ProtoporphyrinogenIXOxidase Inhibitors

The mode of action of Protox herbicides is via an inhibition of the enzyme protoporphyrinogen oxidase, the last common enzyme to both heme and chlorophyll biosynthesis. This chapter discusses the recent developments and challenges in the field of Protox inhibiting agrochemicals. The phenyl tetrahydrophthalimides are primarily post-emergence herbicides used to control broadleaf weeds, although they will show pre-emergence activity at higher rates of application. The chapter also discusses the unusual replacement of the phenyl portion of Protox herbicides with a pyrazole ring to give pyraclonil. Weeds resistant to commonly used herbicides, in particular glyphosate, have become a major problem in important row crops such as corn, soybean, and cotton. For flumioxazin, it was found that exposure to the active ingredient causes protoporphyrin IX accumulation in rat embryos and that the peak period of protoporphyrin IX accumulation correlated to that of developmental effects.