Redox regulation of water stress responses in field-grown plants. Role of hydrogen peroxide and ascorbate

Growth and antioxidant responses to water stress in eggplant MAGIC population parents, F1 hybrids and a subset of recombinant inbred lines

BACKGROUND

The generation of new eggplant (Solanum melongena L.) cultivars with drought tolerance is a main challenge in the current context of climate change. In this study, the eight parents (seven of S. melongena and one of the wild relative S. incanum L.) of the first eggplant MAGIC (Multiparent Advanced Generation Intercrossing) population, together with four F1 hybrids amongst them, five S5 MAGIC recombinant inbred lines selected for their genetic diversity, and one commercial hybrid were evaluated in young plant stage under water stress conditions (30% field capacity; FC) and control conditions (100% FC). After a 21-day treatment period, growth and biomass traits, photosynthetic pigments, oxidative stress markers, antioxidant compounds, and proline content were evaluated.

RESULTS

Significant effects (p < 0.05) were observed for genotype, water treatments and their interaction in most of the traits analyzed. The eight MAGIC population parental genotypes displayed a wide variation in their responses to water stress, with some of them exhibiting enhanced root development and reduced foliar biomass. The commercial hybrid had greater aerial growth compared to root growth. The four F1 hybrids among MAGIC parents differed in their performance, with some having significant positive or negative heterosis in several traits. The subset of five MAGIC lines displayed a wide diversity in their response to water stress.

CONCLUSION

The results show that a large diversity for tolerance to drought is available among the eggplant MAGIC materials, which can contribute to developing drought-tolerant eggplant cultivars.

Mechanistic Insights on Salicylic Acid-Induced Enhancement of Photosystem II Function in Basil Plants under Non-Stress or Mild Drought Stress

Photosystem II (PSII) functions were investigated in basil (Ocimum basilicum L.) plants sprayed with 1 mM salicylic acid (SA) under non-stress (NS) or mild drought-stress (MiDS) conditions. Under MiDS, SA-sprayed leaves retained significantly higher (+36%) chlorophyll content compared to NS, SA-sprayed leaves. PSII efficiency in SA-sprayed leaves under NS conditions, evaluated at both low light (LL, 200 μmol photons m-2 s-1) and high light (HL, 900 μmol photons m-2 s-1), increased significantly with a parallel significant decrease in the excitation pressure at PSII (1-qL) and the excess excitation energy (EXC). This enhancement of PSII efficiency under NS conditions was induced by the mechanism of non-photochemical quenching (NPQ) that reduced singlet oxygen (1O2) production, as indicated by the reduced quantum yield of non-regulated energy loss in PSII (ΦNO). Under MiDS, the thylakoid structure of water-sprayed leaves appeared slightly dilated, and the efficiency of PSII declined, compared to NS conditions. In contrast, the thylakoid structure of SA-sprayed leaves did not change under MiDS, while PSII functionality was retained, similar to NS plants at HL. This was due to the photoprotective heat dissipation by NPQ, which was sufficient to retain the same percentage of open PSII reaction centers (qp), as in NS conditions and HL. We suggest that the redox status of the plastoquinone pool (qp) under MiDS and HL initiated the acclimation response to MiDS in SA-sprayed leaves, which retained the same electron transport rate (ETR) with control plants. Foliar spray of SA could be considered as a method to improve PSII efficiency in basil plants under NS conditions, at both LL and HL, while under MiDS and HL conditions, basil plants could retain PSII efficiency similar to control plants.

Appropriate Drought Training Induces Optimal Drought Tolerance by Inducing Stepwise H2O2 Homeostasis in Soybean

Soybean is considered one of the most drought-sensitive crops, and ROS homeostasis can regulate drought tolerance in these plants. Understanding the mechanism of H2O2 homeostasis and its regulatory effect on drought stress is important for improving drought tolerance in soybean. We used different concentrations of polyethylene glycol (PEG) solutions to simulate the progression from weak drought stress (0.2%, 0.5%, and 1% PEG) to strong drought stress (5% PEG). We investigated the responses of the soybean plant phenotype, ROS level, injury severity, antioxidant system, etc., to different weak drought stresses and subsequent strong drought stresses. The results show that drought-treated plants accumulated H2O2 for signaling and exhibited drought tolerance under the following stronger drought stress, among which the 0.5% PEG treatment had the greatest effect. Under the optimal treatment, there was qualitatively describable H2O2 homeostasis, characterized by a consistent increasing amplitude in H2O2 content compared with CK. The H2O2 signal formed under the optimum treatment induced the capacity of the antioxidant system to remove excess H2O2 to form a primary H2O2 homeostasis. The primary H2O2 homeostasis further induced senior H2O2 homeostasis under the following strong drought and maximized the improvement of drought tolerance. These findings might suggest that gradual drought training could result in stepwise H2O2 homeostasis to continuously improve drought tolerance.

Mechanistic Insights on Salicylic Acid Mediated Enhancement of Photosystem II Function in Oregano Seedlings Subjected to Moderate Drought Stress

Dramatic climate change has led to an increase in the intensity and frequency of drought episodes and, together with the high light conditions of the Mediterranean area, detrimentally influences crop production. Salicylic acid (SA) has been shown to supress phototoxicity, offering photosystem II (PSII) photoprotection. In the current study, we attempted to reveal the mechanism by which SA is improving PSII efficiency in oregano seedlings under moderate drought stress (MoDS). Foliar application of SA decreased chlorophyll content under normal growth conditions, but under MoDS increased chlorophyll content, compared to H₂O-sprayed oregano seedlings. SA improved the PSII efficiency of oregano seedlings under normal growth conditions at high light (HL), and under MoDS, at both low light (LL) and HL. The mechanism by which, under normal growth conditions and HL, SA sprayed oregano seedlings compared to H₂O-sprayed exhibited a more efficient PSII photochemistry, was the increased (17%) fraction of open PSII reaction centers (qp), and the increased (7%) efficiency of these open reaction centers (Fv'/Fm'), which resulted in an enhanced (24%) electron transport rate (ETR). SA application under MoDS, by modulating chlorophyll content, resulted in optimized antenna size and enhanced effective quantum yield of PSII photochemistry (Φ_PSII) under both LL (7%) and HL (25%), compared to non-SA-sprayed oregano seedlings. This increased effective quantum yield of PSII photochemistry (Φ_PSII) was due to the enhanced efficiency of the oxygen evolving complex (OEC), and the increased fraction of open PSII reaction centers (qp), which resulted in an increased electron transport rate (ETR) and a lower amount of singlet oxygen (¹O₂) production with less excess excitation energy (EXC).

Light harvesting regulation: A versatile network of key components operating under various stress conditions in higher plants

Light harvesting is finetuned through two main strategies controlling energy transfer to the reaction centers of photosystems: i) regulating the amount of light energy at the absorption level, ii) regulating the amount of the absorbed energy at the utilization level. The first strategy is ensured by changes in the cross-section, i.e., the size of the photosynthetic antenna. These changes can occur in a short-term (state transitions) or long-term way (changes in antenna protein biosynthesis) depending on the light conditions. The interrelation of these two ways is still underexplored. Regulating light absorption through the long-term modulation of photosystem II antenna size has been mostly considered as an acclimatory mechanism to light conditions. The present review highlights that this mechanism represents one of the most versatile mechanisms of higher plant acclimation to various conditions including drought, salinity, temperature changes, and even biotic factors. We suggest that H₂O₂ is the universal signaling agent providing the switch from the short-term to long-term modulation of photosystem II antenna size under these factors. The second strategy of light harvesting is represented by redirecting energy to waste mainly via thermal energy dissipation in the photosystem II antenna in high light through PsbS protein and xanthophyll cycle. In the latter case, H₂O₂ also plays a considerable role. This circumstance may explain the maintenance of the appropriate level of zeaxanthin not only upon high light but also upon other stress factors. Thus, the review emphasizes the significance of both strategies for ensuring plant sustainability under various environmental conditions.

Functional verification and screening of protein interacting with the slPHB3

slPHB3 was cloned from Salix linearistipularis, the amino acid sequence blast and phylogenetic tree analysis showed that slPHB3 has the most similarity with PHB3 from Populus trichocarpa using DNAMAN software and MEGA7 software. RT-qPCR results confirmed that the expression of slPHB3 was induced obviously under stress treatments. The growth of recombinant yeast cells was better than that of the control group under the stress treatment, indicating that slPHB3 may be involved in the stress response of yeast cells. The transgenic tobacco was treated with different concentrations of NaCl, NaHCO3 and H2O2, fresh weigh of overexpression tobacco were heavier than wild-types. The results showed that transgenic tobacco was more tolerant to salt and oxidation than wild-type tobacco. Expression of important genes including NHX1 and SOS1 in salt stress response pathways are steadily higher in overexpression tobacco than that in wild-types. We identified 17 proteins interacting with slPHB3 by yeast two-hybrid technique, most of these proteins were relation to the stresses. The salt tolerance of slPHB3 expressing yeast and slPHB3 overexpressing plants were better than that of the control. Ten stress-related proteins may interact with slPHB3, which preliminarily indicated that slPHB3 had a certain response relationship with salt stress. The study of slPHB3 under abiotic stress can improve our understanding of PHB3 gene function.

