Bixlozone Metabolism in Crop and Weed Species: A Basis for Selectivity and Evolved Resistance

Bixlozone, a proherbicide that requires in planta activation to its phytotoxic form, 5-ketobixlozone, is used to selectively control annual ryegrass (<i>Lolium rigidum</i>) in Australian cereal and canola crops. Bixlozone resistance has been detected in southern Australian annual ryegrass populations, and this can be increased with recurrent selection. The metabolic fate of bixlozone in young weed and crop seedlings was investigated by liquid chromatography-mass spectrometry to determine if differential metabolism can explain differences in bixlozone response. The observed tolerance of canola and wild radish (<i>Raphanus raphanistrum</i>) was due to the reduced activation of bixlozone to 5-ketobixlozone. In contrast, a resistant annual ryegrass population and tolerant wheat and barley showed preferential conversion of bixlozone to hydroxylated derivatives, whereas susceptible annual ryegrass populations produced more 5-ketobixlozone. Direct application of 5-ketobixlozone to seedlings resulted in an unexpected conversion to hydroxylated metabolites, potentially implicating plant reductases in 5-ketobixlozone metabolism.