Post-Transcriptional Suppression of Cytosolic Ascorbate Peroxidase Expression during Pathogen-Induced Programmed Cell Death in Tobacco

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Abstract
As a means to eliminate pathogen-infected cells and prevent diseases, programmed cell death (PCD) appears to be a defense strategy employed by most multicellular organisms. Recent studies have indicated that reactive oxygen species, such as and H2O2, play a central role in the activation and propagation of pathogen-induced PCD in plants. However, plants contain several mechanisms that detoxify and H2O2 and may inhibit PCD. We found that during viral-induced PCD in tobacco, the expression of cytosolic ascorbate peroxidase (cAPX), a key H2O2 detoxifying enzyme, is post-transcriptionally suppressed. Thus, although the steady state level of transcripts encoding cAPX was induced during PCD, as expected under conditions of elevated H2O2, the level of the cAPX protein declined. In vivo protein labeling, followed by immunoprecipitation, indicated that the synthesis of the cAPX protein was inhibited. Although transcripts encoding cAPX were found to associate with polysomes during PCD, no cAPX protein was detected after in vitro polysome run-off assays. Our findings suggest that viral-induced PCD in tobacco is accompanied by the suppression of cAPX expression, possibly at the level of translation elongation. This suppression is likely to contribute to a reduction in the capability of cells to scavenge H2O2, which in turn enables the accumulation of H2O2 and the acceleration of PCD.