Shotgun proteomic analysis of soybean embryonic axes during germination under salt stress

Seed imbibition and radicle emergence are generally less affected by salinity in soybean than in other crop plants. In order to unveil the mechanisms underlying this remarkable salt tolerance of soybean at seed germination, a comparative label‐free shotgun proteomic analysis of embryonic axes exposed to salinity during germination sensu stricto (GSS) was conducted. The results revealed that the application of 100 and 200 mmol/L NaCl stress was accompanied by significant changes (>2‐fold, P<0.05) of 97 and 75 proteins, respectively. Most of these salt‐responsive proteins (70%) were classified into three major functional categories: disease/defense response, protein destination and storage and primary metabolism. The involvement of these proteins in salt tolerance of soybean was discussed, and some of them were suggested to be potential salt‐tolerant proteins. Furthermore, our results suggest that the cross‐protection against aldehydes, oxidative as well as osmotic stress, is the major adaptive response to salinity in soybean.     

Heat and salt stress in the food pathogen Bacillus cereus

AIMS: The effects of stresses imposed on bacterial contaminants during food processing and treatment of packaging material were evaluated on the food pathogen Bacillus cereus. METHODS AND RESULTS: Conditions were established which allowed the cells to adapt to heat, ethanol and hydrogen peroxide stresses, but not to osmotic shock. Cross protection between stresses indicated a clear hierarchy of resistance with salt protecting against hydrogen peroxide, which protected against ethanol, which protected against heat shock. The cultures were shown to be most sensitive to heat, ethanol and oxidative stress at mid-exponential phase and to become resistant at stationary phase. Adaptive levels of stressor were found to induce synthesis of general stress and stress-specific proteins and differential accumulation of proteins was demonstrated between heat- or salt-stressed and unstressed cells. CONCLUSIONS: Sequencing revealed that a number of glycolytic enzymes were regulated by heat and osmotic shocks and that the chaperone GroEL was induced by heat shock. SIGNIFICANCE AND IMPACT OF THE STUDY: The implications of the physiological data in designing storage and processing conditions for food are discussed. The identification of stress-regulated proteins reveals a clear role for glycolysis in adaptation to heat shock and osmotic stress.     

Autophagy is required for tolerance of drought and salt stress in plants

Autophagy is a protein degradation process in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon the induction of autophagy, a double membrane autophagosome forms around cytoplasmic components and delivers them to the vacuole or lysosome for degradation. In plants, autophagy has been shown previously to be induced during abiotic stresses including nutrient starvation and oxidative stress. In this paper, we demonstrate the induction of autophagy in high salt and osmotic stress conditions, concomitant with the upregulation of expression of an Arabidopsis thaliana autophagy-related gene AtATG18a. Autophagy-defective RNAi-AtATG18a plants are more sensitive to salt and drought conditions than wild-type plants, demonstrating a role for autophagy in the response to these stresses. NADPH oxidase inhibitors block autophagy induction upon nutrient starvation and salt stress, but not during osmotic stress, indicating that autophagy can be activated by NADPH oxidase-dependent or -independent pathways. Together our results indicate that diverse environmental stresses can induce autophagy and that autophagy is regulated by distinct signaling pathways in different conditions.     

核黄素参与水稻非生物胁迫响应的分析

为探究核黄素在水稻非生物胁迫响应中的作用,以粳稻Kitaake和籼稻T98B为试验材料,考察了核黄素对2种材料的盐、高温、渗透、碱和氧化胁迫响应的影响,重点测定了盐和高温胁迫下水稻体内核黄素合成基因的表达和相关生理指标。结果表明,(1)施加外源核黄素有效提高了2种水稻材料的盐和高温胁迫耐受性,降低了渗透胁迫耐受性,而其氧化和碱胁迫耐受性不受影响。(2)逆境胁迫均不同程度地促进了核黄素在2种水稻材料中的积累,尤其在盐和高温胁迫下促进效果最明显。(3)盐和高温胁迫均诱导了核黄素合成酶基因的表达,促进了核黄素的生物合成,改善了水稻的胁迫耐受性。研究表明,非生物逆境胁迫能促进核黄素在水稻体内的合成和积累,外源核黄素也能明显提高水稻对盐和高温胁迫的耐受性,但却降低了其对渗透胁迫的耐受性。     

