Differential regulation of photorespiratory gene expression by moderate and severe salt and drought stress in relation to oxidative stress

A meta-analysis highlights the cross-resistance of plants to drought and salt stresses from physiological, biochemical, and growth levels

In nature, drought and salt stresses often occur simultaneously and affect plant growth at multiple levels. However, the mechanisms underlying plant responses to drought and salt stresses and their interactions are still not fully understood. We performed a meta-analysis to compare the effects of drought, salt, and combined stresses on plant physiological, biochemical, morphological and growth traits, analyze the different responses of C3 and C4 plants, as well as halophytes and non-halophytes, and identify the interactive effects on plants. There were numerous similarities in plant responses to drought, salt, and combined stresses. C4 plants had a more effective antioxidant defense system, and could better maintain above-ground growth. Halophytes could better maintain photosynthetic rate (Pn) and relative water content (RWC), and reduce growth as an adaptation strategy. The responses of most traits (Pn, RWC, chlorophyll content, soluble sugar content, H2O2 content, plant dry weight, etc.) to combined stress were less-than-additive, indicating cross-resistance rather than cross-sensitivity of plants to drought and salt stresses. These results are important to improve our understanding of drought and salt cross-resistance mechanisms and further induce resistance or screen-resistant varieties under stress combination.

Exploration of plant growth promoting traits and regulatory mechanisms of Bacillus anthracis PM21 in enhancing salt stress tolerance in maize

Bacillus species have been reported to reduce the negative effects of salt stress on plants; the involvement of Bacillus anthracis PM21 and the internal mechanisms involved in this process are unclear. The effects of PM21 inoculation on maize plants under salt stress were investigated in this study. The study aimed to assess the ability of Bacillus anthracis PM21 to endure high levels of salinity stress while preserving the concentration of plant growth regulators. The biomass, photosynthetic pigments, relative water content (RWC), antioxidants, osmoprotectants, inorganic ion contents, regulation of plant hormones and expression of antioxidants enzyme encoded genes were investigated under normal and salinity stress conditions. Bacillus anthracis PM21 produced a significant amount of 1-aminocyclopropane-1-carboxylate deaminase enzyme (ACC deaminase) and exopolysaccharides (EPS) under salt stress and normal conditions. PM21 also produced plant growth stimulants including indole acetic acid, gibberellic acid (GA3), kinetin, and siderophore under salinity stress and normal conditions. Under salt stress, PM21 inoculation markedly increased plant growth indices, stimulate antioxidant enzyme mechanisms, osmoprotectants, and chlorophyll content. The use of qRT-PCR to analyze the transcription of targeted genes indicated greater expression of antioxidant-encoded genes and inferred their possible function in salinity stress tolerance. Our findings shed light on the functions of PM21 and its regulatory mechanisms in plant salt stress tolerance, as well as the importance of PM21 in this process. This study will provide a thorough analysis of the theoretical framework for adopting PM21 in agricultural production as an eco-friendly method to enhance crop growth and yield under salinity stress.

Salt stress-induced chloroplastic hydrogen peroxide stimulates pdTPI sulfenylation and methylglyoxal accumulation

High salinity, an adverse environmental factor affecting about 20% of irrigated arable land worldwide, inhibits plant growth and development by causing oxidative stress, damaging cellular components, and disturbing global metabolism. However, whether and how reactive oxygen species disturb the metabolism of salt-stressed plants remain elusive. Here, we report that salt-induced hydrogen peroxide (H2O2) inhibits the activity of plastid triose phosphate isomerase (pdTPI) to promote methylglyoxal (MG) accumulation and stimulates the sulfenylation of pdTPI at cysteine 74. We also show that MG is a key factor limiting the plant growth, as a decrease in MG levels completely rescued the stunted growth and repressed salt stress tolerance of the pdtpi mutant. Furthermore, targeting CATALASE 2 into chloroplasts to prevent salt-induced overaccumulation of H2O2 conferred salt stress tolerance, revealing a role for chloroplastic H2O2 in salt-caused plant damage. In addition, we demonstrate that the H2O2-mediated accumulation of MG in turn induces H2O2 production, thus forming a regulatory loop that further inhibits the pdTPI activity in salt-stressed plants. Our findings, therefore, illustrate how salt stress induces MG production to inhibit the plant growth.

Characterization of the Seed Biopriming, Plant Growth-Promoting and Salinity-Ameliorating Potential of Halophilic Fungi Isolated from Hypersaline Habitats

Salinity stress is one of the major abiotic factors limiting crop yield in arid and semi-arid regions. Plant growth-promoting fungi can help plants thrive in stressful conditions. In this study, we isolated and characterized 26 halophilic fungi (endophytic, rhizospheric, and soil) from the coastal region of Muscat, Oman, for plant growth-promoting activities. About 16 out of 26 fungi were found to produce IAA, and about 11 isolates (MGRF1, MGRF2, GREF1, GREF2, TQRF4, TQRF5, TQRF5, TQRF6, TQRF7, TQRF8, TQRF2) out of 26 strains were found to significantly improve seed germination and seedling growth of wheat. To evaluate the effect of the above-selected strains on salt tolerance in wheat, we grew wheat seedlings in 150 mM, 300 mM NaCl and SW (100% seawater) treatments and inoculated them with the above strains. Our findings showed that fungal strains MGRF1, MGRF2, GREF2, and TQRF9 alleviate 150 mM salt stress and increase shoot length compared to their respective control plants. However, in 300 mM stressed plants, GREF1 and TQRF9 were observed to improve shoot length. Two strains, GREF2 and TQRF8, also promoted plant growth and reduced salt stress in SW-treated plants. Like shoot length, an analogous pattern was observed in root length, and different salt stressors such as 150 mM, 300 mM, and SW reduced root length by up to 4%, 7.5%, and 19.5%, respectively. Three strains, GREF1, TQRF7, and MGRF1, had higher catalase (CAT) levels, and similar results were observed in polyphenol oxidase (PPO), and GREF1 inoculation dramatically raised the PPO level in 150 mM salt stress. The fungal strains had varying effects, with some, such as GREF1, GREF2, and TQRF9, showing a significant increase in protein content as compared to their respective control plants. Under salinity stress, the expression of DREB2 and DREB6 genes was reduced. However, the WDREB2 gene, on the other hand, was shown to be highly elevated during salt stress conditions, whereas the opposite was observed in inoculated plants.

