Antioxidative protection in a drought-resistant maize strain during leaf senescence

Leaves of 7- and 18-day-old plants of two maize strains, one resistant (LIZA) and one sensitive (LG11) to water stress, were floated in 1 m M paraquat and 1 m M H₂O₂ for 12 h in light and in darkness. The aim of this work was to analyse the effects of these substances on the activities of enzymes involved in the scavenging of active oxygen species during senescence. Three senescence parameters; chlorophyll loss, lipid peroxidation and conductivity; showed a general cell damage caused by both oxidative treatments and revealed a higher tolerance of LIZA than LG11 to paraquat and H₂O₂ both in light and in darkness. Activities of antioxidative enzymes increased by the effect of oxidative treatments in young and senescent leaves of the drought-resistant maize strain LIZA. These increases were about 3-to 6-fold in glutathione reductase. 3-to 4-fold in superoxide dismutase and 2-fold in ascorbate peroxidase activities. The possible correlation between water stress resistance. senescence and the potential of antioxidant enzymes was analysed.     

Antioxidant Defences under Hyperoxygenic and Hyperosmotic Conditions in Leaves of Two Lines of Maize with Differential Sensitivity to Drought

The effects of hyperoxygenic and hyperosmotic stress on severalaspects of antioxidant defences were studied in the leaves ofa drought-sensitive (LG11) and a drought-tolerant (LIZA) lineof maize (Zea mays L.). When leaf disks were subjected to theseverest stress conditions (100% O₂ and 0.5 M mannitol), theactivities of antioxidant enzymes, such as superoxide dismutase(SOD), catalase (Cat), ascorbate peroxidase (Asc-Px) and glutathionereductase (GSSG-Red), remained higher in disks of LIZA thanin disks of LG11. The ratios of activities of SOD to Cat, SODto Asc-Px and SOD to GSSG-Red were much higher in leaf disksfrom LG11 than in those from LIZA. Damage, as indicated by,for example, the extend of lipid peroxidation, the destructionof chlorophyll and carotenoids, the decrease in levels of proteinsulfhydryl groups and the leakage of electrolytes from cellswas apparent in leaf disks of both LIZA and LG11 as consequenceof the applied stresses. However, the damage was less markedin LIZA than in LG11. (Received September 2, 1992; Accepted July 20, 1993)     

Hydroxyl Radicals and a Thylakoid-Bound Endopeptidase are Involved in Light- and Oxygen-Induced Proteolysis in Oat Chloroplasts

The hypothesis that light- and oxygen-induced proteolysis inchloroplasts is mediated by active oxygen species was examined.In order to determine whether or not H₂O₂ and/or ^{dot}OH radicalsare involved in these degradative processes we compared thedegradation of proteins in isolated oat chloroplasts exposedto white light at 80 W m^-2 with that in chloroplasts incubatedin darkness in the absence or presence of H₂O₂ or a ^{dot}OH-generatingsystem composed by ascorbic acid, FeCl₃ and H₂O₂ (Asc-Fe-H₂O₂).Light enhanced the rate of degradation of at least 18 polypeptides,while proteolysis was almost negligible in darkness in the abscenceof additives. H₂O₂ had a very small effect. However, Asc-Fe-H₂O₂-treatedchloroplasts in darkness showed a pattern of protein degradationalmost identical to that observed in the light. A thylakoid-boundendopeptidase (EP), the activity of which increased under photooxidativeenvironmental conditions and treatment with an ^{dot}OH-generatingsystem, was partially purified and characterized as a serinetypeprotease. Treatments with inhibitors of serine-type proteaseprevented both light- and Asc- Fe-H₂O₂-induced proteolysis.EP was more active against both soluble and membranous proteinsthat had been pretreated with Asc-Fe-H₂O₂ than against untreatedproteins. It is proposed that a high dose of light irradiationpromotes proteolysis by increasing the formation of ^{dot}OH,which may modify proteins such that they become more susceptibleto EP-catalyzed hydrolysis. ¹Fisiología Vegetal, Dept. de Biología Vegetal,Universidad de Alcalá de Henares, Present address: 28871Alcalá de Henares (Madrid), España.     

