Proteomics reveal cucumber Spd-responses under normal condition and salt stress

Research Advancements in Salt Tolerance of Cucurbitaceae: From Salt Response to Molecular Mechanisms

Soil salinization severely limits the quality and productivity of economic crops, threatening global food security. Recent advancements have improved our understanding of how plants perceive, signal, and respond to salt stress. The discovery of the Salt Overly Sensitive (SOS) pathway has been crucial in revealing the molecular mechanisms behind plant salinity tolerance. Additionally, extensive research into various plant hormones, transcription factors, and signaling molecules has greatly enhanced our knowledge of plants' salinity tolerance mechanisms. Cucurbitaceae plants, cherished for their economic value as fruits and vegetables, display sensitivity to salt stress. Despite garnering some attention, research on the salinity tolerance of these plants remains somewhat scattered and disorganized. Consequently, this article offers a review centered on three aspects: the salt response of Cucurbitaceae under stress; physiological and biochemical responses to salt stress; and the current research status of their molecular mechanisms in economically significant crops, like cucumbers, watermelons, melon, and loofahs. Additionally, some measures to improve the salt tolerance of Cucurbitaceae crops are summarized. It aims to provide insights for the in-depth exploration of Cucurbitaceae's salt response mechanisms, uncovering the roles of salt-resistant genes and fostering the cultivation of novel varieties through molecular biology in the future.

Genome-wide identification of SAMS gene family in Cucurbitaceae and the role of ClSAMS1 in watermelon tolerance to abiotic stress

S-Adenosyl-L-methionine (SAM) is widely involved in plant growth, development, and abiotic stress response. SAM synthetase (SAMS) is the key enzyme that catalyzes the synthesis of SAM from methionine and ATP. However, the SAMS gene family has not been identified and their functions have not been characterized in most Cucurbitaceae plants. Here, a total of 30 SAMS genes were identified in nine Cucurbitaceae species and they were categorized into 3 subfamilies. Physicochemical properties and gene structure analysis showed that the SAMS protein members are tightly conserved. Further analysis of the cis-regulatory elements (CREs) of SAMS genes' promoter implied their potential roles in stress tolerance. To further understand the molecular functions of SAMS genes, watermelon SAMSs (ClSAMSs) were chosen to analyze the expression patterns in different tissues and under various abiotic stress and hormone responses. Among the investigated genes, ClSAMS1 expression was observed in all tissues and found to be up-regulated by abiotic stresses including salt, cold and drought treatments as well as exogenous hormone treatments including ETH, SA, MeJA and ABA. Furthermore, knockdown of ClSAMS1 via virus-induced gene silencing (VIGS) decreased SAM contents in watermelon seedings. The pTRSV2-ClSAMS1 plants showed reduced susceptibility to drought, cold and NaCl stress, indicating a positive role of ClSAMS1 in abiotic stresses tolerance. Those results provided candidate SAMS genes to regulate plant resistance against abiotic stresses in Cucurbitaceae plants.

Polyamines mediate the inhibitory effect of drought stress on nitrogen reallocation and utilization to regulate grain number in wheat

Drought stress poses a serious threat to grain formation in wheat. Nitrogen (N) plays crucial roles in plant organ development; however, the physiological mechanisms by which drought stress affects plant N availability and mediates the formation of grains in spikes of winter wheat are still unclear. In this study, we determined that pre-reproductive drought stress significantly reduced the number of fertile florets and the number of grains formed. Transcriptome analysis demonstrated that this was related to N metabolism, and in particular, the metabolism pathways of arginine (the main precursor for synthesis of polyamine) and proline. Continuous drought stress restricted plant N accumulation and reallocation rates, and plants preferentially allocated more N to spike development. As the activities of amino acid biosynthesis enzymes and catabolic enzymes were inhibited, more free amino acids accumulated in young spikes. The expression of polyamine synthase genes was down-regulated under drought stress, whilst expression of genes encoding catabolic enzymes was enhanced, resulting in reductions in endogenous spermidine and putrescine. Treatment with exogenous spermidine optimized N allocation in young spikes and leaves, which greatly alleviated the drought-induced reduction in the number of grains per spike. Overall, our results show that pre-reproductive drought stress affects wheat grain numbers by regulating N redistribution and polyamine metabolism.

Transcriptomic and Metabolic Profiling of Kenaf Stems under Salinity Stress

Kenaf (Hibiscus cannabinus L.) is an indispensable fiber crop that faces increasing salinity stress. In previous studies regarding the molecular mechanisms of how kenaf may respond to salt stress, no metabolic evidences have been reported. Meanwhile, studies regarding kenaf stems under adverse growth conditions have not been conducted. In the present study, multiple-layer evidences including physiological, transcriptomic, and metabolic data regarding how kenaf stems were affected by the salt stress are provided, wherein the stem growth, especially the lignification process, is retarded. Meanwhile, the transcriptomic data indicated genes involved in the photosynthesis are significantly repressed while the multiple flavonoid metabolism genes are enriched. As to the metabolic data, the content variation for the growth-promotion phytohormones such as IAA and the stress-responding ones including ABA are within or without expectations, implying these phytohormones played complicated roles when the kenaf stems encounter salt stress. However, the metabolite variations did not always agree with the expression levels of corresponding key pathway genes, possibly because the metabolite could be biosynthesized or catabolized in multiple pathways. Collectively, our data may enlighten, more specifically, downstream studies on kenaf responses against salinity and other adverse conditions.

