Abscisic acid induced changes in cell wall peroxidase activity and hydrogen peroxide level in roots of rice seedlings

Facilitating effects of plant extracts on soil health and replanted Panax ginseng growth in recession soil

BACKGROUND

Plant extracts have been shown to be effective agricultural strategies for improving soil fertility and quality, and promoting plant growth in soil degradation remediation. The application of plant extracts improves the material cycle of soil microecology, such as the decomposition of nitrogen, phosphorus, and potassium, while increasing plant resistance. However, there is currently no experiment to demonstrate whether plant extracts have a promoting effect on the growth of ginseng and the mechanism of action.

OBJECTIVES AND METHODS

Pot experiments were carried out to investigate the effects of extracts, namely Rubia cordifolia (RC), Schisandra chinensis (SC), and Euphorbia humifusa (EH) on soil properties, enzyme activities, and plant physiological characteristics were evaluated.

RESULTS

Results showed that compared with CK, plant extract-related treatments increased soil Organic carbon (OC), Available nitrogen (AN), Available phosphorus (AP) contents, and Soil urease activity. (S-UE), Soil sucrase activity (Soil sucrase), Soil acid phosphatase activity. (S-ACP). Meanwhile, plant extract-related treatments significantly increased plant physiological properties and TP (Total protein) content, and decreased the content of MDA (malondialdehyde) by 15.70% -36.59% and PRO (proline) by 30.13% -148.44%. Furthermore, plant extract-related treatments also significantly promote plant growth and reduce plant incidence, the fresh weight of ginseng increased by 27.80% -52.08%, ginseng root activity increased by 45.13% -90.07%, and ginseng incidence rate decreased by 20.00% -46.67%. Through correlation analysis between fresh weight of ginseng and root parameters and soil index, fresh weight is significantly positively correlated with root diameter, fiber root number, root activity, total protein (TP), catalytic activity (CAT) and superoxide dismutase activity (SOD), H, soil urea activity (S-UE), soil sucrose activity (S-SC), soil acid phosphate activity (S-ACP), and soil laccase activity (SL); The fresh weight was significantly negatively correlated with incidence rate, disease severity index, and malondialdehyde content (MDA).

CONCLUSION

In summary, plant extract-related treatments improve soil quality and promote ginseng growth, further enhancing soil health and plant disease resistance. These findings provide new insights into ginseng cultivation and soil health management and highlight a new approach that can be applied to a wider range of agricultural practices and environmental sustainability.

Effect of Different Monochromatic LEDs on the Environmental Adaptability of Spathiphyllum floribundum and Chrysanthemum morifolium

Light-emitting diodes (LEDs) can be programmed to provide specialized light sources and spectra for plant growth. UV-A (397.6 nm), blue (460.6 nm), green (520.7 nm), and red (661.9 nm) LED light sources were used to study the effects of different monochromatic lights on the growth, antioxidant system, and photosynthetic characteristics of Spathiphyllum floribundum 'Tian Jiao' (a shade-loving species) and Chrysanthemum morifolium 'Huang Xiu Qiu' (a sun-loving species). This research revealed that green and blue light could enhance the morphological indicators, Chl a/b, photosynthetic electron transfer chain performance, and photosystem activity of S. floribundum, blue and red light could enhance the solution protein, Chl a, and photosynthetic electron transfer chain performance of C. morifolium, red and UV-A light viewed the highest SOD and CAT activities of S. floribundum (275.56 U·min·g-1; 148.33 U·min·g-1) and C. morifolium (587.03 U·min·g-1; 98.33 U·min·g-1), respectively. Blue and green light were more suitable for the growth and development of the shade-loving plant S. floribundum, while red and blue light were more suitable for the sun-loving plant C. morifolium. UV-A light could be used for their stress research. The research revealed the different adaptation mechanism of different plants to light environmental conditions.

CsPAO2 Improves Salt Tolerance of Cucumber through the Interaction with CsPSA3 by Affecting Photosynthesis and Polyamine Conversion

Polyamine oxidases (PAOs) are key enzymes in polyamine metabolism and are related to the tolerance of plants to abiotic stresses. In this study, overexpression of cucumber (Cucumis sativus L.) PAO2 (CsPAO2) in Arabidopsis resulted in increased activity of the antioxidant enzyme and accelerated conversion from Put to Spd and Spm, while malondialdehyde content (MDA) and electrolyte leakage (EL) was decreased when compared with wild type, leading to enhanced plant growth under salt stress. Photosystem Ⅰ assembly 3 in cucumber (CsPSA3) was revealed as an interacting protein of CsPAO2 by screening yeast two-hybrid library combined with in vitro and in vivo methods. Then, CsPAO2 and CsPSA3 were silenced in cucumber via virus-mediated gene silencing (VIGS) with pV190 as the empty vector. Under salt stress, net photosynthetic rate (Pn) and transpiration rate (Tr) of CsPAO2-silencing plants were lower than pV190-silencing plants, and EL in root was higher than pV190-silencing plants, indicating that CsPAO2-silencing plants suffered more serious salt stress damage. However, photosynthetic parameters of CsPSA3-silencing plants were all higher than those of CsPAO2 and pV190-silencing plants, thereby enhancing the photosynthesis process. Moreover, CsPSA3 silencing reduced the EL in both leaves and roots when compared with CsPAO2-silencing plants, but the EL only in leaves was significantly lower than the other two gene-silencing plants, and conversion from Put to Spd and Spm in leaf was also promoted, suggesting that CsPSA3 interacts with CsPAO2 in leaves to participate in the regulation of salt tolerance through photosynthesis and polyamine conversion.

Role of Plasma Membrane NADPH Oxidase in Response to Salt Stress in Cucumber Seedlings

Plasma membrane NADPH oxidases (RBOHs, EC 1.6.3.1) are known as the main ROS generators involved in plant adaptation to stress conditions. In the present work, regulation of NADPH oxidase was analyzed in cucumber (Cucumis sativus L. var. Krak) seedlings exposed to salinity. RBOH activity and gene expression, as well as H₂O₂ content, were determined in the roots of plants treated with 50 or 100 mM NaCl for 1 h, and 50 mM NaCl for 1 or 6 days. It was found that enzyme activity increased in parallel with an enhancement in the H₂O₂ level in roots exposed to 100 mM NaCl for 1 h, and to 50 mM NaCl for 1 day. The expression of some CsRboh genes was induced by salt. Moreover, an increase in the activity of G6PDH, providing the substrate for the NADPH oxidase, was observed. In seedlings subjected to salinity for a longer time, antioxidant enzymes-including superoxide dismutase, catalase, and ascorbate peroxidase-were activated, participating in maintaining a steady-state H₂O₂ content in the root cells. In conclusion, NADPH oxidase and endogenous H₂O₂ up-regulation seem to be early events in cucumber response to salinity.

Putrescine regulates stomatal opening of cucumber leaves under salt stress via the H2O2-mediated signaling pathway

The stomatal aperture is imperative for photosynthesis in higher plants. The function of polyamines (PAs) in stomatal regulation under a stressful environment has not been fully determined. In this study, we demonstrated the mechanism by which putrescine (Put) regulates stomatal changes in cucumber leaves under salt stress. The results showed that foliar application of Put alleviated the decrease of stomatal aperture and photosynthesis caused by salt stress and promoted plant growth. Exogenous Put caused a significant increase in endogenous PAs and hydrogen peroxide (H₂O₂) levels by 105.43% and 27.97%, respectively, while decreased abscisic acid (ABA) content by 67.68% under salt stress. However, application of inhibitors of aminoguanidine hydrochloride (AG), 1, 8-diaminooctane (1, 8-DO), diphenyleneiodonium chloride (DPI) and salicylhydroxamic acid (SHAM) upregulated the 9-cis-cyclocarotenoid dioxygenase (NCED) gene and downregulated the reduced glutathione synthetase (GSHS) gene. These inhibitors also decreased the stomatal aperture, levels of H₂O₂ and reduced glutathione (GSH), but increased the ABA content under salt stress and Put treatment conditions. The order of influence is AG > 1, 8-DO > DPI > SHAM. However, Put-induced downregulation of ABA content and upregulation of GSH content under salt stress were effectively blocked by N, N'-dimethylthiourea (DMTU, H₂O₂ scavenger) and 1-chloro-2,4-dinitrobenzene (CDNB, GSH scavenger) treatments. Taken together, these results suggest that Put induced the formation of H₂O₂ signaling mediates the degradation of PAs by diamine oxidase (DAO), increasing GSH content and inhibiting the accumulation of ABA in leaves, thereby promoting stomatal opening in salt-stressed cucumber leaves.

