Mechanism Of Herbicide Resistance Research Articles

Molecular Mechanisms of Herbicide Resistance with Special Emphasis on Cytochrome P450 Monooxygenases.

Molecular Mechanisms of Herbicide Resistance with Special Emphasis on Cytochrome P450 Monooxygenases.

Mechanisms of Resistance to Aryloxyphenoxypropionate Herbicides in Two Resistant Biotypes ofAlopecurus myosuroides(blackgrass): Herbicide Metabolism as a Cross-Resistance Mechanism

The mechanisms of AOPP herbicide resistance in twoAlopecurus myosuroidesbiotypes were investigated. Resistant biotype Peldon A1, which is highly resistant to the phenyl-urea chlorotoluron, is moderately resistant to the AOPP herbicides diclofop-methyl, fenoxaprop-ethyl, fluazifop-P-butyl, and the CHD tralkoxydim. Resistant biotype Lincs. E1, which is only moderately resistant to chlorotoluron, is highly resistant to the AOPP herbicide fenoxaprop-ethyl, and moderately resistant to diclofop-methyl, fluazifop-P-butyl, and the CHD tralkoxydim. There is no clear evidence of resistance to the CHD sethoxydim in either biotype. Both Peldon A1 and Lincs. E1 exhibited moderately enhanced metabolism of diclofop-methyl. The approximate half life of diclofop was 8 and 9 HAT, respectively, compared to 17 HAT for the susceptible Rothamsted biotype. Peldon A1 showed moderately enhanced metabolism of fenoxaprop-P-ethyl. However, in the highly resistant Lincs. E1, fenoxaprop-P-ethyl metabolism rates were intermediate between Peldon A1 and the susceptible biotype. Fenoxaprop-P-ethyl metabolism inA. myosuroideswas not significantly reduced by inhibitors of cytochrome P450: PBO, tetcyclasis, or ABT. While enhanced herbicide metabolism can account for the moderate AOPP/CHD resistance observed in Peldon A1in vivo, it cannot account in total for fenoxaprop-ethyl resistance in Lincs. E1. Lincs. E1 may possess one or more additional resistance mechanism.

Old Enzymes for a New Job (Herbicide Detoxification in Plants).

Higher plants are equipped with a remarkably versatile system that protects them from the potentially phytotoxic actions of xenobiotics, i.e. synthetic chemicals present in the plant's environment. Particularly striking is the natural tolerance of certain plants toward herbicides that profoundly affect closely related species. This phenomenon of herbicide selectivity is widely exploited in agriculture to control competing weeds in a field of crop plants that are tolerant to the particular herbicide. Herbicide selectivity is, in most cases, based primarily on the differential ability of plant species to metabolically detoxify the herbicide (Lamoureux et al., 1991; Cole, 1994). Metabolic herbicide inactivation has been employed in the genetic engineering of crops for herbicide tolerance (Hinchee et al., 1993). Finally, enhanced herbicide detoxification is one of the mechanisms of herbicide resistance, in addition to altered target site susceptibility and yet-unknown mechanisms, that may emerge within formerly susceptible weed species upon continuous use of the same herbicide or herbicide class (Holt et al., 1993). It is well documented that plants are able to metabolize and detoxify herbicides by a variety of enzymatic reactions and with extraordinary diversity among species. Furthermore, recent research has revealed that transporters in the vacuolar membrane mediate the energydependent export of herbicide metabolites into the large central vacuole (Martinoia et al., 1993).

Auxinic Herbicide-Resistant and -Susceptible Wild Mustard ( Sinapis arvensis L.) Biotypes: Effect of Auxinic Herbicides on Seedling Growth and Auxin-Binding Activity

The effect of the auxinic herbicides dicamba, mecoprop, MCPA, and picloram on seedling development of an auxinic herbicide-resistant and -susceptible biotype of wild mustard ( Sinapis arvensis L.) was studied. When treated with dicamba, MCPA, and picloram differences in the seedling dose-response characteristics were observed between the two biotypes. Conversely, following mecoprop treatment there were no differences between the response of the herbicide susceptible and resistant biotypes. Some characteristics of the resistant and susceptible wild mustard biotype auxin-binding protein (ABP) preparations were examined. Although, both ABP preparations appear to have similar substrate ([ 3H]indole-3-acetic acid ([ 3H]IAA) binding) and time course profiles, Scatchard analysis revealed differences between the biotypes; the resistant biotype lacks a high-affinity population of ABP which is present in the susceptible biotype. The effects of the auxinic herbicides on specific [ 3H]IAA binding to ABP preparations from the resistant and susceptible biotypes were also examined. Mecoprop and MCPA were the most potent inhibitors of [ 3H]IAA binding to the susceptible biotype ABP preparations, followed by dicamba and picloram. The same pattern of inhibition was also observed when the effect of these herbicides on seedling growth was examined in the susceptible biotype. When picloram and dicamba were used to inhibit [ 3H]IAA binding and seedling growth, the susceptible biotype was significantly more sensitive to inhibition than the resistant biotype ABP preparations; however, no differences between the two biotypes were observed following MCPA or mecoprop treatment. Our results suggest the effects of various auxinic herbicides on the binding of [ 3H]IAA to ABP preparations and seedling growth correlate with the effect of these herbicides on whole plants. This data suggests there is a relationship between ABP binding activity and sensitivity to auxinic herbicides and this relationship may explain the mechanism of auxinic herbicide resistance in this wild mustard biotype.

