Aryloxyphenoxypropionate Herbicides Research Articles

Late Watergrass (Echinochloa phyllopogon): Mechanisms Involved in the Resistance to Fenoxaprop-p-ethyl

Fenoxaprop-p-ethyl (FE), 2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy] propanoate, ethyl ester (R), is an aryloxyphenoxypropionate herbicide for postemergence control of annual and perennial grasses in paddy fields; its site of action is acetyl-coenzyme A carboxylase (ACCase), an enzyme in fatty acids biosynthesis. The possible mechanism(s) of resistance to FE in a resistant biotype of Echinochloa phyllopogon was examined, namely, absorption, translocation, and metabolism of FE and ACCase susceptibility to fenoxaprop acid (FA). Studies of the in vitro inhibition of ACCase discounted any differential active site sensitivity as the basis of resistance to FE. There were differences in absorption rates between biotypes from 3 to 48 h after application (HAA). Biotypes did not differ in either the amounts or the rates of FE translocated; 98% of applied [14C]FE remaining in the treated leaf. However, there was a good correlation between the rate of herbicide metabolism and the plant resistance. The R biotype produced 5-fold less FA and approximately 2-fold more nontoxic (polar) metabolites 48 HAA than the S biotype. Moreover, the higher rate of GSH conjugation in the resistant biotype as compared to the susceptible one indicates that GSH and cysteine conjugation is the major mechanism of resistance of the R biotype against FE toxicity.

Morphological and Molecular Characterization of Different Echinochloa spp. and Oryza sativa Populations

Echinochloa P. Beauv. is an important genus because many of its species are weeds infesting most paddy fields, which can reduce the rice grain production by up to 80%. A controversy exists about the taxonomy of the genus due to the high level of morphological variations found in these species. Cyhalofop-butyl, an aryloxyphenoxy-propionate herbicide, is used to control Echinochloa spp. in paddy fields, although differences in susceptibility were found between different Echinochloa species. E. colona was highly susceptible [ED50= 34 g of active ingredient (ai) ha(-1)]; very similar results were obtained with the remaining species. By contrast, E. oryzicola (170 g of ai ha(-1)) was less sensitive, with the herbicide symptoms appearing later. Because of this differential susceptibility, morphological and molecular studies were carried out. A morphological study, using 21 characters both quantitative and qualitative of spikelets and seedlings, was capable of clearly distinguishing closely related E. crus-galli plants (two populations), E. muricata and E. crus-pavonis, and E. oryzicola, E. utilis, and E. colona species. The resolution of Echinochloa species at the molecular level, based on RAPD analyses, was fairly consistent with morphological analysis results. Among the 60 primers screened, 21 primers exhibited polymorphic bands and produced a total of 136 RAPD markers. Of all the amplified fragments, 90 were found to be polymorphic. E. oryzicola and E. colona were clearly separated, and the RAPD analyses showed that both E. crus-galli populations were 100% related and 51% related to E. utilis, whereas E. crus-pavonis and E. muricata (73% similarity) appeared as being clearly separated from this group.

Basis of selectivity of cyhalofop-butyl in Oryza sativa L.

Cyhalofop-butyl (CB), 2-[4-(4-cyano-2-fluorophenoxy)phenoxy]propanoic acid, butyl ester (R), is an aryloxyphenoxypropionate (AOPP) herbicide for postemergence use in rice to control grasses, mainly Echinochloa spp. Similar to other AOPP and cyclohexanedione herbicides, the site of action of CB is acetyl-coenzyme A carboxylase (ACCase), an enzyme in fatty acid biosynthesis. The mechanisms involved in the selectivity of CB in rice (Oryza sativa L.)-absorption, translocation, metabolism, and ACCase susceptibility-were studied. Studies of in vitro inhibition of ACCase in E. oryzoides and O. sativa L. species discounted any differential active site sensitivity as the basis of tolerance to CB. The O. sativa L. cuticle was uniformly covered by waxes, with predominantly unshaped large waxes randomly distributed, obtaining absorption values of under 30%, 24 h after application (HAA). The E. oryzoides cuticle formed a non-uniform covered reticule, with less wax density and areas lacking in waxes reaching maximum values of absorption rising to 73%, 24 HAA. Translocation studies revealed no significant differences, either between species, or between times, remaining in the treated leaf. There was a good correlation between the rate of metabolism and plant tolerance. Plant metabolism studies demonstrated that tolerant rice inactivated the esterases producing a lack of functionality thus reducing the conversion of CB to cyhalofop acid, which is the active form of the herbicide. Moreover, it increased the metabolism of the herbicide forming non toxic metabolites much faster than E. oryzoides. It was concluded that the basis of rice tolerance to CB was a lack of esterase functionality, a reduced absorption through the cuticle and an increase in cyhalofop acid metabolism.

