Critical Roles for Cytochromes P450 in Sustainable Wheat Production

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The efficient production of wheat, the major crop in the UK and a globally important staple cereal, is highly dependent on the use of selective herbicides that control the growth of competing weeds, particularly troublesome wild grasses such as ryegrass, wild oat, blackgrass and bromes. Failure to control these weeds causes major crop losses that can be crippling to an already hard-pressed sector. A superfamily of enzymes called cytochromes P450 (CYPs) play essential roles in controlling herbicide selectivity, as they can detoxify a wide range of chemistries through catalysing their oxidative metabolism, thereby allowing the compounds to be further modified and safely stored. Normally, in wheat, CYPs rapidly detoxify herbicides, whereas in the competing grasses these reactions occur more slowly and this is the basis of differential toxicity to the weeds. This selectivity can be further enhanced by treating the wheat with safeners, compounds that enhance CYP expression in crops and accelerate herbicide detoxification. Due to the repeated use of a relatively small number of chemistries, grass weeds have started to evolve multiple herbicide resistance (MHR), due to the selection of populations that have elevated expression of detoxifying enzymes. Among these detoxification processes in MHR, CYPs appear to play a dominant role. MHR is now widespread in the UK, particularly in blackgrass and ryegrass and is causing major crop losses and threatens the continued production of winter wheat. In this project, working with the agrochemical company Syngenta we will characterise the complement of CYPs in wheat and competing grass weeds that control selectivity and resistance to herbicides. The approach we have adopted is based on recent advance in the applicant's laboratory that have made it possible to routinely clone and express CYPs from plants and assay them for detoxifying activities toward herbicides using the power of mass spectrometry to sensitively detect metabolites. Using the latest RNA sequencing technology we will characterise the range of CYP genes expressed in wheat and the four problem grasses, with those CYPs associated with MHR in weeds and safening in wheat being of particular interest. After expressing the coding sequences in yeast, we will assay the recombinant enzymes for their activities toward herbicides. Armed with this inventory of characterised CYPs we will then investigate their functions in crops and weeds. Prioritised questions will be which CYPs are particularly associated with safening in crops and what differences there are with the responses seen in weeds and can different patterns of CYP expression account for the distinct sub-types of MHR now being reported in wild grasses. We will then investigate where these enzymes are expressed in wheat and weeds as it relates to where herbicide detoxification occurs in plants and then assess how CYPs function with other enzymes and transporters involved in foreign compound metabolism. Finally we will carry out studies to look for the natural substrates of CYPs, on the premise that they did not originally evolve to detoxify herbicides. At the conclusion of the project we will be in a position, for the first time, to overview the roles of CYPs in herbicide metabolism selectivity and resistance in wheat and competing weeds. The information derived from the study will help in the development of new selective herbicides and resistance busting synergists that are now desperately needed by the UK cereal industry to counteract the steady rise of MHR grass weeds and improve wheat productivity and profitability in the UK.