Na2CO3胁迫下星星草幼苗活性氧及保护酶活性的变化

对低浓度Na₂CO₃胁迫下星星草幼苗相对电导率、O^-₂产生速率、H₂O₂含量以及保护酶CAT、SOD和POD活性的研究结果表明,低盐胁迫1 d后,星星草幼苗细胞膜的通透性、O^-₂产生速率、H₂O₂含量及保护酶活性都随着盐胁迫的加剧而升高,其具体的变化规律与盐胁迫强度和幼苗细胞膜的受损伤程度密切相关,但相关关系的性质上具有差异。     

外源NO对南方红豆杉幼苗光合色素及抗氧化酶的影响

以4年生南方红豆杉幼苗为实验材料,通过对南方红豆杉幼苗喷施不同浓度外源一氧化氮（NO）供体硝普钠溶液（0、0.01、0.1、0.5和1 mmol·L^-1SNP）,测定光合色素含量、抗氧化酶活性、丙二醛（MDA）含量和过氧化氢（H₂O₂）含量等生理指标,以探讨不同浓度外源NO对南方红豆杉叶片光合色素和抗氧化酶的影响。结果表明：喷施低浓度（0.01、0.1 mmol·L^-1）SNP可显著提高南方红豆杉叶片的叶绿素a、叶绿素b、类胡萝卜素和总叶绿素含量,增加叶绿素a/b的比值,而喷施高浓度（0.5、1 mmol·L^-1）SNP降低了叶片的光合色素含量。随着外源NO供体浓度的增加,叶片过氧化氢酶(CAT)活性显著增加,过氧化物酶(POD)活性先增加后降低。此外,处理前期,低浓度SNP处理明显提高了抗坏血酸过氧化物酶(APX)活性,而高浓度SNP处理显著降低了APX活性,处理后期APX活性随SNP浓度的增加而显著下降。喷施低浓度SNP可有效提高超氧化物歧化酶(SOD)活性和增加可溶性蛋白含量,降低MDA和H₂O₂的含量,而喷施高浓度SNP显著增加了MDA和H₂O₂的含量。因此,低浓度的SNP（<0.5 mmol·L^-1）处理南方红豆杉幼苗,可增加其叶绿素含量,提高抗氧化酶活性,降低MDA和H₂O₂的含量,而高浓度的SNP（≥0.5 mmol·L^-1）处理会降低叶绿素含量,提高H₂O₂含量,增加细胞膜质过氧化程度,从而对南方红豆杉幼苗造成一定伤害。     

活性氧在UV-B诱导的玉米幼苗叶片乙烯产生中的作用

 研究了活性氧在UV-B(280～320 nm)诱导的玉米(Zea mays)幼苗叶片乙烯合成中的作用。结果表明,UV-B促进了玉米幼苗活性氧和乙烯的产生;乙 烯合成抑制剂氨氧乙烯基甘氨酸 (AVG)和氨氧乙酸(AOA)能明显减弱UV-B对玉米幼苗乙烯产生的诱导作用,但对活性氧(ROS)的 产生没有明显影 响;ROS的清除剂不但能抑制UV-B诱导的 ROS的产生,而且还可以抑制UV_B诱导的乙烯的产生,但这种抑制作用可以被外源O₂^.-的供体所逆转。这 说明,乙烯的积累不能作为UV-B胁迫下ROS的诱导的因素,相反,ROS的积累则导致了乙烯的积累;因此,ROS可能参与了UV-B胁迫诱导的乙烯的产生 。质膜NADPH氧化酶的抑制剂二苯碘鎓(DPI)和H₂O₂的特异性清除剂过氧化氢酶（CAT）对UV-B胁迫诱导的乙烯积累 几乎没有影响, 这说明H₂O₂ 可能与UV-B诱导的玉米幼苗叶片乙烯的产生无关, 在UV-B诱导的玉米幼苗叶片乙烯的生物合成过程中O₂^.-起着很重要的作用,相关的O₂^.-不是由 NADPH氧化酶催化产生的。     

