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  • Author or Editor: Y. Hafez x
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Pepper fruits were sprayed with benzothiadiazole (BTH) and hydrogen peroxide (H 2 O 2 ) three times after harvest. Three days after the third spray, pepper fruits were inoculated with Botrytis cinerea (10 5 conidia/ml under 20 °C, 99% relative humidity). Under postharvest conditions (without artificial inoculation) BTH at 0.9 mM and H 2 O 2 at 20 or 50 mM reduced the weight loss, natural rot rate and nitrate content, however, ascorbic acid content and the shelf-life of fruits were increased significantly.When pepper fruits were inoculated with the fungus and treated with BTH (0.9 mM) and H 2 O 2 (50 mM) treatments, disease severity was reduced to 29.6 and 34%, respectively, as compared with the control (77.3%). Diameter of necrotic lesions were 1.2 and 1.38 mm, respectively, as compared with the control (2.8 mm).BTH (0.9 mM) and H 2 O 2 (50 mM) increased the level of endogenous H 2 O 2 and total phenolic contents 3–6 days after inoculation (dai) which are considered to play a pivotal role in plant disease resistance. Activities of the antioxidant enzymes dehydroascorbate reductase (DHAR), ascorbate peroxidase (APX), catalase (CAT) and peroxidase (POX) decreased at 3 and 6 dai then increased between 9–15 dai. The effectiveness of BTH treatment was always higher than that of 50 mM H 2 O 2 (high concentration).Low concentration of H 2 O 2 (20 mM) enhanced the antioxidant activities in infected fruits, however, the level of H 2 O 2 and total phenolic compound contents were not altered significantly, thus, the fungus was not inhibited.The results showed that BTH (0.9 mM) and H 2 O 2 (50 mM) treatments enhanced disease resistance against the fungus early after inoculation by elevating the level of endogenous H 2 O 2 and phenolic contents. Low concentration of H 2 O 2 (20 mM) was able to increase activity of antioxidants which may contribute to symptom’s resistance but did not inhibit the pathogen importantly. It would seem that BTH and H 2 O 2 could be suitable to control postharvest diseases.

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Abstract  

Chemical factors such as pH, concentration and temperature affecting the removal of UO2 2+, Th4+, Fe3+, Cu2+, Pb2+,Cd2+, Ni2+, MnO4 - and phenol by Eichornia crassipes aquatic plant from their solutions were examined. Maximum uptake of ions by Eichornia crassipes occurred at pH 4 to 6±0.5 at 25±3 °C. An initial rapid uptake phase for the first 6 hours followed by a slower near linear one was observed. One gram of Eichornia crassipes can accumulate about 25 mg UO2 2+, 5 mg Th4+, 30 mg Fe3+, 10 mg MnO4 -, 15 mg Cu2+, 1.0 mg Pb2+, 1.5 mg Ni2+, 0.7 mg Cd2+ and or 25 mg of phenol.

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Three genotypes of barley (cultivar Ingrid) expressing the genes Mlo (susceptible), Mla12 (resistant with HR symptoms) and mlo5 (resistant without HR) in relation to infection by race A6 of Blumeria graminis f. sp. hordei have been sprayed with a solution of H2O2 after establishment of infection (2-3 days after inoculation). Under the influence of H2O2, leaves of the susceptible Mlo and mlo5-resistant plants exhibited HR-type symptoms with tissue necroses. The Mla12-resistant genotype produced HR earlier and the number of necrotic lesions increased, as compared to untreated control leaves. Treatment with H2O2 before establishment of infection (one day after inoculation), resulted in all the three genotypes in inhibition of the pathogen and symptomless response. It was possible to reverse the inhibitory as well as the HR-producing actions of H2O2 with injection of leaves with a combination of superoxide dismutase (SOD) and catalase (CAT) before treatment with H2O2.  It is suggested that the hypothetical negative regulation of HR-associated resistance in susceptible plants carrying the gene Mlo as well as in barley displaying HR-independent resistance and carrying the gene mlo5, could be associated with the limited production of H2O2 in infected plants. Supplying H2O2 to barley leaves that are either susceptible or display HR-independent resistance after establishment of infection, releases the negative regulation of symptoms of HR-associated resistance. This action of H2O2 is sensitive to antioxidant enzymes, such as SOD and CAT.

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Infection of some leaves of Xanthi-nc tobacco with tobacco mosaic virus (TMV) induces systemic acquired resistance (SAR) in remote leaves of the plant to a second (challenge) infection, and therefore produces only a limited necrotization in the resistant leaves. Here we show that the levels of superoxide and hydrogen peroxide are lower in the remote infected leaves exhibiting the SAR. Treatment of leaves of Xanthi-nc tobacco with benzothiadiazole (BTH) also suppresses tissue necrotization and accumulation of superoxide and hydrogen peroxide upon TMV inoculation. However, both of these reactive oxygen species are up-regulated and tissue necrotization is increased in a transgenic NahG tobacco, which is unable to produce a SAR response. Treatment of TMV-infected NahG leaves with BTH also resulted in a reduced level of necrotization and an attenuated accumulation of superoxide and hydrogen peroxide after inoculation with TMV. Thus, the level of reactive oxygen species seems to be correlated with the size and number of necrotic lesions caused by TMV. It would seem that reactive oxygen species play a pivotal role in TMV-induced cell death response.

