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

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 H 2 O 2 , play a central role in the activation and propagation of pathogen-induced PCD in plants. However, plants contain several mechanisms that detoxify and H 2 O 2 and may inhibit PCD. We found that during viral-induced PCD in tobacco, the expression of cytosolic ascorbate peroxidase (cAPX), a key H 2 O 2 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 H 2 O 2 , 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 H 2 O 2 , which in turn enables the accumulation of H 2 O 2 and the acceleration of PCD. INTRODUCTIONThe antimicrobial response of plants and animals is often accompanied by a coordinated activation of programmed cell death (PCD) and defense mechanisms (Zychlinsky et al., 1992;Hacker and Vaux, 1994;Greenberg, 1996;. In plants, this response is termed the hypersensitive response (HR) and results in the formation of a zone of dead cells around the infection site, the synthesis of salicylic acid (SA), and the accumulation of antimicrobial agents, such as pathogenesis-related (PR) proteins and phytoalexins (Goodman and Novacky, 1994;Dangl et al., 1996; HammondKosack and Jones, 1996). The layers of dead cells that surround the site of pathogen entry are thought to function as a physical barrier that inhibits further proliferation and spread of the pathogen (Goodman and Novacky, 1994).PCD that occurs during the HR is accompanied by an increase in the production of reactive oxygen species (ROS) and lipid peroxidation (Doke and Ohashi, 1988;Mehdy, 1994;Hammond-Kosack and Jones, 1996;May et al., 1996; Yang et al., 1997). Recent studies have indicated that reactive oxygen intermediates in the form of H 2 O 2 and are key mediators of PCD during the HR (Levine et al., 1994, O 2 • Ϫ 1996; Jabs et al., 1996;Draper, 1997;Shirasu et al., 1997) and may function as part of a signal transduction pathway leading to the induction of PR proteins and systemic resistance in infected and noninfected parts of the plant (Chen et al., 1993;Green and Fluhr, 1995). Although the role of ROS in mediating systemic resistance was recently challenged (Bi et al., 1995;Leon et al., 1995;Neuenschwander et al., 1995), considerable evidence i...

Interactions between mitochondrial electron transport, reactive oxygen species, and the susceptibility of Nicotiana tabacum cells to programmed cell death

“…This is in keeping with studies demonstrating that a cell committed to cell death displays a decreased expression and activity of H 2 O 2 scavenging enzymes (Mittler et al 1998;Vacca et al 2004). While this result is usually interpreted to suggest that ROS-scavenging capacity is reduced to boost a deathpromoting ROS signal, another interpretation is that their decline results simply because the ROS signal, likely important to maintain their expression, is declined.…”

“…For example, alteration of ROS metabolism by genetic manipulation of key ROSscavenging components of the cell has been shown to alter susceptibility to diverse PCD-inducing signals (Kokoszka et al 2001;Dat et al 2003;Murgia et al 2004;Pavet et al 2005;Tarantino et al 2005). Similarly, active changes in ROS-scavenging capacity have been noted during PCD activation (Mittler et al 1998;Fath et al 2001Fath et al , 2002Vacca et al 2004). Such studies are broadly in keeping with the concept of ''oxidative signaling'', whereby specific ROS species may act to appropriately adjust the gene expression, metabolism and physiology of cells (Apel and Hirt 2004;Foyer and Noctor 2005).…”

Nitric oxide, induced by wounding, mediates redox regulation in pelargonium leaves

“…(2002). Thus, the suppression of APX and CAT appears common to both the HR (Mittler et al. 1999; Bestwick et al.…”

Seleno‐independent glutathione peroxidases

“…A decrease in gene expression during an incompatible interaction has also been observed for an ascorbate peroxidase, another antioxidant enzyme [137]. This down‐regulation of antioxidant genes favours AOS accumulation, which is necessary to promote the hypersensitive response.…”