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(H₂O₂) in the acclimation of a Mediterranean shrub, Cistus albidusL., to summer drought growing under Mediterranean field conditions.For this purpose, changes in H₂O₂ concentrations and localizationthroughout a year were analysed. H₂O₂ changes in response toenvironmental conditions in parallel with changes in abscisicacid (ABA) and oxidative stress markers, together with ligninaccumulation, xylem and sclerenchyma differentiation, and leafarea were also investigated. During the summer drought, leafH₂O₂ concentrations increased 11-fold, reaching values of 10µmol g^–1 dry weight (DW). This increase occurredmainly in mesophyll cell walls, xylem vessels, and sclerenchymacells in the differentiation stage. An increase in ABA levelspreceded that of H₂O₂, but both peaked at the same time in conditionsof prolonged stress. C. albidus plants tolerated high concentrationsof H₂O₂ because of its localization in the apoplast of mesophyllcells, xylem vessels, and in differentiating sclerenchyma cells.The increase in ABA, and consequently of H₂O₂, in plants subjectedto drought stress might induce a 3.5-fold increase in ascorbicacid (AA), which maintained and even decreased its oxidativestatus, thus protecting plants from oxidative damage. Afterrecovery from drought following late-summer and autumn rainfall,a decrease in ABA, H₂O₂, and AA to their basal levels (60 pmolg^–1 DW, 1 µmol g^–1 DW, and 20 µmol g^–1DW) was observed. Key words: Abscisic acid, ascorbate, ascorbate oxidative status, Cistus albidus, hydrogen peroxide, leaf plasticity, lignin, Mediterranean shrubs, oxidative markers, summer drought Received 29 July 2008; Revised 15 September 2008 Accepted 8 October 2008     

Role of calreticulin in the sensitivity of myocardiac H9c2 cells to oxidative stress caused by hydrogen peroxide

Calreticulin (CRT), a Ca²⁺-binding molecular chaperone in the endoplasmic reticulum, plays a vital role in cardiac physiology and pathology. Oxidative stress is a main cause of myocardiac apoptosis in the ischemic heart, but the function of CRT under oxidative stress is not fully understood. In the present study, the effect of overexpression of CRT on susceptibility to apoptosis under oxidative stress was examined using myocardiac H9c2 cells transfected with the CRT gene. Under oxidative stress due to H₂O₂, the CRT-overexpressing cells were highly susceptible to apoptosis compared with controls. In the overexpressing cells, the levels of cytoplasmic free Ca²⁺ ([Ca²⁺]_i) were significantly increased by H₂O₂, whereas in controls, only a slight increase was observed. The H₂O₂-induced apoptosis was enhanced by the increase in [Ca²⁺]_i caused by thapsigargin in control cells but was suppressed by BAPTA-AM, a cell-permeable Ca²⁺ chelator in the CRT-overexpressing cells, indicating the importance of the level of [Ca²⁺]_i in the sensitivity to H₂O₂-induced apoptosis. Suppression of CRT by the introduction of the antisense cDNA of CRT enhanced cytoprotection against oxidative stress compared with controls. Furthermore, we found that the levels of activity of calpain and caspase-12 were elevated through the regulation of [Ca²⁺]_i in the CRT-overexpressing cells treated with H₂O₂ compared with controls. Thus we conclude that the level of CRT regulates the sensitivity to apoptosis under oxidative stress due to H₂O₂ through a change in Ca²⁺ homeostasis and the regulation of the Ca²⁺-calpain-caspase-12 pathway in myocardiac cells. apoptosis; calcium; endoplasmic reticulum     

Glucose Induced H+ Influx and Transient Currents in Excised Roots: particularly those of Zea mays