The efficacy of Azotobacter chroococcum in altering maize plant-defense responses to armyworm at elevated CO2 concentration

Elevated atmospheric carbon dioxide (eCO₂) concentrations can alter the carbon:nitrogen ratio and palatability of host plants for herbivorous insects, but rhizobacteria likely mitigate the alteration and influence physiological adaptation of insects. In this study, we conducted transcriptomic analysis of maize (Zea mays) response to Azotobacter chroococcum (AC) inoculation under eCO₂ conditions in contrast to ambient CO₂ (aCO₂), and studied the effects of plant-defense change of maize under eCO₂ on the oriental armyworm, Mythimna separata. Results showed that there were 16, 14, 16 and 135 differentially expressed genes that were associated with plant-defense response in maize leaves between aCO₂-CK and aCO₂-AC, eCO₂-CK and eCO₂-AC, aCO₂-CK and eCO₂-CK, aCO₂-AC and eCO₂-AC, respectively. Moreover, A. chroococcum inoculation and eCO₂ influenced plant hormone signal transduction of maize. Interestingly, A. chroococcum inoculation significantly decreased the contents of JA (jasmonic acid) and JA-Ile (isoleucine conjugate of JA) in leaves, but eCO₂ markedly increased contents of JA-Ile, JA and SA (salicylic acid). Compared to aCO₂, eCO₂ significantly decreased activity of protective enzyme (catalase), and increased activities of digestive (lipase and protease), protective (peroxidase) and detoxifying enzymes (carboxylesterase, Mixed-functional oxidase and glutathione s-transferase), prolonged developmental time, and decreased survival rate and body weight of larvae (P < 0.05). A. chroococcum inoculation significantly increased the activity of protective enzyme (catalase), and decreased the activities of detoxifying enzymes (carboxylesterase, glutathione s-transferase and mixed-functional oxidase), thus increased the growth rate and body weight of larvae in comparison with no-inoculation of A. chroococcum (P < 0.05). The indices of M. separata were significantly correlated with the foliar contents of JA, JA-Ile and SA (|r| = 0.44-0.85, P < 0.05), indicating that A. chroococcum inoculation altered the physiological adaptation of M. separata under eCO₂ by disturbing defense substances in maize. Our results in understanding effects of A. chroococcum inoculation on maize resistance to herbivorous insects will be valuable for agricultural pest control in the future at eCO₂ conditions.

Over-expression of the Zea mays phytoglobin (ZmPgb1.1) alleviates the effect of water stress through shoot-specific mechanisms

Water deficit limits plant growth and development by interfering with several physiological and molecular processes both in root and shoot tissues. Through their ability to scavenge nitric oxide (NO), phytoglobins (Pgbs) exercise a protective role during several conditions of stress. While their action has been mainly documented in roots, it is unclear whether Pgb exercises a specific and direct role in shoot tissue. We used a Zea mays root-less system to assess how over-expression or down-regulation of ZmPgb1.1 influences the behavior of shoots exposed to polyethylene glycol (PEG)-simulated water deficit. Relative to their WT and ZmPgb1.1 down-regulating counterparts, PEG-treated shoots over-expressing ZmPgb1.1 exhibited a reduced accumulation of ROS and lipid peroxidation. These effects were ascribed to lower transcript levels of Respiratory Burst Oxidase Homolog (RBOH) genes encoding the ROS generating enzyme complex NADPH oxidase, and a more active antioxidant system. Furthermore, over-expression of ZmPgb1.1 attenuated the reduction in osmotic potential and relative water content experienced during water stress, an observation also demonstrated at a whole plant level, possibly through the retention of the expression of three aquaporins involved in water transfer and implicated in drought tolerance. Pharmacological treatments modulating NO and ethylene levels revealed that the ZmPgb1.1 action was mediated by ethylene synthesis and response, with NO acting as an upstream intermediate. Collectively we provide substantial evidence that ZmPgb1.1 exercises a direct role in shoot tissue, independent from that previously reported in roots, which confers tolerance to water stress.

