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Tobacco Class I and II Catalases Are Differentially Expressed During Elicitor-Induced Hypersensitive Cell Death and Localized Acquired Resistance

Expression of tobacco class I (CAT1) and class II (CAT2) catalases was analyzed in leaves reacting hypersensitively to tobacco mosaic virus (TMV) or to a fungal glycoprotein elicitor. In TMV-infected plants, Cat1 transcript levels declined rapidly while Cat2 transcripts accumulated strongly. The spatial and temporal changes in catalase transcripts, proteins, and activity during the hypersensitive reaction (HR) were further investigated in tobacco leaves infiltrated with a glycoprotein elicitor. Two functionally different zones were discriminated: the infiltrated tissue in which cells undergo the HR, called the HR-zone 1; and the surrounding tissue showing strong induced defense responses, called the LAR (Localized Acquired Resistance)- zone 2. Levels of Cat1 and Cat2 mRNA and proteins and catalase activity decreased in the HR-zone 1. In the LAR-zone 2, Cat1 transcripts became rapidly undetectable, but levels of Cat2 mRNA and protein and catalase activity increased. Catalase expression in elicitorinfiltrated leaves reflected that in TMV-infected leaves. A strong rise in hydrogen peroxide occurred in the HR-zone 1 and paralleled the CAT activity decline. No H₂O₂ increase was measured in the LAR-zone 2. There was no correlation between salicylic acid levels and catalase activity. Modulation of catalase activity in tobacco leaves undergoing the HR appeared predominantly supported by changes in catalase transcripts and proteins. We have shown that neither H₂O₂ nor salicylic acid can be the primary mobile signal diffusing from the HR-zone 1 to the LAR-zone 2 and inducing CAT2 expression. Furthermore, the signaling pathway responsible for decreased CAT2 expression in the HR-zone 1 does not involve reactive oxygen intermediates.