Mechanisms of Resistance to Protoporphyrinogen Oxidase-Inhibiting Herbicides

Resistance to protoporphyrinogen oxidase-inhibiting herbicides can theoretically be generated by at least six different mechanisms. They include: 1) reduction of uptake or sequestration of the herbicide, 2) rapid metabolic destruction of the herbicide, 3) resistance at the site of action, 4) rapid metabolic destruction of protoporphyrinogen IX or protoporphyrin IX to non-photodynamic compounds, 5) inactivation of the herbicide-resistant enzyme that converts protoporphyrinogen IX to protoporphyrin IX, and 6) resistance to toxic oxygen species due to high levels of antioxidants and enzymes that destroy toxic oxygen species. Some of these mechanisms have been reported in cases of naturally-occurring resistance of crops and/or weeds, although resistance at the site of action is not well documented, and mechanism five has not been reported. Despite so many possible mechanisms of resistance, there are no documented cases of evolved resistance to these herbicides as a result of their use, and no crops have been genetically engineered to be resistant to these herbicides.