The molecular bases for resistance to acetyl co-enzyme A carboxylase (ACCase) inhibiting herbicides in two target-based resistant biotypes of annual ryegrass (Lolium rigidum)

Acetyl-CoA carboxylase (ACCase) (EC.6.4.1.2) is an essential enzyme in fatty acid biosynthesis and, in world agriculture, commercial herbicides target this enzyme in plant species. In nearly all grass species the plastidic ACCase is strongly inhibited by commercial ACCase inhibiting herbicides [aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicide chemicals]. Many ACCase herbicide resistant biotypes (populations) of L. rigidum have evolved, especially in Australia. In many cases, resistance to ACCase inhibiting herbicides is due to a resistant ACCase enzyme. Two ACCase herbicide resistant L. rigidum biotypes were studied to identify the molecular basis of ACCase inhibiting herbicide resistance. The carboxyl-transferase (CT) domain of the plastidic ACCase gene was amplified by PCR and sequenced. Amino acid substitutions in the CT domain were identified by comparison of sequences from resistant and susceptible plants. The amino acid residues Gln-102 (CAG codon) and Ile-127 (ATA codon) were substituted with a Glu residue (GAG codon) and Leu residue (TTA codon), respectively, in both resistant biotypes. Amino acid positions 102 and 127 within the fragment sequenced from L. rigidum corresponded to amino acid residues 1756 and 1781, respectively, in the A. myosuroides full ACCase sequence. Allele-specific PCR results further confirmed the mutations linked with resistance in these populations. The Ile-to-Leu substitution at position 1781 has been identified in other resistant grass species as endowing resistance to APP and CHD herbicides. The Gln-to-Glu substitution at position 1756 has not previously been reported and its role in herbicide resistance remains to be established.

Mechanisms of cross and multiple herbicide resistance in Alopecurus myosuroides and Lolium rigidum

Alopecurus myosuroides and Lolium rigidum have developed resistance to herbicides with several modes of action in many herbicide classes. A. myosuroides biotype Peldon A1 from England exhibits non-target site cross resistance to substituted urea and aryloxyphenoxypropionate herbicides (APP) due to enhanced metabolism. L. rigidum biotype SLR 31 from Australia has multiple resistance mechanisms, including both non-target site cross resistance and target site cross resistance. The majority of the SLR 31 population has enhanced metabolism of chlorsulfuron and diclofop-methyl and a mechanism correlated with altered plasma membrane response, which correlates with resistance to some APP and cyclohexanedione (CHD) herbicides. A small proportion of the population also has target site cross resistance to APP and CHD herbicides. While A myosuroides and L. rigidum share common biological elements, they are not unique. Non-target site cross resistance and multiple herbicide resistance is predicted to develop in other weed species. The repercussions of cross and multiple resistance warrant proactive measures to prevent or delay onset.

Cloransulam Antagonizes Annual Grass Control with Aryloxyphenoxypropionate Graminicides But Not Cyclohexanediones