Proofing Direct-Seeded Rice with Better Root Plasticity and Architecture

The underground reserve (root) has been an uncharted research territory with its untapped genetic variation yet to be exploited. Identifying ideal traits and breeding new rice varieties with efficient root system architecture (RSA) has great potential to increase resource-use efficiency and grain yield, especially under direct-seeded rice, by adapting to aerobic soil conditions. In this review, we tried to mine the available research information on the direct-seeded rice (DSR) root system to highlight the requirements of different root traits such as root architecture, length, number, density, thickness, diameter, and angle that play a pivotal role in determining the uptake of nutrients and moisture at different stages of plant growth. RSA also faces several stresses, due to excess or deficiency of moisture and nutrients, low or high temperature, or saline conditions. To counteract these hindrances, adaptation in response to stress becomes essential. Candidate genes such as early root growth enhancer PSTOL1, surface rooting QTL qSOR1, deep rooting gene DRO1, and numerous transporters for their respective nutrients and stress-responsive factors have been identified and validated under different circumstances. Identifying the desired QTLs and transporters underlying these traits and then designing an ideal root architecture can help in developing a suitable DSR cultivar and aid in further advancement in this direction.

Hydrogen Peroxide Increases during Endodormancy and Decreases during Budbreak in Grapevine (Vitis vinifera L.) Buds

Changes in the level of hydrogen peroxide (H₂O₂) is a good indicator to monitor fluctuations in cellular metabolism and in the stress responses. In this study, the changes in H₂O₂ content during bud endodormancy (ED) and budbreak were analysed in grapevine (Vitis vinifera L.). The results showed a gradual increase in the H₂O₂ content during the development of bud ED, which was mainly due to an increase in the activity of peroxidases (PODs). The maximum H₂O₂ content reached in the grapevine buds coincided with the maximum depth of bud ED. In contrast, during budbreak, the H₂O₂ content decreased. As the plant hormones cytokinin (CK) and auxin play an important role in budbreak and growth resumption in grapevine, the effect of exogenous applications of H₂O₂ on the expression of genes involved in CK and auxin metabolism was analysed. The results showed that H₂O₂ represses the expression of the CK biosynthesis genes VvIPT3a and VvLOG1 and induces the expression of the CK-inactivating gene VvCKX3, thus reducing potentially the CK content in the grapevine bud. On the other hand, H₂O₂ induced the expression of the auxin biosynthesis genes VvAMI1 and VvYUC3 and of the auxin transporter gene VvPIN3, thus increasing potentially the auxin content and auxin transport in grapevine buds. In general, the results suggest that H₂O₂ in grapevine buds is associated with the depth of ED and negatively regulates its budbreak.

UDP-Glucose Dehydrogenases: Identification, Expression, and Function Analyses in Upland Cotton (Gossypium hirsutum)

UDP-glucose dehydrogenase (UGD; EC1.1.1.22) is a NAD⁺-dependent enzyme that catalyzes the two-fold oxidation of UDP-glucose (UDP-Glc) to produce UDP-glucuronic acid and plays an important role in plant cell wall synthesis. A total of 42 UGD genes from four Gossypium genomes including G. hirsutum, G. arboretum, G. barbadense, and G. raimondii were identified and found that the UGD gene family has conservative evolution patterns in gene structure and protein domain. The growth of fibers can be effectively promoted after adding the UDP-Glc to the medium, and the GhUGD gene expression enhanced. In addition, the transgenic Arabidopsis lines over-expressing GH_D12G1806 had longer root lengths and higher gene expression level than the wild-type plants of Columbia-0. These results indicated that UGD may play important roles in cotton fiber development and has a guiding significance for dissecting fiber development mechanism.

Photoperiod stress induces an oxidative burst-like response and is associated with increased apoplastic peroxidase and decreased catalase activities

Periodic changes of light and dark regulate numerous processes in plants. Recently, a novel type of stress caused by an extended light period has been described in Arabidopsis thaliana and was named photoperiod stress. Although photoperiod stress causes the induction of numerous stress response genes of which many are indicators of oxidative stress, the exact timing and mechanisms involved in dealing with this stress have not yet been investigated. We describe the response of the cellular redox system in wild-type Arabidopsis, the photoperiod stress sensitive cytokinin receptor mutant ahk2 ahk3 and the clock mutant cca1 lhy. Photoperiod stress caused several changes in the ROS scavenging system including a reduction of the ascorbic acid (AsA) redox status and strong peroxide formation during the night following the extended photoperiod. The changes were associated with reduced catalase (CAT) and increased apoplastic peroxidase (PRX) activities. Consistently, the expression of the apoplastic PRX genes PRX4, PRX33, PRX34 and PRX71 was strongly induced by photoperiod stress. We show that extending the light period by only few hours causes a stress response during the following night suggesting that the photoperiod stress response might occur in a natural setting.

Role of genes and metabolites involved in polyamines synthesis pathways and nitric oxide synthase in stomatal closure on Rosa damascena Mill. under drought stress

In order to evaluate the genes involved in polyamines synthesis pathway and the role of nitric oxide synthase (NOS) and H₂O₂ in stomatal closure under drought stress, a research conducted with three irrigation levels (100, 50 and 25% field capacity) at 1, 3, 6 and 12 days on Rosa damascena Mill. HPLC and qPCR results showed that putrescine (Put) accumulation occurred at first day in both 50 and 25% of field capacity and then decreased the other days. Furthermore, Put accumulation in the indirect pathway (ADC, AIH and CPA) was more effective related to the direct pathway (ODC) under severe stress. Increased expression of genes involved in production of spermidine (Spd) and spermine (Spm) i.e., SAMDC, SPDS and SPMS correlated with the highest accumulation of Spd and Spm under 50% FC at 6 d and 25% FC at 12 d, respectively. Moreover, results showed that Put reduction simultaneously accumulated H₂O₂, which subsequently increased NOS expression suggesting a key signal for stomatal closure.

Biochemical responses of rice roots to cold stress

BACKGROUND

Cold stress is the main factor that reduces rice yield in subtropical areas, especially at the seedling stage. Most of the current studies on cold stress focus the responses of rice shoots to cold stress. Limited studies are available on that of rice roots to cold stress. This study aimed to illustrate the biochemical responses of rice root under cold treatment, and subject to the establishment of cold stress-related biochemical traits for rice breeding or cropping-adjustment.

RESULTS

Our results showed that the growth of rice seedling diminished under cold stress with difference extents among eight rice cultivars of most productive in Taiwan. Under cold treatments, the tested cultivars with higher growth rate had a higher level of hydrogen peroxide (H₂O₂) in the shoots but had a lower level in the roots. In contrast, the tested cultivates with low growth rate had higher levels of H₂O₂ in the roots but a lower level in the shoots. Meanwhile, higher MDA contents and higher cell-damage related electrolyte leakage were also found in the roots not in the shoots, suggesting that cold stress might induce oxidative stress in the roots, not in the shoots. Furthermore, the activity analysis of four antioxidant enzymes, namely superoxide dismutase (SOD), catalase (CAT), ascorbic peroxidase (APX), and glutathione reductase (GR), revealed that cold stress could increase SOD and CAT activities in the roots.

