The sulfonylurea herbicide chlorsulfuron blocks the biosynthesis of the amino acids valine and isoleucine in plants. Addition of these two amino acids to excised pea root (Pisum sativum L. var Alaska) cultures incubated in the presence of chlorsulfuron completely alleviates herbicide-induced growth inhibition. The site of action of chlorsulfuron is the enzyme acetolactate synthase which catalyzes the fist step in the biosynthesis of valine and isoleucine. This enzyme is extremely sensitive to inhibition by chlorsulfuron having Iso values ranging from 18 to 36 nanomolar. In addition, acetolactate synthase from a wide variety of tolerant and sensitive plants species is highly sensitive to inhibition by chlorsulfuron.The selective sulfonylurea herbicide, chlorsulfuron (Fig. 1), represents a major advancement in weed control technology combining high herbicidal activity with outstanding tolerance to cereal crops and very low mammalian toxicity (3). Sensitive weeds are controlled at 10 to 20 g/ha which is equivalent to 1 to 2 tablespoons of active ingredient per acre. Chlorsulfuron acts to inhibit plant growth by blocking some process necessary for plant cell division (7-9). Recent studies have shown that another sulfonylurea herbicide, sulfometuron methyl, which is the active ingredient in Oust'( Weed Killer (Fig. 1 MATERIALS AND METHODS Pea Root Culture. Peas (Pisum sativum L. var Alaska) were surface sterilized by two 3-min soakings with gentle stirring in commercial bleach (5.25% NaOCI). After surface sterilization, the peas were sown in moist sterile vermiculite and incubated under sterile conditions for 72 h at 25 'C. Root sections, 10 mm long, including the root tip were excised from the resulting seedlings and grown in White's medium (12) supplemented with 20 g/l of sucrose as described by Van't Hof (10). All manipulations were carried out under sterile conditions. The pH of the culture medium was adjusted to 6.5 prior to autoclaving. Stock solutions of chlorsulfuron were prepared to a concentration of 500 mg/l in tetrahydrofuran. Aliquots of this stock were taken to dryness under nitrogen in small vials and the chlorsulfuron was then dissolved in 5 mM K2HPO4 (pH 7.5), diluted with phosphate buffer, and filter sterilized. These precautions were taken to avoid introducing organic solvents into the root cultures. Filter sterilized solutions of chlorsulfuron, casein hydrolysate (acid hydrolyzed), and L-amino acids were added to freshly inoculated sterile cultures of pea roots under sterile conditions. Culture inoculum consisted of 10 root sections which were added to 50 ml of White's medium in 250 ml Erlenmeyer flasks (12). The roots were grown in the dark at 25°C with gentle gyrotory shaking (60 rpm) and were collected after 96 h of incubation. Growth was determined by measuring root length. The net growth of the roots was determined by subtracting the initial 10 mm from the final length.Whole Plant Culture. Pea seeds were surface sterilized by two 3-min rinses in full strength commercial bleach (5.25% ...
Several mutants resistant to the herbicides chlorsulfuron and sulfometuron methyl were isolated form cultured cells of Nicotiana tabacum. Resistance was inherited as a single dominant or semidominant mutation in all cases. Linkage analysis of six mutants identified two unlinked genetic loci. Studies of plants homozygous for one mutation showed the mutant plants to be completely resistant to treatment with a concentration of chlorsulfuron 100 times higher than that which produces symptoms of phytotoxicity on normal plants.
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