Physiological and biochemical characterization of glyphosate resistant Ambrosia trifida L.

Giant ragweed (Ambrosia trifida L.) is an annual plant that persists in crop and non-crop areas. Agronomic producers often use glyphosate, the most widely used herbicide in the world, to control giant ragweed in glyphosate resistant crops. Glyphosate inhibits EPSP synthase, the key enzyme of the shikimate pathway. Repeated use of glyphosate has resulted in the occurrence of glyphosate resistant giant ragweed populations in Indiana. Initial studies confirmed glyphosate resistance in a giant ragweed population from Noble County, IN at multiple rates (0.7 kg ae ha−1, 1.4 kg ae ha−1, and 2.8 kg ae ha−1) and growth stages. Prior to glyphosate treatment, no observational differences existed between the sensitive and resistant populations, however, after treatment a unique phenotype with rapid necrosis of mature leaves was observed in resistant plants. To classify resistance, a method was developed using fluorescence to measure photosynthetic activity. Rapid mature leaf necrosis occurred on resistant but not sensitive plants within 12 hours of glyphosate treatment. Biochemical experiments, investigating possible mechanisms for glyphosate resistance, showed no difference in EPSPS protein abundance or transcript levels between resistant and sensitive plants. 14C-glyphosate uptake and translocation showed resistant plants translocated 28% of absorbed glyphosate whereas sensitive plants translocated 54% of absorbed glyphosate. NMR studies using glyphosate infiltrated leaves indicated rapid cell death in resistant plants but not in sensitive plants. To further investigate if the rapid necrosis mechanism was associated with the hypersensitive disease response pathway in plants, mass spectroscopy was used but showed no difference between glyphosate sensitive and resistant giant ragweed plant in salicylic acid and jasmonic acid levels after mechanical wounding. Results show glyphosate resistant giant ragweed plants have a rapid cell death response that limits herbicide translocation to sink tissues allowing plant survival