Exogenous proline and trehalose promote recovery of rice seedlings from salt-stress and differentially modulate antioxidant enzymes and expression of related genes

HS-SPME-GC-MS/MS Method for the Rapid and Sensitive Quantitation of 2-Acetyl-1-pyrroline in Single Rice Kernels

Demand for aromatic rice varieties (e.g., Basmati) is increasing in the US. Aromatic varieties typically have elevated levels of the aroma compound 2-acetyl-1-pyrroline (2AP). Due to its very low aroma threshold, analysis of 2AP provides a useful screening tool for rice breeders. Methods for 2AP analysis in rice should quantitate 2AP at or below sensory threshold level, avoid artifactual 2AP generation, and be able to analyze single rice kernels in cases where only small sample quantities are available (e.g., breeding trials). We combined headspace solid phase microextraction with gas chromatography tandem mass spectrometry (HS-SPME-GC-MS/MS) for analysis of 2AP, using an extraction temperature of 40 °C and a stable isotopologue as internal standard. 2AP calibrations were linear between the concentrations of 53 and 5380 pg/g, with detection limits below the sensory threshold of 2AP. Forty-eight aromatic and nonaromatic, milled rice samples from three harvest years were screened with the method for their 2AP content, and overall reproducibility, observed for all samples, ranged from 5% for experimental aromatic lines to 33% for nonaromatic lines.

The Functional and Regulatory Mechanisms of the Thellungiella salsuginea Ascorbate Peroxidase 6 (TsAPX6) in Response to Salinity and Water Deficit Stresses

Soil salinization is a resource and ecological problem in the world. Thellungiella salsuginea is becoming a new model plant because it resembles its relative species, Arabidopsis thaliana, in small genome and short life cycle. It is highly tolerant to salinity and drought stresses. Ascorbate peroxidase (APX) is an enzyme that clears H₂O₂ in plants. The function and molecular and regulation mechanisms of APX in T. salsuginea have rarely been reported. In this study, an APX gene, TsApx6, was cloned from T. salsuginea and its responses to abiotic stresses in transgenic Arabidopsis were studied. Under high salinity treatment, the expression of TsApx6 was significantly induced. Under drought treatment, overexpression of TsApx6 increased the survival rate and reduced leaf water loss rate in Arabidopsis. Compared to the wild type plants, high salinity treatment reduced the concentrations of MDA, H₂O₂ and proline but elevated the activities of APX, GPX, CAT and SOD in the TsApx6-overexpressing plants. Meanwhile, germination rate, cotyledon greening, and root length were improved in the transgenic plants compared to the wild type plants under salt and water deficit conditions. Based on these findings, TsApx6 has an important function in the resistance of plants to certain abiotic stresses. The TsApx6 promoter sequence was obtained using Genome Walking technology. Bioinformatics analysis indicated that it contains some cis-acting elements related to stress response. The treatments of salt, dehydration, and ABA induced the expression of Gus gene under the regulation of the TsApx6 promoter. Mutation analysis showed that the MBS motif present in the TsApx6 promoter might be a key negative regulatory element which has an important effect on the growth and developmental process of plants.

Effect of Sulfated Chitooligosaccharides on Wheat Seedlings (Triticum aestivum L.) under Salt Stress

In this study, sulfated chitooligosaccharide (SCOS) was applied to wheat seedlings to investigate its effect on the plants' defense response under salt stress. The antioxidant enzyme activities, chlorophyll contents, and fluorescence characters of wheat seedlings were determined at a certain time. The results showed that treatment with exogenous SCOS could decrease the content of malondialdehyde, increase the chlorophyll contents, and modulate fluorescence characters in wheat seedlings under salt stress. In addition, SCOS was able to regulate the activities of antioxidant enzymes containing superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase. Similarly, the mRNA expression levels of several antioxidant enzymes were efficiently modulated by SCOS. The results indicated that SCOS could alleviate the damage of salt stress by adjusting the antioxidant enzyme activities of plant. The effect of SCOS on the photochemical efficiency of wheat seedlings was associated with its enhanced capacity for antioxidant enzymes, which prevented structure degradation of the photosynthetic apparatus under NaCl stress. Furthermore, the effective activities of alleviating salt stress indicated the activities of SCOS were closely related with the sulfate group.

