Selective chemical regulation of plant metabolism with herbicide safeners

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Safeners are used in conjunction with herbicides in major cereal crops such as wheat, maize and rice to enhance their selective weed control. This is achieved by the safeners acting to increase the expression of herbicide detoxifying proteins by up-regulating the expression of the respective genes in the crop. Recently, we have determined that safeners work in a species specific manner, with each crop responding to a specific class of safener chemistry. We have also determined that the effects of safeners are not restricted to herbicide detoxifying proteins, but extend to a subtle modification of endogenous plant metabolism, notably through altering the production of phenolics and thiol antioxidants. Safeners are therefore bioactive synthetic compounds which are able to selectively reprogramme elements of plant metabolism which are important in counteracting stress caused by chemicals and/or oxidative damage. While the mechanism of safener action is partly understood at the level of the metabolic responses, nothing is known as to how these synthetic chemicals are recognised by plants to initiate specific cellular signalling events. Based on chemical principles, we propose that modern safeners can be grouped into three major classes of reactivity, all of which will lead to the selective covalent modification of reactive residues on the surface of proteins. We propose that such modification of regulatory proteins causes changes in their activity or interactions which initiate signalling pathways leading to the selective activation of genes encoding antioxidant proteins. The objectives of this programme are to use a combination of synthetic chemistry and biochemistry to identify which signalling-proteins become selectively labelled with safeners . This will entail synthesising safeners labelled with recognition tags and then using these 'chemical hooks' to identify the proteins targeted, thereby identifying how these chemical selectively intervene in plant biochemistry. We will concentrate our efforts on identifying these initial binding events in the model plant species Arabidopsis thaliana, which uniquely among the plants we have studied is selectively responsive to all three classes of safener chemistry. Using different synthetic strategies, the chemical probes will be designed to work in protein extracts and in whole plants, with the tagged proteins then identified from their sequence using methods based on mass spectrometry. As part of the programme we will confirm the roles of these proteins in safener-recognition in Arabidopsis by determining the effect of knocking-out their expression on their responses to the three chemistries under test. We will also determine whether or not similar proteins become labelled in cereals to confirm our results in agriculturally important species and develop chemical variants of existing safeners to determine how this affects their selective enhancement of antioxidant responses in Arabidopsis and the crop species. At the conclusion of our interdisciplinary programme we will have determined for the first time how an important group of agrochemicals exert their selective action in plants. Defining their site of action has important implications in identifying new generations of safeners with improved activities to ensure the future efficient and sustainable use of herbicides in crop protection. In addition, understanding how safeners work will allow us to extend their use in promoting resistance to chemical and oxidative stress in other plants which has potential for applications in bioremediating contaminated land.