Plastid Alternative Oxidase (PTOX) Promotes Oxidative Stress When Overexpressed in Tobacco

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 oxygenevolving 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. The plastid terminal oxidase (PTOX 2 or IMMUTANS) is a plastid-located plastoquinol:oxygen oxidoreductase (1-3). It is distantly related to the alternative oxidase (AOX) of the mitochondrial inner membrane. The active site of both oxidases, PTOX and AOX, comprises a non-heme di-iron center (4, 5). PTOX is a minor component (ϳ1% of PSII levels in Arabidopsis thaliana) of the thylakoid membrane and is located in the stroma lamellae (6, 7). PTOX plays an important role in carotenoid biosynthesis and seems to be involved in phytoene desaturation reactions (8 -13).The physiological importance of the role of PTOX as plastoquinol oxidase in alternative photosynthetic electron transport pathways is unclear. Evidence that PTOX acts as a plastoquinol oxidase was shown in tobacco plants, which constitutively expressed the A. thaliana PTOX gene (14). It has been suggested that PTOX may serve to keep the photosynthetic electron transport chain relatively oxidized. Exposure of plants to excess light may result in over-reduction of the plastoquinol pool and may lead to photoinhibition (15). However, no major role for PTOX in oxidizing the PQ pool was found in chlorophyll fluorescence assays when thylakoids from wt A. thaliana and the immutans mutant lacking PTOX were compared (16).Recently, 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). Although these findings support the hypothesis that PTOX may serve as a safety valve under stress conditions, t...

Plastid terminal oxidase 2 (PTOX2) is the major oxidase involved in chlororespiration in Chlamydomonas

“…In plants, such a slow oxidation is sufficient to keep the pool oxidized, so that it must be paralleled by an even slower reduction rate. Simply overexpressing PTOX does not provide the expected benefits in Arabidopsis (25) or tobacco (26), whereas the leaf variegation phenotype of im is alleviated at low light intensities by inactivation of NDH and PGR5 (27). Alpine species not only accumulate higher levels of PTOX but also detoxification enzymes (10,11) and NDH (28).…”

Regulation of the photosynthetic apparatus under fluctuating growth light

“…In fact, the thylakoid membrane has an ROS-producing enzyme, the plastid terminal oxidase, which directs electrons from the PQ-pool to molecular oxygen [23]. This reaction, depending on the capacity of the ROS scavenging enzymes, can function either as a safety valve for excess electrons or as a producer of ROS [24,25].…”

Plastid terminal oxidase requires translocation to the grana stacks to act as a sink for electron transport

“…If we could transfer this PTOX activity to crop species, it might be possible to significantly increase abiotic stress tolerance. However, previous attempts at overexpressing PTOX protein have not resulted in a significant increase in electron transport capacity (16)(17)(18) and have in fact been reported to increase oxidative stress (19).…”