Field, greenhouse, and laboratory studies were conducted to examine the potential for antagonism of postemergence graminicides when tank-mixed with cloransulam and to determine the role of herbicide absorption and translocation in observed antagonistic responses. Cloransulam antagonized annual grass control with aryloxyphenoxypropionate herbicides fluazifop-P, quizalofop, and the prepackaged formulation of fluazifop-P plus fenoxaprop. Cloransulam did not affect annual grass control with the cyclohexandiones clethodim and sethoxydim. In the greenhouse, increasing the rate of the graminicides was a more effective strategy for overcoming antagonism for quizalofop than for fluazifop-P or fluazifop-P plus fenoxaprop, and success was species dependent. Annual grass control with clethodim, sethoxydim, and glyphosate was not adversely affected by tank mixtures with cloransulam. Control of large rhizome johnsongrass was initially reduced when cloransulam was mixed with sethoxydim, fluazifop-P plus fenoxaprop, or quizalofop. By 6 wk after treatment, control of rhizome johnsongrass was antagonized only when cloransulam was mixed with sethoxydim. Rainfall within 1.5 h of application reduced johnsongrass control with glyphosate and sethoxydim but did not affect activity of the other herbicides. Absorption of14C-fluazifop-P and14C-quizalofop into broadleaf signalgrass was not affected by cloransulam 6 or 24 h after treatment. Translocation of14C-fluazifop-P to broadleaf signalgrass shoot tissue above and below the treated leaf was decreased when fluazifop-P was combined with cloransulam. Translocation of quizalofop was not affected by cloransulam.

Purification and cloning of an esterase from the weed black-grass (Alopecurus myosuroides), which bioactivates aryloxyphenoxypropionate herbicides.

Carboxyesterases which activate aryloxyphenoxypropionate (AOPP) graminicides to their bioactive herbicidal acids by hydrolysing the respective ester precursors have been identified in black-grass (Alopecurus myosuroides), a problem weed of cereal crops in Northern Europe. The dominant 40 kDa carboxyesterase was purified 1700-fold and identified as a serine hydrolase by affinity labelling with a biotinylated fluorophosphonate suicide substrate. MS-MS sequencing of a peptide digest identified it to be a member of the GDSL family of serine hydrolases. The full-length A. myosuroides hydrolase (Amgdsh1) was cloned by RACE-PCR and expressed in the yeast Pichia pastoris as a secreted enzyme. Expression was associated with activity towards AOPP esters. AmGDSH1 was predicted to be glycosylated and exported to the apoplast in planta. Based on the analysis of related sequences in monocotyledonous plants an alternative classification of the GDSL plant hydrolase superfamily is suggested and their importance in endogenous metabolism and herbicide bioactivation in crops and weeds discussed.

Fluazifop-butyl causes membrane peroxidation in the herbicide-susceptible broad leaf weed bristly starbur ( Acanthospermum hispidum)

In order to clarify the action mechanism of fluazifop-butyl, an aryloxyphenoxypropionate (AOPP) herbicide in bristly starbur ( Acanthospermum hispidum DC.), a unique fluazifop-butyl-susceptible broad-leaved weed, ethylene evolution and membrane lipid peroxidation in the plant seedlings were investigated. Foliar application of fluazifop- P-butyl induced ethylene evolution only from bristly starbur, but not from oat ( Avena sativa L.), another fluazifop-butyl-susceptible species, and two tolerant species, pea ( Pisum sativum L.) and hairy beggarticks ( Bidens pilosa L.). The other AOPP herbicides, quizalofop-ethyl and fenoxaprop-ethyl, and a cyclohexanedione (CHD) herbicide, sethoxydim, did not enhance ethylene production from bristly starbur. This fluazifop-butyl-induced ethylene production in bristly starbur was completely suppressed by aminoethoxyvinylglycine (AVG), a 1-aminocyclopropane-1-carboxylic acid (ACC) synthase inhibitor, but not by p-chlorophenoxyisobutyric acid (PCIB), an anti-auxin compound, suggesting this evolved ethylene was not auxin-induced. Phytotoxic action by fluazifop- P-butyl (5 μM) in bristly starbur was reduced markedly by two lipid-soluble antioxidants, vitamin E, and ethoxyquin. The ethylene production from the plant was also inhibited by these two antioxidants. Content of malondialdehyde, an indicator of lipid peroxidation, increased only by fluazifop- P-butyl in bristly starbur seedlings but not in oat, and this increase was inhibited by ethoxyquin. These results strongly suggest that the primary site of action for fluazifop-butyl in bristly starbur is on the membranes and active oxygen species and/or free radicals are involved in peroxidation. Ethylene evolution is probably induced by these reactive oxygen species.