Transcriptome-based gene expression profiling identifies differentially expressed genes critical for salt stress response in radish (Raphanus sativus L.)

Transcriptome-based gene expression analysis identifies many critical salt-responsive genes in radish and facilitates further dissecting the molecular mechanism underlying salt stress response. Salt stress severely impacts plant growth and development. Radish, a moderately salt-sensitive vegetable crop, has been studied for decades towards the physiological and biochemical performances under salt stress. However, no systematic study on isolation and identification of genes involved in salt stress response has been performed in radish, and the molecular mechanism governing this process is still indistinct. Here, the RNA-Seq technique was applied to analyze the transcriptomic changes on radish roots treated with salt (200 mM NaCl) for 48 h in comparison with those cultured in normal condition. Totally 8709 differentially expressed genes (DEGs) including 3931 up- and 4778 down-regulated genes were identified. Functional annotation analysis indicated that many genes could be involved in several aspects of salt stress response including stress sensing and signal transduction, osmoregulation, ion homeostasis and ROS scavenging. The association analysis of salt-responsive genes and miRNAs exhibited that 36 miRNA-mRNA pairs had negative correlationship in expression trends. Reverse-transcription quantitative PCR (RT-qPCR) analysis revealed that the expression profiles of DEGs were in line with results from the RNA-Seq analysis. Furthermore, the putative model of DEGs and miRNA-mediated gene regulation was proposed to elucidate how radish sensed and responded to salt stress. This study represents the first comprehensive transcriptome-based gene expression profiling under salt stress in radish. The outcomes of this study could facilitate further dissecting the molecular mechanism underlying salt stress response and provide a valuable platform for further genetic improvement of salt tolerance in radish breeding programs.

Salt stress-induced modulations in the shoot proteome of Brassica juncea genotypes

Indian mustard [Brassica juncea (L.) Czern and Coss] is cultivated mainly in the northwestern agroclimatic region of India and suffers huge losses in productivity due to salinization. In an effort to figure out adaptation strategies of Indian mustard to salt stress, we conducted a comparative proteome analysis of shoots of its two genotypes, with contrasting sensitivity to salt stress. Differential expression of 21 proteins was observed during the two-dimensional electrophoresis (2DE). The identified salt-stress-responsive proteins were associated with different functional processes including osmoregulation, photosynthesis, carbohydrate metabolism, ion homeostasis, protein synthesis and stabilization, energy metabolism, and antioxidant defense system. Salt-tolerant genotype (CS-52) showed a relatively higher expression of proteins involved in turgor regulation, stabilization of photosystems and proteins, and salt compartmentalization, as compared to salt-sensitive genotype (Pusa Varuna). Our results suggest that modulating the expression of salt-responsive proteins can pave the way for developing salt tolerance in the Indian mustard plants.

Hexaconazole-Cu complex improves the salt tolerance of Triticum aestivum seedlings

Hexaconazole is one of the triazole complexes that are broadly used as systemic fungicides with non-traditional plant growth regulator properties. Hexaconazole-Cu complex (Hex-Cu) is a new triazole derivative, and the biological effect of Hex-Cu has been rarely studied. In this work, we investigated the functions of Hex-Cu in regulating growth and the response to salt stress in the seedlings of Triticum aestivum. Pretreated with 60μmolL(-1) Hex-Cu, the seedling plants got increased root/shoot ratio by 42.0%, and the contents of chlorophyll and soluble protein were also increased by 38.1% and 27.9%, respectively. Furthermore, Hex-Cu alleviated the growth inhibition caused by salt stress, enabled the seedlings to maintain a higher proline content and lower malondialdehyde accumulation. The functions of Hex-Cu in regulating the expression of proline synthetase (P5CS and P5CR) genes were investigated by quantitative real-time PCR (qPCR). Under 100mmolL(-1) NaCl stress, the expression of P5CS and P5CR in the seedlings by Hex-Cu pretreatment were significantly up-regulated. It attributed to the enhanced salt tolerance in plants.

