Twenty-two amino acid substitutions at seven conserved amino acid residues in the acetohydroxyaci... more Twenty-two amino acid substitutions at seven conserved amino acid residues in the acetohydroxyacid synthase (AHAS) gene have been identified to date that confer target-site resistance to AHAS-inhibiting herbicides in biotypes of field-evolved resistant weed species. However, the effect of resistance mutations on AHAS functionality and plant growth has been investigated for only a very few mutations. This research investigates the effect of various AHAS resistance mutations in Lolium rigidum on AHAS functionality and plant growth. The enzyme kinetics of AHAS from five purified L. rigidum populations, each homozygous for the resistance mutations Pro-197-Ala, Pro-197-Arg, Pro-197-Gln, Pro-197-Ser or Trp-574-Leu, were characterized and the pleiotropic effect of three mutations on plant growth was assessed via relative growth rate analysis. All these resistance mutations endowed a herbicide-resistant AHAS and most resulted in higher extractable AHAS activity, with no-to-minor changes in AHAS kinetics. The Pro-197-Arg mutation slightly (but significantly) increased the K m for pyruvate and remarkably increased sensitivity to feedback inhibition by branched chain amino acids. Whereas the Pro-197-Ser and Trp-574-Leu mutations exhibited no significant effects on plant growth, the Pro-197-Arg mutation resulted in lower growth rates. It is clear that, at least in L. rigidum, these five AHAS resistance mutations have no major impact on AHAS functionality and hence probably no plant resistance costs. These results, in part, explain why so many Pro-197 AHAS resistance mutations in AHAS have evolved and why the Pro-197-Ser and the Trp-574-Leu AHAS resistance mutations are frequently found in many weed species.
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enab... more The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enabled the routine use of an alternative weed control technology at a novel weed control timing in global grain cropping fields. Driven by the significant threat of widespread populations of annual ryegrass (Lolium rigidum) with multiple-herbicide resistance, in the 1990s Australian growers and researchers developed techniques to target, at grain harvest, the seed production of annual ryegrass and other important weed species. The HWSC approach to weed management is now routinely used by a majority of Australian grain producers as an integral component of effective weed control programs. Here we detail the development and introduction of current HWSC systems and describe their efficacy in Australian grain production systems. The use of HWSC has likely contributed to lower annual ryegrass population densities and thus mitigated the impacts of herbicide resistance as well as slowing further evolution of resistance. In addition, low weed densities enable the introduction of site-specific weed control technologies and the opportunity to target specific in-crop weeds with non-selective alternative weed control techniques. With an awareness of the evolutionary potential of weed species to adapt to all forms of weed control, there is an understanding that HWSC treatments need to be judiciously used in grain cropping systems to ensure their ongoing efficacy. The successful use of Australian developed HWSC systems has attracted global interest and there is now a considerable international research effort aimed at introducing this alternative weed control approach and timing into the world's major cropping systems.
Abstract: In 1998, field populations of wild radish suspected of being resistant to chlorsulfuron... more Abstract: In 1998, field populations of wild radish suspected of being resistant to chlorsulfuron were collected and screened for resistance to this herbicide using the Quick-Test technique. This test successfully identified chlorsulfuron-resistant populations of wild radish. Detailed ...
Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for s... more Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for selective control of broadleaf weeds in cereals and transgenic crops. Although the troublesome weed wild radish ( Raphanus raphanistrum) has developed resistance to 2,4-D, no populations have yet displayed an enhanced capacity for metabolic detoxification of the herbicide, with both susceptible and resistant wild radish plants readily metabolizing 2,4-D. Using mass spectrometry and nuclear magnetic resonance, the major 2,4-D metabolite was identified as the glucose ester, and its structure was confirmed by synthesis. As expected, both the endogenous and synthetic compounds retained auxin activity in a bioassay. The lack of detectable 2,4-D hydroxylation in wild radish and the lability of the glucose ester suggest that metabolic 2,4-D resistance is unlikely to develop in this species.