Cross Talk between GlAQP and NOX Modulates the Effects of ROS Balance on Ganoderic Acid Biosynthesis of Ganoderma lucidum under Water Stress

Water stress affects both the growth and development of filamentous fungi; however, the mechanisms underlying their response to water stress remain unclear. In this study, water stress was found to increase intracellular reactive oxygen species (ROS) level, ganoderic acid (GA) content, and NADPH oxidase (NOX) activity of Ganoderma lucidum by 148.45%, 75.32%, and 161.61%, respectively. Water stress induced the expression of the G. lucidum aquaporin (GlAQP) gene, which facilitated water transfer for microbial growth. Compared to wild type (WT), exposure to water stress increased growth inhibition rate, ROS level, and GA content of GlAQP-silenced strains by 37 to 41%, 36 to 38%, and 25%, respectively. Furthermore, at the early stage of fermentation in GlAQP-silenced strains, water stress resulted in 16 to 17% and 9 to 10% lower ROS level and GA content compared to WT, respectively. However, in GlAQP-overexpressing strains, ROS level and GA content were 22 to 24% and 12 to 13% higher than in WT, respectively. In GlAQP-silenced strains, water stress at the late stage resulted in 35 to 37% and 29 to 30% higher ROS level and GA content, respectively, while in GlAQP-overexpressing strains, levels were 16 to 17% and 9% lower than WT, respectively. Cross talk between GlAQP and NOX positively regulated the GA biosynthesis of G. lucidum via ROS under water stress at the early stage but this regulation became negative at the late stage. This study deepens the understanding of fungal signaling transduction under water stress and provides a reference for analyzing environmental factors that influence the regulation of the fungal secondary metabolism. IMPORTANCE Ganoderma lucidum is an advanced basidiomycete that produces medicinally active secondary metabolites (especially ganoderic acid [GA]) with high commercial value. Water stress imposes an important environmental challenge to G. lucidum. The mechanism of GA biosynthesis under water stress and the role of G. lucidum aquaporin (GlAQP) during its biosynthesis remain unclear. Moreover, the effect of the relationship between GlAQP and NADPH oxidase (NOX) on the level of reactive oxygen species and GA production under water stress is unknown. This study provides information on the biological response mechanism of G. lucidum to water stress. A new theory on the cell signaling cascade of G. lucidum tolerance to water stress is provided that also incorporates the biosynthesis of secondary metabolites involved in NOX and GlAQP.

Impact of Biochar Application at Water Shortage on Biochemical and Physiological Processes in Medicago ciliaris

The application of biochar is mostly used to improve soil fertility, water retention capacity and nutrient uptake. The present study was conducted in order to study the impact of biochar at water deficiency conditions on the physiological and biochemical processes of Medicago ciliaris seedlings. Seedlings were cultivated under greenhouse conditions in pots filled with a mixture of soil and sand mixed in the presence or absence of 2% biochar. Plants of uniform size were subjected after a pretreatment phase (72 days) either to low (36% water holding capacity, water potential low) or high soil water potential (60% water holding capacity, water potential high). Pots were weighed every day to control and maintain a stable water holding capacity. In Medicago ciliaris, drought led to a significant reduction in plant growth and an increase in the root/shoot ratio. The growth response was accompanied by a decreased stomatal conductance and a reduction of the net CO₂ assimilation rate and water use efficiency. The associated higher risk of ROS production was indicated by a high level of lipid peroxidation, high antioxidant activities and high proline accumulation. Soil amendment with biochar enhanced the growth significantly and supported the photosynthetic apparatus of Medicago ciliaris species by boosting chlorophyll content and A_net both under well and insufficient watered plants and water use efficiency in case of water shortage. This increase of water use efficiency was correlated with the biochar-mediated decrease of the MDA and proline contents in the leaves buffering the impact of drought on photosynthetic apparatus by increasing the activity of enzymatic antioxidants SOD, APX, GPOX and GR and non-enzymatic antioxidants, such as AsA and DHAsA, giving the overall picture of a moderate stress response. These results confirmed the hypothesis that biochar application significantly reduces both the degree of stress and the negative impact of oxidative stress on Medicago ciliaris plants. These results implied that this species could be suitable as a cash pasture plant in the development of agriculture on dry wasteland in a future world of water shortages.

Over-Expression of Dehydroascorbate Reductase Improves Salt Tolerance, Environmental Adaptability and Productivity in Oryza sativa

Abiotic stress induces reactive oxygen species (ROS) generation in plants, and high ROS levels can cause partial or severe oxidative damage to cellular components that regulate the redox status. Here, we developed salt-tolerant transgenic rice plants that overexpressed the dehydroascorbate reductase gene (OsDHAR1) under the control of a stress-inducible sweet potato promoter (SWPA2). OsDHAR1-expressing transgenic plants exhibited improved environmental adaptability compared to wild-type plants, owing to enhanced ascorbate levels, redox homeostasis, photosynthetic ability, and membrane stability through cross-activation of ascorbate-glutathione cycle enzymes under paddy-field conditions, which enhanced various agronomic traits, including root development, panicle number, spikelet number per panicle, and total grain yield. dhar2-knockdown plants were susceptible to salt stress, and owing to poor seed maturation, exhibited reduced biomass (root growth) and grain yield under paddy field conditions. Microarray revealed that transgenic plants highly expressed genes associated with cell growth, plant growth, leaf senescence, root development, ROS and heavy metal detoxification systems, lipid metabolism, isoflavone and ascorbate recycling, and photosynthesis. We identified the genetic source of functional genomics‒based molecular breeding in crop plants and provided new insights into the physiological processes underlying environmental adaptability, which will enable improvement of stress tolerance and crop species productivity in response to climate change.

The Effects of Hydro-Priming and Colonization with Piriformospora indica and Azotobacter chroococcum on Physio-Biochemical Traits, Flavonolignans and Fatty Acids Composition of Milk Thistle (Silybum marianum) under Saline Conditions

Salinity is an important challenge around the world, effecting all physiological and biochemical processes of plants. It seems that seed priming can diminish the negative impacts of salinity. To study the effects of hydro-priming and inoculation with Piriformospora indica (Pi) and Azotobacter chroococcum (Az) on physio-biochemical traits, flavonolignans and fatty acids composition of milk thistle under saline conditions, a greenhouse experiment was carried out. Our results indicated that under salinity, seed priming, especially Pi, improved physio-biochemical properties in milk thistle. Under 120 mM NaCl, inoculation with Pi increased membrane stability index (MSI) and relative water content (RWC) (by 21.86 and 33.43%, respectively). However, peroxidase (POX) (5.57- and 5.68-fold in roots and leaves, respectively), superoxide dismutase (SOD) (4.74- and 4.44-fold in roots and leaves, respectively), catalase (CAT) (6.90- and 8.50-fold in roots and leaves, respectively) and ascorbate peroxidase (APX) (5.61- and 5.68-fold in roots and leaves, respectively) activities increased with increasing salinity. Contrary to salinity, hydro-priming with Az and Pi positively altered all these traits. The highest content of the osmolytes, adenosine triphosphate (ATP) content and rubisco activity were recorded in Pi treatments under 120 mM NaCl. Stearic acid (20.24%), oleic acid (21.06%) and palmitic acid (10.48%) increased, but oil content (3.81%), linolenic and linoleic acid content (22.21 and 15.07%, respectively) decreased under saline conditions. Inoculations of Pi positively altered all these traits. The present study indicated that seed priming with Pi under 120 mM NaCl resulted in maximum silychristin, taxidolin, silydianin, isosilybin, silybin and silymarin of milk thistle seeds.

Improving oxidative stress resilience in plants

Originally conceived as harmful metabolic byproducts, reactive oxygen species (ROS) are now recognized as an integral part of numerous cellular programs. Thanks to their diverse physicochemical properties, compartmentalized production, and tight control exerted by the antioxidant machinery they activate signaling pathways that govern plant growth, development, and defense. Excessive ROS levels are often driven by adverse changes in environmental conditions, ultimately causing oxidative stress. The associated negative impact on cellular constituents have been a major focus of decade-long research efforts to improve the oxidative stress resilience by boosting the antioxidant machinery in model and crop species. We highlight the role of enzymatic and non-enzymatic antioxidants as integral factors of multiple signaling cascades beyond their mere function to prevent oxidative damage under adverse abiotic stress conditions.