CONCLUSIONS

In summary, low H₂O₂ and low MDA contents along with lower SOD and CAT activities in rice root could be the biochemical traits of cold responses in rice seedlings. The results are hoping to have a contribution to the rice breeding or cropping-adjustment on cold tolerance.

Abscisic acid-mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Proteome of rice roots treated with exogenous proline

Proteomic analysis was conducted to identify the rice root proteins induced by exogenous proline and their involvement in root growth. Proteins were extracted from the root tissues grown under two conditions, T1 (control) and T2 (10 mM proline), and profiled by two-dimensional polyacrylamide gel electrophoresis. Seventeen of 30 differentially expressed proteins were identified by mass spectrometry. Proline-treated rice roots showed up-regulation and down-regulation of nine and eight proteins, respectively, when compared to those in the control. Among the differentially expressed proteins, the down-regulation of glutathione reductase and peroxidase could be involved in the regulation of cellular hydrogen peroxide and reactive oxygen species levels that modulate the root cell wall structure. Differentially expressed proteins identified as pathogenesis-related proteins might be related to stress adaptive mechanisms in response to exogenous proline treatment. In addition, differentially expressed protein identified as the fructose-bisphosphate aldolases and cytochrome c oxidase might be associated with energy metabolism, which is needed during root developmental process. This is the first attempt to study the changes in rice root proteome treated with proline. The acquired information could open new avenues for further functional studies on the involvement of proline in modulating root development and its relation to stress adaptation of plants.

Hazardous gases (CO, NOx, CH4 and C3H8) released from CO2 fertilizer unit lead to oxidative damage and degrades photosynthesis in strawberry plants

CO2 boilers/direct heating systems used in greenhouses often lead to incomplete combustion, which results in the formation of hazardous gases, such as carbon monoxide (CO), nitroxide (NOX) and other hydrocarbons. In this study, strawberry plants that were grown on rockwool cubes were transferred to airtight bottles and treated with CO, NOX, CH4 and C3H8 gases for 1-48 hours. Oxidative damage due to hazardous gases was observed, as indicated by H2O2 and [Formula: see text] determination. Photosynthetic pigments were reduced, and stomatal guard cells were damaged and remained closed compared to the control. The activity of other photosynthetic parameters was negatively related to hazardous gases. Reduction in the expression of multiprotein complexes was highly observed under hazardous gas treatments. This study highlighted that hazardous gases (CO, NOX, CH4 and C3H8) emitted due to incomplete combustion of CO2 fertilization units/or direct heating systems resulted in the formation of ROS in shoots and limited photosynthetic metabolism. We predicted that major steps must be incorporated to improve the efficiency of CO2 boiler/heating systems to decrease the emission of these hazardous gases and other hydrocarbons and to reduce the observed risks that are key to the reduction of crops.

Spermidine-mediated hydrogen peroxide signaling enhances the antioxidant capacity of salt-stressed cucumber roots

Hydrogen peroxide (H₂O₂) is a key signaling molecule that mediates a variety of physiological processes and defense responses against abiotic stress in higher plants. In this study, our aims are to clarify the role of H₂O₂ accumulation induced by the exogenous application of spermidine (Spd) to cucumber (Cucumis sativus) seedlings in regulating the antioxidant capacity of roots under salt stress. The results showed that Spd caused a significant increase in endogenous polyamines and H₂O₂ levels, and peaked at 2 h after salt stress. Spd-induced H₂O₂ accumulation was blocked under salt stress by pretreatment with a H₂O₂ scavenger and respective inhibitors of cell wall peroxidase (CWPOD; EC: 1.11.1.7), polyamine oxidase (PAO; EC: 1.5.3.11) and NADPH oxidase (NOX; EC: 1.6.3.1); among these three inhibitors, the largest decrease was found in response to the addition of the inhibitor of polyamine oxidase. In addition, we observed that exogenous Spd could increase the activities of the enzymes superoxide dismutase (SOD; EC: 1.15.1.1), peroxidase (POD; EC: 1.11.1.7) and catalase (CAT; EC: 1.11.1.6) as well as the expression of their genes in salt-stressed roots, and the effects were inhibited by H₂O₂ scavengers and polyamine oxidase inhibitors. These results suggested that, by regulating endogenous PAs-mediated H₂O₂ signaling in roots, Spd could enhance antioxidant enzyme activities and reduce oxidative damage; the main source of H₂O₂ was polyamine oxidation, which was associated with improved tolerance and root growth recovery of cucumber under salt stress.

Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deficits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.

Early Cold-Induced Peroxidases and Aquaporins Are Associated With High Cold Tolerance in Dajiao (Musa spp. 'Dajiao')

Banana is an important tropical fruit with high economic value. One of the main cultivars ('Cavendish') is susceptible to low temperatures, while another closely related specie ('Dajiao') has considerably higher cold tolerance. We previously reported that some membrane proteins appear to be involved in the cold tolerance of Dajiao bananas via an antioxidation mechanism. To investigate the early cold stress response of Dajiao, here we applied comparative membrane proteomics analysis for both cold-sensitive Cavendish and cold-tolerant Dajiao bananas subjected to cold stress at 10°C for 0, 3, and 6 h. A total of 2,333 and 1,834 proteins were identified in Cavendish and Dajiao, respectively. Subsequent bioinformatics analyses showed that 692 Cavendish proteins and 524 Dajiao proteins were predicted to be membrane proteins, of which 82 and 137 differentially abundant membrane proteins (DAMPs) were found in Cavendish and Dajiao, respectively. Interestingly, the number of DAMPs with increased abundance following 3 h of cold treatment in Dajiao (80) was seven times more than that in Cavendish (11). Gene ontology molecular function analysis of DAMPs for Cavendish and Dajiao indicated that they belong to eight categories including hydrolase activity, binding, transporter activity, antioxidant activity, etc., but the number in Dajiao is twice that in Cavendish. Strikingly, we found peroxidases (PODs) and aquaporins among the protein groups whose abundance was significantly increased after 3 h of cold treatment in Dajiao. Some of the PODs and aquaporins were verified by reverse-transcription PCR, multiple reaction monitoring, and green fluorescent protein-based subcellular localization analysis, demonstrating that the global membrane proteomics data are reliable. By combining the physiological and biochemical data, we found that membrane-bound Peroxidase 52 and Peroxidase P7, and aquaporins (MaPIP1;1, MaPIP1;2, MaPIP2;4, MaPIP2;6, MaTIP1;3) are mainly involved in decreased lipid peroxidation and maintaining leaf cell water potential, which appear to be the key cellular adaptations contributing to the cold tolerance of Dajiao. This membrane proteomics study provides new insights into cold stress tolerance mechanisms of banana, toward potential applications for ultimate genetic improvement of cold tolerance in banana.

In vitro nitro-fatty acid release from Cys-NO2-fatty acid adducts under nitro-oxidative conditions

Stress situations are characterized by a rise in reactive oxygen (ROS) and nitrogen (RNS) species levels. Nitro-fatty acids (NO₂-FAs), or nitroalkenes, are produced by the interaction of RNS and unsaturated fatty acids, stored in cells, mostly as part of protein-adducted NO₂-FAs, and are esterified in complex lipids. These molecules, which have been shown to play a pivotal role as anti-inflammatory and pro-survival players, have been widely characterized in animal systems. Recently, it has been reported that NO₂-FAs play an important role in plant defense against several stress conditions. Furthermore, a significant increase in NO₂-FA levels has been observed under various inflammatory and stressful conditions in both animal and plant systems. In this study, we describe the in vitro release of NO₂-FAs from protein-adducts under nitro-oxidative stress conditions. The findings of this study highlight the ability of hydrogen peroxide and peroxynitrite, as representative ROS and RNS molecules induced under stress conditions, to oxidize cysteine-adducted NO₂-FAs, which is followed by the release of free nitroalkenes. This release may be partly responsible for the increase in NO₂-FA content observed under different stressful conditions in both animal and plant systems as well as the activation of antioxidant and anti-inflammatory properties attributed to these molecules.

