Understanding pesticide metabolism in plants and microorganisms is necessary for pesticide development, for safe and efficient use, as well as for developing pesticide bioremediation strategies for contaminated soil and water. Pesticide biotransformation may occur via multistep processes known as metabolism or cometabolism. Cometabolism is the biotransformation of an organic compound that is not used as an energy source or as a constitutive element of the organism. Individual reactions of degradation–detoxification pathways include oxidation, reduction, hydrolysis, and conjugation. Metabolic pathway diversity depends on the chemical structure of the xenobiotic compound, the organism, environmental conditions, metabolic factors, and the regulating expression of these biochemical pathways. Knowledge of these enzymatic processes, especially concepts related to pesticide mechanism of action, resistance, selectivity, tolerance, and environmental fate, has advanced our understanding of pesticide science, and of plant and microbial biochemistry and physiology. There are some fundamental similarities and differences between plant and microbial pesticide metabolism. In this review, directed to researchers in weed science, we present concepts that were discussed at a symposium of the American Chemical Society (ACS) in 1999 and in the subsequent book Pesticide Biotransformation in Plants and Microorganism: Similarities and Divergences, edited by J. C. Hall, R. E. Hoagland, and R. M. Zablotowicz, and published by Oxford University Press, 2001.

The production and biological activity of selected toxic metabolites of fungal biological control agents are reviewed. These metabolites include destruxins, oxalic salts, trichothecenes, zearalenone, fumonisins, fusaric acid and aflatoxin isolated from Metarhizium anisopliae, Beauveria bassiana, Trichoderma spp., Fusarium spp., Alternaria alternata, F. oxysporum and Aspergillus spp., respectively.

Propanil-resistant barnyardgrass was reported in Poinsett County, AR, in 1990. Greenhouse studies were initiated to determine the distribution of propanil-resistant barnyardgrass in the state and to characterize the resistance. Barnyardgrass seeds were obtained in 1991 and 1992 from fields in 19 of the 38 rice producing counties in Arkansas where propanil treatment at recommended rates gave unsatisfactory barnyardgrass control. Barnyardgrass seedlings from the various sources were treated with propanil at 4.5 kg ai/ha and seedling injury response was compared to the response of seedlings collected from known resistant and susceptible barnyardgrass populations. Propanil-resistance of varying levels was confirmed in 115 (16 counties) out of the 138 Arkansas barnyardgrass seed sources. Propanil I50values (rate of herbicide required to provide 50% injury/control) were determined to be 14, 20, and 39 kg/ha for slightly, moderately, and highly resistant barnyardgrass, respectively. A resistance factor of 20® was found in the highly resistant barnyardgrass category. Development of resistance was highly correlated with crop rotations where rice was grown one out of two, or two out of three years.

Amounts of extractable phenylalanine ammonia-lyase (PAL; E.C. 4.3.1.5) activity increased in the axes of 3-day-old, dark-grown soybean seedlings IGlyeine max (L.) Merr.] shortly after the seedlings were transferred to glyphosate IAf-(phosphonomethyl)-glychie] solutions. The stimulation of PAL activity in herbicide-treated tissue (as compared to control tissue) was detectable as early as 12 h after treatment, whereas growth inhibition (length, fresh weight and dry weight) was not significantly affected until 24 h on a fresh-weight basis and at 48 h on a dry-weight basis. PAL activity increased with time (12-72 h) in herbicide-treated axes when expressed as activity per gram fresh weight, specific activity, and on a per axis basis. PAL activity stimulation correlated positively with glyphosate concentration from 10"* to 10~' M. PAL activity in control tissues remained nearly constant over the sampling period (12-72 h). Total alcohol-soluble hydroxyphenolic compound levels in treated axes were not significantly different from the control at any sampling period. The total soluble amino acid pool showed a general decrease with time in glyphosate-treated tissues. The phenylalanine pool was lowered with treatment time and the ammonia concentration (per g fr. wt, basis) was increased after treatment. No significant differences were noted in the concentration of soluble protein of glyphosate-treated tissue when compared to controls. Visual effects {stunting, lack of secondary root formation, and necrotic areas) of glyphosate were more obvious in the root than in the hypocotyl. Analysis of various chemical constituents substantiated that other glyphosate effects were more clearly demonstrable in the root than in the hypocotyl or in the intact axis. On a per root basis glyphosate markedly increased PAL activity while reducing free phenylalanine, free tyrosine, soluble hydroxyphenolics, total free amino acids and ammonia content. The effect of glyphosate in the root was greatest on phenyl-' Mississippi Agricultural and Forestry Experiment Station »operating. alanine content, reducing it five-fold. The results indicate that glyphosate could exert its effect through either induction of PAL activity and/or inhibition of aromatic amino acid synthesis.Recently, we postulated that glyphosate might reduce growth in maize {Zea mays L.) by inducing phenylalanine 0031-9317/79/080357-10803,00/0 © 1979 Physiologia Plantarum