Differential Responses of Nitrogen-Fixing Cyanobacteria to Salinity and Osmotic Stresses

Two nitrogen-fixing Anabaena strains were found to be differentially tolerant to salinity and osmotic stresses. Anabaena torulosa, a brackish-water, salt-tolerant strain, was relatively osmosensitive. Anabaena sp. strain L-31, a freshwater, salt-sensitive strain, on the other hand, displayed significant osmotolerance. Salinity and osmotic stresses affected nitrogenase activity differently. Nitrogen fixation in both of the strains was severely inhibited by the ionic, but not by the osmotic, component of salinity stress. Such differential sensitivity of diazotrophy to salinity-osmotic stresses was observed irrespective of the inherent tolerance of the two strains to salt-osmotic stress. Exogenously added ammonium conferred significant protection against salinity stress but was ineffective against osmotic stress. Salinity and osmotic stresses also affected stress-induced gene expression differently. Synthesis of several proteins was repressed by salinity stress but not by equivalent or higher osmotic stress. Salinity and osmotic stresses induced many common proteins. In addition, unique salt stress- or osmotic stress-specific proteins were also induced in both strains, indicating differential regulation of protein synthesis by the two stresses. These data show that cyanobacterial sensitivity and responses to salinity and osmotic stresses are distinct, independent phenomena.     

Gene mining in halophytes: functional identification of stress tolerance genes in Lepidium crassifolium

Extremophile plants are valuable sources of genes conferring tolerance traits, which can be explored to improve stress tolerance of crops. Lepidium crassifolium is a halophytic relative of the model plant Arabidopsis thaliana, and displays tolerance to salt, osmotic and oxidative stresses. We have employed the modified Conditional cDNA Overexpression System to transfer a cDNA library from L. crassifolium to the glycophyte A. thaliana. By screening for salt, osmotic and oxidative stress tolerance through in vitro growth assays and non‐destructive chlorophyll fluorescence imaging, 20 Arabidopsis lines were identified with superior performance under restrictive conditions. Several cDNA inserts were cloned and confirmed to be responsible for the enhanced tolerance by analysing independent transgenic lines. Examples include full‐length cDNAs encoding proteins with high homologies to GDSL‐lipase/esterase or acyl CoA‐binding protein or proteins without known function, which could confer tolerance to one or several stress conditions. Our results confirm that random gene transfer from stress tolerant to sensitive plant species is a valuable tool to discover novel genes with potential for biotechnological applications.     

Seed biopriming with drought tolerant isolates of Trichoderma harzianum promote growth and drought tolerance in Triticum aestivum

Green house study was aimed to investigate the effect of seed biopriming with drought tolerant isolates of Trichoderma harzianum, viz. Th 56, 69, 75, 82 and 89 on growth of wheat under drought stress and to explore the mechanism underlying plant water stress resilience in response to Trichoderma inoculation. Measurements of relative water content, osmotic potential, osmotic adjustment, leaf gas exchange, chlorophyll fluorescence and membrane stability index were performed. In addition, analysis of the phenolics, proline, lipid peroxidation and measurements of phenylalanine ammonia‐lyase activity were carried out. Seed biopriming enhanced drought tolerance of wheat as drought induced changes like stomatal conductance, net photosynthesis and chlorophyll fluorescence were delayed. Drought stress from 4 to 13 days of withholding water induced an increase in the concentration of stress induced metabolites in leaves, while Trichoderma colonisation caused decrease in proline, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and an increase in total phenolics. A common factor that negatively affects plants under drought stress conditions is accumulation of toxic reactive oxygen species (ROS), and we tested the hypothesis that seed biopriming reduced damages resulting from accumulation of ROS in stressed plants. The enhanced redox state of colonised plants could be explained by higher l ‐phenylalanine ammonia‐lyase (PAL) activity in leaves after 13 days of drought stress in Trichoderma treated plants. Similar activity was induced in untreated plants in response to drought stress but to a lower extent in comparison to treated plants. Our results support the hypothesis that seed biopriming in wheat with drought tolerant T. harzianum strains increased root vigour besides performing the process of osmoregulation. It ameliorates drought stress by inducing physiological protection in plants against oxidative damage, due to enhanced capacity to scavenge ROS and increased level of PAL, a mechanism that is expected to augment tolerance to abiotic stresses.     