Cd-induced oxidative stress and lignification in the roots of two Vicia sativa L. varieties with different Cd tolerances

We examined the effects of Cd on growth, lipid peroxidation, reactive oxygen species (ROS) accumulation, antioxidant enzymatic activity, and lignin content in the roots of two varieties of Vicia sativa. Treatment with Cd decreased plant growth and increased ROS and lipid peroxidation levels to a greater extent in the Cd-sensitive variety ZM than in the Cd-tolerant variety L3. Most hydrogen peroxide (H2O2) and superoxide anion (O2(•-)) were accumulated in the cell walls and extracellular spaces in response to Cd treatments. Chemical assays and experiments using inhibitors showed that larger increases in H2O2 and O2(•-) production in ZM than in L3 were probably attributed to elevated Cd-induced nicotinamide adenine dinucleotide-peroxidase (NADH-POD) activity. Cd treatment increased the accumulation of lignin and the guaiacol peroxidase (GPOD) activities in the apoplast more significantly in ZM root than in L3. Howerver, root laccase activity was higher in L3 than in ZM. Thus Cd toxicity induced significant lignification in the roots of V. sativa, and increases in H2O2 accumulation and apoplastic GPOD activity were likely responsible for this effect.

A spectrometric method for hydrogen peroxide concentration measurement with a reusable and cost-efficient sensor

In this study we developed a low cost sensor for measuring the concentration of hydrogen peroxide (H₂O₂) in liquids utilizing a spectrometric method. The sensor was tested using various concentrations of a peroxidase enzyme immobilized on a glass substrate. H₂O₂ can be catalyzed by peroxidase and converted into water and oxygen. The reagent 4-amino-phenazone takes up oxygen together with phenol to form a colored product that has absorption peaks at 510 nm and 450 nm. The transmission intensity is strongly related to the hydrogen peroxide concentration, so can be used for quantitative analysis. The measurement range for hydrogen peroxide is from 5 × 10⁻⁵% to 1 × 10⁻³% (0.5 ppm to 10 ppm) and the results show high linearity. This device can achieve a sensitivity and resolution of 41,400 (photon count/%) and 3.49 × 10⁻⁵% (0.35 ppm), respectively. The response time of the sensor is less than 3 min and the sensor can be reused for 10 applications with similar performance.

A novel thylakoid ascorbate peroxidase from Jatrophacurcas enhances salt tolerance in transgenic tobacco

Ascorbate peroxidase (APX) plays an important role in the metabolism of hydrogen peroxide in higher plants. In the present study, a novel APX gene (JctAPX) was cloned from Jatropha curcas L. The deduced amino acid sequence was similar to that of APX of some other plant species. JctAPX has a chloroplast transit peptide and was localized to the chloroplasts by analysis with a JctAPX-green fluorescent protein (GFP) fusion protein. Quantitative polymerase chain reaction (qPCR) analysis showed that JctAPX was constitutively expressed in different tissues from J. curcas and was upregulated by NaCl stress. To characterize its function in salt tolerance, the construct p35S: JctAPX was created and successfully introduced into tobacco by Agrobacterium-mediated transformation. Compared with wild type (WT), the transgenic plants exhibited no morphological abnormalities in the no-stress condition. However, under 200 mM NaCl treatment, JctAPX over-expressing plants showed increased tolerance to salt during seedling establishment and growth. In addition, the transgenic lines showed higher chlorophyll content and APX activity, which resulted in lower H2O2 content than WT when subjected to 400 mM NaCl stress. These results suggest that the increased APX activity in the chloroplasts from transformed plants increased salt tolerance by enhancing reactive oxygen species (ROS)-scavenging capacity under short-term NaCl stress conditions.

Effect of CO, NOx and SO2 on ROS production, photosynthesis and ascorbate-glutathione pathway to induce Fragaria×annasa as a hyperaccumulator

A study was conducted to determine the effect of carbon monoxide (CO), nitroxide (NOx) and sulfur dioxide (SO2) on ROS production, photosynthesis and ascorbate-glutathione pathway in strawberry plants. The results showed that both singlet oxygen (O2(-1)) and hydrogen peroxide (H2O2) content increased in CO, NOx and SO2 treated strawberry leaves. A drastic reduction of primary metabolism of plants (photosynthesis), with the closure of stomata, resulted in a reduction of protein, carbohydrate and sucrose content due to production of reactive oxygen species (ROS) under prolonged exposure of gas stress. The resulting antioxidant enzymes were increased under a low dose of gas stress, whereas they were decreased due to a high dose of gas stress. Our results indicate that increased ROS may act as a signal to induce defense responses to CO, NOx and SO2 gas stress. The increased level of antioxidant enzymes plays a significant role in plant protection due to which strawberry plants can be used as a hyperaccumulator to maintain environmental pollution, however, the defense capacity cannot sufficiently alleviate oxidative damage under prolonged exposure of CO, NOx and SO2 stress.

Abscisic acid induced changes in production of primary and secondary metabolites, photosynthetic capacity, antioxidant capability, antioxidant enzymes and lipoxygenase inhibitory activity of Orthosiphon stamineus Benth

An experiment was conducted to investigate and distinguish the relationships in the production of total phenolics, total flavonoids, soluble sugars, H2O2, O2-, phenylalanine ammonia lyase (PAL) activity, leaf gas exchange, antioxidant activity, antioxidant enzyme activity [ascorbate peroxidase (APX), catalase (CAT), superoxide dismutase (SOD) and Lipoxygenase inhibitory activity (LOX)] under four levels of foliar abscisic acid (ABA) application (0, 2, 4, 6 µM) for 15 weeks in Orthosiphon stamineus Benth. It was found that the production of plant secondary metabolites, soluble sugars, antioxidant activity, PAL activity and LOX inhibitory activity was influenced by foliar application of ABA. As the concentration of ABA was increased from 0 to 6 µM the production of total phenolics, flavonoids, sucrose, H2O2, O2-, PAL activity and LOX inhibitory activity was enhanced. It was also observed that the antioxidant capabilities (DPPH and ORAC) were increased. This was followed by increases in production of antioxidant enzymes APX, CAT and SOD. Under high application rates of ABA the net photosynthesis and stomatal conductance was found to be reduced. The production of primary and secondary metabolites displayed a significant positive relationship with H2O2 (total phenolics, r2 = 0.877; total flavonoids, r2 = 0.812; p ≤ 0.05) and O2- (total phenolics, r2 = 0.778; total flavonoids, r2 = 0.912; p ≤ 0.05). This indicated that increased oxidative stress at high application rates of ABA, improved the production of phytochemicals.

The critical role of potassium in plant stress response

Agricultural production continues to be constrained by a number of biotic and abiotic factors that can reduce crop yield quantity and quality. Potassium (K) is an essential nutrient that affects most of the biochemical and physiological processes that influence plant growth and metabolism. It also contributes to the survival of plants exposed to various biotic and abiotic stresses. The following review focuses on the emerging role of K in defending against a number of biotic and abiotic stresses, including diseases, pests, drought, salinity, cold and frost and waterlogging. The availability of K and its effects on plant growth, anatomy, morphology and plant metabolism are discussed. The physiological and molecular mechanisms of K function in plant stress resistance are reviewed. This article also evaluates the potential for improving plant stress resistance by modifying K fertilizer inputs and highlights the future needs for research about the role of K in agriculture.