Sulfonylurea Selectivity and Safener Activity in ′Landmark′ and ′Merit′ Sweet Corn

The sulfonylurea herbicides primisulfuron and nicosulfuron are phytotoxic to the sweet corn cultivar Merit, whereas most other cultivars, including ′Landmark,′ are relatively resistant. The mechanism of herbicide resistance between ′Merit′ and Landmark was studied. The sensitivity of acetolactate synthase (ALS) activity to primisulfuron and nicosulfuron in protein extracts from Merit and Landmark was compared as a possible mechanism of resistance. The rates of ALS activity from the two varieties were similar, at 0.58 and 0.57 μmol · (mg protein · hr)−1 for Merit and Landmark, respectively. Enzyme activity from both varieties was sensitive to herbicide inhibition. I50 values for primisulfuron were 16 and 12 nM for inhibition of ALS activity from Merit and Landmark, respectively. I50 values for nicosulfuron were 340 and 295 nM for inhibition of ALS activity from Merit and Landmark, respectively. The relative levels of primisulfuron and nicosulfuron metabolism in Merit and Landmark were also considered as a mechanism of resistance. Excised shoots from Landmark had a much greater capacity to metabolize primisulfuron and nicosulfuron than Merit. With Merit, after 8 hr, 8.3 and 13.3% of the extracted radioactivity was found as primisulfuron and nicosulfuron metabolites, respectively, whereas 60.5 and 63.5% of the extracted radioactivity in Landmark was found as primisulfuron and nicosulfuron metabolites, respectively. The effect of the safener BAS 145 138 on ALS activity and herbicide metabolism in both cultivars was also studied. BAS 145 138 did not affect ALS activity or herbicide sensitivity in either species. BAS 145 138 did not affect primisulfuron or nicosulfuron metabolism in Merit, whereas BAS 145 138 increased the metabolism of both herbicides in Landmark. Safener treatment increased the metabolism of primisulfuron from 33% in untreated to 49% in treated Landmark seedlings. The metabolism of nicosulfuron increased from 61% in the untreated Landmark to 84% in the treated seedlings. Because ALS activity from both Merit and Landmark was equally sensitive to both nicosulfuron and primisulfuron, differential sensitivity at the proposed biochemical site of action does not account for the observed selectivity at the whole plant level. The greater level of sulfonylurea metabolism in Landmark than Merit suggests that resistance to these herbicides is due to metabolic detoxication of the parent herbicide. BAS 145 138 enhanced herbicide metabolism in Landmark but not Merit.

Resistant Acetyl-CoA Carboxylase is a Mechanism of Herbicide Resistance in a Biotype of Avena sterilis ssp. ludoviciana

Resistant Acetyl-CoA Carboxylase is a Mechanism of Herbicide Resistance in a Biotype of Avena sterilis ssp. ludoviciana

Pollen Expression of Herbicide Target Site Resistance Genes in Annual Ryegrass (Lolium rigidum).

Herbicide resistance can occur either through target-site insensitivity or by nontarget site-based mechanisms. Two herbicide-resistant biotypes of Lolium rigidum Gaud., one resistant to acetolactate synthase (ALS)-inhibiting herbicides (biotype WLR1) and the other resistant to acetyl CoA carboxylase (ACCase)-inhibiting herbicides (biotype WLR96) through target-site insensitivity at the whole plant and enzymic levels, were found to express this resistance in the pollen. Pollen produced by resistant biotypes grew uninhibited when challenged with herbicide, whereas that from a susceptible biotype was inhibited. A third biotype, SLR31, resistant to ACCase-inhibiting and certain ALS-inhibiting herbicides at the whole plant level through nontarget site-based mechanisms, did not exhibit this expression in the pollen. The technique described may form the basis for a rapid screen for certain nuclear-encoded, target site-based herbicide-resistance mechanisms.

Diclofop and Fenoxaprop Resistance in Wild Oat Is Associated with an Altered Effect on the Plasma Membrane Electrogenic Potential