The application of D-glucose to solutions bathing excised maize,wheat, pea and bean roots causes a rapid depolarization of theelectrical potentials between the cut tops of the roots andthe bathing solutions. Similar effects are observed for theplasma membrane potentials of maize lateral roots. A flow cell apparatus was used to demonstrate qualitative andquantitative relations between glucose induced H⁺ influx andthe transient decrease in current through the root. The currentchanges appear to be due entirely to H⁺ fluxes. Current andH⁺ fluxes are strongly influenced by external pH, the optimumpH for glucose induced current change being about 4.0. A similarpH optimum was found for 3-O-methyl-D-glucopyranoside but 1-O-methyl--D-glucopyranosidedid not significantly affect the trans-root potential at anypH, suggesting a significant role for the anomeric hydroxylgroup of glucose. Compounds which depolarize the trans-root potential also inhibitthe glucose induced depolarization. Surface -SH groups are probablynot involved in the glucose/H⁺ cotransport. Eadie-Hofstee plots relating the depolarization of trans-rootpotential to the concentrations of D-glucose or 3-O-methyl-D-glucopyranosidehave shown that K_m values increase with increasing monosaccharideconcentration and are very similar to reported values of 3-O-methyl-D-glucopyranosideuptake in maize root segments. K_m values for a similar rangeof D-glucose concentrations do not vary significantly with pHor with membrane depolarization due to a 10-fold increase ofKCl concentration. However, V_max is lowered by an increase inexternal pH or a decrease in trans-root potential. It appearsthat both proton and electrical gradients can affect glucoseinduced H⁺ influx. The auxin herbicide, 2, 4-dichlorophenoxyethanoic acid (0.01mM) stimulates the glucose induced depolarizations in a mannerconsistent with an increase in cytoplasmic pH. This is discussedin relation to the reported action of indole-3-acetic acid andfusicoccin on maize root tissue.     

Grain and Bean Lysates Improve Function of Endothelial Progenitor Cells from Human Peripheral Blood: Involvement of the Endogenous Antioxidant Defenses

Increased oxidative stress contributes to the functional impairment of endothelial progenitor cells (EPCs), the pivotal players in the servicing of the endothelial cell lining. Several evidences suggest that decreasing oxidative stress by natural compounds with antioxidant properties may improve EPCs bioactivity. Here, we investigated the effects of Lisosan G (LG), a Triticum Sativum grain powder, and Lady Joy (LJ), a bean lysate, on function of EPCs exposed to oxidative stress. Peripheral blood mononuclear cells were isolated and plated on fibronectin-coated culture dishes; adherent cells, identified as early EPCs, were pre-treated with different concentrations of LG and LJ and incubated with hydrogen peroxide (H₂O₂). Viability, senescence, adhesion, ROS production and antioxidant enzymes gene expression were evaluated. Lysate-mediated Nrf-2 (nuclear factor (erythroid-derived 2)-like 2)/ARE (antioxidant response element) activation, a modulator of oxidative stress, was assessed by immunocytochemistry. Lady Joy 0.35–0.7 mg/ml increases EPCs viability; pre-treatment with either LG 0.7 mg/ml and LJ 0.35–0.7 mg/ml protect EPCs viability against H₂O₂-induced injury. LG 0.7 and LJ 0.35–0.7 mg/ml improve EPCs adhesion; pre-treatment with either LG 0.35 and 0.7 mg/ml or LJ 0.35, 0.7 and 1.4 mg/ml preserve adhesiveness of EPCs exposed to H₂O₂. Senescence is attenuated in EPCs incubated with lysates 0.35 mg/ml. After exposure to H₂O₂, LG pre-treated cells show a lower senescence than untreated EPCs. Lysates significantly decrease H₂O₂-induced ROS generation. Both lysates increase glutathione peroxidase-1 and superoxide dismutase-2 (SOD-2) expression; upon H₂O₂ exposure, pre-treatment with LJ allows higher SOD-2 expression. Heme oxigenase-1 increases in EPCs pre-treated with LG even upon H₂O₂ exposure. Finally, incubation with LG 0.7 mg/ml results in Nrf-2 translocation into the nucleus both at baseline and after the oxidative challenge. Our data suggest a protective effect of lysates on EPCs exposed to oxidative stress through the involvement of antioxidant systems. Lisosan G seems to activate the Nrf-2/ARE pathways.     

An activated-oxygen-mediated role for peroxisomes in the mechanism of senescence of Pisum sativum L. leaves