Effect of Lead and Copper on Photosynthetic Apparatus in Citrus (Citrus aurantium L.) Plants. The Role of Antioxidants in Oxidative Damage as a Response to Heavy Metal Stress

Photosynthetic changes and antioxidant activity to oxidative stress were evaluated in sour orange (Citrus aurantium L.) leaves subjected to lead (Pb), copper (Cu) and also Pb + Cu toxicity treatments, in order to elucidate the mechanisms involved in heavy metal tolerance. The simultaneous effect of Pb^- and Cu on growth, concentration of malondialdehyde (MDA), hydrogen peroxide (H₂O₂), chlorophylls, flavonoids, carotenoids, phenolics, chlorophyll fluorescence and photosynthetic parameters were examined in leaves of Citrus aurantium L. plants. Exogenous application of Pb and Cu resulted in an increase in leaf H₂O₂ and lipid peroxidation (MDA). Toxicity symptoms of both Pb and Cu treated plants were stunted growth and decreased pigments concentration. Furthermore, photosynthetic activity of treated plants exhibited a significant decline. The inhibition of growth in Pb and Cu-treated plants was accompanied by oxidative stress, as indicated by the enhanced lipid peroxidation and the high H₂O₂ concentration. Furthermore, antioxidants in citrus plants after exposure to high Pb and Cu concentrations were significantly increased compared to control and low Pb and Cu treatments. In conclusion, this study indicates that Pb and Cu promote lipid peroxidation, disrupt membrane integrity, reduces growth and photosynthesis and inhibit mineral nutrition. Considering the potential for adverse human health effects associated with high concentrations of Pb and Cu contained in edible parts of citrus plants the study signals that it is important to conduct further research into the accessibility and uptake of the tested heavy metals in the soil and whether they pose risks to humans.

Priming alleviates high temperature induced oxidative DNA damage and repair using Apurinic/apyrimidinic endonuclease (Ape1L) homologue in wheat (Triticum aestivum L.)

Crop plants require an optimum range of temperature for normal growth and development however high temperature can adversely affect the plants, induce oxidative stress and disintegrate biomolecules especially DNA and proteins. In wheat, high temperature stress (35-40 °C) during ripening stage hampers the yield tremendously. In this study, we assessed high temperature (HT) induced oxidative stress, subsequent DNA damage and role of priming in stress tolerance by analyzing DNA repair enzyme Triticum aestivum AP endonuclease (TaApe1L). Sixteen days old seedlings of wheat varieties PBW 550 and PBW 343 were primed with mild drought and exposed to HT (38 °C) for 2, 4, and 6 h. Hydrogen peroxide (H₂O₂) was used as oxidative stress marker and quantified on regular time intervals. DNA damage was analyzed by DNA laddering and TaApe1L gene expression was analyzed using RT PCR and western blotting. Phylogenetic analysis of Ape1 revealed presence of some key amino acids that are evolutionary conserved. A significant increase in H₂O₂ content was observed after 6 h of exposure especially in PBW 343. Similarly, the DNA damage was also increased with HT exposure especially in PBW 343. The TaApe1L mRNA expression increased after priming in both the varieties after 4 h. But APE1 protein expression was higher in PBW 343, which can be correlated with DNA damage and repair. Lastly, it can be concluded that there is varietal difference in the HT sensitivity but 6 h exposure was detrimental to both the varieties. Also, drought priming improved HT tolerance by over expressing APE1.

Endogenous NO Is Involved in Dissimilar Responses to Rehydration and Pb(NO3)2 in Ramalina farinacea Thalli and Its Isolated Phycobionts

Lichens undergo desiccation/rehydration cycles and are permeable to heavy metals, which induce free radicals. Nitrogen monoxide (NO) regulates important cellular functions, but the research on lichen NO is still very scarce. In Ramalina farinacea thalli, NO seems to be involved in the peroxidative damage caused by air pollution, antioxidant defence and regulation of lipid peroxidation and photosynthesis. Our hypothesis is that NO also has a critical role during the rehydration and in the responses to lead of its isolated phycobionts (Trebouxia sp. TR9 and Trebouxia jamesii). Therefore, we studied the intracellular reactive oxygen species (ROS) production, lipid peroxidation and chlorophyll autofluorescence during rehydration of thalli and isolated microalgae in the presence of a NO scavenger and Pb(NO₃)₂. During rehydration, NO scavenging modulates free radical release and chlorophyll autofluorescence but not lipid peroxidation in both thalli and phycobionts. Pb(NO₃)₂ reduced free radical release (hormetic effect) both in the whole thallus and in microalgae. However, only in TR9, the ROS production, chlorophyll autofluorescence and lipid peroxidation were dependent on NO. In conclusion, Pb hormetic effect seems to depend on NO solely in TR9, while is doubtful for T. jamesii and the whole thalli.

Ameliorating effects of biochar on photosynthetic efficiency and antioxidant defence of Phragmites karka under drought stress

Biochar (BC) has been reported to improve growth and drought resistance in many plants. However, adequate information on the drought resistance mechanism mediated of BC on Phragmites karka, a bioenergy plant, is not available. The impact of BC addition (0%, 0.75% and 2.5%) on plant growth and physiology of P. karka under drought was assessed. Soil water-holding capacity and soil water content were significantly improved with 0.75% BC as compared with the un-amended controls. This resulted in improved plant performance under drought conditions. An increase of parameters, such as plant fresh and dry biomass, root to shoot ratio and root mass fraction, was paralleled by an increase of chlorophyll content, net photosynthesis rate and water use efficiency of plants. Plants treated with 0.75% BC experienced less oxidative stress due to higher photosystem II efficiency and stimulated activity of antioxidant defense systems. Our results demonstrate that soil amendment with 0.75% BC allow the potential energy plant P. karka to grow in an arid habitat.

Exploring the Potential of Nitric Oxide and Hydrogen Sulfide (NOSH)-Releasing Synthetic Compounds as Novel Priming Agents against Drought Stress in Medicago sativa Plants

Land plants are continuously exposed to multiple abiotic stress factors like drought, heat, and salinity. Nitric oxide (NO) and hydrogen sulfide (H₂S) are two well-examined signaling molecules that act as priming agents, regulating the response of plants to stressful conditions. Several chemical donors exist that provide plants with NO and H₂S separately. NOSH is a remarkable novel donor as it can donate NO and H₂S simultaneously to plants, while NOSH-aspirin additionally provides the pharmaceutical molecule acetylsalicylic acid. The current study aimed to investigate the potential synergistic effect of these molecules in drought-stressed Medicago sativa L. plants by following a pharmacological approach. Plants were initially pre-treated with both donors (NOSH and NOSH-aspirin) via foliar spraying, and were then subsequently exposed to a moderate water deficit while NO and H₂S inhibitors (cPTIO and HA, respectively) were also employed. Phenotypic and physiological data showed that pre-treatment with NOSH synthetic compounds induced acclimation to subsequent drought stress and improved the recovery following rewatering. This was accompanied by modified reactive-oxygen and nitrogen-species signaling and metabolism, as well as attenuation of cellular damage, as evidenced by altered lipid peroxidation and proline accumulation levels. Furthermore, real-time RT-qPCR analysis revealed the differential regulation of multiple defense-related transcripts, including antioxidant enzymes. Overall, the present study proposed a novel role for NOSH compounds as efficient plant priming agents against environmental constraints through the coordinated regulation of multiple defense components, thus opening new horizons in the field of chemical priming research toward the use of target-selected compounds for stress tolerance enhancement.

Exogenous application of β-sitosterol mediated growth and yield improvement in water-stressed wheat (Triticum aestivum) involves up-regulated antioxidant system

Water stress reduces crop production significantly, and climate change has further aggravated the problem mainly in arid and semi-arid regions. This was the first study on the possible effects of β-sitosterol application in ameliorating the deleterious changes in wheat induced by water stress under field condition and drip irrigation regimes. A field experiment with the split-plot design was conducted, and wheat plants were foliar sprayed with four β-sitosterol (BBS) concentrations (0, 25, 75, and 100 mg L^-1) and two irrigation regimes [50 and 100% of crop evapotranspiration (ET_c)]. Water stress without BBS treatment reduced biological yield, grain yield, harvest index, and photosynthetic efficiency significantly by 28.9%, 42.8%, 19.6%, and 20.5% compared with the well-watered plants, respectively. Proline content increased in water-stressed and BSS-treated plants, owing to a significant role in cellular osmotic adjustment. Application of BSS was effective in reducing the generation of hydrogen peroxide (H₂O₂) and hence the malondialdehyde content significantly in water-stressed and well-watered wheat plants. Application of BSS up-regulated the activity of antioxidant enzymes (SOD, CAT, POD, and APX) significantly and increased the content of tocopherol, ascorbic acid, and carotene thereby reducing the levels of reactive oxygen species. The increased antioxidant system in BSS treated plants was further supported by the expression level of SOD and dehydrin genes in both water-stressed and well-watered plants. In the present study, the application of BBS at 100 mg L^-1 was beneficial and can be recommended for improving the growth and yield of the wheat crop under water stress.