Melatonin-rich transgenic rice plants exhibit resistance to herbicide-induced oxidative stress

To examine whether melatonin-rich plants can defend against oxidative stress, we subjected melatonin-rich transgenic (MRT) rice plants to the singlet-oxygen-generating herbicide butafenacil. Both MRT and transgenic control (TC; expressing the vector only) rice seeds germinated and grew equally well in continuous dark on half-strength Murashige and Skoog (MS) medium containing 0.1 μm butafenacil. However, after transferring the seedlings to light, the TCs rapidly necrotized, whereas the MRT seedlings showed resistant phenotypes. Seven-day-old MRT seedlings treated with 0.1 μm butafenacil were resistant to the herbicide and contained high chlorophyll levels and low malondialdehyde and hydrogen peroxide contents compared with the TCs. As they did before the herbicide treatment, the MRT plants also produced much more melatonin after the herbicide treatment than the TCs. In addition, the MRT plants exhibited higher superoxide dismutase and catalase activities before and after the herbicide treatment compared with the TCs. This is the first report showing that MRT plants exhibit resistance against a peroxidizing herbicide that acts by generating reactive oxygen species (ROS) that kill plants. This result indicates that melatonin scavenges ROS efficiently in vivo in the transgenic plants, leading to oxidative stress resistance.

Overexpression of the wheat aquaporin gene, TaAQP7, enhances drought tolerance in transgenic tobacco

Aquaporin (AQP) proteins have been shown to transport water and other small molecules through biological membranes, which is crucial for plants to combat stress caused by drought. However, the precise role of AQPs in drought stress response is not completely understood in plants. In this study, a PIP2 subgroup gene AQP, designated as TaAQP7, was cloned and characterized from wheat. Expression of TaAQP7-GFP fusion protein revealed its localization in the plasma membrane. TaAQP7 exhibited high water channel activity in Xenopus laevis oocytes and TaAQP7 transcript was induced by dehydration, and treatments with polyethylene glycol (PEG), abscisic acid (ABA) and H(2)O(2). Further, TaAQP7 was upregulated after PEG treatment and was blocked by inhibitors of ABA biosynthesis, implying that ABA signaling was involved in the upregulation of TaAQP7 after PEG treatment. Overexpression of TaAQP7 increased drought tolerance in tobacco. The transgenic tobacco lines had lower levels of malondialdehyde (MDA) and H(2)O(2), and less ion leakage (IL), but higher relative water content (RWC) and superoxide dismutase (SOD) and catalase (CAT) activities when compared with the wild type (WT) under drought stress. Taken together, our results show that TaAQP7 confers drought stress tolerance in transgenic tobacco by increasing the ability to retain water, reduce ROS accumulation and membrane damage, and enhance the activities of antioxidants.

Characterization of physiological response and identification of associated genes under heat stress in rice seedlings

Global warming, which is caused by greenhouse gas emissions, makes food crops more vulnerable to heat stress. Understanding the heat stress-related mechanisms in crops and classifying heat stress-related genes can increase our knowledge in heat-resistant molecular biology and propel developments in molecular design breeding, which can help rice cope with unfavorable temperatures. In this study, we carried out a physiological analysis of rice plants after heat stress. The results show a dramatic increase in malondialdehyde contents and SOD activities. We successfully isolated 11 heat-related rice genes with known function annotation through DNSH, which is an improved SSH method for screening long cDNA fragments. The reanalysis of microarray data from public database revealed that all these genes displayed various expression patterns after heat stress, drought, cold and salt. Quantitative real-time reverse transcription PCR was also performed to validate the expression of these genes after heat stress. The expressions in 10 genes were all significantly changed except for contig 77, which is a CBL-interacting protein kinase. Several reports have been published about the members of the same gene family.

Aluminum induces oxidative burst, cell wall NADH peroxidase activity, and DNA damage in root cells of Allium cepa L

Plants under stress incur an oxidative burst that involves a rapid and transient overproduction of reactive oxygen species (ROS: O(2) (•-) , H(2) O(2) , (•) OH). We hypothesized that aluminum (Al), an established soil pollutant that causes plant stress, would induce an oxidative burst through the activation of cell wall-NADH peroxidase (NADH-PX) and/or plasma membrane-associated NADPH oxidase (NADPH-OX), leading to DNA damage in the root cells of Allium cepa L. Growing roots of A. cepa were treated with Al(3+) (800 μM of AlCl(3) ) for 3 or 6 hr without or with the pretreatment of inhibitors specific to NADH-PX and NADPH-OX for 2 hr. At the end of the treatment, the extent of ROS generation, cell death, and DNA damage were determined. The cell wall-bound protein (CWP) fractions extracted from the untreated control and the Al-treated roots under the aforementioned experimental conditions were also subjected to in vitro studies, which measured the extent of activation of peroxidase/oxidase, generation of (•) OH, and DNA damage. Overall, the present study demonstrates that the cell wall-bound NADH-PX contributes to the Al-induced oxidative burst through the generation of ROS that lead to cell death and DNA damage in the root cells of A. cepa. Furthermore, the in vitro studies revealed that the CWP fraction by itself caused DNA damage in the presence of NADH, supporting a role for NADH-PX in the stress response. Altogether, this study underscores the crucial function of the cell wall-bound NADH-PX in the oxidative burst-mediated cell death and DNA damage in plants under Al stress.

Transgenic Expression of MsHsp23 Confers Enhanced Tolerance to Abiotic Stresses in Tall Fescue

Tall fescue (Festuca arundinacea Schreb.) is an important cool season forage plant that is not well suited to extreme heat, salts, or heavy metals. To develop transgenic tall fescue plants with enhanced tolerance to abiotic stress, we introduced an alfalfa Hsp23 gene expression vector construct through Agrobacterium-mediated transformation. Integration and expression of the transgene were confirmed by polymerase chain reaction, northern blot, and western blot analyses. Under normal growth conditions, there was no significant difference in the growth of the transgenic plants and the non-transgenic controls. However, when exposed to various stresses such as salt or arsenic, transgenic plants showed a significantly lower accumulation of hydrogen peroxide and thiobarbituric acid reactive substances than control plants. The reduced accumulation of thiobarbituric acid reactive substances indicates that the transgenic plants possessed a more efficient reactive oxygen species-scavenging system. We speculate that the high levels of MsHsp23 proteins in the transgenic plants protect leaves from oxidative damage through chaperon and antioxidant activities. These results suggest that MsHsp23 confers abiotic stress tolerance in transgenic tall fescue and may be useful in developing stress tolerance in other crops.

Arabidopsis peroxidase AtPRX53 influences cell elongation and susceptibility to Heterodera schachtii

Cyst nematodes establish and maintain feeding sites (syncytia) in the roots of host plants by altering expression of host genes. Among these genes are members of the large gene family of class III peroxidases, which have reported functions in a variety of biological processes. In this study, we used Arabidopsis-Heterodera schachtii as a model system to functionally characterize peroxidase 53 (AtPRX53). Promoter assays showed that under non-infected conditions AtPRX53 is expressed mainly in the root, the hypocotyl and the base of the pistil. Under infected conditions, the AtPRX53 promoter showed upregulation at the nematode penetration sites and in their migration paths. Interestingly, strong GUS activity was observed in H. schachtii-induced syncytia during the early stage of infection and remained strong in the syncytia of third-stage juveniles. Also, AtPRX53 showed upregulation in response to wounding and jasmonic acid treatments. Manipulation of AtPRX53 expression through overexpression and knockout mutation affected both plant morphology and nematode susceptibility. While AtPRX53 overexpression lines exhibited short hypocotyls, aberrant flower development and reduced nematode susceptibility to H. schachtii, the atprx53 mutant showed long hypocotyls and a 3-carpel silique phenotype as well as a non significant increase of nematode susceptibility. Taken together these data, therefore, indicate diverse roles of AtPRX53 in the wound response, flower development and syncytium formation.

