Investigating the role of reactive oxygen species in herbicidal activity

Abstract

Reactive oxygen species (ROS) are an integral part of stress response found in all celltypes across all organisms. These reactive molecules often act as stress signals and are controlled by a host of antioxidative molecules and enzymes. When a stressor over-induces production of ROS this can result in damage to cells through oxidation of DNA, lipids, and proteins and can result in eventual membrane breakdown and cell death. Many stressors can cause over accumulation of ROS. In this dissertation the main stressors that cause ROS over accumulation are light-dependent herbicides that induce ROS as a primary mode of damage via direct or indirect inhibition photosynthetic electron transport or disruption of the porphyrin pathway. To further our understanding of the role of antioxidants in quenching ROS during herbicidal activity, the first study investigated the response of catalase (CAT) to herbicides in Arabidopsis thaliana. This was done by developing single, double, and triple knockout mutants of the three CAT isoforms in Arabidopsis and comparing their response to different ROS accumulating herbicides. By comparing the cellular localization of the CAT isoforms versus the primary accumulation sites of ROS for the different herbicides we were able to increase our understanding of the differential response of CAT to herbicidal activity. An important part of herbicide physiology research is finding ways to increase the efficacy of herbicide applications, which can be done by the inclusion of surfactants that caniii increase the amount of herbicide that can reach the target site of inhibition. Our study compared three nonionic surfactants to investigate which would provide the greatest increase in fomesafen activity, thereby the greatest increase in protoporphyrinogen oxidase (PPO) inhibition and resulting over accumulation of ROS and membrane breakdown. Use of surfactants increased fomesafen efficacy in dose response, herbicide uptake and contact angle, and increased products of inhibition. The surfactant EXT1649, specifically, showed the highest trend of increase in efficacy for response, uptake, and increased products of inhibition, showcasing that the lowest contact angle does not necessarily result in higher increased efficacy. To increase the efficacy of herbicides, application of multiple herbicides together can be used to target multiple pathways, increase the penetration and absorption of herbicide, or distract the metabolic system from metabolizing an herbicide for continued activity. If this combination of herbicides can result in enhanced weed control, where the efficacy of combination exceeds that of the sum of individual treatments, it can be a useful tool for increased weed control as well as decreased rate of herbicide resistance evolution. Our study investigated the use of a combination of amicarbazone and metribuzin, two photosystem II (PSII) inhibiting herbicides that result in an over accumulation of ROS due to dysfunction of the photosynthetic electron transport chain (PETC). Since these herbicides have the same mode of action, the interaction at the target site level was only additive and was consistently additive at the whole plant level as well. Herbicide detection levels may provide evidence of possible metabolic interactions leading to decreased metabolism of one herbicide over the other when used in combination.