Varietal differences in the growth of rice seedlings exposed to perchlorate and their antioxidative defense mechanisms

A hydroponic experiment was conducted to investigate perchlorate (ClO4 (-) ) phytotoxicity in different rice varieties. Considerable variations were observed when 24 rice varieties were treated with ClO4 (-) . The shoot height, root length, and biomass of most varieties were significantly reduced by ClO4 (-) . The roots were more sensitive than the shoots. Hierarchical clustering analysis demonstrated primarily 4 groups: ClO4 (-) -sensitive, medium ClO4 (-) -sensitive, medium ClO4 (-) tolerant, and ClO4 (-) -tolerant. Gannuoxiang (a ClO4 (-) -tolerant variety) and IR65598-112-2 (a ClO4 (-) -sensitive variety) were chosen to explore their antioxidant response when exposed to 0.2 mmol/L, 2.0 mmol/L, and 4.0 mmol/L ClO4 (-) . The results showed that the activities of superoxide dismutase and catalase increased in the shoots and roots of gannuoxiang with increasing doses of ClO4 (-) , but both of them decreased at higher concentrations of ClO4 (-) in IR65598-112-2. The addition of ClO4 (-) led to a significant increase in peroxidase activities for both of the varieties, whereas the increase was more pronounced in gannuoxiang than in IR65598-112-2. No significant difference was found in malondialdehyde (MDA) contents in gannuoxiang, whereas the addition of ClO4 (-) increased the MDA level significantly in IR65598-112-2. The results indicated that gannuoxiang has higher activities of antioxidant enzymes than IR65598-112-2 to cope with oxidative damage caused by ClO4 (-) stress, which may be the main cause of its high tolerance.

Physiological and biochemical mechanisms associated with trehalose-induced copper-stress tolerance in rice

In this study, we examined the possible mechanisms of trehalose (Tre) in improving copper-stress (Cu-stress) tolerance in rice seedlings. Our findings indicated that pretreatment of rice seedlings with Tre enhanced the endogenous Tre level and significantly mitigated the toxic effects of excessive Cu on photosynthesis- and plant growth-related parameters. The improved tolerance induced by Tre could be attributed to its ability to reduce Cu uptake and decrease Cu-induced oxidative damage by lowering the accumulation of reactive oxygen species (ROS) and malondialdehyde in Cu-stressed plants. Tre counteracted the Cu-induced increase in proline and glutathione content, but significantly improved ascorbic acid content and redox status. The activities of major antioxidant enzymes were largely stimulated by Tre pretreatment in rice plants exposed to excessive Cu. Additionally, increased activities of glyoxalases I and II correlated with reduced levels of methylglyoxal in Tre-pretreated Cu-stressed rice plants. These results indicate that modifying the endogenous Tre content by Tre pretreatment improved Cu tolerance in rice plants by inhibiting Cu uptake and regulating the antioxidant and glyoxalase systems, and thereby demonstrated the important role of Tre in mitigating heavy metal toxicity. Our findings provide a solid foundation for developing metal toxicity-tolerant crops by genetic engineering of Tre biosynthesis.

Ascorbate Peroxidase and Catalase Activities and Their Genetic Regulation in Plants Subjected to Drought and Salinity Stresses

Hydrogen peroxide (H2O2), an important relatively stable non-radical reactive oxygen species (ROS) is produced by normal aerobic metabolism in plants. At low concentrations, H2O2 acts as a signal molecule involved in the regulation of specific biological/physiological processes (photosynthetic functions, cell cycle, growth and development, plant responses to biotic and abiotic stresses). Oxidative stress and eventual cell death in plants can be caused by excess H2O2 accumulation. Since stress factors provoke enhanced production of H2O2 in plants, severe damage to biomolecules can be possible due to elevated and non-metabolized cellular H2O2. Plants are endowed with H2O2-metabolizing enzymes such as catalases (CAT), ascorbate peroxidases (APX), some peroxiredoxins, glutathione/thioredoxin peroxidases, and glutathione sulfo-transferases. However, the most notably distinguished enzymes are CAT and APX since the former mainly occurs in peroxisomes and does not require a reductant for catalyzing a dismutation reaction. In particular, APX has a higher affinity for H2O2 and reduces it to H2O in chloroplasts, cytosol, mitochondria and peroxisomes, as well as in the apoplastic space, utilizing ascorbate as specific electron donor. Based on recent reports, this review highlights the role of H2O2 in plants experiencing water deficit and salinity and synthesizes major outcomes of studies on CAT and APX activity and genetic regulation in drought- and salt-stressed plants.

