Selection of kochia (Kochia scoparia) biotypes resistant to the sulfonylurea herbicide chlorsulfuron has occurred through the continued use of this herbicide in monoculture cereal-growing areas in the United States. The apparent sulfonylurea resistance observed in kochia was confirmed in greenhouse tests. Fresh and dry weight accumulation in the resistant kochia was 2-to >350-fold higher in the presence of four sulfonylurea herbicides as compared to the susceptible biotype. Acetolactate synthase (ALS) activity isolated from sulfonylurea-resistant kochia was less sensitive to inhibition by three classes of ALS-inhibiting herbicides, sulfonylureas, imidazolinones, and sulfonanilides. The decrease in ALS sensitivity to inhibition (as measured by the ratio of resistant 150 to susceptible 150) was 5-to 28-fold, 2-to 6-fold, and 20-fold for sulfonylurea herbicides, imidazolinone herbicides, and a sulfonanilide herbicide, respectively. No differences were observed in the ALS-specific activities or the rates of ["4C]chlorsulfuron uptake, translocation, and metabolism between susceptible and resistant kochia biotypes. The Km values for pyruvate using ALS from susceptible and resistant kochia were 2.13 and 1.74 mm, respectively. Based on these results, the mechanism of sulfonylurea resistance in this kochia biotype is due solely to a less sulfonylurea-sensitive ALS enzyme.The loss of effective control by pesticides of weeds, insects, and fungi because of resistance is a well known, established phenomenon (9). Among weeds, triazine resistance was first reported in 1970 (27). Subsequently, resistance to several other herbicides in addition to the triazines has occurred in weeds (2, 10). The continued use of a single control agent is often a common feature in cases of resistance.The sulfonylureas are highly active herbicides that have been in commercial use since 1982. The mode of action of the sulfonylurea (5,20,26) as well as the imidazolinone (30) and sulfonanilide (18) herbicides is the inhibition of ALS (see abbreviation list in Table I) (also known as acetohydroxyacid synthase, EC 4.1.3.18), the first enzyme common to the biosynthesis of the branched-chain amino acids, Leu, Ile, and Val. Chlorsulfuron, a sulfonylurea herbicide used in cereals, is wheat (Triticum aestivum L.)-tolerant because the crop metabolizes the chlorsulfuron to nonphytotoxic products more quickly than susceptible weeds (32). This differential selectivity based on metabolism is contrasted by selectivity due to differences in target site sensitivity as obtained by selection or production of sulfonylurea-resistant bacteria (20,34), yeast (7) (25) and prickly lettuce (Lactuca serriola L.) (22) in certain cereal-growing areas. In this paper, the mechanism by which kochia is resistant to the sulfonylurea and imidazolinone herbicides is described.
MATERIALS AND METHODS
Whole Plant Experiments
