Hydroxyphenylpyruvate Dioxygenase ( HPPD ): The Herbicide Target

Herbicidal activity through inhibition of phytoene desaturase (PDS) can be easily detected by a striking whitening effect of tissues in newly grown plant leaves in the light. In addition to PDS and Hydroxyphenylpyruvate Dioxygenase (HPPD) inhibitors, other herbicides became known for their bleaching properties: amitrole and clomazone, both inhibiting an early step in carotenoid biosynthesis. This chapter provides an insight into the discovery of the triketone class of herbicides and their continuing development. It discusses a very qualitative picture of structure-activity relationships (SARs), and describes currently commercialized triketones, in terms of their use, weed spectrum, crop selectivity, environmental and toxicological profiles, and manufacture. For simplification, the HPPD inhibitors with Q different from 1,3-cyclohexanedione and its pro-herbicides are summarized as heterocycles. Topramezone is targeted at the post-emergence control of grasses and major broadleaf weeds in corn crops worldwide.