Study on salt tolerance with YHem1 transgenic canola (Brassica napus)

5-Aminolevulinic acid (5-ALA) has been suggested for improving plant salt tolerance via exogenous application. In this study, we used a transgenic canola (Brassica napus), which contained a constituted gene YHem1 and biosynthesized more 5-ALA, to study salt stress responses. In a long-term pot experiment, the transgenic plants produced higher yield under 200 mmol L(-1) NaCl treatment than the wild type (WT). In a short-term experiment, the YHem1 transformation accelerated endogenous 5-ALA metabolism, leading to more chlorophyll accumulation, higher diurnal photosynthetic rates and upregulated expression of the gene encoding Rubisco small subunit. Furthermore, the activities of antioxidant enzymes, including superoxide dismutase, guaiacol peroxidase, catalase and ascorbate peroxidase, were significantly higher in the transgenic plants than the WT, while the levels of O2 ·(-) and malondialdehyde were lower than the latter. Additionally, the Na(+) content was higher in the transgenic leaves than that in the WT under salinity, but K(+) and Cl(-) were significantly lower. The levels of N, P, Cu, and S in the transgenic plants were also significantly lower than those in the WT, but the Fe content was significantly improved. As the leaf Fe content was decreased by salinity, it was suggested that the stronger salt tolerance of the transgenic plants was related to the higher Fe acquisition. Lastly, YHem1 transformation improved the leaf proline content, but salinity decreased rather than increased it. The content of free amino acids and soluble sugars was similarly decreased as salinity increased, but it was higher in the transgenic plants than that in the WT.

Effect of salinity stress on plants and its tolerance strategies: a review

The environmental stress is a major area of scientific concern because it constraints plant as well as crop productivity. This situation has been further worsened by anthropogenic activities. Therefore, there is a much scientific saddle on researchers to enhance crop productivity under environmental stress in order to cope with the increasing food demands. The abiotic stresses such as salinity, drought, cold, and heat negatively influence the survival, biomass production and yield of staple food crops. According to an estimate of FAO, over 6% of the world's land is affected by salinity. Thus, salinity stress appears to be a major constraint to plant and crop productivity. Here, we review our understanding of salinity impact on various aspects of plant metabolism and its tolerance strategies in plants.

Trehalose pretreatment induces salt tolerance in rice (Oryza sativa L.) seedlings: oxidative damage and co-induction of antioxidant defense and glyoxalase systems

Salinity in the form of abiotic stress adversely effects plant growth, development, and productivity. Various osmoprotectants are involved in regulating plant responses to salinity; however, the precise role of trehalose (Tre) in this process remains to be further elucidated. The present study investigated the regulatory role of Tre in alleviating salt-induced oxidative stress in hydroponically grown rice seedlings. Salt stress (150 and 250 mM NaCl) for 72 h resulted in toxicity symptoms such as stunted growth, severe yellowing, and leaf rolling, particularly at 250 mM NaCl. Histochemical observation of reactive oxygen species (ROS; O2 (∙-) and H2O2) indicated evident oxidative stress in salt-stressed seedlings. In these seedlings, the levels of lipoxygenase (LOX) activity, malondialdehyde (MDA), H2O2, and proline (Pro) increased significantly whereas total chlorophyll (Chl) and relative water content (RWC) decreased. Salt stress caused an imbalance in non-enzymatic antioxidants, i.e., ascorbic acid (AsA) content, AsA/DHA ratio, and GSH/GSSG ratio decreased but glutathione (GSH) content increased significantly. In contrast, Tre pretreatment (10 mM, 48 h) significantly addressed salt-induced toxicity symptoms and dramatically depressed LOX activity, ROS, MDA, and Pro accumulation whereas AsA, GSH, RWC, Chl contents, and redox status improved considerably. Salt stress stimulated the activities of SOD, GPX, APX, MDHAR, DHAR, and GR but decreased the activities of CAT and GST. However, Tre-pretreated salt-stressed seedlings counteracted SOD and MDHAR activities, elevated CAT and GST activities, further enhanced APX and DHAR activities, and maintained GPX and GR activities similar to the seedlings stressed with salt alone. In addition, Tre pretreatment enhanced the activities of methylglyoxal detoxifying enzymes (Gly I and Gly II) more efficiently in salt-stressed seedlings. Our results suggest a role for Tre in protecting against salt-induced oxidative damage attributed to reduced ROS accumulation, elevation of non-enzymatic antioxidants, and co-activation of the antioxidative and glyoxalase systems.