Overexpression of a Wheat Aquaporin Gene, TdPIP2;1, Enhances Salt and Drought Tolerance in Transgenic Durum Wheat cv. Maali

In this study, we generated transgenic durum wheat cv. Maali overexpressing the wheat plasma membrane aquaporin TdPIP2;1 gene under the control of PrTdPIP2;1 promoter or under the constitutive PrCaMV35S promoter. Histochemical analysis of the fusion PrTdPIP2;1::TdPIP2;1::GusA in wheat plants showed that the β-glucuronidase (GUS) activity was detected in the leaves, stems and roots of stably transformed wheat T3 plants. Our results showed that transgenic wheat lines overexpressing the TdPIP2;1 gene exhibited improved germination rates and biomass production and retained low Na+ and high K+ concentrations in their shoots under high salt and osmotic stress conditions. In a long-term study under greenhouse conditions on salt or drought stress, transgenic TdPIP2;1 lines produced filled grains, whereas wild-type (WT) plants either died at the vegetative stage under salt stress or showed drastically reduced grain filling under drought stress. Performing real time RT-PCR experiments on wheat plants transformed with the fusion PrTdPIP2;1::GusA, we showed an increase in the accumulation of GusA transcripts in the roots of plants challenged with salt and drought stress. Study of the antioxidant defence system in transgenic wheat TdPIP2;1 lines showed that these lines induced the antioxidative enzymes Catalase (CAT) and Superoxide dismutase (SOD) activities more efficiently than the WT plants, which is associated with lower malondialdehyde and hydrogen peroxide contents. Taken together, these results indicate the high potential of the TdPIP2;1 gene for reducing water evaporation from leaves (water loss) in response to water deficit through the lowering of transpiration per unit leaf area (stomatal conductance) and engineering effective drought and salt tolerance in transgenic TdPIP2;1 lines.

Integrated analysis of the detoxification responses of two Euramerican poplar genotypes exposed to ozone and water deficit: Focus on the ascorbate-glutathione cycle

Ozone (O₃) and drought increase tree oxidative stress. To protect forest health, we need to improve risk assessment, using metric model such as the phytotoxic O₃ dose above a threshold of y nmol·m^-2·s^-1 (PODy), while taking into account detoxification mechanisms and interacting stresses. The impact of drought events on the effect of O₃ pollution deserves special attention. Water deficit may decrease O₃ entrance into the leaves by reducing stomatal opening; however, water deficit also induces changes in cell redox homeostasis. Besides, the behaviour of the cell antioxidative charge in case of stress combination (water deficit and O₃) still remains poorly investigated. To decipher the response of detoxification mechanisms relatively to the Halliwell-Asada-Foyer cycle (HAF), we exposed poplar saplings (Populus nigra × deltoides) composed of two genotypes (Carpaccio and Robusta), to various treatments for 17 days, i.e. i) mild water deficit, ii) 120 ppb O₃, and iii) a combination of these two treatments. Ozone similarly impacted the growth of the two genotypes, with an important leaf loss. Water deficit decreased growth by almost one third as compared to the control plants. As for the combined treatment, water deficit protected the saplings from leaf ozone injury, but with an inhibitory effect on growth. The pool of total ascorbate was not modified by the different treatments, while the pool of total glutathione increased with POD₀. We noticed a few differences between the two genotypes, particularly concerning the activity of monodehydroascorbate reductase and glutathione reductase relatively to POD₀. The expression profiles of genes coding for the dehydroascorbate reductase and glutathione reductase isoforms differed, probably in link with the putative localisation of ROS production in response to water deficit and ozone, respectively. Our result would argue for a major role of MDHAR, GR and glutathione in the preservation of the redox status.

WRKYs, the Jack-of-various-Trades, Modulate Dehydration Stress in Populus davidiana-A Transcriptomic Approach

Populus davidiana, native to Korea and central Asian countries, is a major contributor to the Korean forest cover. In the current study, using high-throughput RNA-seq mediated transcriptome analysis, we identified about 87 P. davidiana WRKY transcription factors (PopdaWRKY TFs) that showed differential expression to dehydration stress in both sensitive and tolerant cultivars. Our results suggested that, on average, most of the WRKY genes were upregulated in tolerant cultivars but downregulated in sensitive cultivars. Based on protein sequence alignment, P. davidiana WRKYs were classified into three major groups, I, II, III, and further subgroups. Phylogenetic analysis showed that WRKY TFs and their orthologs in Arabidopsis and rice were clustered together in the same subgroups, suggesting similar functions across species. Significant correlation was found among qRT-PCR and RNA-seq analysis. In vivo analysis using model plant Arabidopsis showed that atwrky62 (orthologous to Potri.016G137900) knockout mutants were significantly sensitive to dehydration possibly due to an inability to close their stomata under dehydration conditions. In addition, a concomitant decrease in expression of ABA biosynthetic genes was observed. The AtHK1 that regulates stomatal movement was also downregulated in atwrky62 compared to the wild type. Taken together, our findings suggest a regulatory role of PopdaWRKYs under dehydration stress.

Oxidative stress responses and cellular energy allocation changes in microalgae following exposure to widely used human antibiotics

The individual effect of four human antibiotics on the microalgae Raphidocelis subcapitata was investigated following a 120-h exposure. The effects were assessed by analyzing growth, and biochemical parameters related with: 1) antioxidant capacity and oxidative damage by measuring superoxide dismutase (SOD) activity and lipid peroxidation (LPO) levels; and 2) cellular energy allocation (CEA) by quantifying the content in energy reserves, which represents the energy available (Ea), and the electron transport system activity that represents a measure of oxygen and cellular energy consumption (Ec). Growth yield inhibitory concentrations of sulfamethoxazole (18-30%), clarithromycin (28.7%), ciprofloxacin (28%) and erythromycin (17-39%) were found to elicit a considerable increase in Ec, thereby causing a significant decrease in the CEA. The elevated Ec can be a result of the need to respond to oxidative stress occurring under those conditions given the significant increase in SOD activity at these levels. For sulfamethoxazole, erythromycin and ciprofloxacin, the antioxidant responses do not seem to be enough to cope with the reactive oxygen species and prevent oxidative damage, given the elevated LPO levels observed. A stimulatory effect on growth yield was observed (up to 16%) at ciprofloxacin lowest concentration, which highly correlated with the increase in CEA. Based on the no observed effect concentration (NOECs) and/or effective concentration (EC₁₀) results, Ec, SOD and CEA were more sensitive than the classical endpoint of growth rate for all the tested antibiotics. By revealing the antibiotic stress effects in R. subcapitata at the cellular level, this study suggests CEA as a more reliable indicator of the organisms' physiological status.

ABA regulation of antioxidant activity during post-germination desiccation and subsequent rehydration in wheat

ABA regulation of antioxidant activity during post-germination desiccation and subsequent rehydration was studied in two wheat cultivars PBW 644 (ABA-higher sensitive and drought tolerant) and PBW 343 (ABA-lesser sensitive and drought susceptible) where 1 d-germinated seeds were exposed to ABA/ PEG- 6000 for next 1 d, desiccated for 4 d and subsequently rehydrated for 4 d. Ascorbate, dehydrascorbate to ascorbate ratio, malondialdehyde (MDA), hydroxyl radicals, and activities of monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), alcohol dehydrogenase (AlcDH) and aldehyde dehydrogenase (AldDH) were measured in seedlings just before desiccation (2 d old), desiccated (6 d old) and rehydrated (10 d old) stages. ROS/NO signaling was studied under CT and ABA supply by supplying ROS and NO scavengers. During desiccation, both cultivars showed increase of oxidative stress (dehydroascorbate to ascorbate ratio, MDA, hydroxyl radicals) and antioxidant activity in the form of ascorbate content and AldDH activity while other antioxidant enzymes were not increased. PBW 644 showed higher antioxidant activity thus produced less oxidative stress compared to PBW 343. During rehydration, activities of all antioxidant enzymes and levels of ROS (hydroxyl radicals) were increased in both cultivars and MDA was decreased in PBW 343. ABA supply improved desiccation as well as rehydration by improving all parameters of antioxidant activity tested in this study. PEG supply resembled to ABA-supply for its effects. ABA/PEG improvements were seen higher in PBW 644. ROS/NO-signalling was involved under CT as well as under ABA for increasing antioxidant activity during desiccation as well as rehydration in both cultivars.

28-homobrassinolide regulates antioxidant enzyme activities and gene expression in response to salt- and temperature-induced oxidative stress in Brassica juncea

Brassinosteroids (BRs) are a group of naturally occurring plant steroid hormones that can induce plant tolerance to various plant stresses by regulating ROS production in cells, but the underlying mechanisms of this scavenging activity by BRs are not well understood. This study investigated the effects of 28-homobrassinolide (28-HBL) seed priming on Brassica juncea seedlings subjected to the combined stress of extreme temperatures (low, 4 °C or high, 44 °C) and salinity (180 mM), either alone or supplemented with 28-HBL treatments (0, 10⁻⁶, 10⁻⁹, 10⁻¹² M). The combined temperature and salt stress treatments significantly reduced shoot and root lengths, but these improved when supplemented with 28-HBL although the response was dose-dependent. The combined stress alone significantly increased H₂O₂ content, but was inhibited when supplemented with 28-HBL. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APOX), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) increased in response to 28-HBL. Overall, the 28-HBL seed priming treatment improved the plant’s potential to combat the toxic effects imposed by the combined temperature and salt stress by tightly regulating the accumulation of ROS, which was reflected in the improved redox state of antioxidants.

