A new mode of herbicidal action was established by finding specific inhibitors of imidazoleglycerol phosphate dehydratase, an enzyme of histidine (His) biosynthesis. Three triazole phosphonates inhibited the reaction of the enzyme with Ki values of 40 f 6.5, 10 f 1.6, and 8.5 2 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. This effect was completely reversed by the addition of His, proving that the cytotoxicity was primarily caused by the inhibition of His biosynthesis. These inhibitors showed widespectrum, postemergent herbicidal activity at application rates ranging from 0.05 to 2 kg/ha.

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