Tolerance to acetolactate synthase and acetyl-coenzyme A carboxylase inhibiting herbicides in Vulpia bromoides is conferred by two co-existing resistance mechanisms

Vulpia bromoides is a grass species naturally tolerant to acetolactate synthase (ALS) and acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides. The mechanism of tolerance to ALS herbicides was determined as cytochrome P450-monooxygenase mediated metabolic detoxification. The ALS enzyme extract partially purified from V. bromoides shoot tissue was found to be as sensitive as that of herbicide susceptible Lolium rigidum to ALS-inhibiting sulfonylurea (SU), triazolopyrimidine (TP), and imidazolinone (IM) herbicides. Furthermore, phytotoxicity of the wheat-selective SU herbicide chlorsulfuron was significantly enhanced in vivo in the presence of the known P450 inhibitor malathion. In contract, the biochemical basis of tolerance to ACCase inhibiting herbicides was established as an insensitive ACCase. In vitro ACCase inhibition assays showed that, compared to a herbicide susceptible L. rigidum, the V. bromoides ACCase was moderately (4.5- to 9.5-fold) insensitive to the aryloxyphenoxypropionate (APP) herbicides diclofop, fluazifop, and haloxyfop and highly insensitive (20- to >71-fold) to the cyclohexanedione (CHD) herbicides sethoxydim and tralkoxydim. No differential absorption or de-esterification of fluazifop- P-butyl was observed between the two species at 48 h after herbicide application, and furthermore V. bromoides did not detoxify fluazifop acid as rapidly as susceptible L. rigidum. It is concluded that two co-existing resistance mechanisms, i.e., an enhanced metabolism of ALS herbicides and an insensitive target ACCase, endow natural tolerance to ALS and ACCase inhibiting herbicides in V. bromoides.

Diversity of herbicide resistance among wild oat sampled 36 yr apart

The diversity of resistance among wild oat collected before and after commercial introduction of imazamethabenz, difenzoquat, diclofop, fenoxaprop-P, sethoxydim, and tralkoxydim was evaluated. Wild oat sampled in 1964 and 2000 from the Red River Valley of Minnesota and North Dakota were screened for resistance. Nearly 43% of the 1964 collections were susceptible (S) to all six herbicides, whereas only 9% were S by 2000. The frequency of resistance in 2000 compared with 1964 increased for all six herbicides, and 27 phenotype response groups to the six herbicides occurred in 2000 vs. 14 phenotype response groups for the 1964 collection. The proportion of resistant (R) plants increased faster for the aryloxyphenoxypropionate (APP) herbicides, diclofop and fenoxaprop-P, than for the cyclohexanedione (CHD) herbicides, sethoxydim and tralkoxydim. High diversity of resistance responses was observed in wild oat to acetyl-coenzyme A carboxylase–inhibitor herbicides, suggesting that there may be multiple APP herbicide– or CHD herbicide–specific resistance mechanisms in addition to those that confer cross-resistance. The trend of resistance response generally indicates that increased exposure to herbicides in wild oat may confer resistance to newly introduced but unrelated herbicides.

Characterisation of target-site resistance to ACCase-inhibiting herbicides in the weed Alopecurus myosuroides (black-grass).

Resistance to aryloxyphenoxypropionate (AOPP), cyclohexanedione (CHD) and phenylurea herbicides was determined in UK populations of Alopecurus myosuroides Huds. Two populations (Oxford AA1, Notts. A1) were highly resistant (Resistance indices 13-->1000) to the AOPP and CHD herbicides fenoxaprop, diclofop, fluazifop-P and sethoxydim, but only marginally resistant to the phenylurea, chlorotoluron. Analyses of acetyl coenzyme A carboxylase (ACCase) activity showed that an insensitive ACCase conferred resistance to all the AOPP/CHD herbicides investigated. Another population, Oxford S1, showed no resistance to sethoxydim at the population level, but contained a small proportion of plants (<10%) with an insensitive ACCase. Genetic studies on the Notts A1 and Oxford S1 populations demonstrated that target site resistance conferred by an insensitive ACCase is monogenic, nuclearly inherited with the resistant allele showing complete dominance. Investigations of the molecular basis of resistance in the Notts A1 population showed that sethoxydim resistance in A myosuroides was associated with the substitution of an isoleucine in susceptible with a leucine in resistant plants, which has also been found in three other resistant grass-weed species (Setaria viridis (L) Beauv, Avena fatua L, Lolium rigidum Gaud).