A Dehydration-Induced Eukaryotic Translation Initiation Factor iso4G Identified in a Slow Wilting Soybean Cultivar Enhances Abiotic Stress Tolerance in Arabidopsis

Water is usually the main limiting factor for soybean productivity worldwide and yet advances in genetic improvement for drought resistance in this crop are still limited. In the present study, we investigated the physiological and molecular responses to drought in two soybean contrasting genotypes, a slow wilting N7001 and a drought sensitive TJS2049 cultivars. Measurements of stomatal conductance, carbon isotope ratios and accumulated dry matter showed that N7001 responds to drought by employing mechanisms resulting in a more efficient water use than TJS2049. To provide an insight into the molecular mechanisms that these cultivars employ to deal with water stress, their early and late transcriptional responses to drought were analyzed by suppression subtractive hybridization. A number of differentially regulated genes from N7001 were identified and their expression pattern was compared between in this genotype and TJS2049. Overall, the data set indicated that N7001 responds to drought earlier than TJ2049 by up-regulating a larger number of genes, most of them encoding proteins with regulatory and signaling functions. The data supports the idea that at least some of the phenotypic differences between slow wilting and drought sensitive plants may rely on the regulation of the level and timing of expression of specific genes. One of the genes that exhibited a marked N7001-specific drought induction profile encoded a eukaryotic translation initiation factor iso4G (GmeIFiso4G-1a). GmeIFiso4G-1a is one of four members of this protein family in soybean, all of them sharing high sequence identity with each other. In silico analysis of GmeIFiso4G-1 promoter sequences suggested a possible functional specialization between distinct family members, which can attain differences at the transcriptional level. Conditional overexpression of GmeIFiso4G-1a in Arabidopsis conferred the transgenic plants increased tolerance to osmotic, salt, drought and low temperature stress, providing a strong experimental evidence for a direct association between a protein of this class and general abiotic stress tolerance mechanisms. Moreover, the results of this work reinforce the importance of the control of protein synthesis as a central mechanism of stress adaptation and opens up for new strategies for improving crop performance under stress.

Drought stress tolerance strategies revealed by RNA-Seq in two sorghum genotypes with contrasting WUE

BACKGROUND

Drought stress is the major environmental stress that affects plant growth and productivity. It triggers a wide range of responses detectable at molecular, biochemical and physiological levels. At the molecular level the response to drought stress results in the differential expression of several metabolic pathways. For this reason, exploring the subtle differences in gene expression of drought sensitive and drought tolerant genotypes enables the identification of drought-related genes that could be used for selection of drought tolerance traits. Genome-wide RNA-Seq technology was used to compare the drought response of two sorghum genotypes characterized by contrasting water use efficiency.

RESULTS

The physiological measurements carried out confirmed the drought sensitivity of IS20351 and the drought tolerance of IS22330 genotypes, as previously studied. The expression of drought-related genes was more abundant in the drought sensitive genotype IS20351 compared to the tolerant genotype IS22330. Under drought stress Gene Ontology enrichment highlighted a massive increase in transcript abundance in the sensitive genotype IS20351 in "response to stress" and "abiotic stimulus", as well as for "oxidation-reduction reaction". "Antioxidant" and "secondary metabolism", "photosynthesis and carbon fixation process", "lipids" and "carbon metabolism" were the pathways most affected by drought in the sensitive genotype IS20351. In addition, genotype IS20351 showed a lower constitutive expression level of "secondary metabolic process" (GO:0019748) and "glutathione transferase activity" (GO:000004364) under well-watered conditions.

CONCLUSIONS

RNA-Seq analysis proved to be a very useful tool to explore differences between sensitive and tolerant sorghum genotypes. Transcriptomics analysis results supported all the physiological measurements and were essential to clarify the tolerance of the two genotypes studied. The connection between differential gene expression and physiological response to drought unequivocally revealed the drought tolerance of genotype IS22330 and the strategy adopted to cope with drought stress.

Aluminum resistance in wheat involves maintenance of leaf Ca(2+) and Mg(2+) content, decreased lipid peroxidation and Al accumulation, and low photosystem II excitation pressure

The phytotoxic aluminum species (Al(3+)) is considered as the primary factor limiting crop productivity in over 40 % of world's arable land that is acidic. We evaluated the responses of two wheat cultivars (Triticum aestivum L.) with differential Al resistance, cv. Yecora E (Al-resistant) and cv. Dio (Al-sensitive), exposed to 0, 37, 74 and 148 μM Al for 14 days in hydroponic culture at pH 4.5. With increasing Al concentration, leaf Ca(2+) and Mg(2+) content decreased, as well as the effective quantum yield of photosystem II (PSII) photochemistry (Φ PSII ), while a gradual increase in leaf membrane lipid peroxidation, Al accumulation, photoinhibition (estimated as F v /F m ), and PSII excitation pressure (1 - q p ) occurred. However, the Al-resistant cultivar with lower Al accumulation, retained larger concentrations of Ca(2+) and Mg(2+) in the leaves and kept a larger fraction of the PSII reaction centres (RCs) in an open configuration, i.e. a higher ratio of oxidized to reduced quinone A (QA), than plants of the Al-sensitive cultivar. Four times higher Al concentration in the nutrient solution was required for Al-resistant plants (148 μM Al) than for Al-sensitive (37 μM Al), in order to establish the same closed RCs. Yet, the decline in photosynthetic efficiency in the cultivar Dio was not only due to closure of PSII RCs but also to a decrease in the quantum yield of the open RCs. We suggest that Al(3+) toxicity may be mediated by nutrient deficiency and oxidative stress, and that Al-resistance of the wheat cultivar Yecora E, may be due at least partially, from the decreased Al accumulation that resulted to decreased reactive oxygen species (ROS) formation. However, under equal internal Al accumulation (exposure Al concentration: Dio 74 μM, Yecora E 148 μM) that resulted to the same oxidative stress, the reduced PSII excitation pressure and the better PSII functioning of the Al-resistant cultivar was probably due to the larger concentrations of Ca(2+) and Mg(2+) in the leaves. We propose that the different sensitivities of wheat cultivars to Al(3+) toxicity can be correlated to differences in the redox state of QA. Thus, chlorophyll fluorescence measurements can be a promising tool for rapid screening of Al resistance in wheat cultivars.

Variations in physiological and biochemical traits of oak seedlings grown under drought and ozone stress

Despite the huge biodiversity characterizing the Mediterranean environment, environmental constraints, such as high sunlight and high temperatures alongside with dry periods, make plant survival hard. In addition, high irradiance leads to increasing ozone (O3 ) concentrations in ambient air. In this era of global warming, it is necessary to understand the mechanisms that allow native species to tolerate these environmental constraints and how such mechanisms interact. Three Mediterranean oak species (Quercus ilex, Quercus pubescens and Quercus cerris) with different features (drought tolerant, evergreen or deciduous species) were selected to assess their biometrical, physiological and biochemical responses under drought and/or O3 stress (80-100 nl l(-1) of O3 for 5 h day(-1) for 77 consecutive days). Leaf visible injury appeared only under drought stress (alone or combined with O3 ) in all three species. Drought × O3 induced strong reductions in leaf dry weight in Q. pubescens and Q. cerris (-70 and -75%, respectively). Alterations in physiological (i.e. decrease in maximum carboxylation rate) and biochemical parameters (i.e. increase in proline content and build-up of malondialdehyde by-products) occurred in all the three species, although drought represented the major determinant. Quercus ilex and Q. pubescens, which co-occur in dry environments, were more tolerant to drought and drought × O3 . Quercus ilex was the species in which oxidative stress occurred only when drought was applied with O3 . High plasticity at a biochemical level (i.e. proline content) and evergreen habitus are likely on the basis of the higher tolerance of Q. ilex.

Leaf Age-Dependent Photoprotective and Antioxidative Response Mechanisms to Paraquat-Induced Oxidative Stress in Arabidopsis thaliana

Exposure of Arabidopsis thaliana young and mature leaves to the herbicide paraquat (Pq) resulted in a localized increase of hydrogen peroxide (H₂O₂) in the leaf veins and the neighboring mesophyll cells, but this increase was not similar in the two leaf types. Increased H₂O₂ production was concomitant with closed reaction centers (q_P). Thirty min after Pq exposure despite the induction of the photoprotective mechanism of non-photochemical quenching (NPQ) in mature leaves, H₂O₂ production was lower in young leaves mainly due to the higher increase activity of ascorbate peroxidase (APX). Later, 60 min after Pq exposure, the total antioxidant capacity of young leaves was not sufficient to scavenge the excess reactive oxygen species (ROS) that were formed, and thus, a higher H₂O₂ accumulation in young leaves occurred. The energy allocation of absorbed light in photosystem II (PSII) suggests the existence of a differential photoprotective regulatory mechanism in the two leaf types to the time-course Pq exposure accompanied by differential antioxidant protection mechanisms. It is concluded that tolerance to Pq-induced oxidative stress is related to the redox state of quinone A (Q_A).