Research conducted by L Hunt. A Blake completed this article after L Hunt's resignation from DAFW... more Research conducted by L Hunt. A Blake completed this article after L Hunt's resignation from DAFWA. KEY MESSAGES x Pre-emergent herbicide treatments were more effective than post-emergence at controlling slender iceplant (Mesembryanthemum nodiflorum). x The only fully registered option for controlling slender iceplant in WA is dicamba in a mix with 2,4-D amine in non-crop situations. The Liebe group also holds minor use permits for the use of Ally®, Glean® and atrazine for controlling slender iceplant in cereal and pasture paddocks valid until October 2010 (Permit number PER8226). AIMS Slender iceplant is a common weed of saline valley floor areas in the northern wheatbelt of WA but can also be found on non-saline areas. Registered options for controlling slender iceplant are limited. This research aims to identify treatments with potential to be registered for controlling slender iceplant in both crop and pasture situations. METHOD Four trials were conducted in 2008 at two sites, one site near Carnamah and one site near Morowa. The Carnamah site was red cracking clay and received 247 mm of April-October rainfall. The Morowa site was red clay-loam over a red-brown hardpan and received 241mm April-October rainfall. At each site one trial assessed pre emergent herbicide treatment and another trial assessed post emergent herbicide treatments aimed at controlling slender iceplant. The trials were designed as randomised blocks, each with three replicates. Spray strips (20 m x 3 m) were applied on 17 June for pre-emergent trials and on August 12 2008 for the post-emergent trials (60L water volume/ha with 11002 nozzles at 12 km/hr). Control of slender iceplant was visually assessed in August and November and expressed as a percentage reduction in iceplant biomass compared with the unsprayed control. RESULTS Table 1. Percentage reduction in slender iceplant biomass visually assessed at two locations 63 days and 139 days after application of pre-emergent herbicide treatment. Morowa Carnamah Pre-emergent herbicide (rate/ha) 19 August 3 November 19 August 3 November 25 g/ha Broadstrike® (flumetsulam 800 g/kg) 89 90 98 99 15 g/ha Broadstrike® (flumetsulam 800 g/kg) 78 82 93 98 150 mL/ha diflufenican (500 g/L) 100 98 100 99 75 mL/ha diflufenican (500 g/L) 83 48 95 57 1 L/ha diuron (500 g/L) 58 7 99 90 500 mL/ha diuron (500 g/L) 8 0 80 7 3 L/ha Stomp® (pendimethalin 330 g/L) 93 62 100 93 2 L/ha Stomp® (pendimethalin 330 g/L) 73 13 97 85 1 L/ha Stomp® (pendimethalin 330 g/L) 23 0 92 33 200 mL/ha Goal® (oxyfluron 240 g/L) 83 13 93 10 500 mL/ha Goal® (oxyfluron 240 g/L) 100 97 100 98 200 g/ha Lexone ® (Metribuzine 750 g/L) 85 47 93 85 2 L/ha trifluralin (480 g/L) 40 0 83 13 500 mL/ha Dual Gold®
Many herbicide-resistant weed species are polyploids, but far too little about the evolution of r... more Many herbicide-resistant weed species are polyploids, but far too little about the evolution of resistance mutations in polyploids is understood. Hexaploid wild oat (Avena fatua) is a global crop weed and many populations have evolved herbicide resistance. We studied plastidic acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicide resistance in hexaploid wild oat and revealed that resistant individuals can express one, two or three different plastidic ACCase gene resistance mutations (Ile-1781-Leu, Asp-2078-Gly and Cys-2088-Arg). Using ACCase resistance mutations as molecular markers, combined with genetic, molecular and biochemical approaches, we found in individual resistant wild-oat plants that (1) up to three unlinked ACCase gene loci assort independently following Mendelian laws for disomic inheritance, (2) all three of these homoeologous ACCase genes were transcribed, with each able to carry its own mutation and (3) in a hexaploid background, each individual ACCase resistance mutation confers relatively low-level herbicide resistance, in contrast to high-level resistance conferred by the same mutations in unrelated diploid weed species of the Poaceae (grass) family. Low resistance conferred by individual ACCase resistance mutations is likely due to a dilution effect by susceptible ACCase expressed by homoeologs in hexaploid wild oat and/or differential expression of homoeologous ACCase gene copies. Thus, polyploidy in hexaploid wild oat may slow resistance evolution. Evidence of coexisting non-target-site resistance mechanisms among wild-oat populations was also revealed. In all, these results demonstrate that herbicide resistance and its evolution can be more complex in hexaploid wild oat than in unrelated diploid grass weeds. Our data provide a starting point for the daunting task of understanding resistance evolution in polyploids
BACKGROUND: Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene famil... more BACKGROUND: Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target-site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence ⊍-tubulin transcripts. RESULTS: Four ⊍-tubulin isoforms (TUA1, TUA2, TUA3 and TUA4) were identified in L. rigidum. Variations in the number and sequence of transcripts encoding these ⊍-tubulin proteins were found in individuals from the two L. rigidum populations examined. Within and among populations, differences in the 5 0-and 3 0-untranslated regions of cDNA in TUA3 and TUA4 were identified. Furthermore, a novel double mutation, Arg-390-Cys+Asp-442-Glu, in the TUA3 transcript was identified and has the potential to confer dinitroaniline resistance. CONCLUSION: This research reveals the complexity of the ⊍-tubulin gene family in individuals/populations of the crosspollinated weedy species L. rigidum, and highlights the need for better understanding of the molecular architecture of tubulin gene families for detecting resistance point mutations. Although TUA4 is a commonly expressed ⊍-tubulin isoform containing most frequently reported resistance mutations, other mutant tubulin isoforms may also have a role in conferring dinitroaniline resistance.