Imazamethabenz-resistant wild oat ( Avena fatua L.) is resistant to diclofop-methyl

An imazamethabenz-resistant wild oat accession AR1 was found to be resistant to diclofop-methyl. Experiments were conducted to determine the response of this accession to other acetyl-CoA-carboxylase-inhibiting herbicides and the physiological mechanism of its resistance to diclofop-methyl. Diclofop-methyl dose responses in greenhouse experiments indicated an ED 50 of 1.04 kg ha −1 for the AHS2 susceptible accession and 43.8 kg ha −1 for the AR1 resistant accession with a AR1/AHS2 ED 50 ratio of 42. The diclofop-methyl-resistant AR1 accession was susceptible, i.e., not cross resistant, to both aryloxyphenoxypropionate herbicides, fenoxaprop-P and clodinafop, and cyclohexanedione herbicides, sethoxydim, clethodim and tralkoxydim. Diclofop similarly inhibited the acetyl-CoA carboxylase from the AR1 ( I 50=31 μM) and AHS2 ( I 50=35 μM) accessions. Absorption of [ 14 C ]diclofop-methyl was similar for the two accessions for at least the first 12 h after treatment (HAT), but was greater by the AHS2 than the AR1 accession at 24, 72, and 168 HAT. Metabolism of [ 14 C ]diclofop-methyl to diclofop was similar in AR1 and AHS2 wild oat at 24 and 72 HAT. Therefore, the mechanism of resistance to diclofop-methyl in the AR1 wild oat accession is not due to an altered ACCase enzyme nor due to differential metabolic activation of diclofop-methyl to diclofop. Difference in the pattern of absorption of [ 14 C ]diclofop-methyl between AR1 and AHS2 accessions may have a minor role. The role of other possible mechanisms of resistance is discussed.

The Carboxyltransferase Activity of the Apicoplast Acetyl-CoA Carboxylase of Toxoplasma gondii Is the Target of Aryloxyphenoxypropionate Inhibitors

Inhibition of growth of the apicomplexan parasite Toxoplasma gondii by aryloxyphenoxypropionate herbicides has been correlated with the inhibition of its acetyl-CoA carboxylase (ACC) by these compounds. Here, full-length and C-terminal fragments of T. gondii apicoplast ACC as well as C-terminal fragments of the cytosolic ACC were expressed in Escherichia coli. The recombinant proteins that were soluble showed the expected enzymatic activities. Yeast gene-replacement strains depending for growth on the expressed T. gondii ACC were derived by complementation of a yeast ACC1 null mutation. In vitro and in vivo tests with aryloxyphenoxypropionates showed that the carboxyltransferase domain of the apicoplast T. gondii ACC is the target for this class of inhibitors. The cytosolic T. gondii ACC is resistant to aryloxyphenoxypropionates. Both T. gondii isozymes are resistant to cyclohexanediones, another class of inhibitors targeting the ACC of grass plastids.

Susceptibility of a broad‐leaved weed, Acanthospermum hispidum, to the grass herbicide fluazifop‐butyl

Broad‐leaved plants are generally resistant to aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides. In laboratory experiments, however, we confirmed that Acanthospermum hispidum, a Compositae weed, was susceptible to one of the AOPP herbicides, fluazifop‐butyl, but tolerant to other AOPP herbicides (quizalofop‐ethyl and fenoxaprop‐ethyl) and a CHD herbicide (sethoxydim). The symptoms induced by fluazifop‐butyl in A. hispidum (wilting and necrosis) were distinctly different from those induced in oat (chlorosis). The period required to cause seedling death of A. hispidum (48–72 h) was shorter than that of oat (ca 15 days). The (R)‐enantiomer of fluazifop‐butyl was more active on this weed. In oat, lipid biosynthesis and acetyl‐CoA carboxylase (ACCase) activity were inhibited, and electrolyte leakage from the shoots was increased by fluazifop‐butyl and sethoxydim. In the case of A. hispidum, the membrane permeability increased and the lipid biosynthesis was inhibited only by fluazifop‐butyl. These results indicate that A. hispidum is particularly sensitive to fluazifop‐butyl, and its mechanism of action in the plant may be different from its mechanism of action in oat.