Arbuscular Mycorrhizal Fungi Mediate Drought Tolerance and Recovery in Two Contrasting Carob (Ceratonia siliqua L.) Ecotypes by Regulating Stomatal, Water Relations, and (In)Organic Adjustments

Irregular precipitation and drought caused an increase in tree mortality rates in multiple forest biomes with alterations in both ecosystem services and carbon balance. Carob (Ceratonia siliqua) growth and production in arid and semi-arid ecosystems are likely affected by climate change-induced droughts. Understanding the physiological responses of drought-induced early-stage tree death and strategies to enhance drought tolerance and optimize growth will help tree improvement programs. Mycorrhizal inoculation has a pronounced impact on plant growth, water absorption, mineral nutrition, and protection from abiotic stresses. However, a better understanding of these complex interconnected cellular processes and arbuscular mycorrhizal fungi (AMF)-mediated mechanisms regulating drought tolerance in plants will enhance its potential application as an efficient approach for bio-amelioration of stresses. The objectives of this work were to elucidate the different effects of autochthone AMF on inorganic solute and water content uptakes, organic adjustments (sugar and proteins content), leaf gas exchange (stomatal conductance and efficiency of photosystems I and II), and oxidative damage of two contrasting ecotypes of carob seedlings: coastal (southern ecotype (SE)) and in-land (northern ecotype (NE)) under control (C), drought (by cessation of irrigation for 15 days (15D)), and recovery (R) conditions. Our findings showed that AMF promoted growth, nutrient content, and physiological and biochemical parameters in plants of both ecotypes during C, 15D, and R conditions. After four days of recovery, stomatal conductance (gs), the maximum photochemical efficiency of PSII (Fv/Fm), water content, and plant uptake of mineral nutrients (P, K, Na, and Ca) were significantly higher in shoots of mycorrhizal (AM) than non-mycorrhizal (NM) control plants. Consequently, AMF reduced to a greater degree the accumulation of hydrogen peroxide (H2O2) and oxidative damage to lipid (malondialdehyde (MDA)) content in AM than NM plants in NE and SE, after recovery. Altogether, our findings suggest that AMF can play a role in drought resistance of carob trees at an early stage by increasing the inorganic solutes (P, K, Na, and Ca), water content uptake, organic solutes (soluble sugars and protein content), stomatal conductance, and defense response against oxidative damage during re-watering after drought stress.

Comparative Proteomics Unravels the Differences in Salt Stress Response of Own-Rooted and 110R-Grafted Thompson Seedless Grapevines

Thompson Seedless, a commonly grown table grape variety, is sensitive to salinity when grown on its own roots, and therefore, it is frequently grafted onto salinity-tolerant wild grapevine rootstocks. Rising soil salinity is a growing concern in irrigated agricultural systems. The accumulation of salts near the root zone severely hampers plant growth, leading to a decrease in the productive lifespan of grapevine and causing heavy yield losses to the farmer. In the present study, we investigated the differences in response to salinity between own-rooted Thompson Seedless (TSOR) and 110R-grafted Thompson Seedless (TS110R) grapevines, wherein 110R is reported to be a salt-tolerant rootstock. The grapevines were subjected to salt stress by treating them with a 150 mM NaCl solution. The stress-induced changes in protein abundance were investigated using a label-free shotgun proteomics approach at three time-points viz. 6 h, 48 h, and 7 days of salt treatment. A total of 2793 proteins were identified, of which 246 were differentially abundant at various time-points in TSOR and TS110R vines. The abundance of proteins involved in several biological processes such as photosynthesis, amino acid metabolism, translation, chlorophyll biosynthesis, and generation of precursor metabolites was significantly affected by salt stress in both the vines but at different stages of stress. The results revealed that TSOR vines responded fervently to salt stress, while TS110R vines adopted a preventive approach. The findings of this study add to the knowledge of salinity response in woody and grafted plants and hence open the scope for further studies on salt stress-specific differences induced by grafting.

Morphological, physiochemical and antioxidant responses of Maclura pomifera to drought stress

Drought is one of the most important environmental factor limiting the growth of woody and non woody plants. In the present paper, we aimed to explore the performance of Maclura pomifera under a prolonged drought period followed by re-watering. M. pomifera plants were exposed to four different watering regimes (100%, 75%, 50% and 30% of the field capacity (FC)) for three weeks and then rewatered. The exposure to drought affected physiological, morphological and biochemical traits of M. pomifera. Leaf area, relative water content and water potential of leaf decreased in parallel with increased water deficit. Malondialdehyde content increased along with the drought stress experiment. Soluble carbohydrates (sucrose, glucose and fructose) accumulated during drought stress, but decreased after 22 days of water deficit in severe stressed plants (30% FC). Proline and mannitol, two compatible osmolytes, were higher in drought stresses plants than in control plants. Additionally the activity of antioxidant enzymes (SOD, APX, DHAR and GR) resulted affected by drought stress. In the recovery period, the physiological parameters as well as the proline content recovered at control levels, whereas soluble sugars, mannitol and total activity of antioxidant enzymes remained slight higher than in control plants, presumably to allow plants a complete recovery after stress. Our results suggest that M. pomifera has a good adaptive response to drought stress, probably corresponded to decreasing oxidative injury by induction of the antioxidant system and accumulation of stable and protective osmolytes such as proline and mannitol at higher rates.

