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  • Author or Editor: A. Künstler x
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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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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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Fatty acid hydroperoxide-producing lipoxygenase (LOX) and hydroperoxide-degrading glutathione peroxidase (GPOX) enzyme activities were studied in leaves of virus resistant Xanthi-nc and susceptible Samsun-nn tobacco cultivars after inoculation with Tobacco mosaic virus (TMV). Total LOX activity showed a maximum at pH 5.5 in cell-free extracts of uninfected leaves. LOX activity markedly increased at this pH after TMV inoculation, but a substantial induction was detected also in the basic pH range with an emerging peak around pH = 8.5. TMV-elicited LOX induction was weaker and appeared later in Samsun-nn than in Xanthi-nc leaves. GPOX activity was also substantially induced by TMV infection. However, this induction appeared only 4 days post-inoculation in resistant Xanthi-nc plants in tissues surrounding the localized necrotic lesions. In contrast, GPOX activity did not change in TMV-inoculated, susceptible Samsun-nn leaves. Several glutathione S-transferase (GST) isoenzymes also display GPOX activity. The expression of a tau class GST gene was markedly induced by TMV inoculation in Xanthi-nc leaves. This tobacco GST gene was partially cloned and sequenced.

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