Detection of Fenoxaprop-Resistant Smooth Crabgrass (Digitaria ischaemum) in Turf1

Greenhouse studies verified that smooth crabgrass from a golf course tee in southern New Jersey was resistant to fenoxaprop-P, although smooth crabgrass from a nearby untreated rough was susceptible to this herbicide. Fenoxaprop-P-resistant plants were injured, however, when fenoxaprop-P was applied at levels above the maximum use rate for turf. Fenoxaprop-P applied postemergence at 0.76 kg ai/ha, four times the maximum rate, reduced shoot weight of the resistant biotype by 35%, whereas application of 1.52 kg/ha, eight times the maximum rate, reduced shoot weight by 64%. Large crabgrass and the susceptible biotype of smooth crabgrass were controlled by fenoxaprop-P applied at 0.1 kg/ha. Fenoxaprop-susceptible and -resistant biotypes of smooth crabgrass had a similar response to MSMA, dithiopyr, and quinclorac applied postemergence. The fenoxaprop-P–resistant smooth crabgrass biotype was controlled by the cyclohexanedione herbicides sethoxydim and clethodim, but the aryloxyphenoxypropionate herbicides flua...

PCR-based detection of resistance to acetyl-CoA carboxylase-inhibiting herbicides in black-grass (Alopecurus myosuroides Huds) and ryegrass (Lolium rigidum gaud).

A simple method based upon allele-specific PCR was developed to detect an isoleucine-leucine substitution in the gene encoding chloroplastic acetyl-coenzyme A carboxylase (ACCase) in two gramineous weeds: Lolium rigidum Gaud and Alopecurus myosuroides Huds. Analysis of 1800 A myosuroides and 750 L rigidum seedlings showed that the presence of ACCase leucine allele(s) conferred cross-resistance to the cyclohexanedione herbicide cycloxydim and to the aryloxyphenoxypropionate herbicides fenoxaprop-P-ethyl and diclofop-methyl. Seedlings containing ACCase leucine allele(s) could be either sensitive or resistant to the aryloxyphenoxypropionate herbicides haloxyfop-P-methyl and clodinafop-propargyl. Successful detection of resistant plants in a field population of A myosuroides was achieved using this PCR assay. Using it with basic molecular biology laboratory equipment, the presence of resistant leucine ACCase allele(s) can be detected within one working day.

The mechanism of resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides in a johnsongrass biotype

Acetyl-coenzyme A carboxylase (ACCase) assays and absorption, translocation, and metabolism experiments were conducted to investigate the mechanism(s) responsible for resistance in a johnsongrass biotype that exhibited low levels of resistance to the cyclohexanedione (CHD) herbicide sethoxydim and the aryloxyphenoxypropionate (APP) herbicides quizalofop-P and fluazifop-P. The rate of [14C]quizalofop-ethyl absorption was significantly higher in the resistant compared to the susceptible biotype 8, 24, and 48 h after treatment (HAT), but by 72 HAT, there was no significant difference in the amount of [14C]quizalofop-ethyl detected in either biotype. Additionally, little or no differences in the translocation of [14C]quizalofop-ethyl were observed in the resistant and susceptible biotypes at any time interval after application. In [14C]quizalofop-ethyl metabolism experiments, similar levels of quizalofop-ethyl and quizalofop metabolites were observed in the resistant and susceptible biotypes 8, 24, 48, and 72 HAT, but slightly higher levels of quizalofop acid were detected in the resistant biotype 48 and 72 HAT. In ACCase assays, the concentrations of quizalofop-P, clethodim, and sethoxydim that inhibited ACCase activity by 50% (I50) were statistically similar in the two biotypes, indicating that the resistant johnsongrass biotype contains an ACCase that is sensitive to the APP and CHD herbicides. In the absence of APP or CHD herbicides, however, the specific activity of ACCase in the resistant biotype was two to three times greater than that of the susceptible biotype. The specific activity of ACCase in the resistant biotype was also significantly greater than that of the susceptible biotype in the presence of all concentrations of quizalofop-P and sethoxydim and in the presence of 0.1, 1, and 10 µM clethodim. These results suggest that resistance to quizalofop-P and sethoxydim is conferred by an overproduction of ACCase in the resistant johnsongrass biotype.

Biochemical mechanisms of cross‐resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides in populations of Avena spp.

Aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicides are used extensively in the UK to control grass weeds, including Avena spp. (wild‐oats). Reports of resistance to APP and CHD herbicides are a particular concern for the agricultural community. In this study, the responses of four UK Avena populations were characterized towards the APP herbicides fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl, and towards the CHD herbicides cycloxydim and tralkoxydim. An A. sterilis ssp. ludoviciana population (T/41) was found to be highly resistant to fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl, but did not show cross‐resistance to cycloxydim and tralkoxydim. In contrast, one A. sterilis ssp. ludoviciana (T/11) and one A. fatua population (Dorset) showed partial resistance to both APP herbicides and also showed cross‐resistance to the CHD herbicide tralkoxydim, but not to cycloxydim. Before this study, the biochemical mechanisms that confer resistance to the APP and CHD herbicides in UK Avena populations were unknown. Results from the present study show that an enhanced rate of metabolism of fenoxaprop‐P‐ethyl was found to confer resistance in the two partially resistant Avena populations (T/11 and Dorset), and the presence of an insensitive form of the target enzyme, ACCase, was responsible for target site resistance to fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl in the highly resistant population T/41. Cross‐resistance to the CHD herbicide tralkoxydim in the T/11 and Dorset populations was not conferred by insensitive ACCase, and was most probably caused by enhanced metabolism. This is the first report that resistance to fenoxaprop‐P‐ethyl can be conferred by enhanced metabolism in Avena spp.

A pollen test to detect ACCase target‐site resistance within Alopecurus myosuroides populations

This study was conducted to determine a suitable medium for in vitro germination of Alopecurus myosuroides pollen and to develop a reliable test for the rapid screening of ACCase target site‐resistant plants within populations. The assay is based upon germination of pollen in a medium supplemented with ACCase inhibitors. A 0.25% agar medium, containing 200 mg L–1 CaNO3, 100 mg L–1 H3BO3, 200 g L–1 sucrose, was selected as a suitable medium for in vitro pollen germination. At 25 °C, this medium supported a mean germination rate of 85% within two hours. Plants highly resistant (Rh) to aryloxyphenoxypropionate (APP), owing to the expression of an insensitive ACCase, were found to express this resistance in their pollen. In contrast, plants moderately resistant (Rm) to APP herbicides, owing to an enhanced capacity to detoxify herbicides, did not exhibit this resistance in their pollen. Concentrations of 120 μM fenoxaprop and 1000 μM clodinafop were selected as the best for reliable discrimination of the target‐site‐resistant biotypes. At these concentrations there was more than 50% germination of the Rh pollen grains whereas less than 10% of the S and Rm pollen grains germinated. This test, using haploid material, may also permit distinction between homozygous‐ and heterozygous‐resistant individuals.

Dicotyledons lacking the multisubunit form of the herbicide-target enzyme acetyl coenzyme A carboxylase may be restricted to the family Geraniaceae

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 The aryloxyphenoxypropionate herbicide haloxyfop is transported in the phloem to the sink tissue where, in certain species, it disrupts the production of lipids that are essential for the functioning of membranes and organelles involved in the assimilation, partitioning and transport of carbon. Haloxyfop inhibits a key regulatory enzyme of lipid synthesis, acetyl coenzyme A carboxylase (ACCase), in species that lack a herbicide-insensitive multisubunit (MS) form of ACCase found in most plants. The absence of MS-ACCase, and sensitivity to haloxyfop, was considered to be restricted to monocotyledons in the family Gramineae but has recently been demonstrated for the dicotyledon Erodium moschatum (Geraniaceae). Species related to E. moschatum were examined to determine how widespread this phenomenon is among dicotyledons. In the two families most closely related to the Geraniaceae, four species in the Oxalidaceae and one species in the Tropaeolaceae respectively retained MS-ACCase. Within the family Geraniaceae, certain species in the genera Erodium and Pelargonium, but not those in the genus Geranium, have lost MS-ACCase, indicating that this phenomenon may be restricted to Erodium and Pelargonium. When treated with 104 g ai ha–1 haloxyfop-ethoxyethyl, plants of all 15 species retaining MS-ACCase were resistant while 8 out of 13 species lacking MS-ACCase were susceptible. It is noteworthy that five species lacking MS-ACCase were nonetheless resistant. The mechanism(s) of resistance in such species remains to be determined.