Arbuscular Mycorrhizal Fungi Alleviate Drought Stress in C3 (Leymus chinensis) and C4 (Hemarthria altissima) Grasses via Altering Antioxidant Enzyme Activities and Photosynthesis

As one of the most important limiting factors of grassland productivity, drought is predicted to increase in intensity and frequency. Greenhouse studies suggest that arbuscular mycorrhizal fungi (AMF) can improve plant drought resistance. However, whether AMF can improve plant drought resistance in field conditions and whether the effects of AMF on drought resistance differ among plants with different photosynthetic pathways remain unclear. To evaluate the effect of indigenous AMF on plant drought resistance, an in situ rainfall exclusion experiment was conducted in a temperate meadow in northeast China. The results showed that AMF significantly reduced the negative effects of drought on plant growth. On average, AMF enhanced plant biomass, photosynthetic rate (A), stomatal conductance (g s), intrinsic water use efficiency (iWUE), and superoxide dismutase (SOD) activity of the C3 species Leymus chinensis by 58, 63, 38, 15, and 45%, respectively, and reduced levels of malondialdehyde (MDA) by 32% under light and moderate drought (rainfall exclusion of 30 and 50%, respectively). However, under extreme drought (rainfall exclusion of 70%), AMF elevated only aboveground biomass and catalase (CAT) activities. Averagely, AMF increased the aboveground biomass, A, and CAT activity of Hemarthria altissima (C4) by 37, 28, and 30%, respectively, under light and moderate droughts. The contribution of AMF to plant drought resistance was higher for the C3 species than that for the C4 species under both light and moderate drought conditions. The results highlight potential photosynthetic type differences in the magnitude of AMF-associated enhancement in plant drought resistance. Therefore, AMF may determine plant community structure under future climate change scenarios by affecting the drought resistance of different plant functional groups.

Photorespiration is complemented by cyclic electron flow and the alternative oxidase pathway to optimize photosynthesis and protect against abiotic stress

Optimization of photosynthetic performance and protection against abiotic stress are essential to sustain plant growth. Photorespiratory metabolism can help plants to adapt to abiotic stress. The beneficial role of photorespiration under abiotic stress is further strengthened by cyclic electron flow (CEF) and alternative oxidase (AOX) pathways. We have attempted to critically assess the literature on the responses of these three phenomena-photorespiration, CEF and AOX, to different stress situations. We emphasize that photorespiration is the key player to protect photosynthesis and upregulates CEF as well as AOX. Then these three processes work in coordination to protect the plants against photoinhibition and maintain an optimal redox state in the cell, while providing ATP for metabolism and protein repair. H₂O₂ generated during photorespiratory metabolism seems to be an important signal to upregulate CEF or AOX. Further experiments are necessary to identify the signals originating from CEF or AOX to modulate photorespiration. The mutants deficient in CEF or AOX or both could be useful in this regard. The mutual interactions between CEF and AOX, so as to keep their complementarity, are also to be examined further.

Responses of antioxidant enzymes, photosynthetic pigments and carbohydrates in micropropagated Pitcairnia encholirioides L.B. Sm. (Bromeliaceae) under ex vitro water deficit and after rehydration

Abstract In this study, the activities of antioxidant enzymes, photosynthetic pigments, proline and carbohydrate contents in Pitcairnia encholirioides under ex vitro conditions of water deficit were evaluated. Results show that plants under progressive water stress, previously in vitro cultured in media supplemented with 30 g L-1 sucrose and GA3, accumulated more proline and increased peroxidase (POD) activity and the contents of photosynthetic pigments and carbohydrates. For plants previously in vitro cultured with 15 g L-1 sucrose and NAA, no differences were found for proline content and there were reductions in activities of peroxidase (POD), catalase (CAT) and poliphenoloxidase (PPO), and in contents of carbohydrates, with progress of ex vitro water deficit. After rehydration, plants showed physiological recovery, with enzymatic activities and contents of metabolites similar to those found in the controls not submitted to dehydration, regardless of the previous in vitro culture conditions. These results show that micropropagated P. encholirioides has high tolerance to dehydration once in ex vitro conditions, which can ensure the survival of plants from tissue culture when transferred to its natural environment, emphasizing the importance of such biotechnology for the propagation of endangered species.

Salt tolerance of Glycine max.L induced by endophytic fungus Aspergillus flavus CSH1, via regulating its endogenous hormones and antioxidative system

Abiotic stress resistance strategies are powerful approaches to sustainable agriculture because they reduce chemical input and enhance plant productivity. In current study, an endophytic fungus, Aspergillus flavus CHS1 was isolated from Chenopodium album Roots. CHS1 was initially screened for growth promoting activities like siderphore, phosphate solubilization, and the production of indole acetic acid and gibberellins and were further assayed for its ability to promote the growth of mutant Waito-C rice. The results revealed that different plant growth characteristic such as chlorophyll content, root-shoot length, and biomass production were significantly promoted during CHS1 treatment. This growth promotion action was due to the presence of various types of GAs and IAA in the endophyte culture filtrate. Significant up regulation with respect to levels in the control was observed in all endogenous plant GAs, after treatment with CHS1. Furthermore, to evaluate the potential of CHS1 against NaCl stress up to 400 mM, it was tested for its ability to improve soybean plant growth under NaCl stress. In endophyte-soybean interaction, CHS1 association significantly increased plant growth and attenuated the NaCl stress by down regulating ABA and JA synthesis. Similarly, it significantly elevated antioxidant activities of enzymes catalase, polyphenoloxidase, superoxide dismutase and peroxidase as compared to non-inoculated salt stress plants. Thus, CHS1 ameliorated the adverse effect of high NaCl stress and rescued soybean plant growth by regulating the endogenous plant hormones and antioxidative system. We conclude that CHS1 isolate could be exploited to increase salt resistant and yield in crop plants.

