Plastid terminal oxidase - a route to improving food security

The most important challenge facing humanity in the 21st century ensuring that the growing world population can continue to be fed in a reliable way, against a backdrop of changes in climate induced by human activities, especially the burning of fossil fuels. Crops are especially vulnerable to fluctuations in weather conditions, with events such as droughts, heat waves and floods all liable to reduce yields. As such weather events are expected to increase in the future, we urgently need to breed plants with a greater capacity to tolerate these.

The process of photosynthesis is at the forefront of plant interactions with their environment. Plants absorb light energy using molecules called chlorophylls, and use that energy to make sugars which feed the plant. Extreme weather does not stop light being absorbed, but it can stop it being used. When this occurs, the unused energy can instead cause damage to the leaf. In particular, it can give rise to the formation of highly reactive molecules, such as the bleach hydrogen peroxide. These can, if not controlled, destroy the leaf.

The aim of this project is to investigate a new mechanism which protects plants from excess light. We have been working with a plant called salt cress (Eutrema salsugineum) which is a species found growing salt marshes. Salt cress is a relative of important crop plants such as cabbage and oil seed. We have identified a protein in this plant called the Plastid Terminal Oxidase (or PTOX for short) which we have shown can act as a safety valve for photosynthesis, dissipating excess light energy, when this cannot be used. PTOX has the potential therefore to improve stress tolerance in crop plants. When we take the gene for PTOX from salt cress and transfer it to another species, however, we are unable to detect any activity. We have recently shown that activity requires a change in the localisation of the PTOX protein in the leaf cells, moving it to a different compartment where it has not previously been seen.

The aim of this project is to understand the processes involved in the relocalisation of PTOX in Eutrema and to attempt to transfer these to another species. We will use a combination of state of the art protein and gene expression analysis, combined with electron microscopy and genetic engineering to characterise the mechanisms involved in moving PTOX and to identify new genes which maybe required for this. At the end of the project we will have gathered new information to define the processes involved and will have tested certain genes to see if they play a role in the activation of PTOX.

Photosynthesis is a key target to ensure food security. We need to increase stress tolerance in crops, making them better able to tolerate extreme conditions, such as drought and heat stress. Environmental stress can result in the production of reactive oxygen species (ROS) in the chloroplast, as light absorption exceeds the ability to assimilate the energy captured.

Plastid terminal oxidase (PTOX) has been identified as a candidate to provide protection against environmental stress. PTOX is a plastoquinone oxidase shown to act, in some species, as a sink for electron transport from Photosystem (PS) II, limiting ROS production. Transferring activity into stress-sensitive species is suggested as a pathway to induce stress tolerance, however, various attempts in different species have so far failed to demonstrate significant activity of over expressed protein.

Recently, we obtained exciting data which both explains the previous failures and opens up new avenues to generate stress tolerant crops. In the halophyte Eutrema salsugineum we detect a substantial flux of electrons from PSII to PTOX under salt stress conditions. Over expression of PTOX does not induce activity but the induction rate and extent of activity is increased in these plants when exposed to salt, implying an activation step is required. We have shown that this activation involves the translocation of the protein in the chloroplast from the stromal lamellae to the grana membranes.

The aim of this project is to investigate the mechanism of activation of PTOX and to identify candidate genes involved. We will use a combination of microscopy, mass spectrometry and transcriptomics to characterise the activaiton process and to identify candidate genes involved in this activation. Selected candidates will be validated in Eutrema and Arabidopsis.

Who will benefit from this project? This project aims to elucidate the mechanisms by which the model stress tolerant plant Salt Cress (Eutrema salsugineum) is able to tolerate environmental stresses such as salt, drough and extreme temperatures. It will contribute to the identification of novel processes which it may then be possible to breed into crop species to improve their stress tolerance, so contributing to improving global food security. As such, it will be of direct relevance to crop breeders and, through them to farmers. Ultimately the work will benefit not only farmers, but the whole community, as it will help towards ensuring a plentiful and safe food supply in times of changing climate. It will contribute to meeting the objectives of the research councils, relating to food security and living with climate change, and as such will benefit government and policy makers.

How might they benefit from this project? This work will be contributing new knowledge and datasets which will be widely available through open access peer reviewed scientific papers. As such the data will be directly available to applied scientists in the crop breeding sector, by providing novel traits, markers and genes which might be used in breeding programmes using conventional or GM strategies. In the long term this will lead to the generation of new crop varieties with greater stress tolerance and which can be grown in more marginal locations, allowing more secure incomes for farmers especially in developing world countries. The general public will benefit then through the secured availability of food crops.

In addition to the long term potential of the work to contribute to food security, activities undertaken as part of the impact plan will directly contribute to improved public understanding of plant responses to environmental stress and of stress tolerance in wild and crop plants. This will be achieved by running events for the public which showcase the work being carried in the wider context of food security.