The possible involvement of peroxisomes and their activated-oxygen metabolism in the mechanism of leaf senescence was investigated in detached pea (Pisum sativum L.) leaves which were induced to senesce by incubation in complete darkness for up to 11 d. At days 0, 3, 8, and 11 of senescence, peroxisomes were purified from leaves and the activities of different peroxisomal and glyoxysomal enzymes were measured. Xanthine-oxidoreductase activity increased with senescence, especially the O ₂ ^{. -} -producing xanthine oxidase (EC 1.1.3.22). The activities of H₂O₂-generating Mn-superoxide dismutase (EC 1.15.1.1) and urate oxidase (EC 1.7.3.3) were also enhanced by senescence, whereas catalase (EC 1.11.1.6) activity was severely depressed. Hydrogen peroxide concentrations increased significantly in senescent leaf peroxisomes. During the progress of senescence, glycollate oxidase (EC 1.1.3.1) and hydroxypyruvate reductase (EC 1.1.1.81), two marker enzymes of photorespiratory metabolism, gradually decreased in activity and disappeared. At the same time, the activities of malate synthase (EC 4.1.3.2) and isocitrate lyase (EC 4.1.3.1), key enzymes of the glyoxylate cycle, which were undetectable in presenescent leaves, increased dramatically upon induction of senescence. Ultrastructural studies of intact leaves showed that the population of peroxisomes and mitochondria increased with senescence. Results indicate that peroxisomes could play a role, mediated by activated oxygen species, in the oxidative mechanism of leaf senescence, and further support the idea, proposed by other authors, that foliar senescence is associated with the transition of leaf peroxisomes into glyoxysomes.Abbreviation Mn-SOD (manganese-containing) superoxide dismutase The authors thank Dr. A.J. Sánchez-Raya (Unidad de Fisiología Vegetal, Estación Experimental del Zaidín, Granada, Spain) for his valuable help in measuring ethylene production, and Dr. G. Barja de Quiroga (Departamento de Biología Animal II, Universidad Complutense, Madrid, Spain) for carrying out the malondialdehyde determinations by HPLC. This work was supported by grant PB87-0404-01 from the DGICYT and the Junta de Andaluc'ia (Research Group # 3315), Spain.     

Synthesis of Isozymes of Superoxide Dismutase in Maize Leaves in Response to O3, SO2 and Elevated O2

Matters, G. L. and Scandalios, J. G. 1987. Synthesis of isozymesof superoxide dismutase in maize leaves in response to O₃ SO₂and elevated O₂.—J. exp. Bot 38: 842–852. The activities of the enzymes superoxide dismutase (SOD) andcatalase were determined in maize leaves treated with O₃or SO₂for8 h, or with elevated levels of oxygen for up to 96 h. NeitherO₃nor SO₂significantly increased the levels of superoxide dismutaseor catalase activity. However, after 72 h in an atmosphere containing90% oxygen, superoxide dismutase activity was increased, butnot the activities of catalase, ascorbate pcroxidase, and malatedehydrogenase. Immunological analysis showed that amounts ofthe cytosolic superoxide dismutase isozymes, SOD-2 and SOD-4,were increased by the elevated oxygen but not the chroloplast(SOD-1) or mitochondrial (SOD-3) isozymes. Immunoprecipitationof translation products of leaf polysomes indicated that thehigher levels of SOD-2 and SOD-4 were due to increased amountsof polysome-bound mRNA coding for these proteins. The specificresponse of SOD-2 and SOD-4 to 90% oxygen treatments contrastswith the increase in all SOD isozymes in maize leaves treatedwith the herbicide paraquat. Key words: Air pollutants, maize, oxidative stress, oxygen, superoxide dismutase     

Why are literature data for H2O2 contents so variable? A discussion of potential difficulties in the quantitative assay of leaf extracts

Leaf metabolism produces H₂O₂ at high rates, but current conceptssuggest that the potent signalling effects of this oxidant requirethat concentrations be controlled by a battery of antioxidativeenzymes. The extent to which H₂O₂ is allowed to accumulate remainsunclear. There is little consensus on leaf H₂O₂ values in theliterature and measured concentrations in unstressed conditionsrange from 50–5000 nmol g^–1 fresh weight, a differencethat probably reflects technical inaccuracies as much as biologicalvariability. This article uses new experimental and literaturedata to examine some of the difficulties in accurately measuringH₂O₂ in leaf extracts. Potential problems relate to sensitivity,interference from other redox-active compounds, and H₂O₂ stabilityduring sample preparation. Particular attention is drawn tothe influence of tissue mass/extraction volume in the quantitativeestimation of H₂O₂ contents, and the possibility that this factorcould contribute to the variability of literature data. Key words: Ascorbate, chemiluminescence, oxidative stress, redox signalling, xylenol orange Received 4 June 2007; Revised 18 June 2007 Accepted 23 June 2007     