Antioxidant metabolism variation associated with alkali-salt tolerance in thirty switchgrass (Panicum virgatum) lines

Soil salinization is a major factor limiting crop growth and development in many areas. Switchgrass (Panicum virgatum L.) is an important warm-season grass species used for biofuel production. The objective of this study was to investigate antioxidant metabolism, proline,and protein variation associated with alkali-salt tolerance among 30 switchgrass lines and identify metabolic markers for evaluating alkali-salt tolerance of switchgrass lines. The grass lines were transplanted into plastic pots containing fine sand. When the plants reached E5 developmental stage, they were subjected to either alkali-salt stress treatment (150 mM Na⁺ and pH of 9.5) or control (no alkali-salt stress) for 20 d. The 30 switchgrass lines differed in alkali-salt tolerance as determined by the level of leaf malondialdehyde (MDA), antioxidant enzyme activity [(superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX)], proline and protein. Alkali-salt stress increased MDA, proline, SOD, reduced CAT activity, but its effect on protein and APX varied depending on lines. Wide variations in the five parameters existed among the 30 lines. In general, the lines with higher CAT activity and lower proline content under alkali-salt stress had less MDA, exhibiting better alkali-salt tolerance. Among the five parameters, CAT can be considered as valuable metabolic markers for assessment of switchgrass tolerance to alkali-salt stress.

Mitochondrial Biogenesis in Diverse Cauliflower Cultivars under Mild and Severe Drought. Impaired Coordination of Selected Transcript and Proteomic Responses, and Regulation of Various Multifunctional Proteins

Mitochondrial responses under drought within Brassica genus are poorly understood. The main goal of this study was to investigate mitochondrial biogenesis of three cauliflower (Brassica oleracea var. botrytis) cultivars with varying drought tolerance. Diverse quantitative changes (decreases in abundance mostly) in the mitochondrial proteome were assessed by two-dimensional gel electrophoresis (2D PAGE) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Respiratory (e.g., complex II, IV (CII, CIV) and ATP synthase subunits), transporter (including diverse porin isoforms) and matrix multifunctional proteins (e.g., components of RNA editing machinery) were diversely affected in their abundance under two drought levels. Western immunoassays showed additional cultivar-specific responses of selected mitochondrial proteins. Dehydrin-related tryptic peptides (found in several 2D spots) immunopositive with dehydrin-specific antisera highlighted the relevance of mitochondrial dehydrin-like proteins for the drought response. The abundance of selected mRNAs participating in drought response was also determined. We conclude that mitochondrial biogenesis was strongly, but diversely affected in various cauliflower cultivars, and associated with drought tolerance at the proteomic and functional levels. However, discussed alternative oxidase (AOX) regulation at the RNA and protein level were largely uncoordinated due to the altered availability of transcripts for translation, mRNA/ribosome interactions, and/or miRNA impact on transcript abundance and translation.

A molecular approach to drought-induced reduction in leaf CO2 exchange in drought-resistant Quercus ilex

Drought-induced reduction of leaf gas exchange entails a complex regulation of the plant leaf metabolism. We used a combined molecular and physiological approach to understand leaf photosynthetic and respiratory responses of 2-year-old Quercus ilex seedlings to drought. Mild drought stress resulted in glucose accumulation while net photosynthetic CO₂ uptake (P_n ) remained unchanged, suggesting a role of glucose in stress signaling and/or osmoregulation. Simple sugars and sugar alcohols increased throughout moderate-to-very severe drought stress conditions, in parallel to a progressive decline in P_n and the quantum efficiency of photosystem II; by contrast, minor changes occurred in respiration rates until drought stress was very severe. At very severe drought stress, 2-oxoglutarate dehydrogenase complex gene expression significantly decreased, and the abundance of most amino acids dramatically increased, especially that of proline and γ-aminobutyric acid (GABA) suggesting enhanced protection against oxidative damage and a reorganization of the tricarboxylic cycle acid cycle via the GABA shunt. Altogether, our results point to Q. ilex drought tolerance being linked to signaling and osmoregulation by hexoses during early stages of drought stress, and enhanced protection against oxidative damage by polyols and amino acids under severe drought stress.

Relative contribution of photorespiration and antioxidative mechanisms in Caragana korshinskii under drought conditions across the Loess Plateau

The drought-tolerant plant Caragana korshinskii Kom. was used to investigate the relative contribution of photorespiration and antioxidative mechanisms to water-stress adaptations across the Loess Plateau. The samples were collected from Shenmu, Yulin and Dongsheng along with the reduction of rainfall. The results showed the lower leaf water potential and the lower content of O2•- and malondialdehyde (MDA) were found in the plants in drier zone. H2O2 didn't show a significant difference among these sampling sites. Both photorespiratory rate (PR) and net photosynthetic rate (PN) increased with the decrease of rainfall. Low rainfall upregulated the gene expression and activities of photorespiratory enzymes. In addition, ascorbate peroxidase (APX), glutathione peroxidase (GPX) and superoxide dismutase (SOD) activity and reduced glutathione (GSH), ascorbic acid (AsA) contents increased with the decrease of rainfall, whereas catalase (CAT) activity decreased. These results indicate photorespiration could play a protective role for the photosynthetic apparatus from photoinhibition and photodamage under low rainfall levels, which could also act together with the antioxidation system to improve the tolerance to drought in C. korshinskii.