Genome-Wide Identification of the SAMS Gene Family in Upland Cotton (Gossypium hirsutum L.) and Expression Analysis in Drought Stress Treatments

Cotton is an important commercial crop whose growth and yield are severely affected by drought. S-adenosylmethionine (SAM) is widely involved in the plant stress response and growth regulation; however, the role of the S-adenosylmethionine synthase (SAMS) gene family in this process is poorly understood. Here, we systematically analyzed the expression of SAMS genes in Upland Cotton (Gossypium hirsutum L.). A total of 16 SAMS genes were identified, each with a similar predicted structure. A large number of cis-acting elements involved in the response to abiotic stress were predicted based on promoter analysis, indicating a likely important role in abiotic stress responses. The results of qRT-PCR validation showed that GhSAMS genes had different expression patterns after drought stress and in response to drought stress. Analysis of a selected subset of GhSAMS genes showed increased expression in cultivar Xinluzhong 39 (drought resistant) when compared to cultivar Xinluzao 26 (drought-sensitive) upland cotton. This study provides important relevant information for further study of SAMS genes in drought resistance research of upland cotton, which is helpful for drought-resistance improvement of upland cotton.

CsCDPK6, a CsSAMS1-Interacting Protein, Affects Polyamine/Ethylene Biosynthesis in Cucumber and Enhances Salt Tolerance by Overexpression in Tobacco

S-adenosylmethionine synthetase (SAMS) plays a crucial role in regulating stress responses. In a recent study, we found that overexpression of the cucumber gene CsSAMS1 in tobacco can affect the production of polyamines and ethylene, as well as enhancing the salt stress tolerance of tobacco, but the exact underlying mechanisms are elusive. The calcium-dependent protein kinase (CDPK) family is ubiquitous in plants and performs different biological functions in plant development and response to abiotic stress. We used a yeast two-hybrid system to detect whether the protein CDPK6 could interact with SAMS1 and verified their interaction by bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP) assays. To further explore the function of cucumber CDPK6, we isolated and characterized CsCDPK6 in cucumber. CsCDPK6 is a membrane protein that is highly expressed under various abiotic stresses, including salt stress. It was also observed that ectopic overexpression of CsCDPK6 in tobacco enhanced salt tolerance. Under salt stress, CsCDPK6-overexpressing lines enhanced the survival rate and reduced stomatal apertures in comparison to wild-type (WT) lines, as well as lowering malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents and causing less relative electrolyte leakage. Moreover, repression of CsCDPK6 expression by virus-induced gene silencing (VIGS) in cucumber seedling cotyledons under salt stress increased ethylene production and promoted the transformation from putrescine (Put) to spermidine (Spd) and spermine (Spm). These findings shed light on the interaction of CsSAMS1 and CsCDPK6, which functions positively to regulate salt stress in plants.

Biologia futura: the role of polyamine in plant science

Polyamines (PAs) are positively charged amines such as putrescine, spermidine and spermine that ubiquitously exist in all organisms. They have been considered as a new type of plant biostimulants, with pivotal roles in many physiological processes. Polyamine levels are controlled by intricate regulatory feedback mechanisms. PAs are directly or indirectly regulated through interaction with signaling metabolites (H₂0₂, NO), aminobutyric acid (GABA), phytohormones (abscisic acid, gibberellins, ethylene, cytokinins, auxin, jasmonic acid and brassinosteroids) and nitrogen metabolism (maintaining the balance of C:N in plants). Exogenous applications of PAs enhance the stress resistance, flowering and fruit set, synthesis of bioactive compounds and extension of agricultural crops shelf life. Up-regulation of PAs biosynthesis by genetic manipulation can be a novel strategy to increase the productivity of agricultural crops. Recently, the role of PAs in symbiosis relationships between plants and beneficial microorganisms has been confirmed. PA metabolism has also been targeted to design new harmless fungicides.

Effects of compound growth regulators on the anatomy of Jujube Leaf and Fruit

Effects of three compound growth regulators formulated with hypersensitivity protein, spermidine, salicylic acid and DA-6 (diethyl aminoethanol hexanoate) were tested on Xinjiang Jun Jujube. The doses of compound growth regulators were named as A (Hypersensitivity protein + spermidine + salicylic acid at the rate of 30 mg/L, 0.1 mmol/L and 0.25 mmol/L, respectively), B (Hypersensitive protein + spermidine + DA-6 at the rate of 30 mg/L, 0.1 mmol/L and 30 mg/L, respectively) and C (Spermidine + salicylic acid + DA-6 at the rate of 0.1 mmol/L, 0.25 mmol/L and 30 mg/L, respectively) versus a control group CK (contained only water). Fruit anatomical structures were compared after spraying. The results indicated that after spraying, the thickness of the upper and lower epidermal cells and the stratum corneum were increased. However, the upper epidermal stratum corneum became significantly thicker than the lower epidermis. Spraying with A improved the thickness of upper and lower epidermal cells, stratum corneum, the central vein and mesophyll. The cumulative effects of all these changes in leaf and fruit anatomical structures provided the resistance of the experimental fruit plant to stress. While the B and C regulators had inhibitory effects. So, the results obtained after spraying A category were beneficial to improve the stress resistance of the fruits. The length and cell area of pericarp and sarcocarp cells in the treatment groups were not changed significantly. But the length, number of sarcocarp cells and number of gaps were lower than those in the CK. This study can provide new measures for improving plant resistance in jujube production.

