The development of a novel herbicide

In the next 50 years the global population is expected to increase by 50% and exceed 9 billion. The increased agricultural output needed to feed this growing population cannot be met by bringing more land into food production as no further suitable arable land is available. Therefore, meeting this challenge will require the adoption of highly efficient and sustainable agriculture practices. Efficient agriculture requires the removal of weeds. Current arable farming relies on ploughing to remove weeds by burying them as the soil is turned over. However, ploughing (tillage) removes vegetation cover, disrupts the soil structure and leads to soil erosion by water, wind or both. The Dust Bowl inter-war era in the mid-western United States graphically illustrated the vulnerability of plough-based agriculture as wind blew away precious topsoil from the drought ravaged southern plains whose soil structure had been weakened by ploughing. No-till farming is a branch of conservation agriculture in which the ground is permanently covered by a cover-crop that is left on the fields over winter to protect the soil. The roots of the cover crop hold the soil particles together, strengthening the structure and foliage intercepts rain water to lessen its impact on the soil. In this method of farming, herbicides, rather than the plough, are used for weed control and, prior to planting, the cover crop is degraded by the application of a broad spectrum herbicide and the dead organic matter is left on the surface to degrade naturally. Seeds are then drilled into the ground through the residue in a process that results in minimal disturbance of the soil and improved water retention. A further benefit of no-till is that the cover crop acts as a carbon sink removing CO2 from the atmosphere during photosynthesis with the non-harvested residue being converted to soil organic matter. It has been estimated that approximately half of the overall potential for U.S. croplands to sequester soil carbon comes from conservation tillage. Moreover, as no-till is less mechanised, agricultural diesel use is reduced by 50-80%. No-till therefore offers the potential to off-set fossil fuel emissions in other sectors. It has been suggested that the global introduction of no-till could result in the annual sequestration of 1 billion tons of carbon, 15% of that required to limit atmospheric CO2 to a trajectory that avoids further increases over today's levels. This saving is equivalent to that saved from the global doubling of nuclear capacity replacing coal, or to the combined annual emissions of all the road transport currently in use on the planet. Whilst the benefits of no-till in decreasing soil erosion and promoting carbon capture are clear, its practice relies on the availability of effective herbicides. More importantly, in order to avoid damage to the emerging crop, the herbicide has to have the property of being inactivated when it contacts the soil so that available residues are minimised. However, reliance on compounds that act by inhibiting a limited number of processes in plants has encouraged the emergence of herbicide resistant weeds. Sustainable agriculture therefore requires the development of herbicides having novel modes of action. The search to find new herbicides is now an important area of research. In this project we aim to examine the prospect of developing a new herbicide, that will be targeted against an enzyme, called imidazoleglycerol-phosphate dehydratase, which is responsible for a key step in the synthesis of histidine, an amino acid that is essential for plant growth. We will determine the atomic structures of a number of chemicals that bind to this enzyme, and use this information in a process of design, to chemically synthesize new compounds that will both inhibit the enzyme activity and be inactivated on soil contact. These new compounds will thus kill the weeds in a new way and facilitate sustainable agriculture practices.

The market for herbicides is dominated by compounds which act at a limited number of biological targets and the emergence of herbicide resistant weeds is now recognized as a growing problem in agriculture. Sustainable agriculture therefore requires not only the rotation of herbicides that target different modes of action but also, given the current paucity of biological targets, the development of herbicides having novel modes of action. Histidine is an essential dietary nutrient for animals but is synthesized de novo by plants and microorganisms and thus the pathway of histidine biosynthesis is a potential target for herbicide development. Imidazoleglycerol-phosphate dehydratase (IGPD; EC 4.2.1.19) is a metallo enzyme that catalyses the dehydration of imidazoleglycerol phosphate (IGP) to imidazoleacetol phosphate (IAP) and is a key step in the pathway for the biosynthesis of histidine. Our structural work has shown that the IGPD active site, which lies in the interface between three subunits in the 24-mer, has a di-manganese cluster and a separate phosphate binding site. The enzyme is the target for a novel class of broad spectrum, phloem-mobile herbicides, the triazole phosphonates, that exploit both the manganese cluster, and the phosphate site, which is partially built from an ordering of the C-terminus of the enzyme on substrate or inhibitor binding. In this grant we propose to develop these compounds into potent inhibitors, using a structure based inhibitor design approach, fully exploiting the different molecular aspects of the active site. We will synthesize new variants based on triazole inhibitor compounds, produce site directed and C-terminal truncation mutants of the enzyme, determine the structures of the complete range of inhibitor/enzyme complexes, and characterize the inhibitors biochemically and thermodynamically, using synthetic chemistry routes, molecular biology techniques, X-ray crystallography, spectrophotmetry and ITC calorimetry.

This research proposal is focussed on producing a soil inactivated, phloem mobile, broad spectrum herbicide, acting on a novel target (IGPD). Currently, the market for herbicides is dominated by compounds which act at a limited number of biological targets, with glyphosate as the market leader with annual sales of $4 billion. The emergence of herbicide resistant weeds, including weeds resistant to glyphosate is now recognized as a growing problem in agriculture. There is thus a pressing requirement for the development of novel herbicides to tackle the problem of herbicide resistance. Should a new herbicide be developed as part of this programme, the proposal has the potential to make a significant contribution to the economy of UK PLC. One of the major markets for such herbicides is in no-till agriculture. No-till farming is a branch of conservation agriculture in which the ground is permanently covered by a cover-crop that is left on the fields over winter to protect the soil. Prior to planting, the cover crop is degraded by the application of a broad spectrum herbicide and crop seeds are drilled into the ground through the residue in a process that results in minimal disturbance of the soil and improved water retention. The practice results in equivalent crop yields, dramatically reduced soil erosion, and, as the cover crop acts as a carbon sink removing carbon dioxide from the atmosphere during photosynthesis with the non-harvested residue being converted to soil organic matter, reduces atmospheric carbon dioxide. If no-till agriculture was to be widely adopted then it has been estimated to have a potential to sequester 1 billion tons of carbon a year, the equivalent to the combined emissions of global road transport. In addition, no-till agriculture offers the potential to reduce soil erosion counteracting current losses of one of the World's natural resources. The proposed programme of research, addressing both the fundamental knowledge of how an inhibitor binds to an enzyme, and also the development of a novel herbicide, lies within many of the BBSRC priority areas, will have impacts in a number of diverse fields, industrially, academically, and for society, including (but not restricted to): Global Security Protecting food supply and security. Living with environmental change Developing novel practices to enhance efficiency of agricultural production sustainability in response to environmental change and managing the effects of environmental change on soil systems. Crop Science (Food Security) Fundamental science underpinning the effective and sustainable exploitation of cultivated plants. Economic and Social Impact Raising UK industrial competitiveness, research in collaboration with industrial partners. Impact on Public Policy Mitigating the effects of climate change on UK farming and food security, promoting more sustainable farming