Knock-down of stress inducible OsSRFP1 encoding an E3 ubiquitin ligase with transcriptional activation activity confers abiotic stress tolerance through enhancing antioxidant protection in rice

E3 ubiquitin ligases are involved in a variety of physiological processes. This study demonstrated the function of a previously unknown rice RING finger E3 ligase, Oryza sativa Stress-related RING Finger Protein 1 (OsSRFP1) in stress responses in rice. OsSRFP1 was ubiquitously expressed in various rice organs, with the higher expression levels in roots, panicles and culm nodes. The transcript of OsSRFP1 was induced by cold, dehydration, salt, H2O2 and abscisic acid treatments. Interestingly, the OsSRFP1-overexpressing plants were less tolerant to salt, cold and oxidative stresses than wild type plants; while the RNA interference silencing of OsSRFP1 plants were more tolerant than wild type without yield penalty. Compared with the wild type, amounts of free proline and activities of antioxidant enzymes were increased in the RNAi plants but decreased in the overexpression plants under cold stress, which were inversely correlated with the malondialdehyde and hydrogen peroxide (H2O2) levels in the tested lines. Microarray analysis showed that expression of numerous genes involving in ROS homeostasis was altered in the OsSRFP1-overexpressing plants under normal and cold conditions. In vitro ubiquitination assays showed that OsSRFP1 possessed E3 ubiquitin ligase activity and the intact RING domain was essential for the activity. Moreover, OsSRFP1 might function in transcriptional regulation with nuclear localization. Taken together, our results demonstrate that OsSRFP1 may have dual functions in post-translational and transcriptional regulations in modulating abiotic stress responses in rice, at least in part, by negatively regulating antioxidant enzymes-mediated reactive oxygen species removal.

Role of Ulva lactuca extract in alleviation of salinity stress on wheat seedlings

Seaweeds are potentially excellent sources of highly bioactive materials that could represent useful leads in the alleviation of salinity stress. The effects of presoaking wheat grains in water extract of Ulva lactuca on growth, some enzymatic activities, and protein pattern of salinized plants were investigated in this study. Algal presoaking of grains demonstrated a highly significant enhancement in the percentage of seed germination and growth parameters. The activity of superoxide dismutase (SOD) and catalase (CAT) increased with increasing the algal extract concentration while activity of ascorbate peroxidase (APX) and glutathione reductase (GR) was decreased with increasing concentration of algal extract more than 1% (w/v). The protein pattern of wheat seedling showed 12 newly formed bands as result of algal extract treatments compared with control. The bioactive components in U. lactuca extract such as ascorbic acid, betaine, glutathione, and proline could potentially participate in the alleviation of salinity stress. Therefore, algal presoaking is proved to be an effective technique to improve the growth of wheat seedlings under salt stress conditions.

Salinity and drought tolerant OsACA6 enhances cold tolerance in transgenic tobacco by interacting with stress-inducible proteins