Mitochondrial proteomic analysis reveals that proteins relate to oxidoreductase activity play a central role in pollen fertility in cotton

Cotton (Gossypium hirsutum L.) is an important economic crop. Cytoplasm male sterility (CMS) has been used to develop hybrid system and to produce hybrid seeds in cotton, but the molecular mechanism of CMS remains unclear. Mitochondria are semi-autonomous organelles, which play an important role in the reproduction of flowering plants. Male sterility has been proved associated with mitochondrial dysfunction in plants. In present study, a new strategy of proteomic sequencing data-independent acquisition (DIA) was used to analysis protein abundance across CMS lines 2074A (cytoplasm of Gossypium harknessii, D_2-2) and 2074S (cytoplasm of G. hirsutum, AD₁), and their maintainer 2074B. Comparing with transcriptome results showed that there is little consistence between proteome and transcriptome. A total of 2095 protein species were identified in three materials, and 186 and 161 differentially proteins were detected in the comparisons of 2074A vs 2074B, and 2074S vs 2074B, respectively. Among them, 49 and 50 proteins were specific existed in anther, and mainly participated in oxidoreductase activity, carbohydrate metabolism, fatty acid metabolism, cell aging, wax or cutin deposition and signal transduction. Gh_A07G0770 and Gh_D05G1908 were specific up-regulated in sterility lines, and the other genes Gh_D08G1196, Gh_D12G1971, Gh_A11G1250, Gh_D08G0388 were down-regulated, which presented similar expression tendency verified by qRT-PCR, transcriptome and proteome results. These six genes related to lipid synthesis, response to oxidative stress and cell aging, suggested them being involved in CMS occurrence. Using virus-induced gene silencing (VIGS) system, sterility obtained demonstrated the silencing Gh_A11G1250 in maintainer 2074B led to partial anthers abortion. Gh_A11G1250 encoded a mitochondrial localization of peroxisomal-like protein, participated in response to reactive oxygen species (ROS). Twenty-two proteins interacting with Gh_A11G1250 mainly related to chlorophyll biosynthetic process, photoperiodism and flowering, which showed different expression pattern between the male sterile line 2074A and maintainer 2074B. This novel research based on mitochondrial proteomics comparison confirmed that DAPs related to oxidative stress are critical to pollen abortion. BIOLOGICAL SIGNIFICANCE: Cytoplasm male sterility (CMS) system is utilized widely for hybrid production in cotton. However, the genetic and molecular mechanisms of CMS still need to be further elucidated. Up till now, fewer comprehensive comparisons of the mitochondrial proteomes from cotton CMS line and maintainer line have been reported. In this study, we performed a novel comparison of mitochondrial protein profiles in two CMS lines and their common maintainer line. Based on our results, we found a potential protein related to oxidative stress led to the anthers abortion. These results accumulate data to interpret the molecular mechanisms of CMS in cotton.

Gibberellin mediates spermidine-induced salt tolerance and the expression of GT-3b in cucumber

Numerous studies have demonstrated that spermidine (Spd) plays a critical role in salt tolerance in plants, but the associated mechanism remains largely unknown. In this study, we investigated the role of gibberellin (GA) in Spd-induced salt tolerance and the expression of GT-3b transcription factor in cucumber. The results showed that exogenous Spd significantly increased the salt tolerance of cucumber plants, while its effects were compromised in the presence of methylglyoxal bis-guanylhydrazone (MGBG), an inhibitor of Spd biosynthesis. Interestingly, the expression of GT-3b was significantly induced by Spd under salt stress, and the promoter sequence of GT-3b was predicted to contain cis-acting regulatory elements that could respond to phytohormones, such as GA, salicylic acid (SA), and methyl jasmonate (MeJA). The application of GA3, SA and MeJA as foliar spray could induce the expression of GT-3b. In addition, exogenous Spd dramatically increased the expression of genes related to GA biosynthesis, the activity of gibberellin oxidase, and the accumulation of GA3, whereas these effects were attenuated in the MGBG-treated plants. Furthermore, the application of GA3 increased GT-3b expression and salt tolerance, whereas these effects were blocked when the plants were treated with paclobutrazol (PAZ), a GA biosynthesis inhibitor. Similarly, the Spd-induced salt tolerance was compromised in the PAZ-treated plants. Our results suggest that GA mediates Spd-induced salt tolerance and the expression of GT-3b in cucumber. These results provide a new perspective for understanding the molecular mechanism of Spd-regulated salt tolerance in plants.

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.

