Vernon A. Wittenbach

Basis for soybean tolerance to thifensulfuron methyl

Abstract Thifensulfuron methyl (methyl 3-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbonyl]aminosulfonyl]-2-thiophenecarboxylate, formerly DPX-M6316) is a sulfonylurea herbicide for postemergence broadleaf weed control in wheat, barley, and soybeans. Plant response data are presented demonstrating > 12-fold soybean tolerance to thifensulfuron methyl relative to sensitive weeds, including Amaranthus retroflexus L., Chenopodium album L., and Abutilon theophrasti Medik. Studies of foliar uptake and in vitro inhibition of acetolactate synthase (ALS) discount differential uptake or differential active site sensitivity as the basis for soybean tolerance to this herbicide. Instead, excised soybean ( Glycine max , cv “Williams”) seedlings are found to rapidly metabolize a transpired pulse of thifensulfuron methyl with a half-life of 4–6 hr while sensitive weeds metabolize this herbicide much more slowly. The primary metabolite of thifensulfuron methyl in soybean seedlings is its deesterified free acid (thifensulfuron acid), which is herbicidally inactive and inactive against ALS. Metabolism rates and pathways of thifensulfuron methyl, chlorimuron ethyl, and metsulfuron methyl in soybean seedlings are compared and discussed. It is concluded that soybean tolerance to thifensulfuron methyl is based on rapid metabolic deesterification and that this deesterification reaction is specific to certain sulfonylurea herbicides.

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 purified enzyme from pea (Pisum sativum L.) shows a broad pH optimum of 7.8 to 9.1 and has Km values for 3-IPM and NAD of 18 and 40 [mu]M, respectively. O-Isobutenyl oxalylhydroxamate (O-IbOHA) has been discovered to be an excellent inhibitor of the pea IPMDH, with an apparent inhibitor constant of 5 nM. As an herbicide, O-IbOHA showed only moderate activity on a variety of broadleaf and grass species. We characterized the herbicidal activity of O-IbOHA on corn (Zea mays L.), a sensitive species; giant foxtail (Setaria faberi) and morning glory (Ipomoea purpurea [L.] Roth), moderately tolerant species; and soybean [Glycine max L. Merr.), a tolerant species. Differences in tolerance among the species were not due to differences in the sensitivity of IPMDH. Studies with [14C]O-IbOHA suggested that uptake and translocation were not major limitations for herbicidal activity, nor were they determinants of tolerance. Moreover, metabolism could not account for the difference in tolerance of corn, foxtail, and morning glory, although it might account for the tolerance of soybean. Herbicidal activity on all four species was correlated with the accumulation of 3-IPM in the plants.

Isolation of mesophyll and paraveinal mesophyll protoplasts from soybean leaves

Abstract Mesophyll (MP) and paraveinal mesophyll protoplasts (PVMP) have been prepared with high yields from primary and trifoliate leaves of soybean, using a Cellulase CEL or CELF and Pectolyase Y-23 enzyme mixture. The PVMP, which are specialized for nitrogen metabolism and storage, are larger than MP and contain very few chloroplasts. This gives rise to considerable difference in the buoyant density of the two protoplast types, which has been exploited to obtain pure preparations of each. The MP and PVMP preparations were free of cellular debri and were stable for several days on ice. The purity of the preparations was further indicated by radial immunodiffusion assay using antibody to a glycoprotein specifically located in the paraveinal mesophyll (PVM) vacuole.