Plant Ca(2+)ATPases regulate many signalling pathways which are important for plant growth, development and abiotic stress responses. Our previous work identified that overexpression of OsACA6 confers salinity and drought tolerance in tobacco. In the present work we report, the function of OsACA6 in cold stress tolerance in transgenic tobacco plants. The expression of OsACA6 was induced by cold stress. The promoter-GUS fusion analyses in the different tissues of transgenic tobacoco confirmed that OsACA6 promoter is cold stress-inducible. Transgenic tobacco plants overexpressing OsACA6 exhibited cold tolerance compared to the wild type (WT) controls. The enhanced tolerance was confirmed by phenotypic analyses as well as by measuring germination, survival rate, chlorophyll content, cell membrane stability, malondialdehyde and proline content. Compared to the WT, the expression of catalase, ascorbate peroxidase and superoxide dismutase increased in the OsACA6 overexpressing plants, which was inversely correlated with the levels of H2O2 in the transgenic lines. We also identified interacting proteins of OsACA6 by using yeast two-hybrid screening assay. Most of the interacting partners of OsACA6 are associated with the widespread biological processes including plant growth, development, signalling and stress adaptation. Furthermore, we also confirmed that OsACA6 is able to self-interact. Overall, these results suggest that OsACA6 plays an important role in cold tolerance at least in part, by regulating antioxidants-mediated removal of reactive oxygen species or by interacting with different calcium signal decoders including calmodulin-like proteins (CaM) calcium/calmodulin dependent protein kinases (CDPKs) and receptor-like protein kinases (RLKs).

Exogenous proline and glycine betaine mediated upregulation of antioxidant defense and glyoxalase systems provides better protection against salt-induced oxidative stress in two rice (Oryza sativa L.) varieties

The present study investigates the roles of exogenous proline (Pro, 5 mM) and glycine betaine (GB, 5 mM) in improving salt stress tolerance in salt sensitive (BRRI dhan49) and salt tolerant (BRRI dhan54) rice (Oryza sativa L.) varieties. Salt stresses (150 and 300 mM NaCl for 48 h) significantly reduced leaf relative water (RWC) and chlorophyll (chl) content and increased endogenous Pro and increased lipid peroxidation and H2O2 levels. Ascorbate (AsA), glutathione (GSH) and GSH/GSSG, ascorbate peroxidae (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), catalase (CAT), and glyoxalase I (Gly I) activities were reduced in sensitive variety and these were increased in tolerant variety due to salt stress. The glyoxalase II (Gly II), glutathione S-transferase (GST), and superoxide dismutase (SOD) activities were increased in both cultivars by salt stress. Exogenous Pro and GB application with salt stress improved physiological parameters and reduced oxidative damage in both cultivars where BRRI dhan54 showed better tolerance. The result suggests that exogenous application of Pro and GB increased rice seedlings' tolerance to salt-induced oxidative damage by upregulating their antioxidant defense system where these protectants rendered better performance to BRRI dhan54 and Pro can be considered as better protectant than GB.

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.

Overexpression of PtSOS2 Enhances Salt Tolerance in Transgenic Poplars

Protein kinases are major signal transduction factors that have a central role in mediating acclimation to environmental changes in eukaryotic organisms. In this study, we cloned and identified three salt overly sensitive 2 (SOS2) genes in the woody plant Populus trichocarpa, designated as PtSOS2.1, PtSOS2.2, and PtSOS2.3, which were transformed into hybrid poplar clone T89 (Populus tremula× Populus tremuloides Michx clone T89) mediated by Agrobacterium tumefaciens. Southern and northern blot analyses verified that the three genes integrated into the plant genome, and were expressed at a stable transcription level. Meanwhile, overexpression of all three PtSOS2 genes did not retard the growth of plants under normal conditions. Instead, it promoted growth in both agar-medium and soil conditions in response to salinity stress. Under salt stress, overexpression of PtSOS2.1, PtSOS2.2, and PtSOS2.3 increased the concentrations of proline and photosynthetic pigments, and the relative water content (RWC), and the activity of antioxidant enzymes, and decreased the malondialdehyde (MDA) concentrations in transgenic lines compared to the control. These results suggest that overexpression of PtSOS2 plays a significant role in improving the salt tolerance of poplars, reducing the damage to membrane structures, and enhancing osmotic adjustment and antioxidative enzyme regulation under salt stress.