Isolation and characterization of S-Adenosylmethionine synthase gene from cucumber and responsive to abiotic stress

S-adenosylmethionine synthetase (SAMS) catalyzes methionine and ATP to generate S-adenosyl-L-methionine (SAM). In plants, accumulating SAMS genes have been characterized and the majority of them are reported to participate in development and stress response. In this study, two putative SAMS genes (CsSAMS1 and CsSAMS2) were identified in cucumber (Cucumis Sativus L.). They displayed 95% similarity and had a high identity with their homologous of Arabidopsis thaliana and Nicotiana tabacum. The qRT-PCR test showed that CsSAMS1 was predominantly expressed in stem, male flower, and young fruit, whereas CsSAMS2 was preferentially accumulated in stem and female flower. And they displayed differential expression profiles under stimuli, including NaCl, ABA, SA, MeJA, drought and low temperature. To elucidate the function of cucumber SAMS, the full-length CDS of CsSAMS1 was cloned, and prokaryotic expression system and transgenic materials were constructed. Expressing CsSAMS1 in Escherichia coli BL21 (DE3) improved the growth of the engineered strain under salt stress. Overexpression of CsSAMS1 significantly increased MDA content, H2O2 content, and POD activity in transgenic lines under non-stress condition. Under salt stress, however, the MDA content of transgenic lines was lower than that of the wild type, the H2O2 content remained high, the polyamine and ACC synthesis in transgenic lines exhibited a CsSAMS1-expressed dependent way. Taken together, our results suggested that both CsSAMS1 and CsSAMS2 were involved in plant development and stress response, and a proper increase of expression level of CsSAMS1 in plants is benificial to improving salt tolerance.

Salt-adaptive strategies in oil seed crop Ricinus communis early seedlings (cotyledon vs. true leaf) revealed from proteomics analysis

Soil salinity is a major abiotic stress affecting crop growth and productivity. Ricinus communis has good salt tolerance and is also an important oilseed crop throughout the world. Early seedling stage (such as cotyledon expansion stage) is the most vulnerable period for plant under stresses. However, little information exist concerning the physiological and molecular mechanisms of Ricinus communis seedlings and the role play by cotyledons and true leaf under salt stress. In the present study, biomass, photosynthesis, chlorophyll fluorescence, inorganic ions and organic solutes contents were measured, and two dimensional gel electrophoresis-based proteomic technology was employed to identify the differentially abundant proteins in the salt-treated Ricinus communis cotyledons and true leaves. The results showed that salt stress reduced growth and photosynthesis in the seedlings. With increasing salinity, the Na⁺ content increased and K⁺ content decreased in both cotyledons and leaves, but the true leaves had lower Na⁺ and higher K⁺ contents. Soluble sugars and proline are the primary organic solutes to cope with osmotic stress. In addition, proteomic analysis revealed 30 and 42 differentially accumulated protein spots in castor cotyledon and true leaf under salt stress, respectively. Most of the identified proteins were involved in carbohydrate and energy metabolism, photosynthesis, genetic information process, reactive oxygen species metabolism, amino acid metabolism and cell structure. The physiological and proteomic results highlighted that cotyledons accumulated a large number of Na⁺ and provided more energy to help true leaves cope with salt stress. The true leaves saved carbon structures to synthesize osmotic substances, and the enhancement of chlorophyll synthesis and electron transfer in true leaves could also maintain photosynthesis under salt stress. These findings provide new insights into different physiological mechanisms in cotyledon and true leaf of Ricinus communis response to salt stress during early seedling stage.

Proteomic and physiological analyses reveal the role of exogenous spermidine on cucumber roots in response to Ca(NO3)2 stress

The mechanism of exogenous Spd-induced Ca(NO₃)₂ stress tolerance in cucumber was studied by proteomics and physiological analyses. Protein-protein interaction network revealed 13 key proteins involved in Spd-induced Ca(NO₃)₂ stress resistance. Ca(NO₃)₂ stress is one of the major reasons for secondary salinization that limits cucumber plant development in greenhouse. The conferred protective role of exogenous Spd on cucumber in response to Ca(NO₃)₂ stress cues involves changes at the cellular and physiological levels. To investigate the molecular foundation of exogenous Spd in Ca(NO₃)₂ stress tolerance, a proteomic approach was performed in our work. After a 9 days period of Ca(NO₃)₂ stress and/or exogenous Spd, 71 differential protein spots were confidently identified. The resulting proteins were enriched in seven different categories of biological processes, including protein metabolism, carbohydrate and energy metabolism, ROS homeostasis and stress defense, cell wall related, transcription, others and unknown. Protein metabolism (31.2%), carbohydrate and energy metabolism (15.6%), ROS homeostasis and stress defense (32.5%) were the three largest functional categories in cucumber root and most of them were significantly increased by exogenous Spd. The Spd-responsive protein interaction network revealed 13 key proteins, whose accumulation changes could be critical for Spd-induced resistance; all 13 proteins were upregulated by Spd at transcriptional and protein levels in response to Ca(NO₃)₂ stress. Furthermore, accumulation of antioxidant enzymes, non-enzymatic antioxidant and polyamines, along with reduction of H₂O₂ and MDA, were detected after exogenous Spd application during Ca(NO₃)₂ stress. The results of these proteomic and physiological analyses in cucumber root may facilitate a better understanding of the underlying mechanism of Ca(NO₃)₂ stress tolerance mediated by exogenous Spd.