The Senescence Process in Oat Leaves and its Regulation by Oxygen Concentration and Light Irradiance

The regulation of senescence by oxygen-concentration, lightirradiance and H₂O₂ has been studied in leaf segments of Avenasativa L. cv. Suregrain. The development of the components of the senescence process,for example chlorophyll breakdown, proteolysis (as soluble aminoacids), hydroperoxides (as malondi-aldehyde) and permeability(as conductivity) is accelerated in light as the O₂-tensionincreases. In darkness, 0.3% O₂ accelerates increases in hydroperoxides,permeability and proteolysis and delays the chlorophyll break-down,but 0.0005% O₂ delays all the components studied. In every casethe hydroperoxide content, permeability and proteolysis areclosely related. Any treatment inducing an increase in membranepermeability causes chlorophyll bleaching (photo-oxidation)if leaf segments are then treated with light in an atmospherecontaining oxygen. Light has a modulating effect on the senescenceprocess. An irradiance lower or higher than 40 W m^–2 hasan accelerating effect on the senescence process. (Received September 7, 1985; Accepted July 30, 1985)     

Antioxidant Efficiency during Callus Initiation from Mature Rice Embryo

The changes in the activities of active oxygen scavenging enzymesand lipid peroxidation during callus formation from germinatingmature rice embryo was investigated. Superoxide dismutase (SOD)and catalase (CAT) activities were much lower during callusformation than during seedling growth indicating the decliningcapacity of the callus tissue to scavenge O^.-₂ and H₂O₂ respectively.Other H₂O₂-utilizing enzymes such as guaiacol peroxidase (GPOX)and ascorbate peroxidase (APOX) had also much lower activitiesduring the initial period of callus formation than during normalgermination but soon these enzyme activities were rapidly enhancedduring callus growth but declined during seedling growth. SinceH₂O₂ level was quite high in the callus tissue, it is probablethat GPOX and APOX are not efficient in decomposing H₂O₂ inthis tissue. Water soluble non-protein -SH compounds of whichGSH is the major component increased more rapidly during seedlinggrowth than during callus formation. This was reflected by thehigher activity of glutathione reductase (GR) in the seedlingtissue than in the callus tissue. Although peroxide and malondialdehydedid not accumulate during the callus initiation period, thefast decrease in the SOD and CAT activities indicates that duringthis transition period the tissue has increasing tendency towardsan oxidative state because of the weakening of the scavengingmechanism. The cellular environment, thereafter, becomes moreoxidizing during callus growth when compared with the normalseedling development in the absence of 2,4-D. (Received September 8, 1994; Accepted February 16, 1995)     

Inactivation and Degradation of CuZn-SOD by Active Oxygen Species in Wheat Chloroplasts Exposed to Photooxidative Stress

Changes in CuZn-SOD actvity and content in isolated wheat chloroplastsunder the light, and the involvement of protease(s) and/or activeoxygen species in this process were studied. Both SOD activityand content decayed with exposure time to photooxidative stress.Ascorbate, a H₂O₂ scavenger, prevented photooxidation-associatedinactivation of SOD, while benzoate, a OH scavenger, preventedSOD degradation. Wheat chloroplasts incubated in the dark didnot hydrolyze exogenous or endogenous SOD, either H₂O₂-pretreatedor not. Protease inhibitors did not prevent SOD degradationunder photooxidative treatment, suggesting that plastid protease(s)did not participate in this process. Purified chloroplast CuZn-SODwas exposed to H₂O₂ and or OH-generating systems. had no effect on either SOD activity or stability (estimated bynative PAGE). H₂O₂ up to 700µM inhibited SOD in a dose-dependentmanner and induced charge/mass changes as seen by native PAGE.OH also reduced SOD activity by inducing its fragmentation.High levels of active oxygen, as can be generated under strongstress conditions, could directly inactivate and degrade chloroplasticSOD. ₁ Present address: Departamento de Biología Vegetal,Univer-sidad de Alcalá de Henares, Alcalá de Henares,E-28871 Madrid, Spain     

