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  • Author or Editor: Y. M. Hafez x
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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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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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Authors: M. Pogány, B. D. Harrach, Y. M. Hafez, B. Barna, Z. Király and E. Páldi

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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Authors: A. A. Czelleng, Z. Bozsó, P. G. Ott, E. Besenyei, G. J. Varga, Á. Szatmári, Y. M. Hafez and Z. Klement

Pseudomonas viridiflava is an opportunistic, post-harvest pathogenic bacterium that causes soft rot of fruits and vegetables. In vivo expression technology was used to identify genes that participate in the pathogenicity of P. viridiflava. Genetic loci that are induced in planta were identified. Ten such loci were partially sequenced and annotated. Here we describe five of them, which influence the pathogen's stress tolerance in planta. Three of the identified ORFs that show sequence identity to known genes encode membrane proteins, the remaining two encode enzymes in catabolic pathways.

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