growth chambers for 6 days. Phenotypic and physiological results indicated that exogenous CTS significantly improved drought resistance of white clover. Metabolome results showed that exogenous CTS induced a significant increase in endogenous CTS content during dehydration accompanied by the maintenance of greater accumulation of sugars, sugar alcohols, amino acids, organic acids, and other metabolites (ascorbate, glutathione, flavonoids, putrescine, and spermidine). These compounds are associated with osmotic adjustment, antioxidant defense, stress signaling, and energy metabolism under stress condition. Similarly, transcriptome revealed that many genes in relation to amino acid and carbohydrate metabolism, energy production and conversion, and ascorbate-glutathione and flavonoid metabolism were significantly up-regulated by CTS in response to dehydration stress. CTS-induced drought resistance was associated with the accumulation of stress protective metabolites, the enhancement of ascorbate-glutathione and tricarboxylic acid cycle, and increases in the γ-aminobutyric acid shunt, polyamine synthesis, and flavonoids metabolism contributing to improved osmotic adjustment, antioxidant capacity, stress signaling, and energy production for stress defense, thereby maintaining metabolic homeostasis under dehydration stress.

NaCl stress induces CsSAMs gene expression in Cucumis sativus by mediating the binding of CsGT-3b to the GT-1 element within the CsSAMs promoter

MAIN CONCLUSION

The CsSAMs promoter is a salt-stress-inducible promoter containing three GT-1 elements that are sufficient for the salt-stress response. The transcription factor CsGT-3b was found to bind to the GT-1 element. The S-adenosyl-L-methionine synthase (SAMs) gene is among the functional genes induced during environmental stress. However, little is known about the regulatory mechanism and upstream regulators of this salt-inducible gene in cucumber plants. Thus, it is necessary to understand the characteristics of the SAMs gene by analyzing its promoter and transcription factors. In this study, we isolated and functionally analyzed a 1743-bp flanking fragment of the CsSAMs gene from Cucumis sativus. To examine promoter activity, the full-length promoter, as well as different promoter fragments, were fused to the β-glucuronidase (GUS) reporter gene and introduced into the tobacco genome. The full-length promoter displayed maximal promoter activity, whereas the P4 promoter, containing 321 bp of upstream sequence, showed no basal promoter activity. In addition, the CsSAMs promoter exhibited stress-inducible regulation rather than tissue-specific activity in transgenic tobacco. Histochemical analysis revealed strong GUS staining in leaves, stems, and roots, especially in the veins of leaves, the vascular bundle of stems, and root tip zones following NaCl stress. A transient expression assay confirmed that the 242-bp region (-1743 to -1500) was sufficient for the NaCl-stress response. Yeast one-hybrid assays further revealed interaction between the NaCl-response protein CsGT-3b and the GT-1 (GAAAAA) element within the 242-bp region. Taken together, we revealed the presence of four salt-stress-responsive elements (GT-1 cis-elements) in the CsSAMs promoter and identified a transcription factor, CsGT-3b, that specifically binds to this sequence. These results might help us better understand the intricate regulatory network of the cucumber SAMs gene.

Comparative analysis of polyamine metabolism in wheat and maize plants

In the present work changes in polyamine contents were investigated after various hydroponic polyamine treatments (putrescine, spermidine and spermine at 0.1, 0.3 and 0.5 mM concentrations) in two different crop species, wheat and maize. In contrast to putrescine, higher polyamines (spermidine and spermine) induced concentration-dependent oxidative damage in both crops, resulting in decreased biomass. The unfavourable effects of polyamines were more pronounced in the roots, and maize was more sensitive than wheat. The adverse effects of polyamine treatment were proportional to the accumulation of polyamine and the plant hormone salicylic acid in the leaves and roots of both plant species. Changes in polyamine content and catabolism during osmotic stress conditions were also studied after beneficial pre-treatment with putrescine. The greater positive effect of putrescine in wheat than in maize can be explained by differences in the polyamine metabolism under normal and osmotic stress conditions, and by relationship between polyamines and salicylic acid. The results demonstrated that changes in the polyamine pool are important for fine tuning of polyamine signalling, which influences the hormonal balance required if putrescine is to exert a protective effect under stress conditions.