BACKGROUND: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers ... more BACKGROUND: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers to control Lolium rigidum Gaud. and other weed species in trazine tolerant (TT)-canola fields. A L. rigidum population (R) collected from a TT-canola field from Western Australia showed multiple resistance to PSII, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors. The mechanisms of multiple resistance in this R population were determined. RESULTS: The R population showed a low-level (about 3.0-fold) resistance to the PSII-inhibiting herbicides metribuzin and atrazine. Sequencing of the psbA gene revealed no differences between the R and susceptible (S) sequences. Furthermore, [ 14 C]metribuzin experiments found no significant difference in metribuzin foliar uptake and translocation between the R and S plants. However, [ 14 C]-metribuzin metabolism in R plants was 2.3-fold greater than in S plants. The cytochrome P450 monooxygenase inhibitor piperonyl butoxide (PBO) enhanced plant mortality response to metribuzin and atrazine in both R and S populations. In addition, multiple resistance to ALS and ACCase inhibitors are due to known resistance mutations in ALS and ACCase genes. CONCLUSION: The results demonstrate that enhanced metribuzin metabolism likely involving cytochrome P450 monooxygenase contributes to metribuzin resistance in Lolium rigidum. This is the first report of metabolic resistance to the PSII-inhibiting herbicide metribuzin in Australian Lolium rigidum.
A random survey involving 260 paddocks was conducted to determine the extent of herbicide resista... more A random survey involving 260 paddocks was conducted to determine the extent of herbicide resistant annual ryegrass (Lolium rigidum Gaud.) in the WA cropping belt. Cropped paddocks were randomly selected from within eight adjoining areas determined by rainfall and latitude. These included Badgingarra in the northwest area through to Lake King in the southeast area and were based on the Agriculture WA crop variety testing areas H2, M2, L2, M3, L3, H4, M4 and L4. Surveying took place prior to harvest in 1998 from a 50 m × 50 m area within each paddock. Ryegrass was easily found in 87% of these paddock sampling areas. A visual assessment of ryegrass density was made and seed was collected from paddocks where adequate ryegrass was present. Samples were stored under shelter but exposed to ambient temperatures over summer. Seed was then germinated on agar before being transplanted into trays containing potting mix. 185 populations were grown outdoors for testing in May-June 1999. Initial resistance testing was performed using the recommended field rate of 375 g a.i. ha-l diclofopmethyl (1L ha-l Hoegrass with non-ionic surfactant). Populations which were found to have greater than 25% of ryegrass plants surviving were classified as resistant. The percentage of paddocks containing a herbicide resistant population was found to vary significantly between agronomic areas. Further work is establishing the extent of resistance to clethodim (Select ®) and the relationship between resistance status and observed ryegrass density. Results will be presented.
One-sentence summary: The plant metabolic enzyme aldo-keto reductase has evolved to metabolize gl... more One-sentence summary: The plant metabolic enzyme aldo-keto reductase has evolved to metabolize glyphosate in a glyphosate-resistant weed species
An overreliance on herbicides in several important grain- and cotton-producing regions of the wor... more An overreliance on herbicides in several important grain- and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. We use the term herbicide-only syndrome (HOS) for this quasi-addiction to herbicides. Growers and their advisers focus on herbicide technology, unaware of or ignoring basic evolutionary principles or the necessary diversity provided by other methods of weed control. Diversity in weed control practices disrupts resistance evolution. Significant challenges exist to implementing diversity, including how to address information so that producers choose to alter existing behaviors (HOS) and take calculated risks by attempting new and more complex strategies. Herbicide resistance management in the long term will require creativity in many sectors...
Making crops and weeds research interesting, understandable, and accessible to all. Colleagues, S... more Making crops and weeds research interesting, understandable, and accessible to all. Colleagues, Since the late 1980's it has been evident to us that herbicide resistance in Lolium populations involves non-target site enhanced rates of herbicide metabolism as a very widespread and important resistance mechanism. Of course, Lolium exhibits multiple resistance mechanisms, embracing non-target site resistance mechanisms and target site mutations. As knowledge and technology developed it was possible to identify target site ALS, ACCase mutations, EPSPS gene mutations in resistant Lolium. However, we have been far less successful in identifying non-target site resistance genes.
