An enzyme-directed approach has led to finding specific inhibitors of imidazoleglycerolphosphate dehydratase (IGPD), which catalyzes a dehydration reaction of imidazoleglycerol phosphate (IGP) to form imidazoleacetol phosphate in the biosynthesis of histidine. These newly synthesized IGPD inhibitors showed wide-spectrum, postemergence herbicidal activity at application rates ranging from 0.05 to 2 kg ai ha−1. IGPD from wheat germ was purified to apparent homogeneity and characterized, and IGPD cDNAs were isolated from Arabidopsis and wheat cDNA libraries. Both the purified plant enzyme and the recombinant enzymes, which have been prepared by expressing the isolated IGPD cDNAs in the baculovirus/insect cell system, were used for establishment of an in vitro enzyme assay for inhibitor evaluation. Enzyme inhibitors were designed and synthesized by considering the structure of the substrate, IGP, and the inhibition activities were enhanced by a systematic structure—activity relationship study in vitro. Three triazole phosphonates, IRL 1695, IRL 1803, and IRL 1856, inhibited the reaction of the enzyme with Ki values of 40 ± 6.5 nM, 10 ± 1.6 nM, and 8.5 ± 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. Plant cell growth inhibition was completely reversed only by l-histidine, proving that the cytotoxicity was primarily caused by the inhibition of histidine biosynthesis. These new IGPD inhibitors have demonstrated that the specific inhibition of plant IGPD is lethal to plants. Further, treatment of Arabidopsis with IRL 1803 affected gene expression of eight enzymes involved in the biosynthesis of aromatic amino acids, histidine, lysine, and purines. These effects were reversed by the addition of l-histidine, indicating that the changes in gene expression were caused by the inhibition of l-histidine biosynthesis. These results indicate that higher plants, like microorganisms, are capable of cross-pathway metabolic regulation.

The use of winter cover crops is beneficial to agriculture. We have tried to design a cover crop that self-destructs in response to an environmental cue, thereby eliminating the use of herbicides and tillage to remove the cover crop in late spring. Here, this novel concept is tested in a model system. The onset of summer brings with it elevated temperatures. Using this as the environmental cue, a self-destruction cassette was designed and tested in tobacco. A heat-shock–responsive promoter was used to direct expression of the ribonuclease Barnase. Because Barnase is extremely toxic to cells, it was necessary to coexpress its inhibitor, Barstar, whose expression was under the control of the CaMV 35S promoter. The wild-type and two mutated Barnase genes, one missense and one translation attenuated, were tested. Our results indicated that the translation-attenuated version of the Barnase gene was most effective in causing heat-shock–regulated plant death. Analysis of the T2 progeny of a transgenic plant carrying this Barnase mutant showed that the Barnase gene expression was sixfold higher in heat-shock–treated plants compared with untreated plants. This level of Barnase gene expression was sufficient to kill transgenic plants.