Determination of Posttranslational Modifications by 2D PAGE: Applications to Polyamines

Polyamines not only affect transcription and translation but also may induce a number of posttranslational modifications. The identification of polyamine-induced posttranslational modifications can be performed by 2D PAGE analyses. Here, we provide a protocol for 2D-gel electrophoresis that has been optimized for plants. The combined use of this protocol with epitope-tagged proteins expressed in plants enables the detailed analysis of posttranslational modifications induced by different polyamines in vivo.

Exogenous applications of Polyamines modulate drought responses in wheat through osmolytes accumulation, increasing free polyamine levels and regulation of polyamine biosynthetic genes

Polyamines (PAs) can improve drought stress tolerance in plants; however, very limited information is available on the mechanism of action of exogenous application by different methods under drought stress in wheat. The present study investigates the mechanism through which seed priming and foliar spraying with PAs protect wheat plants from drought stress. 10 days old wheat seedlings were exposed to drought stress by withholding water alone or with 100 μM PAs solutions (putrescine, Put; spermine, Spm; and mixture of Put and Spm for 10 h seed-priming or three foliar sprays during withholding water. Drought stress impaired the wheat growth and altered the osmoprotectants, endogenous PAs levels, PAs biosynthetic genes expression and weight of 1000 grains compared to the corresponding control values. Exogenously applied PAs improved cell water status, accumulated osmoprotectants and PAs and up-regulated PAs biosynthetic genes, ADC, arginine decarboxylase; DHS, deoxyhypusine synthase; ODC, ornithine decarboxylase and SAMDC, S-adenosyl methionine decarboxylase. Put significantly regulate the endogenous PAs by both methods of application, however, Spm and mixture of Put and Spm could positively regulate the endogenous PAs and the biosynthetic gene expression by foliar spraying rather than seed priming. The data provide evidence that maintenance of water economy through stabilized cellular structure is an important strategy of drought tolerance by PAs in wheat.

Proteomic Analysis Reveals the Positive Effect of Exogenous Spermidine in Tomato Seedlings' Response to High-Temperature Stress

Polyamines are phytohormones that regulate plant growth and development as well as the response to environmental stresses. To evaluate their functions in high-temperature stress responses, the effects of exogenous spermidine (Spd) were determined in tomato leaves using two-dimensional electrophoresis and MALDI-TOF/TOF MS. A total of 67 differentially expressed proteins were identified in response to high-temperature stress and/or exogenous Spd, which were grouped into different categories according to biological processes. The four largest categories included proteins involved in photosynthesis (27%), cell rescue, and defense (24%), protein synthesis, folding and degradation (22%), and energy and metabolism (13%). Exogenous Spd up-regulated most identified proteins involved in photosynthesis, implying an enhancement in photosynthetic capacity. Meanwhile, physiological analysis showed that Spd could improve net photosynthetic rate and the biomass accumulation. Moreover, an increased high-temperature stress tolerance by exogenous Spd would contribute to the higher expressions of proteins involved in cell rescue and defense, and Spd regulated the antioxidant enzymes activities and related genes expression in tomato seedlings exposed to high temperature. Taken together, these findings provide a better understanding of the Spd-induced high-temperature resistance by proteomic approaches, providing valuable insight into improving the high-temperature stress tolerance in the global warming epoch.

Proteomic Analysis Reveals the Positive Roles of the Plant-Growth-Promoting Rhizobacterium NSY50 in the Response of Cucumber Roots to Fusarium oxysporum f. sp. cucumerinum Inoculation

Plant-growth-promoting rhizobacteria (PGPR) can both improve plant growth and enhance plant resistance against a variety of environmental stresses. To investigate the mechanisms that PGPR use to protect plants under pathogenic attack, transmission electron microscopy analysis and a proteomic approach were designed to test the effects of the new potential PGPR strain Paenibacillus polymyxa NSY50 on cucumber seedling roots after they were inoculated with the destructive phytopathogen Fusarium oxysporum f. sp. cucumerinum (FOC). NSY50 could apparently mitigate the injury caused by the FOC infection and maintain the stability of cell structures. The two-dimensional electrophoresis (2-DE) approach in conjunction with MALDI-TOF/TOF analysis revealed a total of 56 proteins that were differentially expressed in response to NSY50 and/or FOC. The application of NSY50 up-regulated most of the identified proteins that were involved in carbohydrate metabolism and amino acid metabolism under normal conditions, which implied that both energy generation and the production of amino acids were enhanced, thereby ensuring an adequate supply of amino acids for the synthesis of new proteins in cucumber seedlings to promote plant growth. Inoculation with FOC inhibited most of the proteins related to carbohydrate and energy metabolism and to protein metabolism. The combined inoculation treatment (NSY50+FOC) accumulated abundant proteins involved in defense mechanisms against oxidation and detoxification as well as carbohydrate metabolism, which might play important roles in preventing pathogens from attacking. Meanwhile, western blotting was used to analyze the accumulation of enolase (ENO) and S-adenosylmethionine synthase (SAMs). NSY50 further increased the expression of ENO and SAMs under FOC stress. In addition, NSY50 adjusted the transcription levels of genes related to those proteins. Taken together, these results suggest that P. polymyxa NSY50 may promote plant growth and alleviate FOC-induced damage by improving the metabolism and activation of defense-related proteins in cucumber roots.