Genotypic differences in physiological and biochemical responses to salinity stress in melon (Cucumis melo L.) plants: Prospects for selection of salt tolerant landraces

Melon (Cucumis melo L.) is one of the most important horticultural crops in Iran often cultivated in arid and semiarid regions of the country with salinity problems. The objective of this work was to better understand the mechanisms of physiological and biochemical responses to salinity stress of five Iranian melon landraces "Samsuri", "Kashan", "Khatouni", "Suski-e-Sabz", and "Ghobadlu" from different geographical origins, and "Galia" F1 cultivar. Plants were grown under greenhouse conditions and irrigated with half-strength Hoagland solution containing 0, 30, 60, or 90 mM NaCl for 60 days. Increase in the external salt concentration was accompanied by an obvious depression in leaf relative water content, membrane stability index, chlorophyll a and b and carotenoid contents, stomata and trichome density, leaf area, specific leaf area, biomass, leaf and stem K⁺ concentrations as well as leaf and stem K⁺/Na⁺ ratios in all landraces studied. In contrast, hydrogen peroxide, lipid peroxidation, proline and soluble carbohydrate contents, activity of antioxidant enzymes as well as leaf and stem Na⁺ and Cl^- concentrations, all increased significantly with increasing stress over all plants. Moreover, carbon isotope discrimination (Δ¹³C), determined on leaf organic matter, was found to be associated with evaluated traits. For example, a highly positive correlation between Δ¹³C and both biomass production and salt tolerance index was notable when all saline treatments were averaged (r = 0.998 and 0.998, respectively). Also, scatter plot and clustering analysis showed that "Suski-e-Sabz" and "Ghobadlu" were placed close to "Galia" F1, a salt tolerant cultivar, indicating that their similar behavior under salinity. Overall, the present results indicated a significant genetic variability for most of the traits studied, suggesting that "Suski-e-Sabz" and "Ghobadlu" could be introduced as the superior landraces and the most promising tolerant parents in the future melon breeding programs due to their suitable performance, in terms of responses to salt stress as compared with other landraces. Also, Δ¹³C can be used as a powerful criterion in melon breeding programs aimed at selection of salt tolerant landraces.

Beneficial Soil Bacterium Pseudomonas frederiksbergensis OS261 Augments Salt Tolerance and Promotes Red Pepper Plant Growth

Soil salinity, being a part of natural ecosystems, is an increasing problem in agricultural soils throughout the world. Pseudomonas frederiksbergensis OS261 has already been proved to be an effective bio-inoculant for enhancing cold stress tolerance in plants, however, its effect on salt stress tolerance is unknown. The main aim of the present study was to elucidate P. frederiksbergensis OS261 mediated salt stress tolerance in red pepper. The plants were exposed to a salt stress using NaCl at the concentrations of 50, 100, and 150 mM after 12 days of transplantation, while plant growth and enzyme activity were estimated 50 days after sowing. The height in P. frederiksbergensis OS261 inoculated plants was significantly increased by 19.05, 34.35, 57.25, and 61.07% compared to un-inoculated controls at 0, 50, 100, and 150 mM of NaCl concentrations, respectively, under greenhouse conditions. The dry biomass of the plants increased by 31.97, 37.47, 62.67, and 67.84% under 0, 50, 100, and 150 mM of NaCl concentrations, respectively. A high emission of ethylene was observed in un-inoculated red pepper plants under salinity stress. P. frederiksbergensis OS261 inoculation significantly reduced ethylene emission by 20.03, 18.01, and 20.07% at 50, 100, and 150 mM of NaCl concentrations, respectively. Furthermore, the activity of antioxidant enzymes (ascorbate peroxidase, superoxide dismutase, and catalase) also varied in the inoculated red pepper plants. Salt stress resistance in the bacterized plants was evident from the improved antioxidant activity in leaf tissues and the decreased hydrogen ion concentration. Thus, we conclude that P. frederiksbergensis OS261 possesses stress mitigating property which can enhance plant growth under high soil salinity by reducing the emission of ethylene and regulating antioxidant enzymes.

The Synergistic Responses of Different Photoprotective Pathways in Dwarf Bamboo (Fargesia rufa) to Drought and Subsequent Rewatering

Dwarf bamboo-dominated forests are often subjected to temporary periods of drought due to rising air temperature and decreasing rainfall. Nevertheless, the relationship among CO2 assimilation, photoprotective pathways and metabolism of reactive oxygen species (ROS) remains unexplored in bamboo species. Changes in leaf gas exchange, chlorophyll fluorescence, energy partitioning, antioxidative system and compounds related to ROS metabolism in Fargesia rufa plants subjected to drought and subsequent rewatering were analyzed. Drought resulted in a reversible inhibition of photochemistry, particularly net CO2 assimilation, and lipid peroxidation due to ROS accumulation. Meanwhile, photoprotective pathways, including the water-water cycle (especially for moderate drought), and adjustment in antenna pigments, thermal dissipation and antioxidative defense capacity at organelle levels (especially for severe drought), were up-regulated at the stress phase. Conversely, photorespiration was down-regulated after drought stress. As a result, rewatering restored most of the photochemical activity under drought, especially moderate drought. Moreover, thermal dissipation under severe drought was still operated for avoiding high ROS levels after rewatering. Therefore, the synergistic function of these photoprotective pathways except photorespiration can protect the photosynthetic apparatus from oxidative damage in response to varying intensities of drought stress when CO2 assimilation is restricted. This is helpful for the gradual recovery of photosynthetic capacity after rewatering. Thus, F. rufa plants can withstand drought and is capable of survival in such environment.

