Mechanism of Herbicidal Action and Rice Selectivity of Iptriazopyrid: A Novel Azole Carboxamide-Based Inhibitor of 4-Hydroxyphenylpyruvate Dioxygenase

4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are widely used as herbicides. However, the emergence of herbicide-resistant weeds necessitates the development of new herbicides with more diverse chemical structures. Therefore, we evaluated the herbicidal and HPPD inhibitory activities of iptriazopyrid, a novel azole carboxamide compound. Phytotoxic tests on <i>Echinochloa crus-galli</i> demonstrated that iptriazopyrid caused chlorosis at a concentration approximately 10 times lower than that required for the common commercial HPPD inhibitor mesotrione. Furthermore, iptriazopyrid showed strong selectivity for <i>Oryza sativa</i> over the weed in the greenhouse-scale experiment. X-ray crystallography and <i>in vitro</i> inhibitory assays revealed that iptriazopyrid was bound to the enzyme active pocket of HPPD and worked as a slow-binding inhibitor. These findings indicate that this inhibitor has potent herbicidal activity with a chemical structure different from those of existing HPPD inhibitors. Thus, it has potential applications as a novel scaffold in herbicide development.