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  • Author or Editor: Z. Király x
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One of the most typical and best studied type of plant resistance is the hypersensitive response (HR), a form of localized programmed cell death associated with restriction or even killing of pathogens that often leads to macroscopically visible localized tissue necrosis. It is generally assumed that cell death and resistance within the HR are physiologically and genetically linked. However, there has been considerable speculation about whether cell death is an absolute requirement for resistance conditioned by the HR. This review discusses the relation of cell death and resistance in the HR, in particular, the importance of cell death in this process. We intend to focus on the increasing amount of research evidence showing that in several plant-pathogen interactions, the two main components of the HR-resistance and cell death-can be physiologically, genetically and temporally uncoupled. In other words, HR should be considered as a combination of resistance and cell death responses, where cell death may be dispensable for the plant resistance response. The varying contribution of these two components (i.e. cell death and resistance) generates an array of defense strategies - ranging from extreme resistance to“systemic HR”- applied by resistant host plants in the battle against pathogen infections.

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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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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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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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The present study investigated how sighted and blind sensory assessors evaluated the quality of various apple varieties: Regal Prince (Gala Must), Jonagold Schneica (Jonica), Watson Jonathan, Golden Reinders and Generos. A total of 80 sighted and 12 blind people participated in the study, in three groups (sighted people without blindfolds, sighted people with blindfolds, blind people). The preference of sensory attributes (size, shape, colour, texture, taste and odour) was evaluated on a six-category scale. The data were analysed using single-factor (ANOVA, LSD95%, 99%) and multi-factor (Cluster analysis, ANOVA, LSD95%, 99%) evaluation. The results showed that blind and sighted assessors made similar judgements on external attributes perceptible by touch (size, shape), but differed in their evaluation of certain quality factors (flavour, taste), resulting in opponent patterns of preference on some apple varieties (Generos, Jonagold Schneica). A further conclusion of the experiment is that there was less deviation between the judgements of sighted assessors when they saw the apples than when they were blindfolded. The background of this phenomenon might be explained by the uncertainty and disturbed perception, which lead to inconsistent judgements.

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Summary  

Excitation functions of proton induced nuclear reactions on natural Te were investigated up to 18 MeV. Cross sections for production of 121,123,124,126,128,130gI and 121gTe were measured. The new experimental data were compared with the results of ALICE-IPPE model calculations and with data found in the literature and measured on natural or enriched Te targets. The new data can be effectively used for validation of recommended cross sections of medically relevant 123I and 124I.

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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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