Deep Sequencing of Suppression Subtractive Hybridisation Drought and Recovery Libraries of the Non-model Crop Trifolium repens L

White clover is a short-lived perennial whose persistence is greatly affected by abiotic stresses, particularly drought. The aim of this work was to characterize its molecular response to water deficit and recovery following re-hydration to identify targets for the breeding of tolerant varieties. We created a white clover reference transcriptome of 16,193 contigs by deep sequencing (mean base coverage 387x) four Suppression Subtractive Hybridization (SSH) libraries (a forward and a reverse library for each treatment) constructed from young leaf tissue of white clover at the onset of the response to drought and recovery. Reads from individual libraries were then mapped to the reference transcriptome and processed comparing expression level data. The pipeline generated four robust sets of transcripts induced and repressed in the leaves of plants subjected to water deficit stress (6,937 and 3,142, respectively) and following re-hydration (6,695 and 4,897, respectively). Semi-quantitative polymerase chain reaction was used to verify the expression pattern of 16 genes. The differentially expressed transcripts were functionally annotated and mapped to biological processes and pathways. In agreement with similar studies in other crops, the majority of transcripts up-regulated in response to drought belonged to metabolic processes, such as amino acid, carbohydrate, and lipid metabolism, while transcripts involved in photosynthesis, such as components of the photosystem and the biosynthesis of photosynthetic pigments, were up-regulated during recovery. The data also highlighted the role of raffinose family oligosaccharides (RFOs) and the possible delayed response of the flavonoid pathways in the initial response of white clover to water withdrawal. The work presented in this paper is to our knowledge the first large scale molecular analysis of the white clover response to drought stress and re-hydration. The data generated provide a valuable genomic resource for marker discovery and ultimately for the improvement of white clover.

Differential Responses of Polyamines and Antioxidants to Drought in a Centipedegrass Mutant in Comparison to Its Wild Type Plants

Centipedegrass (Eremochloa ophiuroides [Munro] Hack.) is an important warm-season turfgrass species with low turf maintenance requirements. However, our knowledge on physiological adaptation of centipedegrass to drought stress is limited. Physiological responses to drought in a gamma-ray-induced mutant 22-1 as compared with two wild type (WT) lines were analyzed for understanding of drought tolerance mechanism of centipedegrass. The mutant showed an elevated drought tolerance with higher levels of relative water content, net photosynthetic rate (A) and stomatal conductance (g_s) and lower levels of ion leakage and malondialdehyde (MDA) under drought stress as compared with WT plants. A showed significant correlation with g_s and MDA. Higher levels of antioxidant enzymes activities, non-enzyme antioxidants, and polyamines including putrescine (Put), spermidine (Spd), and spermine (Spm) were maintained in 22-1 than in WT plants. Superoxide dismutase (SOD), catalase (CAT), ascorbate-peroxidase (APX), and glutathione reductase (GR) activities and ascorbic acid (AsA) content were significantly correlated with both Put and Spd levels, and reduced glutathione level was correlated with Put during drought stress. Exogenous application of Put, Spd, and Spm increased drought tolerance and activities of SOD, CAT, APX, and GR in WT plants. The results suggest that higher levels of polyamines and antioxidant defense system are associated with the elevated drought tolerance in 22-1, which may improve protection on photosynthesis against drought induced oxidative damage.