HWSC systems that target weed seed production during harvest have been in use in Australian crop ... more HWSC systems that target weed seed production during harvest have been in use in Australian crop production systems for over 30 years. Until recently, though, grower adoption of these systems has been relatively low. It is now apparent with the introduction of a range of new weed seed targeting systems that there is renewed grower interest in the use of this approach to weed control. With the aim of determining the current adoption and use of HWSC systems, 600 crop producers from throughout Australia’s cropping regions were interviewed on their adoption and use of these systems. This survey established that 43% of Australian growers are now routinely using HWSC to target weed seed production during grain harvest. The adoption of narrow-windrow burning (30%) was considerably greater than the other currently available techniques of chaff tramlining (7%), chaff carts (3%), bale-direct system (3%), and the Harrington Seed Destructor (HSD) (<1%). When growers were asked about their fu...
BACKGROUND: The auxinic herbicide 2,4-D amine is known, in vitro, as a cytochrome P450 inducer. T... more BACKGROUND: The auxinic herbicide 2,4-D amine is known, in vitro, as a cytochrome P450 inducer. The current study uses 2,4-D pre-treatment, at the whole plant level, to study mechanism(s) of non-target site based herbicide resistance to the ACCase-inhibiting herbicide diclofop-methyl in Lolium rigidum. RESULTS: The 2,4-D pre-treatment caused up to 10-fold shift in LD50 and GR50 in dose–response to subsequently applied diclofop-methyl in a herbicide susceptible L. rigidum population. Foliar uptake and translocation of 14C-diclofop-methyl did not differ in 2,4-D pre-treated versus untreated plants. HPLC analysis revealed that de-esterification of diclofop-methyl to toxic diclofop acid was similar, but further metabolism of diclofop acid to non-toxic metabolites was significantly (1.8-fold) faster in 2,4-D pre-treated than untreated plants. HPLC profile of major polar metabolites was similar when L. rigidum and diclofop-methyl tolerant wheat were compared, but wheat metabolised diclofo...
Twenty-two amino acid substitutions at seven conserved amino acid residues in the acetohydroxyaci... more Twenty-two amino acid substitutions at seven conserved amino acid residues in the acetohydroxyacid synthase (AHAS) gene have been identified to date that confer target-site resistance to AHAS-inhibiting herbicides in biotypes of field-evolved resistant weed species. However, the effect of resistance mutations on AHAS functionality and plant growth has been investigated for only a very few mutations. This research investigates the effect of various AHAS resistance mutations in Lolium rigidum on AHAS functionality and plant growth. The enzyme kinetics of AHAS from five purified L. rigidum populations, each homozygous for the resistance mutations Pro-197-Ala, Pro-197-Arg, Pro-197-Gln, Pro-197-Ser or Trp-574-Leu, were characterized and the pleiotropic effect of three mutations on plant growth was assessed via relative growth rate analysis. All these resistance mutations endowed a herbicide-resistant AHAS and most resulted in higher extractable AHAS activity, with no-to-minor changes in AHAS kinetics. The Pro-197-Arg mutation slightly (but significantly) increased the K m for pyruvate and remarkably increased sensitivity to feedback inhibition by branched chain amino acids. Whereas the Pro-197-Ser and Trp-574-Leu mutations exhibited no significant effects on plant growth, the Pro-197-Arg mutation resulted in lower growth rates. It is clear that, at least in L. rigidum, these five AHAS resistance mutations have no major impact on AHAS functionality and hence probably no plant resistance costs. These results, in part, explain why so many Pro-197 AHAS resistance mutations in AHAS have evolved and why the Pro-197-Ser and the Trp-574-Leu AHAS resistance mutations are frequently found in many weed species.
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enab... more The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enabled the routine use of an alternative weed control technology at a novel weed control timing in global grain cropping fields. Driven by the significant threat of widespread populations of annual ryegrass (Lolium rigidum) with multiple-herbicide resistance, in the 1990s Australian growers and researchers developed techniques to target, at grain harvest, the seed production of annual ryegrass and other important weed species. The HWSC approach to weed management is now routinely used by a majority of Australian grain producers as an integral component of effective weed control programs. Here we detail the development and introduction of current HWSC systems and describe their efficacy in Australian grain production systems. The use of HWSC has likely contributed to lower annual ryegrass population densities and thus mitigated the impacts of herbicide resistance as well as slowing further evolution of resistance. In addition, low weed densities enable the introduction of site-specific weed control technologies and the opportunity to target specific in-crop weeds with non-selective alternative weed control techniques. With an awareness of the evolutionary potential of weed species to adapt to all forms of weed control, there is an understanding that HWSC treatments need to be judiciously used in grain cropping systems to ensure their ongoing efficacy. The successful use of Australian developed HWSC systems has attracted global interest and there is now a considerable international research effort aimed at introducing this alternative weed control approach and timing into the world's major cropping systems.
