No abstract

A population of common ragweed not controlled by an acetolactate synthase (ALS)-inhibiting herbicide, cloransulam-methyl, was sampled near Dunkirk, IN, the first year of the herbicide's commercialization in 1998. Resistance in the Dunkirk population was confirmed by treating greenhouse-grown seedlings with cloransulam-methyl. ALS activity assays and DNA sequencing were used to identify the resistance mechanism. ALS isolated from plants of the Dunkirk population exhibited an R/S ratio for cloransulam-methyl of >5,000 when compared to ALS activity of populations from Claire City, SD, and V & J Seed Farms. R/S ratios of 4,100 and 110 were observed for two other ALS-inhibiting herbicides, chlorimuron and imazaquin, respectively. DNA sequencing revealed that an inferred leucine for tryptophan substitution at amino acid position 574 in ALS was responsible for the observed herbicide resistance. Additionally, DNA sequencing revealed significant variability among common ragweed ALS alleles. Two fragments of ALS were sequenced from three plants each of the Claire City and Dunkirk populations, totaling 688 nucleotide base pairs, of which 72 were polymorphic.

Pyrimidine bases are rapidly catabolized in growing plant tissues. The final enzyme of the catabolic pathway, ␤-ureidopropionase (␤-UP; EC 3.5.1.6), was partially purified from the shoots of etiolated maize (Zea mays) seedlings. The enzyme had a K m for ␤-ureidopropionate (the substrate derived from uracil) of 11 m. Only one enantiomer of racemic ␤-ureidoisobutyrate (derived from thymine) was processed with a K m of 6 m. The enzyme was inactivated by dialysis against 1,10-phenanthroline and activity could be partially restored by addition of Zn 2ϩ . Maize ␤-UP was very sensitive to inactivation by iodoacetamide. This could be prevented by addition of substrate, indicating the presence of an active site Cys. The enzyme was strongly inhibited by short chain aliphatic acids and aryl propionates, the most potent inhibitor of which was 2-(2, 6-dinitrophenoxy)-propionate (I 50 ϭ 0.5 m). A gene for Arabidopsis ␤-UP encodes a polypeptide of 405 amino acids and has about 55% homology with the enzymes from other eukaryotic organisms. Several highly conserved residues link the plant ␤-UP with a larger class of prokaryotic and eukaryotic amidohydrolases. An Arabidopsis cDNA truncated at the N terminus by 14 residues was cloned and overexpressed in Escherichia coli. The recombinant enzyme (43.7 kD) was soluble, functional, and purified to homogeneity with yields of 15 to 20 mg per 30 g fresh weight of E. coli cells. The recombinant enzyme from Arabidopsis and the native enzyme from maize had molecular masses of approximately 440 kD, indicating the enzyme is a decamer at pH 7.

Acetolactate synthase (ALS) was isolated from a field population of cocklebur (Xanthium strumarium) that developed resistance to the herbicide Scepter following three consecutive years of application. lhe active ingredient of Scepter, imazaquin, gave an inhibitor concentration required to produce 50% inhibition of the enzyme activity that was more than 300 times greater for the resistant enzyme than for the wild-type cocklebur ALS. Tests with flumetsulam and chlorimuron show that the resistant ALS was not crossresistant to these two other classes of ALS inhibitors.

A multiyear effort to identify new natural products was built on a hypothesis that both phytotoxins from plant pathogens and antimicrobial compounds might demonstrate herbicidal activity. The discovery of one such compound, mevalocidin, is described in the current report. Mevalocidin was discovered from static cultures of two unrelated fungal isolates designated Rosellinia DA092917 and Fusarium DA056446. The chemical structure was confirmed by independent synthesis. Mevalocidin demonstrated broad spectrum post-emergence activity on grasses and broadleaves and produced a unique set of visual symptoms on treated plants suggesting a novel mode of action. Mevalocidin was rapidly absorbed in a representative grass and broadleaf plant. Translocation occurred from the treated leaf to other plant parts including roots confirming phloem as well as xylem mobility. By 24 hr after application, over 20 % had been redistributed through-out the plant. Mevalocidin is a unique phytotoxin based on its chemistry, with the uncommon attribute of demonstrating both xylem and phloem mobility in grass and broadleaf plants.