HERBICIDAL ACTIVITY OF AN ISOPROPYLMALATE DEHYDROGENASE INHIBITOR

Isopropylmalate dehydrogenase (IPMDH) is the third enzyme specific to leucine biosynthesis. It catalyzes the oxidative decarboxylation of 3- isopropylmalate (3-IPM) to 2-ketoisocaproic acid. The partially purifi ed enzyme from pea (Pisum sativum L.) shows a broad pH optimum of 7.8 to 9.1 and has K-m values for 3-IPM and NAD of 18 and 40 mu M, respect ively. O-Isobutenyl oxalyIhydroxamate (O-IbOHA) has been discovered to be an excellent inhibitor of the pea IPMDH, with an apparent inhibito r constant of 5 nM. As an herbicide, O-IbOHA showed only moderate acti vity on a variety of broadleaf and grass species. We characterized the herbicidal activity of O-IbOHA on corn (Zea mays L.), a sensitive spe cies; giant foxtail (Setaria faberi) and morning glory (Ipomoea purpur ea [L.] Roth), moderately tolerant species; and soybean (Glycine mdu L . Merr.), a tolerant species. Differences in tolerance among the speci es were not due to differences in the sensitivity of IPMDH. Studies wi th [C-14]O-IbOHA suggested that uptake and translocation were not majo r limitations for herbicidal activity, nor were they determinants of t olerance. Moreover, metabolism could not account for the dif ference i n tolerance of corn, foxtail, and morning glory, although it might acc ount for the tolerance of soybean. Herbicidal activity on all four spe cies was correlated with the accumulation of 3-IPM in the plants.