Ultrastructural and physiological responses of potato (Solanum tuberosum L.) plantlets to gradient saline stress

Salinity is one of the major abiotic stresses that impacts plant growth and reduces the productivity of field crops. Compared to field plants, test tube plantlets offer a direct and fast approach to investigate the mechanism of salt tolerance. Here we examined the ultrastructural and physiological responses of potato (Solanum tuberosum L. c.v. "Longshu No. 3") plantlets to gradient saline stress (0, 25, 50, 100, and 200 mM NaCl) with two consequent observations (2 and 6 weeks, respectively). The results showed that, with the increase of external NaCl concentration and the duration of treatments, (1) the number of chloroplasts and cell intercellular spaces markedly decreased, (2) cell walls were thickened and even ruptured, (3) mesophyll cells and chloroplasts were gradually damaged to a complete disorganization containing more starch, (4) leaf Na and Cl contents increased while leaf K content decreased, (5) leaf proline content and the activities of catalase (CAT) and superoxide dismutase (SOD) increased significantly, and (6) leaf malondialdehyde (MDA) content increased significantly and stomatal area and chlorophyll content decline were also detected. Severe salt stress (200 mM NaCl) inhibited plantlet growth. These results indicated that potato plantlets adapt to salt stress to some extent through accumulating osmoprotectants, such as proline, increasing the activities of antioxidant enzymes, such as CAT and SOD. The outcomes of this study provide ultrastructural and physiological insights into characterizing potential damages induced by salt stress for selecting salt-tolerant potato cultivars.

Proline accumulation in leaves of Periploca sepium via both biosynthesis up-regulation and transport during recovery from severe drought

Drought resistance and recovery ability are two important requisites for plant adaptation to drought environments. Proline (Pro) metabolism has been a major concern in plant drought tolerance. However, roles of Pro metabolism in plant recovery ability from severe drought stress are largely unexplored. Periploca sepium Bunge has gained increasing attention for its adaptation to dry environments. Here, we investigated Pro metabolism in different tissues of P. sepium seedlings in the course of drought stress and recovery. We found that leaf Pro metabolism response during post-drought recovery was dependant on drought severity. Pro biosynthesis was down-regulated during recovery from -0.4 MPa but increased continually and notably during recovery from -1.0 MPa. Significant correlation between Pro concentration and Δ1-pyrroline-5-carboxylate synthetase activity indicates that Glutamate pathway is the predominant synthesis route during both drought and re-watering periods. Ornithine δ-aminotransferase activity was up-regulated significantly only during recovery from -1.0 MPa, suggesting positive contribution of ornithine pathway to improving plant recovery capacity from severe drought. In addition to up-regulation of biosynthesis, Pro transport from stems and roots also contributed to high Pro accumulation in leaves and new buds during recovery from -1.0 MPa, as indicated by the combined analysis of Pro concentration and its biosynthesis in stems, roots and new buds. Except its known roles as energy, carbon and nitrogen sources for plant rapid recovery, significant positive correlation between Pro concentration and total antioxidant activity indicates that Pro accumulation can also promote plant damage repair ability by up-regulating antioxidant activity during recovery from severe drought stress.

A basic helix-loop-helix transcription factor, PtrbHLH, of Poncirus trifoliata confers cold tolerance and modulates peroxidase-mediated scavenging of hydrogen peroxide

The basic helix-loop-helix (bHLH) transcription factors are involved in a variety of physiological processes. However, plant bHLHs functioning in cold tolerance and the underlying mechanisms remain poorly understood. Here, we report the identification and functional characterization of PtrbHLH isolated from trifoliate orange (Poncirus trifoliata). The transcript levels of PtrbHLH were up-regulated under various abiotic stresses, particularly cold. PtrbHLH was localized in the nucleus with transactivation activity. Overexpression of PtrbHLH in tobacco (Nicotiana tabacum) or lemon (Citrus limon) conferred enhanced tolerance to cold under chilling or freezing temperatures, whereas down-regulation of PtrbHLH in trifoliate orange by RNA interference (RNAi) resulted in elevated cold sensitivity. A range of stress-responsive genes was up-regulated or down-regulated in the transgenic lemon. Of special note, several peroxidase (POD) genes were induced after cold treatment. Compared with the wild type, POD activity was increased in the overexpression plants but decreased in the RNAi plants, which was inversely correlated with the hydrogen peroxide (H2O2) levels in the tested lines. Treatment of the transgenic tobacco plants with POD inhibitors elevated the H2O2 levels and greatly compromised their cold tolerance, while exogenous replenishment of POD enhanced cold tolerance of the RNAi line. In addition, transgenic tobacco and lemon plants were more tolerant to oxidative stresses. Yeast one-hybrid assay and transient expression analysis demonstrated that PtrbHLH could bind to the E-box elements in the promoter region of a POD gene. Taken together, these results demonstrate that PtrbHLH plays an important role in cold tolerance, at least in part, by positively regulating POD-mediated reactive oxygen species removal.