Redox proteomics and physiological responses in Cistus albidus shrubs subjected to long-term summer drought followed by recovery

The interaction between enzymatic and non-enzymatic antioxidants, endogenous levels of ABA and ABA-GE, the rapid recuperation of photosynthetic proteins under re-watering as well the high level of antioxidant proteins in previously drought-stressed plants under re-watering conditions, will contribute to drought resistance in plants subjected to a long-term drought stress under Mediterranean field conditions. This work provides an overview of the mechanisms of Cistus albidus acclimation to long-term summer drought followed by re-watering in Mediterranean field conditions. To better understand the molecular mechanisms of drought resistance in these plants, a proteomic study using 2-DE and MALDI-TOF/TOF MS/MS was performed on leaves from these shrubs. The analysis identified 57 differentially expressed proteins in water-stressed plants when contrasted to well watered. Water-stressed plants showed an increase, both qualitatively and quantitatively, in HSPs, and downregulation of photosynthesis and carbon metabolism enzymes. Under drought conditions, there was considerable upregulation of enzymes related to redox homeostasis, DHA reductase, Glyoxalase, SOD and isoflavone reductase. However, upregulation of catalase was not observed until after re-watering was carried out. Drought treatment caused an enhancement in antioxidant defense responses that can be modulated by ABA, and its catabolites, ABA-GE, as well as JA. Furthermore, quantification of protein carbonylation was shown to be a useful marker of the relationship between water and oxidative stress, and showed that there was only moderate oxidative stress in C. albidus plants subjected to water stress. After re-watering plants recovered although the levels of ABA-GE and antioxidant enzymes still remain higher than in well-watered plants. We expect that our results will provide new data on summer acclimation to drought stress in Mediterranean shrubs.

Lichen rehydration in heavy metal-polluted environments: Pb modulates the oxidative response of both Ramalina farinacea thalli and its isolated microalgae

Lichens are adapted to desiccation/rehydration and accumulate heavy metals, which induce ROS especially from the photobiont photosynthetic pigments. Although their mechanisms of abiotic stress tolerance are still to be unravelled, they seem related to symbionts' reciprocal upregulation of antioxidant systems. With the aim to study the effect of Pb on oxidative status during rehydration, the kinetics of intracellular ROS, lipid peroxidation and chlorophyll autofluorescence of whole Ramalina farinacea thalli and its isolated microalgae (Trebouxia TR1 and T. TR9) was recorded. A genetic characterization of the microalgae present in the thalli used was also carried out in order to assess possible correlations among the relative abundance of each phycobiont, their individual physiological responses and that of the entire thallus. Unexpectedly, Pb decreased ROS and lipid peroxidation in thalli and its phycobionts, associated with a lower chlorophyll autofluorescence. Each phycobiont showed a particular pattern, but the oxidative response of the thallus paralleled the TR1's, agreeing with the genetic identification of this strain as the predominant phycobiont. We conclude that: (1) the lichen oxidative behaviour seems to be modulated by the predominant phycobiont and (2) Pb evokes in R. farinacea and its phycobionts strong mechanisms to neutralize its own oxidant effects along with those of rehydration.

Molecular, physiological and biochemical responses of Theobroma cacao L. genotypes to soil water deficit

Six months-old seminal plants of 36 cacao genotypes grown under greenhouse conditions were subjected to two soil water regimes (control and drought) to assess, the effects of water deficit on growth, chemical composition and oxidative stress. In the control, soil moisture was maintained near field capacity with leaf water potentials (ΨWL) ranging from -0.1 to -0.5 MPa. In the drought treatment, the soil moisture was reduced gradually by withholding additional water until ΨWL reached values of between -2.0 to -2.5 MPa. The tolerant genotypes PS-1319, MO-20 and MA-15 recorded significant increases in guaiacol peroxidase activity reflecting a more efficient antioxidant metabolism. In relation to drought tolerance, the most important variables in the distinguishing contrasting groups were: total leaf area per plant; leaf, stem and total dry biomass; relative growth rate; plant shoot biomass and leaf content of N, Ca, and Mg. From the results of these analyses, six genotypes were selected with contrasting characteristics for tolerance to soil water deficit [CC-40, C. SUL-4 and SIC-2 (non-tolerant) and MA-15, MO-20, and PA-13 (tolerant)] for further assessment of the expression of genes NCED5, PP2C, psbA and psbO to water deficit. Increased expression of NCED5, PP2C, psbA and psbO genes were found for non-tolerant genotypes, while in the majority of tolerant genotypes there was repression of these genes, with the exception of PA-13 that showed an increased expression of psbA. Mutivariate analysis showed that growth variables, leaf and total dry biomass, relative growth rate as well as Mg content of the leaves were the most important factor in the classification of the genotypes as tolerant, moderately tolerant and sensitive to water deficit. Therefore these variables are reliable plant traits in the selection of plants tolerant to drought.

Compartment-specific importance of glutathione during abiotic and biotic stress

The tripeptide thiol glutathione (γ-L-glutamyl-L-cysteinyl-glycine) is the most important sulfur containing antioxidant in plants and essential for plant defense against abiotic and biotic stress conditions. It is involved in the detoxification of reactive oxygen species (ROS), redox signaling, the modulation of defense gene expression, and the regulation of enzymatic activities. Even though changes in glutathione contents are well documented in plants and its roles in plant defense are well established, still too little is known about its compartment-specific importance during abiotic and biotic stress conditions. Due to technical advances in the visualization of glutathione and the redox state through microscopical methods some progress was made in the last few years in studying the importance of subcellular glutathione contents during stress conditions in plants. This review summarizes the data available on compartment-specific importance of glutathione in the protection against abiotic and biotic stress conditions such as high light stress, exposure to cadmium, drought, and pathogen attack (Pseudomonas, Botrytis, tobacco mosaic virus). The data will be discussed in connection with the subcellular accumulation of ROS during these conditions and glutathione synthesis which are both highly compartment specific (e.g., glutathione synthesis takes place in chloroplasts and the cytosol). Thus this review will reveal the compartment-specific importance of glutathione during abiotic and biotic stress conditions.

Leaf developmental stage modulates metabolite accumulation and photosynthesis contributing to acclimation of Arabidopsis thaliana to water deficit

We examined whether young and mature leaves of Arabidopsis thaliana in their response to mild water deficit (MiWD) and moderate water deficit (MoWD), behave differentially, and whether photosynthetic acclimation to water deficit correlates with increased proline and sugar accumulation. We observed that with increasing water deficit, leaf relative water content decreased, while proline and sugar accumulation increased in both leaf-developmental stages. Under both MiWD and MoWD, young leaves showed less water loss and accumulated higher level of metabolites compared to mature leaves. This, leaf age-related increase in metabolite accumulation that was significantly higher under MoWD, allowed young leaves to cope with oxidative damage by maintaining their base levels of lipid peroxidation. Thus, acclimation of young leaves to MoWD, involves a better homeostasis of reactive oxygen species (ROS), that was achieved among others by (1) increased sugar accumulation and (2) either increased proline synthesis and/or decreased proline catabolism, that decrease the NADPH/NADP(+) ratio, resulting in a higher level of oxidized state of quinone A and thus in a reduced excitation pressure, and by (3) stimulation of the photoprotective mechanism of non-photochemical quenching, that reflects the dissipation of excess excitation energy in the form of harmless heat, thus protecting the plant from the damaging effects of ROS.

RNA-Seq transcriptome profiling of upland cotton (Gossypium hirsutum L.) root tissue under water-deficit stress

An RNA-Seq experiment was performed using field grown well-watered and naturally rain fed cotton plants to identify differentially expressed transcripts under water-deficit stress. Our work constitutes the first application of the newly published diploid D5 Gossypium raimondii sequence in the study of tetraploid AD1 upland cotton RNA-seq transcriptome analysis. A total of 1,530 transcripts were differentially expressed between well-watered and water-deficit stressed root tissues, in patterns that confirm the accuracy of this technique for future studies in cotton genomics. Additionally, putative sequence based genome localization of differentially expressed transcripts detected A2 genome specific gene expression under water-deficit stress. These data will facilitate efforts to understand the complex responses governing transcriptomic regulatory mechanisms and to identify candidate genes that may benefit applied plant breeding programs.

Effect of nitric oxide signaling in bacterial-treated soybean plant under salt stress

To understand protective roles of nitric oxide against salt stress, the effects of exogenous sodium nitroprusside on activities of lipoxygenase, peroxidase, phenylalanine ammonialyase, catalase, superoxide dismutase enzymes, proline accumulation, and distribution of sodium in soybean plants under salt were determined. Application of sodium nitroprusside + bacterium enhanced plant growth-promotion characteristics, activities of different enzymes, and proline accumulation in the presence of sodium nitroprusside under salt stress. Treatment with NaCl at 200 mM and sodium nitroprusside (0.1 mM) reduced Na⁺ levels but increased K⁺ levels in leaves in comparison with the NaCl-treated plants. Correspondingly, the plants treated with exogenous sodium nitroprusside and NaCl maintained a lower ratio of [Na⁺]/[K⁺] in NaCl-stressed plants.