Resistance of Photosynthesis to Hydrogen Peroxide in Algae

The effects of H₂O₂ on the photosynthetic fixation of CO₂ andon thiol-modulated enzymes involved in the photosynthetic reductionof carbon in algae were studied in a comparison with those inchloroplasts isolated from spinach leaves. In both systems,H₂O₂-scavenging enzymes were inhibited by addition of 0.1 mMNaN₃ 1 h prior to the addition of H₂O₂. A concentration (10^-4M) of H₂O₂ caused strong inhibition of the CO₂ fixation by intactspinach chloroplasts, as observed by Kaiser [(1976) Biochim.Biophys. Acta 440: 476], but not that by Euglena and Chlamydomonascells. The same results were also obtained with cells of thecyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803in the presence of 1 mM hydroxylamine. These results indicatethat algal photosynthesis is rather resistant to H₂O₂. The insusceptibilityto H₂O₂ of thiolmodulated enzymes, namely, fructose-1,6-bisphosphatase,NADP-glyceraldehyde-3-phosphate dehydrogenase, and ribulose-5-phosphatekinase, was also observed in the chloroplasts of Euglena andChlamydomonas and in cyanobacterial cells. It seems likely thatthe resistance of photosynthesis to H₂O₂ is due in part to theinsusceptibility of the algal thiol-modulated enzymes to H₂O₂. (Received April 22, 1995; Accepted June 29, 1995)     

外源亚精胺可缓解荇菜镉毒害

研究了外施浓度为0.01 mmol.L^-1的亚精胺(Spd)对不同浓度镉(Cd²⁺⁾胁迫下荇菜(Nymphoides peltatum)叶片的叶绿体结构、叶绿素含量、可溶性蛋白含量、超氧阴离子(O₂^-.)产生速率和丙二醛(MDA)含量, 以及保护酶——超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT) 活性的影响。结果表明, (1) Cd²⁺胁迫可使荇菜细胞的叶绿体结构遭到破坏,叶绿素含量减少。外施Spd则可有效地保护叶绿体结构, 减少叶绿素的流失。(2) 在单一Cd²⁺处理条件下, 随着Cd²⁺浓度的升高, 叶绿素含量呈现先升后降的趋势, 可溶性蛋白含量则逐渐下降。外源Spd处理显著提高了二者的含量, 并延缓了它们的下降速度。(3) 在单一Cd²⁺处理条件下, SOD、POD和CAT活性分别在Cd²⁺浓度为1、 1和2 mg.L^-1时达到最高值, 而后随着Cd²⁺浓度的增加其活性逐渐下降。外施Spd使它们的活性分别提高了5.8%、37.5%和3.3%, 并降低了O₂^-.产生速率和MDA的含量。上述结果表明, Spd增强了荇菜对Cd²⁺毒害的抗性, 并在一定程度上缓解了Cd²⁺对荇菜的毒害。     

Heat shock protein 70 regulates the abscisic acid-induced antioxidant response of maize to combined drought and heat stress

We investigated the interaction between heat shock protein 70 (HSP70) and abscisic acid (ABA)-induced antioxidant response of maize to the combination of drought and heat stress. First, the increased activities of enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), induced by drought were less than those by heat or combined drought and heat stress, except some individual cases (e.g. CAT in leaves, GR in roots). Second, both HSP70 synthesis and H₂O₂ production increased prominently under drought, heat or their combination stress; the increase in leaves induced by drought and heat combination was the highest, followed by heat and by drought, while the increase in roots had not visible difference. Third, either in leaves or roots, pretreatment with ABA inhibitor, HSP70 inhibitor and H₂O₂ scavenger, significantly arrested the stress-induced increase of antioxidant enzyme activities, and ABA inhibitor and H₂O₂ scavenger obviously suppressed HSP70 synthesis, while HSP70 inhibitor slightly heightened H₂O₂ accumulation. Finally, 100 μM ABA significantly enhanced the activities of antioxidant enzymes, HSP70 expression and H₂O₂ production under stresses in comparison with ABA-deficient mutant vp5 maize plants without pretreatment. Thus, ABA-induced H₂O₂ production enhances the HSP70 synthesis and up-regulates the activities of antioxidant enzymes, resulting in the suppression of cellular reactive oxygen species (ROS) levels. Our results suggest that HSP70 may play a crucial role in ABA-induced antioxidant defense of maize to drought and heat combination.