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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 O2.- and H2O2, play a central role in the activation and propagation of pathogen-induced PCD in plants. However, plants contain several mechanisms that detoxify O2.- 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.

Application of exogenous ethylene, irrespective of the method of application, caused intensification of mesocarp discoloration in avocado fruit (Persea americana Mill.) during cold storage of all cultivars tested. 'Ettinger' fruit treated with Ethrel (2-chloroethyl phosphonic acid) prior to packing and storage developed severe chilling injury (CI) symptoms, expressed as mesocarp discoloration after 3 weeks at 5°C. 'Fuerte' fruit treated with ethylene gas (100 ml l − 1 ) for 24 h at 20°C prior to storage at 5°C exhibited mesocarp discoloration, which increased dramatically during shelf life at 20°C. 'Fuerte' fruit treated in cold storage with a continuous low ethylene dose (4 ml l − 1 ) developed severe browning in the fruit pulp after 3 weeks at 5°C. 'Hass' fruit treated with 50 ml l − 1 ethylene, for 12, 24 or 48 h at 5°C showed a gradual increase in mesocarp discoloration after 3 weeks in cold storage plus shelf life; the 48 h ethylene-treated fruit exhibited the most severe pulp browning. Use of absorbent sachets that removed ethylene from modified atmosphere (MA) packaging reduced mesocarp discoloration and decay development in 'Hass' fruit after 5 weeks storage at 5°C. Application of 1-methylcyclopropene (1-MCP), reduced mesocarp discoloration, decay development and polyphenol oxidase activity, whereas this enzyme activity was induced in ethylene-treated fruits that were cold stored for 4 weeks.

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