Assessing the regulation of leaf redox status under water stress conditions in Arabidopsis thaliana: Col-0 ecotype (wild-type and vtc-2), expressing mitochondrial and cytosolic roGFP1

Using Arabidopsis plants Col-0 and vtc2 transformed with a redox sensitive green fluorescent protein, (c-roGFP) and (m-roGFP), we investigated the effects of a progressive water stress and re-watering on the redox status of the cytosol and the mitochondria. Our results establish that water stress affects redox status differently in these two compartments, depending on phenotype and leaf age, furthermore we conclude that ascorbate plays a pivotal role in mediating redox status homeostasis and that Col-0 Arabidopsis subjected to water stress increase the synthesis of ascorbate suggesting that ascorbate may play a role in buffering changes in redox status in the mitochondria and the cytosol, with the presumed buffering capacity of ascorbate being more noticeable in young compared with mature leaves. Re-watering of water-stressed plants was paralleled by a return of both the redox status and ascorbate to the levels of well-watered plants. In contrast to the effects of water stress on ascorbate levels, there were no significant changes in the levels of glutathione, thereby suggesting that the regeneration and increase in ascorbate in water-stressed plants may occur by other processes in addition to the regeneration of ascorbate via the glutathione. Under water stress in vtc2 lines it was observed stronger differences in redox status in relation to leaf age, than due to water stress conditions compared with Col-0 plants. In the vtc2 an increase in DHA was observed in water-stressed plants. Furthermore, this work confirms the accuracy and sensitivity of the roGFP1 biosensor as a reporter for variations in water stress-associated changes in redox potentials.

SAPs as novel regulators of abiotic stress response in plants

Stress associated proteins (SAPs), novel A20/AN1 zinc-finger domain-containing proteins, are fast emerging as potential candidates for biotechnological approaches in order to improve abiotic stress tolerance in plants - the ultimate aim of which is crop-yield protection. Until relatively recently, such proteins had only been identified in humans, where they had been shown to be key regulators of innate immunity. Their phylogenetic relationship and recruitment of diverse protein domains reflect an architectural and mechanistic diversity. Emerging evidence suggests that SAPs may act as ubiquitin ligase, redox sensor, and regulator of gene expression during stress. Here, we evaluate the new knowledge on SAPs with a view to understand their mechanism of action. Furthermore, we set an agenda for investigating hitherto unexplored roles of these proteins.

A drought-sensitive barley variety displays oxidative stress and strongly increased contents in low-molecular weight antioxidant compounds during water deficit compared to a tolerant variety

Barley displays a great genetic diversity, constituting a valuable source to delineate the responses of contrasted genotypes to environmental constraints. Here, we investigated the level of oxidative stress and the participation of antioxidant systems in two barley genotypes: Express, a variety known to be sensitive to drought, and Saïda, an Algerian landrace selected for its tolerance to water deficit. Soil-grown 15-day-old plants were subjected to water deficit for 8 days and then rewatered. We observed that upon water stress Express exhibits compared to Saïda accelerated wilting and a higher level of oxidative stress evaluated by HPLC measurements of lipid peroxidation and by imaging techniques. In parallel, Express plants also display lower levels of catalase and superoxide dismutase activity. No great difference was observed regarding peroxiredoxins and methionine sulfoxide reductases, enzymes detoxifying peroxides and repairing oxidized proteins, respectively. In contrast, upon water stress and recovery, much higher contents and oxidation ratios of glutathione and ascorbate were measured in Express compared to Saïda. Express also shows during water deficit greater increases in the pools of lipophilic antioxidants like xantophyll carotenoids and α-tocopherol. Altogether, these data show that the differential behavior of the two genotypes involves distinct responses regarding antioxidant mechanisms. Indeed, the drought sensitivity of Express compared with Saïda is associated with oxidative damage and a lower enzymatic ROS-scavenging capacity, but in parallel with a much stronger enhancement of most mechanisms involving low-molecular weight antioxidant compounds.

Hydrogen sulfide induces systemic tolerance to salinity and non-ionic osmotic stress in strawberry plants through modification of reactive species biosynthesis and transcriptional regulation of multiple defence pathways

Hydrogen sulfide (H2S) has been recently found to act as a potent priming agent. This study explored the hypothesis that hydroponic pretreatment of strawberry (Fragaria × ananassa cv. Camarosa) roots with a H2S donor, sodium hydrosulfide (NaHS; 100 μM for 48 h), could induce long-lasting priming effects and tolerance to subsequent exposure to 100mM NaCI or 10% (w/v) PEG-6000 for 7 d. Hydrogen sulfide pretreatment of roots resulted in increased leaf chlorophyll fluorescence, stomatal conductance and leaf relative water content as well as lower lipid peroxidation levels in comparison with plants directly subjected to salt and non-ionic osmotic stress, thus suggesting a systemic mitigating effect of H2S pretreatment to cellular damage derived from abiotic stress factors. In addition, root pretreatment with NaHS resulted in the minimization of oxidative and nitrosative stress in strawberry plants, manifested via lower levels of synthesis of NO and H(2)O(2) in leaves and the maintenance of high ascorbate and glutathione redox states, following subsequent salt and non-ionic osmotic stresses. Quantitative real-time RT-PCR gene expression analysis of key antioxidant (cAPX, CAT, MnSOD, GR), ascorbate and glutathione biosynthesis (GCS, GDH, GS), transcription factor (DREB), and salt overly sensitive (SOS) pathway (SOS2-like, SOS3-like, SOS4) genes suggests that H2S plays a pivotal role in the coordinated regulation of multiple transcriptional pathways. The ameliorative effects of H2S were more pronounced in strawberry plants subjected to both stress conditions immediately after NaHS root pretreatment, rather than in plants subjected to stress conditions 3 d after root pretreatment. Overall, H2S-pretreated plants managed to overcome the deleterious effects of salt and non-ionic osmotic stress by controlling oxidative and nitrosative cellular damage through increased performance of antioxidant mechanisms and the coordinated regulation of the SOS pathway, thus proposing a novel role for H2S in plant priming, and in particular in a fruit crop such as strawberry.

Drought stress has contrasting effects on antioxidant enzymes activity and phenylpropanoid biosynthesis in Fraxinus ornus leaves: an excess light stress affair?

The experiment was conducted using Fraxinus ornus plants grown outside under full sunlight irradiance, and supplied with 100% (well-watered, WW), 40% (mild drought, MD), or 20% (severe drought, SD) of the daily evapotranspiration demand, with the main objective of exploring the effect of excess light stress on the activity of antioxidant enzymes and phenylpropanoid biosynthesis. Net CO₂ assimilation rate at saturating light and daily assimilated CO₂ were significantly smaller in SD than in WW and MD plants. Xanthophyll-cycle pigments supported nonphotochemical quenching to a significantly greater extent in SD than in MD and WW leaves. As a consequence, the actual efficiency of PSII (Φ(PSII)) was smaller, while the excess excitation-energy in the photosynthetic apparatus was greater in SD than in WW or MD plants. The concentrations of violaxanthin-cycle pigments relative to total chlorophyll (Chl(tot)) exceeded 200 mmol mol⁻¹ Chl(tot) in SD leaves at the end of the experiment. This leads to hypothesize for zeaxanthin a role not only as nonphotochemical quencher, but also as chloroplast antioxidant. Reductions in ascorbate peroxidase and catalase activities, as drought-stress progressed, were paralleled by greater accumulations of esculetin and quercetin 3-O-glycosides, both phenylpropanoids having effective capacity to scavenge H₂O₂. The drought-induced accumulation of esculetin and quercetin 3-O-glycosides in the vacuoles of mesophyll cells is consistent with their putative functions as reducing agents for H₂O₂ in excess light-stressed leaves. Nonetheless, the concentration of H₂O₂ and the lipid peroxidation were significantly greater in SD than in MD and WW leaves. It is speculated that vacuolar phenylpropanoids may constitute a secondary antioxidant system, even on a temporal basis, activated upon the depletion of primary antioxidant defences, and aimed at keeping whole-cell H₂O₂ within a sub-lethal concentration range.

Leaf senescence and abiotic stresses share reactive oxygen species-mediated chloroplast degradation

Leaf senescence is a genetically programmed decline in various cellular processes including photosynthesis and involves the hydrolysis of macromolecules such as proteins, lipids, etc. It is governed by the developmental age and is induced or enhanced by environmental stresses such as drought, heat, salinity and others. Internal factors such as reproductive structures also influence the rate of leaf senescence. Reactive oxygen species (ROS) generation is one of the earliest responses of plant cells under abiotic stresses and senescence. Chloroplasts are the main targets of ROS-linked damage during various environmental stresses and natural senescence as ROS detoxification systems decline with age. Plants adapt to environmental stresses through the process of acclimation, which involves less ROS production coupled with an efficient antioxidant defence. Chloroplasts are a major site of protein degradation, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is rapidly and selectively degraded during senescence and stress. The process of protein degradation is initiated by ROS and involves the action of proteolytic enzymes such as cysteine and serine proteases. The mechanism of Rubisco degradation still remains to be elucidated. The molecular understanding of leaf senescence was achieved through the characterization of senescence-associated genes and various senescence mutants of Arabidopsis, which is a suitable model plant showing monocarpic senescence. The regulation of senescence involves many regulatory elements composed of positive and negative elements to fine-tune the initiation and progression of senescence. This review gives an overview on chloroplast protein degradation during leaf senescence and abiotic stresses and also highlights the role of ROS management in both processes.