COPPER AMINE OXIDASE1 (CuAO1) of Arabidopsis thaliana contributes to abscisic acid- and polyamine-induced nitric oxide biosynthesis and abscisic acid signal transduction

Polyamines (PA), polyamine oxidases, copper amine oxidases, and nitric oxide (NO) play important roles in physiology and stress responses in plants. NO biosynthesis as a result of catabolism of PA by polyamine oxidases and copper amine oxidases may explain in part PA-mediated responses. Involvement of a copper amine oxidase gene, COPPER AMINE OXIDASE1 (CuAO1), of Arabidopsis was tested for its role in stress responses using the knockouts cuao1-1 and cuao1-2. PA-induced and ABA-induced NO production investigated by fluorometry and fluorescence microscopy showed that the cuao1-1 and cuao1-2 are impaired in NO production, suggesting a function of CuAO1 in PA and ABA-mediated NO production. Furthermore, we found a PA-dependent increase in protein S-nitrosylation. The addition of PA and ABA also resulted in H(2)O(2) increases. cuao1-1 and cuao1-2 showed less sensitivity to exogenous ABA supplementation during germination, seedling establishment, and root growth inhibition as compared to wild-type. In response to ABA treatment, expression levels of the stress-responsive genes RD29A and ADH1 were significantly lower in the knockouts. These observations characterize cuao1-1 and cuao1-2 as ABA-insensitive mutants. Taken together, our findings extend the ABA signal transduction network to include CuAO1 as one potential contributor to enhanced NO production by ABA.

Actin microfilaments and vacuoles are downstream targets of H2O2 signalling pathways in hyperosmotic stress-induced stomatal closure

Changes in osmotic pressure can induce stomatal closure to reduce transpirational water loss from plants. In the present work, we investigated the mechanism underlying the perception and transduction of extracellular changes in osmotic pressure in Vicia faba L. guard cells. Using an epidermal strip bioassay and laser-scanning confocal microscopy, we provide evidence that hyperosmotic stress treatment led to stomatal closure and the rapid promotion of hydrogen peroxide (H2O2) production in V. faba guard cells. The effects were largely reduced by H2O2 scavengers ASA, CAT, NADPH oxidase inhibitor DPI and cell wall peroxidase inhibitor SHAM. These results indicate that hyperosmotic stress induces stomatal closure by promoting H2O2 production. Cytochalasin B (CB), latrunculin B (Lat B) and jasplakinolide (JK) inhibited stomatal closure induced by hyperosmotic stress but didn't prevent the increase of endogenous H2O2 levels, suggesting that microfilaments reorganisation participates in stomatal closure induced by hyperosmotic stress, and may act downstream of H2O2 signalling processes. In addition, we observed splitting of big vacuoles into many small vacuoles in response to hyperosmotic stress and H2O2 treatment, and CB inhibited these changes of vacuoles; stomatal closure was also inhibited. Taken together these results indicate that the stomatal closure in response to hyperosmotic stress may initiate H2O2 generation, and that reorganisation of microfilaments and the changing of vacuoles occurs downstream of H2O2 signalling processes.

Ozone stress-induced proteomic changes in leaf total soluble and chloroplast proteins of soybean reveal that carbon allocation is involved in adaptation in the early developmental stage

Considerable soybean yield losses caused by ozone (O3) stress have been demonstrated by large-scale meta-analyses of free-gas concentration enrichment systems. In this study, comparative proteomic approach was employed to explore the differential changes of proteins in O3 target structures such as leaf and chloroplasts of soybean seedlings. Acute O3 exposure (120 parts-per-billion) for 3 days did not cause any visible symptoms in developing leaves. However, higher amounts of ROS and lipid peroxidation indicated that severe oxidative burst occurred. Immunoblot analysis of O3-induced known proteins revealed that proteins were modulated before symptoms became visible. Proteomic analysis identified a total of 20 and 32 differentially expressed proteins from O3-treated leaf and chloroplast, respectively. Proteins associated with photosynthesis, including photosystem I/II and carbon assimilation decreased following exposure to O3. In contrast, proteins involved in antioxidant defense and carbon metabolism increased. The activity of enzymes involved in carbohydrate metabolism increased following exposure to O3, which is consistent with the decrease in starch and increase in sucrose concentrations. Taken together, these results suggest that carbon allocation is tightly programmed, and starch degradation probably feeds the tricarboxylic acid cycle while the photosynthesis pathway is severely affected during O3 stress.

Transcriptional regulatory network triggered by oxidative signals configures the early response mechanisms of japonica rice to chilling stress

BACKGROUND

The transcriptional regulatory network involved in low temperature response leading to acclimation has been established in Arabidopsis. In japonica rice, which can only withstand transient exposure to milder cold stress (10 degrees C), an oxidative-mediated network has been proposed to play a key role in configuring early responses and short-term defenses. The components, hierarchical organization and physiological consequences of this network were further dissected by a systems-level approach.

RESULTS

Regulatory clusters responding directly to oxidative signals were prominent during the initial 6 to 12 hours at 10 degrees C. Early events mirrored a typical oxidative response based on striking similarities of the transcriptome to disease, elicitor and wounding induced processes. Targets of oxidative-mediated mechanisms are likely regulated by several classes of bZIP factors acting on as1/ocs/TGA-like element enriched clusters, ERF factors acting on GCC-box/JAre-like element enriched clusters and R2R3-MYB factors acting on MYB2-like element enriched clusters.Temporal induction of several H2O2-induced bZIP, ERF and MYB genes coincided with the transient H2O2 spikes within the initial 6 to 12 hours. Oxidative-independent responses involve DREB/CBF, RAP2 and RAV1 factors acting on DRE/CRT/rav1-like enriched clusters and bZIP factors acting on ABRE-like enriched clusters. Oxidative-mediated clusters were activated earlier than ABA-mediated clusters.

CONCLUSION

Genome-wide, physiological and whole-plant level analyses established a holistic view of chilling stress response mechanism of japonica rice. Early response regulatory network triggered by oxidative signals is critical for prolonged survival under sub-optimal temperature. Integration of stress and developmental responses leads to modulated growth and vigor maintenance contributing to a delay of plastic injuries.

Changes in cucumber hypocotyl cell wall dynamics caused by Azospirillum brasilense inoculation

We previously reported that Azospirillum brasilense induced a more elastic cell wall and a higher apoplastic water fraction in both wheat coleoptile and flag leaf. These biophysical characteristics could permit increased growth. Knowledge of the biochemical effects the bacteria could elicit in plant cell walls and how these responses change plant physiology is still scarce. The objective of this work was to analyze whether A. brasilense Sp245 inoculation affected elongation and extensibility of growing cucumber (Cucumis sativus) hypocotyls and ionically bound cell wall peroxidase activities. Hypocotyl tip and basal segments were excised from A. brasilense Sp245-inoculated cucumber seedlings growing in darkness under hydroponic conditions. Elongation, cell wall extensibility, cell wall peroxidase activities against ferulic acid and guaiacol and NADH oxidase activities were analyzed. Azospirillum-inoculated cucumber seedlings grew bigger than non-inoculated ones. Dynamic cell wall differences were detected between inoculated and non-inoculated hypocotyls. They included greater acid-induced cell wall extension and in vivo elongation when incubated in distilled water. Although there was no difference between treatments in either region of the hypocotyl NADH oxidase and ferulic acid peroxidase activities were lower in both regions in inoculated seedlings. These lesser activities could be delaying the stiffening of cell wall in inoculated seedlings. These results showed that the cell wall is a target for A. brasilense growth promotion.