Physiological and proteomic analysis of selenium-mediated tolerance to Cd stress in cucumber (Cucumis sativus L.)

Selenium can mitigate cadmium toxicity in plants. However, the mechanism of this alleviation has not been fully understood. In the present study, the role of Se in inducing tolerance to Cd stress in cucumber was elucidated. Results showed that Se significantly alleviated Cd-induced growth inhibition, reduced Cd concentration, increased SPAD value and improved photosynthetic performance. Through proteomic analysis by two-dimensional gel electrophoresis (2-DE) coupled with mass spectrometry, 26 protein spots were identified, which were significantly influenced by Cd stress and/or Se application. Among these proteins, the abundance of 21 spots (10 in leaves and 11 in roots) were repressed in Cd-treated and up-accumulated or no-changed in Cd+Se-treated cucumber. These altered proteins were involved in the response to stress, metabolism, photosynthesis and storage, they were including glutathione S-transferase F8, heat shock protein STI-like, peroxidase, ascorbate oxidase, fructose-bisphosphate aldolase 2, NiR, Rieske type ion sulfur subunit and PsbP domain-containing protein 6. Furthermore, we identified five proteins with an increase in relative abundance after Cd treatment, they were involved in the functional groups active in response to stress and transport. The present study provided novel insights into Se-mediated tolerance of cucumber seedlings against Cd toxicity at the proteome level.

Root proteomics reveals cucumber 24-epibrassinolide responses under Ca(NO3)2 stress

The application of exogenous 24-epibrassinolide promotes Brassinosteroids intracellular signalling in cucumber, which leads to differentially expressed proteins that participate in different life process to relieve Ca(NO 3 ) 2 damage. NO3 (-) and Ca(2+) are the main anion and cation of soil secondary salinization during greenhouse cultivation. Brassinosteroids (BRs), steroidal phytohormones, regulate various important physiological and developmental processes and are used against abiotic stress. A two-dimensional electrophoresis gel coupled with MALDI-TOF/TOF MS was performed to investigate the effects of exogenous 24-epibrassinolide (EBL) on proteomic changes in cucumber seedling roots under Ca(NO3)2 stress. A total of 80 differentially accumulated protein spots in response to stress and/or exogenous EBL were identified and grouped into different categories of biological processes according to Gene Ontology. Under Ca(NO3)2 stress, proteins related to nitrogen metabolism and lignin biosynthesis were induced, while those related to cytoskeleton organization and cell-wall neutral sugar metabolism were inhibited. However, the accumulation of abundant proteins involved in protein modification and degradation, defence mechanisms against antioxidation and detoxification and lignin biosynthesis by exogenous EBL might play important roles in salt tolerance. Real-time quantitative PCR was performed to investigate BR signalling. BR signalling was induced intracellularly under Ca(NO3)2 stress. Exogenous EBL can alleviate the root indices, effectively reduce the Ca(2+) content and increase the K(+) content in cucumber roots under Ca(NO3)2 stress. This study revealed the differentially expressed proteins and BR signalling-associated mRNAs induced by EBL in cucumber seedling roots under Ca(NO3)2 stress, providing a better understanding of EBL-induced salt resistance in cucumber seedlings. The mechanism for alleviation provides valuable insight into improving Ca(NO3)2 stress tolerance of other horticultural plants.

Proteomics, photosynthesis and salt resistance in crops: An integrative view

Salinity is a stressful condition that causes a significant decrease in crop production worldwide. Salt stress affects several photosynthetic reactions, including the modulation of several important proteins. Despite these effects, few molecular-biochemical markers have been identified and evaluated for their importance in improving plant salt resistance. Proteomics is a powerful tool that allows the analysis of multigenic events at the post-translational level that has been widely used to evaluate protein modulation changes in plants exposed to salt stress. However, these studies are frequently fragmented and the results regarding photosynthesis proteins in response to salinity are limited. These constraints could be related to the low number of important photosynthetic proteins differently modulated in response to salinity, as has been commonly revealed by conventional proteomics. In this review, we present an evaluation and perspective on the integrated application of proteomics for the identification of photosynthesis proteins to improve salt resistance. We propose the use of phospho-, thiol- and redox-proteomics, associated with the utilization of isolated chloroplasts or photosynthetic sub-organellar components. This strategy may allow the characterization of essential proteins, providing a better understanding of photosynthesis regulation. Furthermore, this may contribute to the selection of molecular markers to improve salt resistance in crops.

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.

Proteome Dynamics and Physiological Responses to Short-Term Salt Stress in Brassica napus Leaves

Salt stress limits plant growth and crop productivity and is an increasing threat to agriculture worldwide. In this study, proteomic and physiological responses of Brassica napus leaves under salt stress were investigated. Seedlings under salt treatment showed growth inhibition and photosynthesis reduction. A comparative proteomic analysis of seedling leaves exposed to 200 mM NaCl for 24 h, 48 h and 72 h was conducted. Forty-four protein spots were differentially accumulated upon NaCl treatment and 42 of them were identified, including several novel salt-responsive proteins. To determine the functional roles of these proteins in salt adaptation, their dynamic changes in abundance were analyzed. The results suggested that the up-accumulated proteins, which were associated with protein metabolism, damage repair and defense response, might contribute to the alleviation of the deleterious effect of salt stress on chlorophyll biosynthesis, photosynthesis, energy synthesis and respiration in Brassica napus leaves. This study will lead to a better understanding of the molecular basis of salt stress adaptation in Brassica napus and provides a basis for genetic engineering of plants with improved salt tolerance in the future.