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In plants, recognition of a pathogen as an invader may result in the formation of hypersensitive response (HR) lesions, i.e. localized programmed cell and tissue death associated with restriction of the pathogen to the infection site. A transient suppression of antioxidants is known to occur during relatively early stages of the HR. Here we show that the transient suppression of a catalase and an alternative oxidase gene during virusinduced local lesion formation (HR) has similar kinetics in different hosts regardless of the extent of leaf necrotization. Both Nicotiana edwardsonii var. Columbia and a paraquat tolerant N. tabacum biotype display significantly less and smaller necrotic lesions in response to inoculation by two viruses ( Tobacco mosaic virus and Tobacco necrosis virus ) in comparison to control plants ( N. edwardsonii and N. tabacum cv. Samsun, respectively). We found that all of these plant hosts display a transient suppression of catalase and alternative oxidase transcript levels starting within six hours after virus inoculation. Our results suggest that the transient decline in antioxidant activity during early stages of an HR does not significantly influence the extent of localized cell death around infection sites.

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In spite of the enormous information from research on genetics of plant disease resistance, the question still remains unresolved: what is directly inhibiting or killing pathogens and suppressing symptoms in resistant plants? This is particularly true for resistance to viral infections. Here we show that externally applied reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) or ROS-producing (O 2 ·− [superoxide] and H2O2) chemical systems infiltrated into tobacco leaves 2 hours after inoculation suppress replication of Tobacco mosaic virus (TMV) in the susceptible Samsun (nn) cultivar. This was determined by a biological and a real-time PCR method. Infiltration of leaves of the resistant Xanthi (NN) cultivar with the ROS-producing chemicals and H2O2 significantly suppressed local necrotic lesions (i.e. the hypersensitive response) after inoculation of tobacco leaves with TMV. Accordingly, an early accumulation or external application of ROS, such as O 2 ·− and H2O2, in tobacco may contribute to the development of resistance to TMV infection.

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Similarities and differences in the immune systems of plants and animals are discussed in relation to non-specific and specific immunity (resistance), systemic acquired resistance (immune memory), transgenerational immune memory and gene silencing. Furthermore, we attempt to answer the question “what is inhibiting or killing pathogens during the immune (resistance) process”? Therefore, the possible roles of reactive oxygen species and antioxidants in pathogen inhibition are evaluated in different types of plant disease resistance.

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High concentrations of the reactive oxygen species (ROS) superoxide (O2 •−) and hydrogen peroxide (H2O2) contribute to the induction of plant cell and tissue death (necrosis). In an effort to create transgenic plants with high antioxidant capacity that could resist necrotic symptoms we produced two transgenic tobacco (Nicotiana tabacum cv. SR1) lines (S1 and S2) overexpressing a tomato chloroplast superoxide dismutase (SlChSOD). SOD genes encode for antioxidant enzymes that dismutate superoxide to hydrogen peroxide. Therefore, SOD-overproducing plants may contain high levels of hydrogen peroxide and are sensitive to stress-related necrosis unless sufficient degradation of hydrogen peroxide is conferred by elevated expression of antioxidants like e.g. catalases and peroxidases. Indeed, line S1 displayed elevated expression of a glutathione peroxidase (NtGPX) and a glutathione S-transferase (NtGSTU1b), as compared to wild type plants. Interestingly, however, expression of a catalase (NtCAT1) was repressed in both SOD-overexpressing lines. This predicts that such plants could be sensitive to localized necrosis (HR) caused by virus infection, since repression of NtCAT1 has been shown to occur during virus-induced HR (e.g. Dorey et al., 1998; Künstler et al., 2007). To elucidate whether other catalases might play a role in resistance to virus induced HR-type necrotic symptoms, a maize catalase (ZmCat2) was transiently overexpressed in Nicotiana edwardsonii and N. edwardsonii var. Columbia plants by agroinfiltration. Inoculation of agroinfiltrated plants with Tobacco mosaic virus (TMV) revealed that ZmCat2 confers enhanced resistance to HR-type necrosis during TMV infection. It seems that catalases may play different roles in influencing resistance to virus-induced hypersensitive necrosis.

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It was shown that reactive oxygen species (ROS) produced by two chemical systems or applied directly can alter symptom expression and block pathogen growth in planta. This was demonstrated for diseases caused by four obligate and three facultative pathogens, respectively. When ROS were applied to the infected plants very early after inoculation, symptoms were fully suppressed. If application of ROS to leaves inoculated with biotrophic pathogens occurred 2-4 days after inoculation, hypersensitive type necrotic symptoms (HR) characteristic for resistant plants appeared in the leaves of susceptible cultivars instead of normal pustules containing mycelia. In the case of diseases caused by facultative pathogens only the size of the necrotic spots were diminished or in some cases no visible necroses were produced. The action of ROS were reversed in some host-pathogen combinations by the application of antioxidants, such as superoxide dismutase (SOD) or catalase and resulted in the development of normal disease symptoms. This indicated that superoxide (O2-) and hydrogen peroxide (H2O2) were the most important ROS involved in the inhibition of pathogen growth in planta and in symptom development.different tobacco lines, including transgenic ones, are grown and exposed to natural infection.

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Biotic and abiotic stresses induce increased formation of reactive oxygen species (ROS) through distinct pathways: pathogen infections activate specific ROS-producing enzymes (i.e. NADPH oxidase, cell wall peroxidases), which results in accumulation of cellular or intercellular ROS, such as superoxide or hydrogen peroxide. Abiotic stresses, on the other hand, cause elevated ROS production principally through an impairment of photosynthetic and respiratory electron transport pathways. Also, these two types of stresses have diverse effects on the antioxidant system of the plant. Results of experiments studying the interaction of abiotic and biotic stresses largely depend on the degree of the applied abiotic stress treatment, the compatible or incompatible host-pathogen interaction and the timing of inoculation in relation to the timing of a preceding abiotic stress treatment.

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