Effects of iso-osmotic NaCl and mannitol on growth, proline content, and antioxidant defense in Mammillaria gracilis Pfeiff. in vitro-grown cultures

Effects of iso-osmotic concentrations of NaCl and mannitol were studied in Mammilaria gracilis (Cactaceae) in both calli and tumors grown in vitro. In both tissues, relative growth rates were reduced under osmotic stress, which were accompanied by a decrease in both tissue water and K⁺ content. However, growth was inhibited to a lesser extent after exposure to NaCl, when accumulation of Na⁺ ions was observed. In calli, only salinity increased proline content, whereas with tumors proline accumulated after both osmotic stresses. Osmotic stresses also induced oxidative damage in both cactus tissues, although higher oxidative injury was caused by mannitol in calli and by salt in tumors. Low iso-osmotic concentrations of NaCl (75 mM) and mannitol (150 mM) increased peroxidase, ascorbate peroxidase, and esterase activities, whereas elevated catalase activity was recorded only after mannitol treatment in both tissues. High osmotic stress generally decreased enzymatic activities. However, in calli, esterase activity increased in response to high salinity, whereas ascorbate peroxidase activity was enhanced after high mannitol stress. In conclusion, both in vitro-grown cactus tissues were found to be sensitive to osmotic stress caused by either mannitol or NaCl, but accumulation of Na⁺ ions in response to salt somewhat contributed to osmotic adjustment. However, more prominent oxidative damage induced by NaCl compared to mannitol in tumor could be related to ion toxicity. The mechanisms that mediate responses to salt- and mannitol-induced osmotic stresses differed and were dependent on tissue type.     

RpoS involvement and requirement for exogenous nutrient for osmotically induced cross protection in Vibrio vulnificus

Vibrio vulnificus is an opportunistic human pathogen which is the causative agent of food-borne disease and wound infections. V. vulnificus is able to adapt to a variety of potentially stressful environmental changes, such as osmotic, nutrient, and temperature variations in estuarine environments, as well as oxidative, osmotic, and acidity differences following infection of a human host. After exposure to sub-lethal levels of a particular environmental stress, many bacteria become resistant to unrelated stresses, a phenomenon termed cross protection. In this study, we examined the ability of osmotic shock to cross protect V. vulnificus to high temperature as well as oxidative stress. Log phase cells of V. vulnificus strain C7184o were cross protected by prior osmotic shock to both heat and oxidative challenge, but only when exogenous nutrient was present during the osmotic upshift. Further, and unlike other bacteria, nutrient starvation alone did not result in cross protection against either stress. When small amounts of nutrient were present during osmotic shock, cross protection to an otherwise lethal heat challenge developed extremely rapidly, with significant protection seen within 10 min. Cross protection to oxidative stress was slower to develop, requiring several hours. Although stationary phase alone conferred some cross protection to heat and oxidative stress, the alternate sigma factor RpoS was required for complete cross protection of log phase cells to oxidative stress but not for resistance to heat challenge. Together these findings suggest that the cross protective response in V. vulnificus is complex and appears to involve multiple mechanisms.     

Effect of thermal,oxidative, acidic,osmotic, or nutritional stresses on subsequent culturability of Escherichia coli in seawater

Survival of stressed Escherichia coli with or without the rpoS gene was assessed after 2 and 6 days in sterile seawater. Cells were submitted to thermal (48°C), acidic (pH 5.1), oxidative (H₂O₂ 1mm), nutritional (C, N, P starvation), or osmotic (NaCl 0.5m) stresses for periods ranging from 0 to 4 h. We found a stress-mediated cross protection against seawater relative to controls. Viability was higher when cells were acid, oxidatively, nutritionally or osmotically stressed. Survival increased in cells stressed at 37°C as compared with 20°C. With the exception of osmotic stress, we found that this stress-induced cross protection was rpoS dependent.Correspondence to: P.M. Munro.     