We have examined the mechanism of herbicide resistance in a biotype of wild oat ( Avena fatua L.) that is resistant to diclofop-methyl and many other acetyl-coenzyme A carboxylase (ACCase) inhibitors. Resistance to diclofop-methyl and fenoxaprop-ethyl was not based on reduced uptake nor on enhanced metabolism of the herbicides to inactive products. In in vitro assays of crude or partially purified preparations, ACCase from the resistant (UM-1) and susceptible (UM-5) biotypes was equally sensitive to diclofop, with I 50 values of 6.1 and 7.3 μ M for UM-1 and UM-5, respectively. Corresponding values for fenoxaprop were 2.5 and 1.0 μ M. These results suggest that the high level of resistance observed toward these herbicides is not based on an altered target enzyme. Root tissue from both UM-1 and UM-5 acidified an unbuffered bathing solution. Addition of 100 μ M diclofop or fenoxaprop prevented acidification of the bathing medium by UM-1, but alkalinization occurred rapidly with UM-5. When diclofop was removed from the treatment solution, UM-1 resumed acidification of the solution, whereas the pH of the UM-5 bathing solution continued to increase. Diclofop (50 μ M) rapidly depolarized the cell membrane in peeled coleoptile sections, with no difference between UM-1 and UM-5. However, when diclofop was removed from the treatment solution, the electrogenic membrane potential was quickly reestablished in UM-1, but remained collapsed in UM-5. These results provide support for the hypothesis that the effect of diclofop on the plasma membrane potential is an important component of its herbicidal activity. The reversibility of the effect of diclofop and fenoxaprop on transmembrane proton flux in UM-1 appears to be associated with resistance to these herbicides.

Plant Cell and Tissue Culture Techniques for Weed Science Research

Tissue and cell culture offer weed scientists many opportunities to research herbicide effects on plants. This review will discuss examples in which plant cells grown in vitro have been used to study herbicide action. Plant cell and tissue culture have many advantages over the use of whole plants; however, several disadvantages that exist are discussed. Cell cultures can be established for most plant species and provide a relatively homogeneous system for studying herbicide action. Responses of plant cells to herbicides are usually correlated with responses at the whole plant level, and cells have the advantage of posing fewer physical barriers to herbicide uptake and translocation. Cell culture techniques discussed include: screening candidate herbicide compounds; investigating herbicide efficacy, mechanism of action, metabolism, and uptake; and ascertaining mechanisms of herbicide resistance, selecting for resistance, and regenerating crops.

The Mechanism of Herbicide Resistance in Tobacco Cells with a New Mutation in the QB Protein

The Mechanism of Herbicide Resistance in Tobacco Cells with a New Mutation in the Q_B Protein

The Mechanism of Herbicide Resistance in Tobacco Cells with a New Mutation in the QB Protein

A new mutant of the psbA gene conferring resistance to 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) was obtained by selection of photomixotrophic tobacco (Nicotiana tabacum cv Samsun NN) cells. The 264th codon AGT (serine) in the wild psbA gene was changed to ACT (threonine) in these mutant tobacco cells. All other higher plants resistant to atrazine exhibit a change to GGT (glycine) in this codon. Measurements of Hill reaction activity and chlorophyll fluorescence showed that the threonine 264-containing plastoquinone serving as secondary stable electron acceptor of PSII (Q(B) protein) had not only strong resistance to triazine-type herbicides but also moderate resistance to substituted urea-type herbicides. Threonine-type Q(B) protein showed especially strong resistance to methoxylamino derivatives of the substituted urea herbicides. The projected secondary structures of the mutant Q(B) proteins indicate that the cross-resistance of threonine 264 Q(B) protein to triazine and urea herbicides is mainly due to a conformational change of the binding site for the herbicides. However, the glycine 264 Q(B) protein is resistant to only triazine herbicides because of the absence of an hydroxyl group and not because of a conformational change.

Molecular Mechanism of Herbicide Resistance in Plants

Molecular Mechanism of Herbicide Resistance in Plants

Chloroplast membrane alterations in triazine-resistant Amaranthus retroflexus biotypes

The effectiveness of diuron, atrazine, procyazine, and cyanazine were compared in controlling growth of redroot pigweed (Amaranthus retroflexus L.) in hydroponic culture. A very marked differential inhibition response was observed for atrazine between resistant and susceptible biotypes. Procyazine and cyanazine exhibited less dramatic differential responses, whereas diuron was equally effective in controlling growth in both biotypes. Photosystem II activity of chloroplasts from both triazine-resistant and triazine-susceptible biotypes was inhibited by diuron but only the chloroplasts from triazine-susceptible biotypes were inhibited significantly by atrazine. The photochemical activity of chloroplasts from triazine-resistant biotypes was partially resistant to procyazine or cyanazine inhibition. The parallel lack of diuron differential effects, partial procyazine and cyanazine differential response, and very marked atrazine differential response in both whole plant and chloroplast assays indicates that the chloroplast is the site of selective herbicide tolerance in these triazine-resistant redroot pigweed biotypes.Photosystem II photochemical properties were characterized by analysis of chlorophyll fluorescence transients in the presence or absence of herbicides. Data with susceptible chloroplasts indicated that both diuron and atrazine inhibit electron flow very near the primary electron acceptor of photosystem II. Only diuron altered the fluorescence transient in resistant chloroplasts. In untreated preparations there were marked differences in the fast phases of the fluorescence increase in resistant vs. susceptible chloroplasts; these data are interpreted as showing that the resistant plastids have an alteration in the rate of reoxidation of the primary photosystem II electron acceptor. Electrophoretic analysis of chloroplast membrane proteins of the two biotypes showed small changes in the electrophoretic mobilities of two polypeptide species. The data provide evidence for the following herbicide resistance mechanism: genetically controlled modification of the herbicide target site.