Inducing Plastid Terminal Oxidase for Photoprotection

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Plastid terminal oxidase (PTOX) is a plastoquinone oxygen oxidoreductase, localised to the thylakoid membranes of higher plants, algae and cyanobacteria. In plants, PTOX is essential for normal leaf development and PTOX knockout mutants develop with mottled white leaves (IMMUTANS or GHOST mutants). PTOX is believed to be essential as a plastoquinol oxidase during the synthesis of carotenoids. In a small number of extremophile plant species, PTOX has additionally been shown to act as an important sink for electron transport from Photosystem II (PSII), protecting this from oxidative damage and photoinhibition. Attempts have been made to induce this activity in other plants, by overexpressing PTOX, however these have not only failed to induce activity, they have resulted in plants with increased stress sensitivity.

In a BBSRC funded project, we have identified a pathway inducing PTOX activity in a new species, by targeting PTOX to the lumen of the thylakoid. Previously, we showed that PTOX activity is significant in the model halophyte Eutrema salsugineum, a close Arabidopsis relative. In Eutrema, we were able to show that PTOX activity correlated with a relocalisation of PTOX, from the unstacked stromal lamaellae fraction of the thylakoid to the grana stacks. We hypothesised that this relocalisation was necessary to bring PTOX into proximity with PSII, facilitating plastoquinol diffusion. In Arabidopsis, there was no evidence for PTOX migration or either the native or the Eutrema PTOX. We suggest this is due the tight stacking of the grana, preventing PTOX migration. In our preliminary studies, we have seen that targeting PTOX to the lumen overcomes this problem, allowing PTOX to access PSII. Preliminary data not only shows the lumen PTOX can be activated but also that it protects against stress. In this project we will investigate the processes controlling lumen PTOX activity and attempt to transfer this activity to crop species.