Ascorbate-Glutathione Cycle Alteration in Cadmium Sensitive Rice Mutant cadB-1

A rice cadmium （Cd） sensitive mutant cadB-1 was obtained using Agrobacterium tumefaciens mediated system.After exposure of cadB-1 and wild type （WT） rice seedlings to a range of Cd concentrations for 10 d,Cd accumulated to higher levels in roots,stems and leaves of both cadB-1 and WT with increasing external Cd concentrations,and the inhibition of seedling growth in cadB-1 was more serious than in WT.Hydrogen peroxide accumulation was higher in leaves and roots of cadB-1.The ratios of reduced glutathione （GSH）/oxidized glutathione （GSSG）,ascorbate （ASC）/dehydroascorbate （DHA） and reduced nicotinamide adenine dinucleotide phosphate （NADPH）/oxidized nicotinamide adenine dinucleotide phosphate （NADP＋） were lower in cadB-1 than in WT both in leaves and roots under high Cd levels.The activities of ascorbate peroxidase （APX）,glutathione peroxidase （GR）,dehydroascorbate reductase （DHAR） and monodehydroascorbate reductase （MDHAR） were also lower in cadB-1 than in WT both in leaves and roots under the treatment of high levels of Cd.Our results suggest that under Cd stress,the ASC-GSH cycle was more seriously inhibited in cadB-1 than in WT,indicating that the mutant cadB-1 is less able to scavenge reactive oxygen species and sensitive to Cd.

Toxicity of Nonylphenol on Microcystis aeruginosa Mediated by the Ascorbate-glutathione (AsA-GSH ) Cycle

Nonylphenol( NP) is a stable metabolic product of nonylphenol ethoxylates,which is widely used as an industrial surfactant. NP has been classified as an endocrine disrupter,and its toxicity to organisms can be biomagnified through the food chain. As compared with the endocrine disrupting effect,the toxicity of NP to organisms has not been studied intensively,and the toxicity mechanisms have often been ignored. In the present study,Microcystis aeruginosa,a freshwater alga belonging to the first level of the trophic chain,was chosen to detect the toxicity of NP. The mechanisms of toxicity mediated by the AsA-GSH cycle were explored. The acute toxicity of NP to M. aeruginosa within 96 h was studied and an EC_(50) concentration of 3. 45 mg/L was found. Further,the results showed that the toxicity of NP increased with the increase in concentration and exposure time. As compared with that in the control,the APX and MDHAR activities mostly increased,whereas DHAR activity fluctuated.However,the AsA content elevated at first,but decreased significantly after 72 h. For the GSH system,GR activity was always higher than that in the control. Nevertheless,the reduced GSH content was mostly inhibited. Therefore,the performance of AsA-GSH antioxidant defense system could explain the results of NP toxicity: the enzyme activities and antioxidant molecules increased initially,but an overall decline appeared after exposure for 24 h. This research is helpful for estimating the toxicity of NP integrally and improves people's understanding of mechanisms of NP toxicity in algae.

外源脯氨酸对盐胁迫下甜瓜幼苗根系抗坏血酸-谷胱甘肽循环的影响

以2个不同耐盐强度的甜瓜品种‘玉皇’(耐盐性强)和‘雪美’(耐盐性弱)为材料,采用营养液栽培方法,研究外源脯氨酸对盐胁迫下甜瓜幼苗根系抗坏血酸-谷胱甘肽循环的影响。结果显示:(1)盐胁迫下,2个甜瓜品种根系内的还原型抗坏血酸(ASA)、还原型谷胱甘肽(GSH)含量降低,氧化型谷胱甘肽(GSSG)含量升高,且‘雪美’变化幅度大于‘玉皇’;(2)盐胁迫下,施用外源脯氨酸提高了2个甜瓜品种根系中ASA和GSH的含量,降低了GSSG含量,同时也提高了GSH/GSSG的比值,且对‘雪美’的作用大于‘玉皇’;(3)盐胁迫处理3 d时,2个甜瓜品种根系的抗坏血酸过氧化物酶(APX)、脱氢抗坏血酸还原酶(DHAR)、谷胱甘肽还原酶(GR)活性均下降,且‘雪美’下降的幅度较大;随着胁迫时间的延长(5 d时),‘玉皇’幼苗根系内APX、DHAR、GR活性有所上升,‘雪美’根系中这3种酶活性则进一步降低;(4)盐胁迫下,施用外源脯氨酸提高了2个甜瓜品种根系内的APX、DHAR和GR的酶活性,且对‘雪美’的作用大于‘玉皇’。本研究结果表明,外源脯氨酸可以通过增加非酶促抗氧化物质ASA、GSH的含量和抗氧化酶活性,提高抗坏血酸-谷胱甘肽循环清除活性氧的能力,从而缓解盐胁迫对甜瓜植株的伤害。