Genome-wide identification of differentially expressed genes under water deficit stress in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Cotton is the world's primary fiber crop and is a major agricultural commodity in over 30 countries. Like many other global commodities, sustainable cotton production is challenged by restricted natural resources. In response to the anticipated increase of agricultural water demand, a major research direction involves developing crops that use less water or that use water more efficiently. In this study, our objective was to identify differentially expressed genes in response to water deficit stress in cotton. A global expression analysis using cDNA-Amplified Fragment Length Polymorphism was conducted to compare root and leaf gene expression profiles from a putative drought resistant cotton cultivar grown under water deficit stressed and well watered field conditions.

RESULTS

We identified a total of 519 differentially expressed transcript derived fragments. Of these, 147 transcript derived fragment sequences were functionally annotated according to their gene ontology. Nearly 70 percent of transcript derived fragments belonged to four major categories: 1) unclassified, 2) stress/defense, 3) metabolism, and 4) gene regulation. We found heat shock protein-related and reactive oxygen species-related transcript derived fragments to be among the major parts of functional pathways induced by water deficit stress. Also, twelve novel transcripts were identified as both water deficit responsive and cotton specific. A subset of differentially expressed transcript derived fragments was verified using reverse transcription-polymerase chain reaction. Differential expression analysis also identified five pairs of duplicated transcript derived fragments in which four pairs responded differentially between each of their two homologues under water deficit stress.

CONCLUSIONS

In this study, we detected differentially expressed transcript derived fragments from water deficit stressed root and leaf tissues in tetraploid cotton and provided their gene ontology, functional/biological distribution, and possible roles of gene duplication. This discovery demonstrates complex mechanisms involved with polyploid cotton's transcriptome response to naturally occurring field water deficit stress. The genes identified in this study will provide candidate targets to manipulate the water use characteristics of cotton at the molecular level.

Global regulation of reactive oxygen species scavenging genes in alfalfa root and shoot under gradual drought stress and recovery

Reactive oxygen species (ROS) production and scavenging in plants under drought stress have been studied intensively in recent years. Here we report a global analysis of gene expression for the major ROS generating and scavenging proteins in alfalfa root and shoot under gradual drought stress followed by one-day recovery. Data from two alfalfa varieties, one drought tolerant and one drought sensitive, were compared and no qualitative differences in ROS gene regulation between the two were found. Conserved, tissue-specific patterns of gene expression in response to drought were observed for several ROS-scavenging gene families, including ascorbate peroxidase, monodehydroascorbate reductase, and peroxiredoxin. In addition, differential gene expression within families was observed. Genes for the ROS-generating enzyme, NADPH oxidase were generally induced under drought, while those for glycolate oxidase were repressed. Among the ROS-scavenging protein genes, Ferritin, Cu/Zn superoxide dismutase (SOD), and the majority of the glutathione peroxidase family members were induced under drought in both roots and shoots of both alfalfa varieties. In contrast, Fe-SOD, CC-type glutaredoxins, and thoiredoxins were downregulated.

Differential gene expression in different types of Hevea brasiliensis roots

Three types of roots (taproots, first order laterals and second order laterals) were functionally characterized on 7-month-old in vitro plantlets regenerated by somatic embryogenesis in Hevea brasiliensis. A histological analysis revealed different levels of differentiation depending on root diameter. A primary structure was found in first and second order lateral roots, while taproots displayed a secondary structure. The expression of 48 genes linked to some of the regulatory pathways acting in roots was compared in leaves, stems and the different types of roots by real-time RT-PCR. Thirteen genes were differentially expressed in the different organs studied in plants grown under control conditions. Nine additional other genes were differentially regulated between organs under water deficit conditions. In addition, 10 genes were significantly regulated in response to water deficit, including 8 regulated mainly in lateral roots types. Our results suggest that the regulation of gene expression in lateral roots is different than that in taproots, which have a main role in nutrient uptake and transport, respectively.

Comparison of ascorbate metabolism in fruits of two citrus species with obvious difference in ascorbate content in pulp

Citrus fruit is widely consumed and provides ascorbate for human health. The ascorbate content in pulp is generally higher in orange (Citrus sinensis Osb.) than in Satsuma mandarin (Citrus unshiu Marc.). However, what contributes to such difference is still unknown. In the present study, ascorbate accumulation, expression profiles of genes involved in L-galactose pathway and activity changes of enzymes related with L-ascorbic acid (AA) oxidation and recycling were investigated during fruit development and ripening in fruit pulp of Satsuma mandarin and orange. As fruit ripens, total ascorbate (T-ASC) or AA content increased in mandarin whereas fluctuated on a relatively high level in orange. Concentrations of T-ASC or AA in pulp of orange were over 1.5-fold higher than that in pulp of Satsuma mandarin during fruit ripening. Further analysis showed that each transcript of four genes (encoding GDP-D-mannose-3',5'-epimerase, GDP-L-galactose-pyrophosphatase, L-galactose dehydrogenase and L-galactono-1,4-lactone dehydrogenase respectively) in orange was almost on a higher level and the activities of oxidation enzymes (ascorbate oxidase and ascorbate peroxidase) were lower during fruit ripening as compared with Satsuma mandarin. As ascorbate pool size is decided by the combination of biosynthesis, oxidation and recycling, therefore, higher expression of four genes along with lower activity of oxidation enzymes should contribute at least partially to the higher ASC accumulation in orange pulp.

NFX1-LIKE2 (NFXL2) suppresses abscisic acid accumulation and stomatal closure in Arabidopsis thaliana

The NFX1-LIKE1 (NFXL1) and NFXL2 genes were identified as regulators of salt stress responses. The NFXL1 protein is a nuclear factor that positively affects adaptation to salt stress. The nfxl1-1 loss-of-function mutant displayed reduced survival rates under salt and high light stress. In contrast, the nfxl2-1 mutant, defective in the NFXL2 gene, and NFXL2-antisense plants exhibited enhanced survival under these conditions. We show here that the loss of NFXL2 function results in abscisic acid (ABA) overaccumulation, reduced stomatal conductance, and enhanced survival under drought stress. The nfxl2-1 mutant displayed reduced stomatal aperture under all conditions tested. Fusicoccin treatment, exposition to increasing light intensities, and supply of decreasing CO(2) concentrations demonstrated full opening capacity of nfxl2-1 stomata. Reduced stomatal opening presumably is a consequence of elevated ABA levels. Furthermore, seedling growth, root growth, and stomatal closure were hypersensitive to exogenous ABA. The enhanced ABA responses may contribute to the improved drought stress resistance of the mutant. Three NFXL2 splice variants were cloned and named NFXL2-78, NFXL2-97, and NFXL2-100 according to the molecular weight of the putative proteins. Translational fusions to the green fluorescent protein suggest nuclear localisation of the NFXL2 proteins. Stable expression of the NFXL2-78 splice variant in nfxl2-1 plants largely complemented the mutant phenotype. Our data show that NFXL2 controls ABA levels and suppresses ABA responses. NFXL2 may prevent unnecessary and costly stress adaptation under favourable conditions.

Plant responses to water stress: role of reactive oxygen species

Responses of plants to water stress may be assigned as either injurious change or tolerance index. One of the primary and cardinal changes in response to drought stress is the generation of reactive oxygen species (ROS), which is being considered as the cause of cellular damage. However, recently a signaling role of such ROS in triggering the ROS scavenging system that may confer protection or tolerance against stress is emerging. Such scavenging system consists of antioxidant enzymes like SOD, catalase and peroxidases, and antioxidant compounds like ascorbate, reduced glutathione; a balance between ROS generation and scavenging ultimately determines the oxidative load. As revealed in case of defence against pathogen, signaling via ROS is initiated by NADPH oxidase-catalyzed superoxide generation in the apoplastic space (cell wall) followed by conversion to hydrogen peroxide by the activity of cell wall-localized SOD. Wall peroxidase may also play role in ROS generation for signaling. Hydrogen peroxide may use Ca2+ and MAPK pathway as downstream signaling cascade. Plant hormones associated with stress responses like ABA and ethylene play their role possibly via a cross talk with ROS towards stress tolerance, thus projecting a dual role of ROS under drought stress.

Effect of nonylphenol on response of physiology and photosynthesis-related gene transcription of Chlorella vulgaris

Nonylphenol (NP) is regarded as a kind of persistent organic pollutant which exists ubiquitously in the environment. The objective of this study was to evaluate the effects of NP on Chlorella vulgaris physiological indices and gene transcription. The results showed that NP stress inhibited algal growth in short-term bioassay. NP also decreased chlorophyll content, including chl a, chl b, and total chlorophyll. NP caused oxidant hurt by overproducing reactive oxygen species (ROS), which might destroy the overall membrane system to cause malondialdehyde content increase. NP inhibited photosynthesis-related gene transcription in C. vulgaris after 24 to 48 h exposure. The lowest transcript levels of psaB, psbA, and rbcL in C. vulgaris decreased to only 18.5%, 7%, and 4% of the control, respectively. Taken together, our results demonstrate that NP is toxic to fresh algae growth by affecting the photosynthesis-related genes transcription and overproducing ROS to disrupt cell structure in a short period.