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

Greenhouse and growth chamber experiments were conducted to examine glyphosate [isopropylamine salt of N-(phosphonomethyl) glycine] effects on growth, chlorophyll content, nodulation, and nodule leghemoglobin content of glyphosate-resistant and susceptible soybean (Glycine max [L.] Merr.) varieties. In susceptible soybean, a single application of 0.28 kg/ha reduced chlorophyll content (49%), and shoot and root dry weight (50 and 57%, respectively) at 2 wk after treatment. In glyphosate-resistant soybean, there were no significant effects on these parameters by single application up to 1.12 kg/ha, but 2.24 kg/ha reduced shoot and root dry weight by 25 to 30%. Application of glyphosate 1.12 kg/ha, followed by sequential applications at 0.56 or 1.12 kg/ha, did not affect plant growth and chlorophyll content, but application of 2.24 kg/ha followed by sequential application of 2.24 kg/ha reduced root growth. In glyphosate-resistant soybean, an application of 1.12 kg/ha 3 wk after planting did not affect nodule number or Krishna N. Reddy is Plant Physiologist, Robert E. Hoagland is Research Chemist, and Robert M. Zablotowicz is Soil Microbiologist, Southern

Bioherbicides can be defined as plant pathogens, phytotoxins derived from pathogens or other microorganisms, augmentatively applied to control weeds. Although many pathogens with bioherbicidal potential have been discovered, most lack sufficient aggressiveness to overcome weed defenses to achieve adequate control. Plants use various physical and biochemical mechanisms to defend against pathogen infectivity, including callose deposition, hydroxyproline-rich glycoprotein accumulation, pathogenesis-related proteins (PR-proteins), phytoalexin production, lignin and phenolic formation, and free radical generation. Some herbicides, plant growth regulators, specific enzyme inhibitors, and other chemicals can alter these defenses. Various pathogens also produce chemical suppressors of plant defenses. Secondary plant metabolism is a major biochemical pathway related to several defense processes. Increased activity of a key enzyme of this pathway, phenylalanine ammonia-lyase (PAL), is often a response to pathogen attack, as demonstrated in two weeds and their associated bioherbicidal pathogens:Alternaria cassiaeon sicklepod andA. crassaon jimsonweed. Weakening of physical and biochemical defenses, and lowering of resistance to pathogen attack, may result from reduced production of phenolics, lignin, and phytoalexins caused by herbicides and other chemicals that affect cuticular component biosynthesis and/or key aspects of secondary plant metabolism. Potent PAL inhibitors [aminooxyacetic acid, α-aminooxy-β-phenylpropionic acid, and (l-amino-2-phenylethyl)phosphonic acid] have some regulatory action on secondary plant metabolism and pathogenicity. Various herbicides and other chemicals dramatically affect extractable PAL activity levels and/or substantially alter PAL product production. Some non-pathogenic organisms can alter herbicide efficacy, and some herbicides influence disease development in plants. Research has shown some synergistic interactions of microbes and chemicals with relevance to weed control. Further research on pathogen interactions with agrochemicals (or other chemicals/regulators) could result in increased efficacy of pathogen-herbicide combinations, reduction of herbicide and pathogen levels required for weed control, and expanded pathogen host range.

Propanil-resistant barnyardgrass populations, previously veriÐed in Arkansas rice Ðelds and in greenhouse tests, were examined in the laboratory to ascertain if the resistance mechanism in this weed biotype was herbicide metabolism. Propanil-resistant barnyardgrass was controlled [95% in the greenhouse when carbaryl (an aryl acylamidase inhibitor) was applied two days prior to propanil. Laboratory studies with 14C-radiolabelled propanil indicated that the herbicide was hydrolysed in propanil-resistant barnyardgrass and rice to form 3,4-dichloroaniline, but no detectable hydrolysis occurred in susceptible barnyardgrass. Two additional polar metabolites were detected in propanil-resistant barnyardgrass and rice and tentatively identiÐed by thin layer chromatography. Overall, metabolites in the resistant barnyardgrass had values similar to those R f in rice, indicating similar metabolism for both species. These data, coupled with data from a previous report on the resistant biotype showing no di †erential absorption/translocation or molecular modiÐcation of the herbicide binding site in the resistant biotype, indicate that the resistance mechanism is metabolic degradation of propanil.

The growth regulator, glyphosine [N,N-bis(phosphonomethyl)glycine], and other possible metabolites of glyphosine and glyphosate [N-(phosphonomethyl)glycine] [glycine, sarcosine, and aminomethylphosphonic acid (AMPA)] were tested individually (0.5 mM) or as a mixture (each at 0.5 mM) for their effects on growth, extractable phenylalanine ammonia-lyase (PAL) activity, hydroxyphenolic-compound production, chlorophyll and anthocyanin contents, and on soluble-protein levels in soybean [Glycine max(L.) Merr. ‘Hill’] seedlings. Most chemical treatments caused some inhibition of growth either on fresh weight accumulation or on root elongation in the light and dark over 72 h. Glyphosine was generally the most inhibitory and caused the greatest inhibition on axis dry-weight accumulation. Glyphosine significantly increased extractable PAL activity in axes of light- and dark-grown soybeans to a lesser extent than did glyphosate. AMPA had some inhibitory effects on extractable PAL activity whereas other compounds had little influence on the enzyme. These compounds had little effect on total soluble protein in axes or on soluble protein in PAL preparations from 12 to 72 h in light-or dark-grown seedlings. No in vitro effect of the chemicals on PAL activity was found at concentrations up to 0.5 mM. Hydroxyphenolic compound levels increased within 24 to 72 h (per gram fresh weight basis) in light- or dark-grown soybean axes treated with glyphosine, AMPA, or a metabolite mixture (AMPA, sarcosine, and glycine). Anthocyanin content was decreased by glyphosate and to a lesser extent by glyphosine, but was increased by AMPA and the mixture. Glyphosate significantly increased the chlorophylla/bratio and decreased total chlorophyll, but glyphosine decreased the chlorophyll content to a lesser degree.