Plasma membrane Ca²+ transporters mediate virus-induced acquired resistance to oxidative stress

This paper reports the phenomenon of acquired cross‐tolerance to oxidative stress in plants and investigates the activity of specific Ca²⁺ transport systems mediating this phenomenon. Nicotiana benthamiana plants were infected with Potato virus X (PVX) and exposed to oxidative [either ultraviolet (UV‐C) or H₂O₂] stress. Plant adaptive responses were assessed by the combined application of a range of electrophysiological (non‐invasive microelectrode ion flux measurements), biochemical (Ca²⁺‐ and H⁺‐ATPase activity), imaging (fluorescence lifetime imaging measurements of changes in intracellular Ca²⁺ concentrations), pharmacological and cytological transmission electrone microscopy techniques. Virus‐infected plants had a better ability to control UV‐induced elevations in cytosolic‐free Ca²⁺ and prevent structural and functional damage of chloroplasts. Taken together, our results suggest a high degree of crosstalk between UV and pathogen‐induced oxidative stresses, and highlight the crucial role of Ca²⁺ efflux systems in acquired resistance to oxidative stress in plants.     

A step towards understanding plant responses to multiple environmental stresses: a genome‐wide study

In natural habitats, especially in arid areas, plants are often simultaneously exposed to multiple abiotic stresses, such as salt, osmotic and heat stresses. However, most analyses of gene expression in stress responses examine individual stresses. In this report, we compare gene expression in individual and combined stresses. We show that combined stress treatments with salt, mannitol and heat induce a unique pattern of gene expression that is not a simple merge of the individual stress responses. Under multiple stress conditions, expression of most heat and salt stress‐responsive genes increased to levels similar to or higher than those measured in single stress conditions, but osmotic stress‐responsive genes increased to lower levels. Genes up‐regulated to higher levels under multiple stress condition than single stress conditions include genes for heat shock proteins, heat shock regulators and late embryogenesis abundant proteins (LEAs), which protect other proteins from damage caused by stresses, suggesting their importance in multiple stress condition. Based on this analysis, we identify candidate genes for engineering crop plants tolerant to multiple stresses.     

Recent methods of drought stress tolerance in plants

In the climate change scenario the drought has been diagnosed as major stress affecting crop productivity. This review demonstrates some recent findings on the amelioration of drought stress. Nanoparticles, synthetic growth regulators viz. Trinexapac-ethyl, and Biochar addition helps to economize the water budget of plants, enhances the bioavailability of water and nutrients as well as overcomes drought induced osmotic and oxidative stresses. Besides ABA, SA and JA are also involved in inducing tolerance to drought stress through modulation of physiological and biochemical processes in plants. Plant growth promoting rhizobacteria (PGPR) offer new opportunities in agricultural biotechnology. These beneficial microorganisms colonize the rhizosphere/endo-rhizosphere of plants and impart drought tolerance by improving root architechture, enhancing water use efficiency, producing exopolysaccharides, phytohormones viz, ABA, SA and IAA and volatile compounds. Further PGPR also play positive role in combating osmotic and oxidative stresses induced by drought stress through enhancing the accumulation of osmolytes, antioxidants and upregulation or down regulation of stress responsive genes. In transgenic plants stress inducible genes enhanced abiotic stress tolerance by encoding key enzymes regulating biosynthesis of compatible solutes. The role of genes/cDNAs encoding proteins involved in regulating other genes/proteins, signal transduction process and strategies to improve drought stress tolerance have also been discussed.     

Over-expression of StAPX in tobacco improves seed germination and increases early seedling tolerance to salinity and osmotic stresses

Ascorbate peroxidase plays a key role in scavenging reactive oxygen species under environmental stresses and in protecting plant cells against toxic effects. The Solanum lycopersicum thylakoid-bound ascorbate peroxidase gene (StAPX) was introduced into tobacco under the control of the cauliflower mosaic virus 35S promoter. Transformants were selected for their ability to grow on medium containing kanamycin. RNA gel blot analysis confirmed that StAPX was transferred into the tobacco genome and StAPX was induced by salt and osmotic stresses in tomato leaves. Over-expression of StAPX in tobacco improved seed germination rate and elevated stress tolerance during post-germination development. Two transgenic lines showed higher APX activity and accumulated less hydrogen peroxide than wild-type plants after stress treatments. The photosynthetic rates, the root lengths, the fresh and dry weights of the transgenic lines were distinctly higher than those of wild-type plants under stress conditions. Results indicated that the over-expression of StAPX had enhanced tolerance to salt stress and osmotic stress in transgenic tobacco plants.