Abstract
Photoinhibition and production of reactive oxygen species were studied in tobacco plants overexpressing the plastid terminal oxidase (PTOX). In high light, these plants was more susceptible to photoinhibition than wild-type plants. Also oxygen-evolving activity of isolated thylakoid membranes from the PTOX-overexpressing plants was more strongly inhibited in high light than in thylakoids from wild-type plants. In contrast in low light, in the PTOX overexpressor, the thylakoids were protected against photoinhibition while in wild type they were significantly damaged. The production of superoxide and hydroxyl radicals was shown by EPR spin-trapping techniques in the different samples. Superoxide and hydroxyl radical production was stimulated in the overexpressor. Two-thirds of the superoxide production was maintained in the presence of DNP-INT, an inhibitor of the cytochrome b(6)f complex. No increase of the SOD content was observed in the overexpressor compared with the wild type. We propose that superoxide is produced by PTOX in a side reaction and that PTOX can only act as a safety valve under stress conditions when the generated superoxide is detoxified by an efficient antioxidant system.
Highlights
The plastid terminal oxidase (PTOX2 or IMMUTANS) is a plastid-located plastoquinol:oxygen oxidoreductase [1,2,3]
When attached leaves were exposed to photoinhibitory light (1500 mol quanta mϪ2 sϪ1) a similar loss of variable fluorescence was observed for both, wt and PTOXϩ plants
To investigate whether the PTOXϩ plants were more susceptible to photoinhibition, leaves of wt and PTOXϩ were incubated for 4 h in lincomycin to block the synthesis of D1 and thereby the repair of damaged PSII centers
Summary
Several groups reported that the PTOX level increased under natural stress conditions in several species specialized to harsh environmental conditions This was the case in Ranunculus glacialis, an alpine plant, when it was acclimated to high light and low temperature [17]; in the halophyte Thellungiella halophila when it was exposed to salt stress [18]; and in Brassica fruticulosa when it was exposed to elevated temperature and high light [19]. These findings support the hypothesis that PTOX may serve as a safety valve under stress conditions, they are in direct conflict with the data of Rosso et al [20]. We suggest, based on Western blots, that PTOX can only act as a safety valve and protect against photoinhibition when the level of SOD is adjusted to the actual level of PTOX
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