Hydrogen Peroxide and Polyamines Act as Double Edged Swords in Plant Abiotic Stress Responses

The specific genetic changes through which plants adapt to the multitude of environmental stresses are possible because of the molecular regulations in the system. These intricate regulatory mechanisms once unveiled will surely raise interesting questions. Polyamines and hydrogen peroxide have been suggested to be important signaling molecules during biotic and abiotic stresses. Hydrogen peroxide plays a versatile role from orchestrating physiological processes to stress response. It helps to achieve acclimatization and tolerance to stress by coordinating intra-cellular and systemic signaling systems. Polyamines, on the other hand, are low molecular weight polycationic aliphatic amines, which have been implicated in various stress responses. It is quite interesting to note that both hydrogen peroxide and polyamines have a fine line of inter-relation between them since the catabolic pathways of the latter releases hydrogen peroxide. In this review we have tried to illustrate the roles and their multifaceted functions of these two important signaling molecules based on current literature. This review also highlights the fact that over accumulation of hydrogen peroxide and polyamines can be detrimental for plant cells leading to toxicity and pre-mature cell death.

Physiological and iTRAQ-Based Proteomic Analyses Reveal the Function of Spermidine on Improving Drought Tolerance in White Clover

Endogenous spermidine interacting with phytohormones may be involved in the regulation of differentially expressed proteins (DEPs) associated with drought tolerance in white clover. Plants treated with or without spermidine (50 μM) were subjected to 20% PEG 6000 nutrient solution to induce drought stress (50% leaf-relative water content). The results showed that increased endogenous spermidine induced by exogenous spermidine altered endogenous phytohormones in association with improved drought tolerance, as demonstrated by the delay in water-deficit development, improved photosynthesis and water use efficiency, and lower oxidative damage. As compared to untreated plants, Spd-treated plants maintained a higher abundance of DEPs under drought stress involved in (1) protein biosynthesis (ribosomal and chaperone proteins); (2) amino acids synthesis; (3) the carbon and energy metabolism; (4) antioxidant and stress defense (ascorbate peroxidase, glutathione peroxidase, and dehydrins); and (5) GA and ABA signaling pathways (gibberellin receptor GID1, ABA-responsive protein 17, and ABA stress ripening protein). Thus, the findings of proteome could explain the Spd-induced physiological effects associated with drought tolerance. The analysis of functional protein-protein networks further proved that the alteration of endogenous spermidine and phytohormones induced the interaction among ribosome, photosynthesis, carbon metabolism, and amino acid biosynthesis. These differences could contribute to improved drought tolerance.

Proteomic and Physiological Analyses Reveal Putrescine Responses in Roots of Cucumber Stressed by NaCl

Soil salinity is a major environmental constraint that threatens agricultural productivity. Different strategies have been developed to improve crop salt tolerance, among which the effects of polyamines have been well-reported. To gain a better understanding of the cucumber (Cucumis sativus L.) responses to NaCl and unravel the underlying mechanism of exogenous putrescine (Put) alleviating salt-induced damage, comparative proteomic analysis was conducted on cucumber roots treated with NaCl, and/or Put for 7 days. The results showed that exogenous Put restored the root growth inhibited by NaCl. Sixty-two differentially expressed proteins implicated in various biological processes were successfully identified by MALDI-TOF/TOF MS. The four largest categories included proteins involved in defense response (24.2%), protein metabolism (24.2%), carbohydrate metabolism (19.4%), and amino acid metabolism (14.5%). Exogenous Put up-regulated most identified proteins involved in carbohydrate metabolism, implying an enhancement in energy generation. Proteins involved in defense response and protein metabolism were differently regulated by Put, which indicated the roles of Put in stress resistance and proteome rearrangement. Put also increased the abundance of proteins involved in amino acid metabolism. Meanwhile, physiological analysis showed that Put could further up-regulated the levels of free amino acids in salt stressed-roots. In addition, Put also improved endogenous polyamines contents by regulating the transcription levels of key enzymes in polyamine metabolism. Taken together, these results suggest that Put may alleviate NaCl-induced growth inhibition through degradation of misfolded/damaged proteins, activation of stress defense, and the promotion of carbohydrate metabolism to generate more energy.