Inducing Plastid Terminal Oxidase for Photoprotection
Lead Research Organisation:
University of Manchester
Department Name: Earth Atmospheric and Env Sciences
Abstract
Food security is one of the greatest challenges facing humanity. Growing populations and changing diets are increasing food demand at a time when human-induced climate change is making weather less predictable, threatening crop production. Episodes of drought, flooding, high and low temperatures, even for relatively short periods, can all undermine final crop yields. Against this background, there is an urgent need to breed crops which combine high productivity with the ability to tolerate environmental stress.
One of the main primary targets of environmental stress is photosynthesis. Photosynthesis is the process by which plants capture light energy and use that energy to fix carbon dioxide from the air, producing sugars. Photosynthesis is the ultimate source of all the food we eat. When plants are stressed, imbalances can occur between the amount of energy a leaf absorbs and the amount that can be used in photosynthesis. When this happens, the excess energy can result in the production of harmful molecules called reactive oxygen species (ROS; including for example the bleach, hydrogen peroxide). These ROS can damage the cell, destroying membranes, proteins and DNA.
AAcross the plant kingdom we see a range of mechanisms that help protect plants from ROS. Plants contain high concentrations of antioxidants, such as Vitamins A and E, which are essential components of the human diet. They also possess regulatory mechanisms that prevent ROS production. One example, so far only seen naturally in a handful of extreme stress tolerant plants, is called the Plastid Terminal Oxidase, or PTOX. In stress tolerant plants, such as the cabbage relative salt cress (Eutrema salsugineum, in the brassica family), PTOX acts as a safety valve for photosynthesis, dissipating excess energy harmlessly as water, avoiding ROS production. PTOX has not however been seen in common crop species. Previous attempts to use genetic modification to induce PTOX in other species have not only failed, they have made matters worse, increasing rather than preventing stress.
In a recent breakthrough, we have shown it is possible to induce activity of PTOX in a new species, by targeting the protein to a particular cellular compartment called the thylakoid lumen. Lumen-targeted PTOX is not constitutively active, but becomes active under stress conditions. We have shown that this activity, seen previously in salt cress, can be transfered to another brassica species, thale cress. In this grant, we will examine the factors that are necessary for the stress-induced activation of lumen-targeted PTOX. We will also attempt, using the same approach, to induce PTOX in important crop species - oilseed rape (another brassica), soybean (a legume) and wheat and barley (grasses). If successful, this approach will pave the way to generate crop plants with improved stress tolerance, increasing crop yields under extreme environmental conditions.
One of the main primary targets of environmental stress is photosynthesis. Photosynthesis is the process by which plants capture light energy and use that energy to fix carbon dioxide from the air, producing sugars. Photosynthesis is the ultimate source of all the food we eat. When plants are stressed, imbalances can occur between the amount of energy a leaf absorbs and the amount that can be used in photosynthesis. When this happens, the excess energy can result in the production of harmful molecules called reactive oxygen species (ROS; including for example the bleach, hydrogen peroxide). These ROS can damage the cell, destroying membranes, proteins and DNA.
AAcross the plant kingdom we see a range of mechanisms that help protect plants from ROS. Plants contain high concentrations of antioxidants, such as Vitamins A and E, which are essential components of the human diet. They also possess regulatory mechanisms that prevent ROS production. One example, so far only seen naturally in a handful of extreme stress tolerant plants, is called the Plastid Terminal Oxidase, or PTOX. In stress tolerant plants, such as the cabbage relative salt cress (Eutrema salsugineum, in the brassica family), PTOX acts as a safety valve for photosynthesis, dissipating excess energy harmlessly as water, avoiding ROS production. PTOX has not however been seen in common crop species. Previous attempts to use genetic modification to induce PTOX in other species have not only failed, they have made matters worse, increasing rather than preventing stress.
In a recent breakthrough, we have shown it is possible to induce activity of PTOX in a new species, by targeting the protein to a particular cellular compartment called the thylakoid lumen. Lumen-targeted PTOX is not constitutively active, but becomes active under stress conditions. We have shown that this activity, seen previously in salt cress, can be transfered to another brassica species, thale cress. In this grant, we will examine the factors that are necessary for the stress-induced activation of lumen-targeted PTOX. We will also attempt, using the same approach, to induce PTOX in important crop species - oilseed rape (another brassica), soybean (a legume) and wheat and barley (grasses). If successful, this approach will pave the way to generate crop plants with improved stress tolerance, increasing crop yields under extreme environmental conditions.
Technical Summary
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.
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.