Comparative Study on Antioxidative System in Normal and Vitrified Shoots of Populus suaveolens in Tissue Culture

To explore the physiological and biochemical mechanism of the occurrence of vitrified shoots of Populus suaveolens in tissue culture, the changes in water, chlorphyll, lignin, H2O2, phenylalanine ammonialyase (PAL), malonaldehyde (MDA), protective enzymatic systems, and some key enzymes involved in the ascorbate- glutathione cycle were comparatively studied in both normal and vitrified shoots of P. suaveolens. The results show that the lower activities of peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and PAL, and the less contents of chlorphyll, lignin, ascorbate (ASA) and reduced glutathione (GSH) as well as the lower ratios of ASA / DHA and GSH / GSSG are observed in vitrified shoots than in normal ones during the whole culture period. While in comparison with normal shoots, the higher activity of superoxide dismutase (SOD) and the more concentrations of water, H2O2, MDA, dehydroascorbate (DHA) and oxidized glutathione (GSSG) are found in vitrified shoots. Statistical analysis indicates that the enhanced activity of SOD and the decreased activities of CAT and POD as well as some enzymes involved in the ascorbate-glutathione cycle might be closely correlated to the accumulation of H2O2. The less regeneration of ASA and GSH and the lower capacity of the ascorbate-glutathione cycle observed in vitrified shoots might be due to a significant decrease in APX, MDAR, DHAR and GR activities and a decline in redox status of ASA and GSH. The decreases in chlorphyll content might result in a decline in photosynthesis. The lower activities of POD and PAL could result in the decrease of lignin synthesis and cell wall ligination, which might be the key factor leading to the increase in water content. It is concluded that the deficiency of detoxification capacity caused by the lower capacity of the ascorbate-glutathione pathway and the decreased activity of protective enzymatic system might lead to the large accumulation of H2O2 and the enhancement of membrane lipid peroxidation, which might be the main cause leading to the occurrence of vitrifying shoots of P. suaveolens in tissue culture.

Different Responses of Plant Growth and Antioxidant System to the Combination of Cadmium and Heat Stress in Transgenic and Non-transgenic Rice

A comparative study of just cadmium （Cd） or heat and their combination treatments on some physiological parameters and the antioxidant systems in transgenic rice （Oryza sativa L. cv. Zhonghua No.11） carrying glutathione-S-transferase （GST, EC. 2.5.1.18） and catalasel （CAT1, EC. 1.11.1.6） and non-transgenics was conducted. The results revealed improved resistance in the transgenics to Cd and the combined Cd and heat stress than non-transgenics. Data showed that the activities of CAT, GST, superoxide dismutase （EC.1.15.1.1） and all components of the ascorbate-glutatbione cycle measured in the stressed transgenics shoots are significantly different from those of non-transgenics. Results indicated that co-expression of GST and CAT1 had an important effect on the antioxidant system, in particular, the whole ascorbate-glutathione cycle. The less oxidative damage induced by Cd and the stress combination in the transgenics resulted not only from the GST and CAT1 transgene but also from the coordination of the whole ascorbate-glutathione cycle.

Decreased glutathione reductase2 leads to early leaf senescence in Arabidopsis

Glutathione reductase（GR） catalyzes the reduction of glutathione disulfide（GSSG） to reduced glutathione（GSH）and participates in the ascorbate-glutathione cycle, which scavenges H_2O_2. Here, we report that chloroplastic/mitochondrial GR2 is an important regulator of leaf senescence. Seed development of the homozygous gr2 knockout mutant was blocked at the globular stage. Therefore, to investigate the function of GR2 in leaf senescence, we generated transgenic Arabidopsis plants with decreased GR2 using RNAi. The GR2 RNAi plants displayed early onset of age-dependent and darkand H2O2-induced leaf senescence, which was accompanied by the induction of the senescence-related marker genes SAG12 and SAG13. Furthermore, transcriptome analysis revealed that genes related to leaf senescence, oxidative stress, and phytohormone pathways were upregulated directly before senescence in RNAi plants. In addition, H2O2 accumulated to higher levels in RNAi plants than in wild-type plants and the levels of H_2O_2 peaked in RNAi plants directly before the early onset of leaf senescence. RNAi plants showed a greater decrease in GSH/GSSG levels than wild-type plants during leaf development. Our results suggest that GR2 plays an important role in leaf senescence by modulating H_2O_2 and glutathione signaling in Arabidopsis.