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.

Characterization of the Transcriptome of the Xerophyte Ammopiptanthus mongolicus Leaves under Drought Stress by 454 Pyrosequencing

BACKGROUND

Ammopiptanthus mongolicus (Maxim. Ex Kom.) Cheng f., an endangered ancient legume species, endemic to the Gobi desert in north-western China. As the only evergreen broadleaf shrub in this area, A. mongolicus plays an important role in the region's ecological-environmental stability. Despite the strong potential of A. mongolicus in providing new insights on drought tolerance, sequence information on the species in public databases remains scarce. To both learn about the role of gene expression in drought stress tolerance in A. mongolicus and to expand genomic resources for the species, transcriptome sequencing of stress-treated A. mongolicus plants was performed.

RESULTS

Using 454 pyrosequencing technology, 8,480 and 7,474 contigs were generated after de novo assembly of RNA sequences from leaves of untreated and drought-treated plants, respectively. After clustering using TGICL and CAP3 programs, a combined assembly of all reads produced a total of 11,357 putative unique transcripts (PUTs). Functional annotation and classification of the transcripts were conducted by aligning the 11,357 PUTs against the public protein databases and nucleotide database (Nt). Between control and drought-treated plants, 1,620 differentially expressed genes (DEGs) were identified, of which 1,106 were up-regulated and 514 were down-regulated. The differential expression of twenty candidate genes in metabolic pathways and transcription factors families related to stress-response were confirmed by quantitative real-time PCR. Representatives of several large gene families, such as WRKY and P5CS, were identified and verified in A. mongolicus for the first time.

CONCLUSIONS

The additional transcriptome resources, gene expression profiles, functional annotations, and candidate genes provide a more comprehensive understanding of the stress response pathways in xeric-adapted plant species such as A. mongolicus.

Deciphering the protective role of spermidine against saline-alkaline stress at physiological and proteomic levels in tomato

In this research, the protective effect of spermidine (Spd) in mitigating saline-alkaline stress in tomato (Solanum lycopersicum L.) at physiological and proteomic levels were examined. The results showed that saline-alkaline stress induced accumulation of H2O2 and O2(-*), and increased the activities of antioxidase (SOD, CAT, and POD). Spermidine efficiently alleviated the inhibitory role of saline-alkaline on plant growth and inhibited saline-alkaline stress-induced H2O2 and O2(-*) accumulation. Proteomics investigations of the leaves of tomato seedlings, responding to a 75 mM saline-alkaline solution and 0.25 mM Spd, were performed. Maps of the proteome of leaf extracts were obtained by two-dimensional gel electrophoresis. An average of 49, 47 and 34 spots, which appeared repeatedly and that significantly altered the relative amounts of polypeptides by more than twofold, were detected for seedlings treated with saline-alkaline solution (S) compared to normal solution (CK), saline-alkaline plus spermidine (MS) compared to CK, or S versus MS, respectively. Thirty-nine of these proteins were identified by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and were classified into five functional categories, including energy and metabolism, signal transduction, amino acid metabolism, protein metabolism, and stress-defense response. Proteomics analysis coupled with bioinformatics indicated that Spd treatment helps tomato seedlings combat saline-alkaline stress by modulating the defense mechanism of plants and activating cellular detoxification, which protect plants from oxidative damage induced by saline-alkaline stress.

Effects of grafting with pumpkin rootstock on carbohydrate metabolism in cucumber seedlings under Ca(NO3)2 stress

This study investigated the effects of grafting on the carbohydrate status and the enzymes of carbohydrate metabolism in self-grafted and grafted cucumber seedlings using the salt-tolerant pumpkin rootstock 'Qingzhen 1' (Cucurbita maxima × Cucurbita moschata) under 80 mM Ca(NO3)2 stress for 6 d. The growth of self-grafted seedlings was significantly inhibited after the treatment of Ca(NO3)2 stress, whereas the inhibition of growth was alleviated in pumpkin rootstock-grafted seedlings. Ca(NO3)2 stress increased the contents of the total soluble sugar, sucrose and fructose, but decreased the starch content in rootstock-grafted leaves. However, compared with self-grafted plants, rootstock-grafted seedlings were observed with a higher content of sucrose and total soluble sugar (TSS) under salt stress. Rootstock-grafted seedlings exhibited higher activities of acid invertase (AI), neutral invertase (NI) and phosphate sucrose synthase (SPS) of sucrose metabolism in leaves than that of self-grafted seedlings under salinity. Moreover, the activities of fructokinase (FK), hexokinase (HK), phosphofructokinase (PFK) and pyruvate kinase (PK) of glycolysis were maintained at a higher level in leaves of rootstock-grafted seedlings after Ca(NO3)2 stress. Additionally, rootstock-grafting decrease the high percentage enhancement of key enzymes gene expression in glycolysis in the scion leaves of cucumber seedlings induced by salt stress. These results suggest that the rootstock-grafting improved salt tolerance, which might play a role in elevated sucrose metabolism and a glycolytic pathway regulated by the pumpkin rootstock.