HIGHLIGHTS

1. The effects of drought and subsequent rewatering on Fargesia rufa were studied.2. Drought resulted in a reversible inhibition of photochemistry.3. Photoprotective pathways except photorespiration were up-regulated at the drought phase.4. Rewatering rapidly restored photochemical activity, especially under moderate drought.5. Fargesia rufa plant is capable of resisting and surviving drought environment.

Photoprotective and antioxidative mechanisms against oxidative damage in Fargesia rufa subjected to drought and salinity

Drought and salinity are the two most common and frequently co-occurring abiotic stresses limiting plant productivity worldwide, yet it remains unclear whether bamboo species possess effective mechanisms to protect against oxidative damage caused by drought and salinity, either alone or in combination. In this study, we utilised Fargesia rufa Yi, a species important to forest carbon sequestration and endangered giant pandas, to evaluate physiological, biochemical and ultrastructural responses to drought, salinity and their combination. Under drought alone, F. rufa exhibited reduced water loss from leaves, photochemistry inhibition, pigment degradation, reactive oxygen species accumulation, lipid peroxidation, and damage to organelles compared with salinity and combined stress treatments. The superior performance under drought alone was attributed to greater thermal dissipation and the water-water cycle capacities, increased SOD/AsA-GSH cycle enzymes activities, and a favourable redox balance of antioxidants. Therefore, relative to salinity alone and drought+salinity, F. rufa plants under drought exhibit highly efficient mechanisms to protect against oxidative damage, which most likely allow accelerated recovery of photosynthetic plasticity once the stress is removed.

Plant Growth-Promoting Rhizobacteria Enhance Salinity Stress Tolerance in Okra through ROS-Scavenging Enzymes

Salinity is a major environmental stress that limits crop production worldwide. In this study, we characterized plant growth-promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase and examined their effect on salinity stress tolerance in okra through the induction of ROS-scavenging enzyme activity. PGPR inoculated okra plants exhibited higher germination percentage, growth parameters, and chlorophyll content than control plants. Increased antioxidant enzyme activities (SOD, APX, and CAT) and upregulation of ROS pathway genes (CAT, APX, GR, and DHAR) were observed in PGPR inoculated okra plants under salinity stress. With some exceptions, inoculation with Enterobacter sp. UPMR18 had a significant influence on all tested parameters under salt stress, as compared to other treatments. Thus, the ACC deaminase-containing PGPR isolate Enterobacter sp. UPMR18 could be an effective bioresource for enhancing salt tolerance and growth of okra plants under salinity stress.

Physiological, Ultrastructural and Proteomic Responses in the Leaf of Maize Seedlings to Polyethylene Glycol-Stimulated Severe Water Deficiency

After maize seedlings grown in full-strength Hoagland solution for 20 days were exposed to 20% polyethylene glycol (PEG)-stimulated water deficiency for two days, plant height, shoot fresh and dry weights, and pigment contents significantly decreased, whereas malondialdehyde (MDA) content greatly increased. Using transmission electron microscopy, we observed that chloroplasts of mesophyll cells in PEG-treated maize seedlings were swollen, with a disintegrating envelope and disrupted grana thylakoid lamellae. Using two-dimensional gel electrophoresis (2-DE) method, we were able to identify 22 protein spots with significantly altered abundance in the leaves of treated seedlings in response to water deficiency, 16 of which were successfully identified. These protein species were functionally classified into signal transduction, stress defense, carbohydrate metabolism, protein metabolism, and unknown categories. The change in the abundance of the identified protein species may be closely related to the phenotypic and physiological changes due to PEG-stimulated water deficiency. Most of the identified protein species were putatively located in chloroplasts, indicating that chloroplasts may be prone to damage by PEG stimulated-water deficiency in maize seedlings. Our results help clarify the molecular mechanisms of the responses of higher plants to severe water deficiency.

Transgenic poplar expressing the pine GS1a show alterations in nitrogen homeostasis during drought

Transgenic hybrid poplars engineered to express ectopically the heterologous pine cytosolic GS1a display a number of significant pleiotropic phenotypes including enhanced growth, enhanced nitrogen use efficiency, and resistance to drought stress. The present study was undertaken in order to assess mechanisms whereby ectopic expression of pine GS1a in transgenic poplars results in enhanced agronomic phenotypes. Microarray analysis using the Agilent Populus whole genome array has allowed identification of genes differentially expressed between wild type (WT) and GS transgenics in four tissues (sink leaves, source leaves, stems, and roots) under three growth conditions (well-watered, drought, and recovery). Analysis revealed that differentially expressed genes in functional categories related to nitrogen metabolism show a trend of significant down-regulation in GS poplars compared to the WT, including genes encoding nitrate and nitrite reductases. The down-regulation of these genes was verified using qPCR, and downstream effects were further tested using NR activity assays. Results suggest that higher glutamine levels in GS transgenics regulate nitrate uptake and reduction. Transcript levels of nitrogen-related genes in leaves, including GS/GOGAT cycle enzymes, aspartate aminotransferase, GABA shunt enzymes, photorespiration enzymes, asparagine synthetase, phenylalanine ammonia lyase, isocitrate dehydrogenase, and PII, were also assessed using qPCR revealing significant differences between GS poplars and the WT. Moreover, metabolites related to these differentially expressed genes showed alterations in levels, including higher levels of GABA, hydroxyproline, and putrescine in the GS transgenic. These alterations in nitrogen homeostasis offer insights into mechanisms accounting for drought tolerance observed in GS poplars.

