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- 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.