Evidence of parietal amine oxidase activity in Solanum torvum Sw. stem calli after Ralstonia solanacearum inoculation

Calli induced from Solanum torvum stem explants were inoculated with Ralstonia solanacearum under partial vacuum. All calli showed a hypersensitive response after infiltration. Furthermore, amine oxidase activity with aldehyde and H(2)O(2) production was detected in semi-purified cell walls of calli infiltrated by the bacteria. Due to its preferential affinity for monoamines, this enzyme is supposed to have monoamine oxidase-like (MAO-like) activity. Moreover, the presence of hydroxyl radicals in the aromatic cycle alters the oxidative deamination kinetics of potential substrates. Indeed, the oxidation of dopamine (+2, OH) was shown to be faster than that of tyramine (+1, OH), which in turn was faster than that of phenylethylamine (0, OH). The MAO-like catalytic activity was significantly inhibited by some reducing agents such as sodium bisulphite and cysteine, and also by tryptamine under anaerobiosis. This latter result suggested that the prosthetic group of the MAO-like enzyme could be a tyrosine-derived 6-hydroxytopaquinone structure. Finally, the sigmoid kinetics of the MAO-like enzyme in semi-purified cell walls did not correspond to that expected for a purified MAO, suggesting that the kinetics were affected by some factors present in cell walls.

Hydrogen peroxide is involved in the acclimation of the Mediterranean shrub, Cistus albidus L., to summer drought

This study evaluated the possible role of hydrogen peroxide (H(2)O(2)) in the acclimation of a Mediterranean shrub, Cistus albidus L., to summer drought growing under Mediterranean field conditions. For this purpose, changes in H(2)O(2) concentrations and localization throughout a year were analysed. H(2)O(2) changes in response to environmental conditions in parallel with changes in abscisic acid (ABA) and oxidative stress markers, together with lignin accumulation, xylem and sclerenchyma differentiation, and leaf area were also investigated. During the summer drought, leaf H(2)O(2) concentrations increased 11-fold, reaching values of 10 micromol g(-1) dry weight (DW). This increase occurred mainly in mesophyll cell walls, xylem vessels, and sclerenchyma cells in the differentiation stage. An increase in ABA levels preceded that of H(2)O(2), but both peaked at the same time in conditions of prolonged stress. C. albidus plants tolerated high concentrations of H(2)O(2) because of its localization in the apoplast of mesophyll cells, xylem vessels, and in differentiating sclerenchyma cells. The increase in ABA, and consequently of H(2)O(2), in plants subjected to drought stress might induce a 3.5-fold increase in ascorbic acid (AA), which maintained and even decreased its oxidative status, thus protecting plants from oxidative damage. After recovery from drought following late-summer and autumn rainfall, a decrease in ABA, H(2)O(2), and AA to their basal levels (approximately 60 pmol g(-1) DW, approximately 1 micromol g(-1) DW, and approximately 20 micromol g(-1) DW) was observed.

Rewatering plants after a long water-deficit treatment reveals that leaf epidermal cells retain their ability to expand after the leaf has apparently reached its final size

BACKGROUND AND AIMS

Leaves expand during a given period of time until they reach their final size and form, which is called determinate growth. Duration of leaf expansion is stable when expressed in thermal-time and in the absence of stress, and consequently it is often proposed that it is controlled by a robust programme at the plant scale. The usual hypothesis is that growth cessation occurs when cell expansion becomes limited by an irreversible tightening of cell wall, and that leaf size is fixed once cell expansion ceases. The objective of this paper was to test whether leaf expansion could be restored by rewatering plants after a long soil water-deficit period.

METHODS

Four experiments were performed on two different species (Arabidopsis thaliana and Helianthus annuus) in which the area of leaves that had apparently reached their final size was measured upon reversal of water stresses of different intensities and durations.

KEY RESULTS

Re-growth of leaves that had apparently reached their final size occurred in both species, and its magnitude depended only on the time elapsed from growth cessation to rewatering. Leaf area increased up to 186% in A. thaliana and up to 88% in H. annuus after rewatering, with respect to the leaves of plants that remained under water deficit. Re-growth was accounted for by cell expansion. Increase in leaf area represented actual growth and not only a reversible change due to increased turgor.

CONCLUSIONS

After the leaf has ceased to grow, leaf cells retain their ability to expand for several days before leaf size becomes fixed. A response window was identified in both species, during which the extent of leaf area recovery decreased with time after the 'initial' leaf growth cessation. These results suggest that re-growth after rewatering of leaves having apparently attained their final size could be a generalized phenomenon, at least in dicotyledonous plants.

Hydrogen peroxide generated by copper amine oxidase is involved in abscisic acid-induced stomatal closure in Vicia faba

H(2)O(2) is an essential signal in absicic acid (ABA)-induced stomatal closure. It can be synthesized by several enzymes in plants. In this study, the roles of copper amine oxidase (CuAO) in H(2)O(2) production and stomatal closure were investigated. Exogenous ABA stimulated apoplast CuAO activity, increased H(2)O(2) production and [Ca(2+)](cyt) levels in Vicia faba guard cells, and induced stomatal closure. These processes were impaired by CuAO inhibitor(s). In the metabolized products of CuAO, only H(2)O(2) could induce stomatal closure. By the analysis of enzyme kinetics and polyamine contents in leaves, putrescine was regarded as a substrate of CuAO. Putrescine showed similar effects with ABA on the regulation of H(2)O(2) production, [Ca(2+)](cyt) levels, as well as stomatal closure. The results suggest that CuAO in V. faba guard cells is an essential enzymatic source for H(2)O(2) production in ABA-induced stomatal closure via the degradation of putrescine. Calcium messenger is an important intermediate in this process.

Simultaneous overexpression of both CuZn superoxide dismutase and ascorbate peroxidase in transgenic tall fescue plants confers increased tolerance to a wide range of abiotic stresses

To mitigate the oxidative damage inflicted by biotic or abiotic stresses, plants have evolved complex anti-oxidative defense mechanisms that involve induction of antioxidant and anti-oxidative enzymes, such as superoxide dismutase (SOD) and ascorbate peroxidase (APX). To determine whether overexpression of the genes encoding copper-zinc SOD (CuZnSOD) and APX in plants is capable of decreasing reactive oxygen species (ROS) produced in response to abiotic stresses, we generated transgenic tall fescue plants expressing the CuZnSOD and APX genes in chloroplasts under the control of the oxidative stress-inducible promoter, sweet potato peroxidase anionic 2 (SWPA2). Transgenic plants were generated by Agrobacterium-mediated genetic transformation, and genotypes were confirmed by DNA blot analysis. Transgenic plants were exposed to several ROS-generating abiotic stresses, such as methyl viologen (MV), H(2)O(2), and the heavy metals copper, cadmium, and arsenic, and their tolerance was evaluated. High levels of CuZnSOD and APX gene transcripts in the transgenic plants under these treatments suggested that the transgenes were functionally expressed. Compared to transgenic plants, higher amounts of ROS were generated in the leaves of control plants exposed to abiotic stresses, resulting in increased thiobarbituric acid reactive substances (TBARS), ion leakage, and chlorophyll degradation. These parameters were significantly lower in transgenic plants. Enzyme activity assays and native polyacrylamide gel electrophoresis (PAGE) showed that total SOD and APX were highly active in transgenic plants under the abiotic stresses examined. We conclude that one of the mechanisms of increased anti-oxidative defense in transgenic tall fescue plants is overexpression of the CuZnSOD and APX genes, which are utilized in scavenging ROS and thus provide improved tolerance to abiotic stresses.