Emerging concept for the role of photorespiration as an important part of abiotic stress response

When plants are exposed to stress, generation of reactive oxygen species (ROS) is often one of the first responses. In order to survive, cells attempt to down-regulate the production of ROS, while at the same time scavenging ROS. Photorespiration is now appreciated as an important part of stress responses in green tissues for preventing ROS accumulation. Photorespiratory reactions can dissipate excess reducing equivalents and energy either directly (using ATP, NAD(P)H and reduced ferredoxin) or indirectly (e.g., via alternative oxidase (AOX) and providing an internal CO2 pool). Photorespiration, however, is also a source of H2 O2 that is possibly involved in signal transduction, resulting in modulation of gene expression. We propose that photorespiration can assume a major role in the readjustment of redox homeostasis. Protection of photosynthesis from photoinhibition through photorespiration is well known. Photorespiration can mitigate oxidative stress under conditions of drought/water stress, salinity, low CO2 and chilling. Adjustments to even mild disturbances in redox status, caused by a deficiency in ascorbate, AOX or chloroplastic NADP-malate dehydrogenase, comprise increases in photorespiratory components such as catalase, P-protein of glycine decarboxylase complex (GDC) and glycine content. The accumulation of excess reducing equivalents or ROS in plant cells also affects mitochondria. Therefore, a strong interaction between the chloroplast redox status and photorespiration is not surprising, but highlights interesting properties evident in plant cells. We draw attention to the fact that a complex network of multiple and dynamic systems, including photorespiration, prevents oxidative damage while optimising photosynthesis. Further experiments are necessary to identify and validate the direct targets of redox signals among photorespiratory components.

Salt stress or salt shock: which genes are we studying?

Depending on the method of NaCl application, whether gradual or in a single step, plants may experience either salt stress or salt shock, respectively. The first phase of salt stress is osmotic stress. However, in the event of salt shock, plants suffer osmotic shock, leading to cell plasmolysis and leakage of osmolytes, phenomena that do not occur with osmotic stress. Patterns of gene expression are different in response to salt stress and salt shock. Salt stress initiates relatively smooth changes in gene expression in response to osmotic stress and a more pronounced change in expression of significant numbers of genes related to the ionic phase of salt stress. There is a considerable time delay between changes in expression of genes related to the osmotic and ionic phases of salt stress. In contrast, osmotic shock results in strong, rapid changes in the expression of genes with osmotic function, and fewer changes in ionic-responsive genes that occur earlier. There are very few studies in which the effects of salt stress and salt shock are described in parallel experiments. However, the patterns of changes in gene expression observed in these studies are consistently as described above, despite the use of diverse plant species. It is concluded that gene expression profiles are very different depending the method of salt application. Imposition of salt stress by gradual exposure to NaCl rather than salt shock with a single application of a high concentration of NaCl is recommended for genetic and molecular studies, because this more closely reflects natural incidences of salinity.

Induction of drought tolerance in cucumber plants by a consortium of three plant growth-promoting rhizobacterium strains

Our previous work showed that a consortium of three plant growth-promoting rhizobacterium (PGPR) strains (Bacillus cereus AR156, Bacillus subtilis SM21, and Serratia sp. XY21), termed as BBS for short, was a promising biocontrol agent. The present study investigated its effect on drought tolerance in cucumber plants. After withholding watering for 13 days, BBS-treated cucumber plants had much darker green leaves and substantially lighter wilt symptoms than control plants. Compared to the control, the BBS treatment decreased the leaf monodehydroascorbate (MDA) content and relative electrical conductivity by 40% and 15%, respectively; increased the leaf proline content and the root recovery intension by 3.45-fold and 50%, respectively; and also maintained the leaf chlorophyll content in cucumber plants under drought stress. Besides, in relation to the control, the BBS treatment significantly enhanced the superoxide dismutase (SOD) activity and mitigated the drought-triggered down-regulation of the expression of the genes cAPX, rbcL, and rbcS encoding cytosolic ascorbate peroxidase, and ribulose-1,5-bisphosphate carboxy/oxygenase (Rubisco) large and small subunits, respectively, in cucumber leaves. However, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity was undetected in none of the culture solutions of three BBS constituent strains. These results indicated that BBS conferred induced systemic tolerance to drought stress in cucumber plants, by protecting plant cells, maintaining photosynthetic efficiency and root vigor and increasing some of antioxidase activities, without involving the action of ACC deaminase to lower plant ethylene levels.

Model analysing the antioxidant responses of leaves and roots of switchgrass to NaCl-salinity stress

Salinity is a major environmental stress and a substantial constraint on plant production. The objective of this research was to better understand the mechanisms of the antioxidant responses of leaves and roots to NaCl-salinity stress in switchgrass (Panicum virgatum L.). The effects of NaCl-salinity stress (1, 3, 5, 10 and 15gNaCl/kgdrysoil) on lipid peroxidation and antioxidant defences in switchgrass leaves and roots were investigated, and a model analysis was conducted. The results of the correlation analyses indicated that CAT, POD and SOD were more important than APX, GSH and MDA, whereas the model analyses suggested that the latter three biochemical indices were more sensitive than the former three indices to NaCl-salinity in switchgrass. The connective models of APX, GSH, MDA, CAT, SOD and POD between leaves and roots were obtained, respectively. The new models can be used to closely predict the biochemical index values in switchgrass roots or leaves.