GsSRK, a G-type lectin S-receptor-like serine/threonine protein kinase, is a positive regulator of plant tolerance to salt stress

Receptor-like protein kinases (RLKs) play vital roles in sensing outside signals, yet little is known about RLKs functions and roles in stress signal perception and transduction in plants, especially in wild soybean. Through the microarray analysis, GsSRK was identified as an alkaline (NaHCO3)-responsive gene, and was subsequently isolated from Glycine soja by homologous cloning. GsSRK encodes a 93.22kDa protein with a highly conserved serine/threonine protein kinase catalytic domain, a G-type lectin region, and an S-locus region. Real-time PCR results showed that the expression levels of GsSRK were largely induced by ABA, salt, and drought stresses. Over expression of GsSRK in Arabidopsis promoted seed germination, as well as primary root and rosette leaf growth during the early stages of salt stress. Compared to the wild type Arabidopsis, GsSRK overexpressors exhibited enhanced salt tolerance and higher yields under salt stress, with higher chlorophyll content, lower ion leakage, higher plant height, and more siliques at the adult developmental stage. Our studies suggest that GsSRK plays a crucial role in plant response to salt stress.

Genetic transformation and analysis of rice OsAPx2 gene in Medicago sativa

The OsAPx2 gene from rice was cloned to produce PBI121::OsAPx2 dual-expression plants, of which expression level would be increasing under stressful conditions. The enzyme ascorbate peroxidase (APX) in the leaves and roots of the plants increased with increasing exposure time to different sodium chloride (NaCl) and hydrogen peroxide (H(2)O(2))concentrations, as indicated by protein gel blot analysis. The increased enzyme yield improved the ability of the plants to resist the stress treatments. The OsAPx2 gene was localized in the cytoplasm of epidermal onion cells as indicated by the instantaneous expression of green fluorescence. An 80% regeneration rate was observed in Medicago sativa L. plants transformed with the OsAPx2 gene using Agrobacterium tumefaciens, as indicated by specific primer PCR. The OsAPx2 gene was expressed at the mRNA level and the individual M. sativa (T#1,T#2,T#5) were obtained through assaying the generation of positive T2 using RNA gel blot analysis. When the seeds of the wild type (WT) and the T2 (T#1,T#5) were incubated in culture containing MS with NaCl for 7 days, the results as shown of following: the root length of transgenic plant was longer than WT plants, the H(2)O(2) content in roots of WT was more than of transgenic plants, the APX activity under stresses increased by 2.89 times compared with the WT, the malondialdehyde (MDA) content of the WT was higher than the transgenic plants, the leaves of the WT turned yellow, but those of the transgenic plants remained green and remained healthy. The chlorophyll content in the WT leaves was less than in the transgenic plants, after soaking in solutions of H(2)O(2), sodium sulfite (Na(2)SO(3)), and sodium bicarbonate (NaHCO(3)). Therefore, the OsAPx2 gene overexpression in transgenic M. sativa improves the removal of H(2)O(2) and the salt-resistance compared with WT plants. A novel strain of M. sativa carrying a salt-resistance gene was obtained.

Recent molecular advances on downstream plant responses to abiotic stress

Abiotic stresses such as extremes of temperature and pH, high salinity and drought, comprise some of the major factors causing extensive losses to crop production worldwide. Understanding how plants respond and adapt at cellular and molecular levels to continuous environmental changes is a pre-requisite for the generation of resistant or tolerant plants to abiotic stresses. In this review we aimed to present the recent advances on mechanisms of downstream plant responses to abiotic stresses and the use of stress-related genes in the development of genetically engineered crops.