Proteomic analysis of anthers from wild-type and photosensitive genetic male sterile mutant cotton (Gossypium hirsutum L.)

BACKGROUND

Male sterility is a common phenomenon in flowering plant species, and it has been successfully developed in several crops by taking advantage of heterosis. Using space mutation breeding of upland cotton, a novel photosensitive genetic male sterile (PGMS) mutant was isolated. To take advantage of the PGMS lines in cotton hybrid breeding, it is of great importance to study the molecular mechanisms of its male sterility.

RESULTS

Delayed degradation of the PGMS anther tapetum occurred at different developmental stages as shown by analysis of anther cross-sections. To gain detailed insights into the cellular defects that occurred during PGMS pollen development, we used a differential proteomic approach to investigate the protein profiles of mutant and wild-type anthers at the tetrad, uninucleate and binucleate pollen stages. This approach identified 62 differentially expressed protein spots, including 19 associated with energy and metabolic pathways, 7 involved with pollen tube growth, 5 involved with protein metabolism, and 4 involved with pollen wall development. The remaining 27 protein spots were classified into other functional processes, such as protein folding and assembly (5 spots), and stress defense (4 spots). These differentially expressed proteins strikingly affected pollen development in the PGMS mutant anther and resulted in abnormal pollen grain formation, which may be the key reason for its male sterility.

CONCLUSIONS

This work represents the first study using comparative proteomics between fertile and PGMS cotton plants to identify PGMS-related proteins. The results demonstrate the presence of a complicated metabolic network in anther development and advance our understanding of the molecular mechanisms of microgamete formation, providing insights into the molecular mechanisms of male sterility.

The roles of polyamines during the lifespan of plants: from development to stress

Compelling evidence indicates that free polyamines (PAs) (mainly putrescine, spermidine, spermine, and its isomer thermospermine), some PA conjugates to hydroxycinnamic acids, and the products of PA oxidation (hydrogen peroxide and γ-aminobutyric acid) are required for different processes in plant development and participate in abiotic and biotic stress responses. A tight regulation of PA homeostasis is required, since depletion or overaccumulation of PAs can be detrimental for cell viability in many organisms. In plants, homeostasis is achieved by modulation of PA biosynthesis, conjugation, catabolism, and transport. However, recent data indicate that such mechanisms are not mere modulators of PA pools but actively participate in PA functions. Examples are found in the spermidine-dependent eiF5A hypusination required for cell division, PA hydroxycinnamic acid conjugates required for pollen development, and the involvement of thermospermine in cell specification. Recent advances also point to implications of PA transport in stress tolerance, PA-dependent transcriptional and translational modulation of genes and transcripts, and posttranslational modifications of proteins. Overall, the molecular mechanisms identified suggest that PAs are intricately coordinated and/or mediate different stress and developmental pathways during the lifespan of plants.

Comparative proteomics of Euphorbia kansui Liou milky sap at two different developmental stages

Euphorbia kansui Liou is a unique traditional Chinese medicinal herb. Its milky sap proteins play important roles in laticifer development, synthesis and transport of its biologically active substances. A proteomic approach was applied to analyze the E. kansui latex proteins related to laticifer development and secondary metabolite synthesis by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional polyacrylamide gel electrophoresis. A total of 125 milky sap proteins associated with development of laticifers, disease and defense, and general metabolism were identified, and 19 differentially expressed proteins at two different developmental stages of laticifers were successfully detected. Peroxidase, cytochrome P450 mono-oxygenase superfamily, lipoxygenase, and multidrug resistance protein ABC transporter family may be involved in laticifer development, secondary metabolite synthesis and transport, and plant physiology.

Mechanism of salinity tolerance in plants: physiological, biochemical, and molecular characterization

Salinity is a major abiotic stress limiting growth and productivity of plants in many areas of the world due to increasing use of poor quality of water for irrigation and soil salinization. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, and molecular or gene networks. A comprehensive understanding on how plants respond to salinity stress at different levels and an integrated approach of combining molecular tools with physiological and biochemical techniques are imperative for the development of salt-tolerant varieties of plants in salt-affected areas. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress.

Improvement of plant abiotic stress tolerance through modulation of the polyamine pathway

Polyamines (mainly putrescine (Put), spermidine (Spd), and spermine (Spm)) have been widely found in a range of physiological processes and in almost all diverse environmental stresses. In various plant species, abiotic stresses modulated the accumulation of polyamines and related gene expression. Studies using loss-of-function mutants and transgenic overexpression plants modulating polyamine metabolic pathways confirmed protective roles of polyamines during plant abiotic stress responses, and indicated the possibility to improve plant tolerance through genetic manipulation of the polyamine pathway. Additionally, putative mechanisms of polyamines involved in plant abiotic stress tolerance were thoroughly discussed and crosstalks among polyamine, abscisic acid, and nitric oxide in plant responses to abiotic stress were emphasized. Special attention was paid to the interaction between polyamine and reactive oxygen species, ion channels, amino acid and carbon metabolism, and other adaptive responses. Further studies are needed to elucidate the polyamine signaling pathway, especially polyamine-regulated downstream targets and the connections between polyamines and other stress responsive molecules.

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.