Abscisic Acid is a Key Inducer of Hydrogen Peroxide Production in Leaves of Maize Plants Exposed to Water Stress

The histochemical and cytochemical localization of water stress-induced H(2)O(2) production in the leaves of ABA-deficient vp5 mutant and wild-type maize (Zea mays L.) plants were examined, using 3,3-diaminobenzidine and CeCl(3) staining, respectively, and the roles of endogenous ABA in the production of H(2)O(2) induced by water stress were assessed. Water stress induced by polyethylene glycol resulted in the accumulation of H(2)O(2) in mesophyll cells, bundle-sheath cells and vascular bundles of wild-type maize leaves, and the accumulation was substantially blocked in the mutant maize leaves exposed to water stress. Pre-treatments with several apoplastic H(2)O(2) manipulators abolished the majority of H(2)O(2) accumulation induced by water stress in the wild-type leaves. The subcellular localization of H(2)O(2) production was demonstrated in the cell walls, xylem vessels, chloroplasts, mitochondria and peroxisomes in the leaves of wild-type maize plants exposed to water stress, and the accumulation of H(2)O(2) induced by water stress in the cell walls and xylem vessels, but not in the chloroplasts, mitochondria and peroxisomes, was arrested in the leaves of the ABA mutant or the ABA biosynthesis inhibitor (tungstate)-pre-treated maize plants. Pre-treatments with the apoplastic H(2)O(2) manipulators also blocked the apoplastic but not the intracellular H(2)O(2) accumulation induced by water stress in the leaves of wild-type plants. These data indicate that under water stress, the apoplast is the major source of H(2)O(2) production and ABA is a key inducer of apoplastic H(2)O(2) production. These data also suggest that H(2)O(2) generated in the apoplast could not diffuse freely into subcellular compartments.

Induction of an anionic peroxidase in cowpea leaves by exogenous salicylic acid

Two isoperoxidases were detected in cowpea (Vigna unguiculata) leaves. Treatment of the primary leaves with 10mM salicylic acid increased the total peroxidase activity contributed by the anionic isoform. To isolate both the anionic and cationic peroxidases the leaf crude extract was loaded on a Superose 12 HR 10/30 column followed by chromatography on Mono-Q HR 5/5. Both enzymes were stable in a pH range from 5 to 7. The optimum-temperatures for the cationic and anionic peroxidase isoforms were, respectively, 20-30 degrees C and 30 degrees C. The dependence of guaiacol oxidation rate varying its concentration at constant H(2)O(2) concentration showed, for both enzymes, Michaelis-Menten-type kinetic. Apparent K(m)(s) were 0.8 and 4.8 microM for the cationic and anionic isoperoxidases, respectively.

Mitogen-activated protein kinase is involved in abscisic acid-induced antioxidant defense and acts downstream of reactive oxygen species production in leaves of maize plants

The role of mitogen-activated protein kinase (MAPK) in abscisic acid (ABA)-induced antioxidant defense was investigated in leaves of maize (Zea mays) plants. Treatments with ABA or H(2)O(2) induced the activation of a 46-kD MAPK and enhanced the expression of the antioxidant genes CAT1, cAPX, and GR1 and the total activities of the antioxidant enzymes catalase, ascorbate peroxidase, glutathione reductase, and superoxide dismutase. Such enhancements were blocked by pretreatment with several MAPK kinase inhibitors and reactive oxygen species inhibitors or scavengers. Pretreatment with MAPK kinase inhibitors also substantially arrested the ABA-induced H(2)O(2) production after 2 h of ABA treatment, but did not affect the levels of H(2)O(2) within 1 h of ABA treatment. Pretreatment with several inhibitors of protein tyrosine phosphatase, which is believed to be a negative regulator of MAPK, only slightly prevented the ABA-induced H(2)O(2) production, but did not affect the ABA-induced MAPK activation and ABA-enhanced antioxidant defense systems. These results clearly suggest that MAPK but not protein tyrosine phosphatase is involved in the ABA-induced antioxidant defense, and a cross talk between H(2)O(2) production and MAPK activation plays a pivotal role in the ABA signaling. ABA-induced H(2)O(2) production activates MAPK, which in turn induces the expression and the activities of antioxidant enzymes. The activation of MAPK also enhances the H(2)O(2) production, forming a positive feedback loop.

Abscisic acid-induced apoplastic H2O2 accumulation up-regulates the activities of chloroplastic and cytosolic antioxidant enzymes in maize leaves

The histochemical and cytochemical localization of abscisic acid (ABA)-induced H(2)O(2) production in leaves of maize (Zea mays L.) plants were examined, using 3,3-diaminobenzidine (DAB) and CeCl(3) staining, respectively, and the relationship between ABA-induced H(2)O(2) production and ABA-induced subcellular activities of antioxidant enzymes was studied. H(2)O(2) generated in response to ABA treatment was detected within 0.5 h in major veins of the leaves and maximized at about 2-4 h. In mesophyll and bundle sheath cells, ABA-induced H(2)O(2) accumulation was observed only in apoplast, and the greatest accumulation occurred in the walls of mesophyll cells facing large intercellular spaces. Meanwhile, ABA treatment led to a significant increase in the activities of the leaf chloroplastic and cytosolic antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), and pretreatment with the NADPH oxidase inhibitor diphenyleneiodonium (DPI), the O (2) (-) scavenger Tiron and the H(2)O(2) scavenger dimethylthiourea (DMTU) almost completely arrested the increase in the activities of these antioxidant enzymes. Our results indicate that the accumulation of apoplastic H(2)O(2) is involved in the induction of the chloroplastic and cytosolic antioxidant enzymes. Moreover, an oxidative stress induced by paraquat (PQ), which generates O (2) (-) and then H(2)O(2) in chloroplasts, also up-regulated the activities of the chloroplastic and cytosolic antioxidant enzymes, and the up-regulation was blocked by the pretreatment with Tiron and DMTU. These data suggest that H(2)O(2) produced at a specific cellular site could coordinate the activities of antioxidant enzymes in different subcellular compartments.

Hydrogen peroxide is necessary for abscisic acid-induced senescence of rice leaves

The role of H2O2 in abscisic acid (ABA)-induced rice leaf senescence is investigated. ABA treatment resulted in H2O2 production in rice leaves, which preceded the occurrence of leaf senescence. Dimethylthiourea, a chemical trap for H2O2, was observed to be effective in inhibiting ABA-induced senescence, ABA-increased matondialdehyde (MDA) content, ABA-increased antioxidative enzyme activities (superoxide dismutase, ascorbate peroxidase, glutathione reductase and catalase), and ABA-decreased antioxidant contents (ascorbic acid and reduced glutathione) in rice leaves. Diphenyteneiodonium chloride (DPI) and imidazole (IMD), inhibitors of NADPH oxidase, and KCN and NaN3, inhibitors of peroxidase, prevented ABA-induced H2O2 production, suggesting NADPH oxidase and peroxidase are H2O2-generating enzymes in ABA-treated rice leaves. DPI, IMD, KCN, and NaN3 also inhibited ABA-promoted senescence, ABA-increased MDA contents, ABA-increased antioxidative enzyme activities, and ABA-decreased antioxidants in rice leaves. These results suggest that H2O2 is involved in ABA-induced senescence of rice leaves.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Spermine accumulation under salt stress

Polyamines have long been recognized to be linked to stress situations, and it is generally accepted that they have protective characteristics. However, little is known about their physiological relevance in plants subjected to long-term salt stress. In order to precise their importance, two rice (Oryza sativa) cultivars differing in their salt tolerance were salinized for 7, 14 and 21 days. The activities of some of the enzymes involved in polyamine metabolism, free polyamines and proline contents were evaluated. Arginine decarboxylase and S-adenosyl-L-methionine decarboxylase activities were reduced in both cultivars as a consequence of salt treatment. However, spermidine synthase activity was reduced in the salt tolerant cultivar (var Giza) but not in the salt sensitive (var El Paso), while no polyamine oxidase activity was detected. During the salinization period, putrescine and spermidine levels decreased in both cultivars, although less dramatically in Giza. Simultaneously, spermine accumulations occur in both varieties, while proline accumulation was major in the sensitive one. However, spermine accumulation induced by treatment with spermidine synthase inhibitor cyclohexylamine, determined no reduction in leaf injury associated with salt stress in both cultivars. The data presented suggest that spermine accumulation is not a salt tolerance trait.