Abstract: In 1998, field populations of wild radish suspected of being resistant to chlorsulfuron... more Abstract: In 1998, field populations of wild radish suspected of being resistant to chlorsulfuron were collected and screened for resistance to this herbicide using the Quick-Test technique. This test successfully identified chlorsulfuron-resistant populations of wild radish. Detailed ...
Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for s... more Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for selective control of broadleaf weeds in cereals and transgenic crops. Although the troublesome weed wild radish ( Raphanus raphanistrum) has developed resistance to 2,4-D, no populations have yet displayed an enhanced capacity for metabolic detoxification of the herbicide, with both susceptible and resistant wild radish plants readily metabolizing 2,4-D. Using mass spectrometry and nuclear magnetic resonance, the major 2,4-D metabolite was identified as the glucose ester, and its structure was confirmed by synthesis. As expected, both the endogenous and synthetic compounds retained auxin activity in a bioassay. The lack of detectable 2,4-D hydroxylation in wild radish and the lability of the glucose ester suggest that metabolic 2,4-D resistance is unlikely to develop in this species.
Research conducted by L Hunt. A Blake completed this article after L Hunt's resignation from DAFW... more Research conducted by L Hunt. A Blake completed this article after L Hunt's resignation from DAFWA. KEY MESSAGES x Pre-emergent herbicide treatments were more effective than post-emergence at controlling slender iceplant (Mesembryanthemum nodiflorum). x The only fully registered option for controlling slender iceplant in WA is dicamba in a mix with 2,4-D amine in non-crop situations. The Liebe group also holds minor use permits for the use of Ally®, Glean® and atrazine for controlling slender iceplant in cereal and pasture paddocks valid until October 2010 (Permit number PER8226). AIMS Slender iceplant is a common weed of saline valley floor areas in the northern wheatbelt of WA but can also be found on non-saline areas. Registered options for controlling slender iceplant are limited. This research aims to identify treatments with potential to be registered for controlling slender iceplant in both crop and pasture situations. METHOD Four trials were conducted in 2008 at two sites, one site near Carnamah and one site near Morowa. The Carnamah site was red cracking clay and received 247 mm of April-October rainfall. The Morowa site was red clay-loam over a red-brown hardpan and received 241mm April-October rainfall. At each site one trial assessed pre emergent herbicide treatment and another trial assessed post emergent herbicide treatments aimed at controlling slender iceplant. The trials were designed as randomised blocks, each with three replicates. Spray strips (20 m x 3 m) were applied on 17 June for pre-emergent trials and on August 12 2008 for the post-emergent trials (60L water volume/ha with 11002 nozzles at 12 km/hr). Control of slender iceplant was visually assessed in August and November and expressed as a percentage reduction in iceplant biomass compared with the unsprayed control. RESULTS Table 1. Percentage reduction in slender iceplant biomass visually assessed at two locations 63 days and 139 days after application of pre-emergent herbicide treatment. Morowa Carnamah Pre-emergent herbicide (rate/ha) 19 August 3 November 19 August 3 November 25 g/ha Broadstrike® (flumetsulam 800 g/kg) 89 90 98 99 15 g/ha Broadstrike® (flumetsulam 800 g/kg) 78 82 93 98 150 mL/ha diflufenican (500 g/L) 100 98 100 99 75 mL/ha diflufenican (500 g/L) 83 48 95 57 1 L/ha diuron (500 g/L) 58 7 99 90 500 mL/ha diuron (500 g/L) 8 0 80 7 3 L/ha Stomp® (pendimethalin 330 g/L) 93 62 100 93 2 L/ha Stomp® (pendimethalin 330 g/L) 73 13 97 85 1 L/ha Stomp® (pendimethalin 330 g/L) 23 0 92 33 200 mL/ha Goal® (oxyfluron 240 g/L) 83 13 93 10 500 mL/ha Goal® (oxyfluron 240 g/L) 100 97 100 98 200 g/ha Lexone ® (Metribuzine 750 g/L) 85 47 93 85 2 L/ha trifluralin (480 g/L) 40 0 83 13 500 mL/ha Dual Gold®
Many herbicide-resistant weed species are polyploids, but far too little about the evolution of r... more Many herbicide-resistant weed species are polyploids, but far too little about the evolution of resistance mutations in polyploids is understood. Hexaploid wild oat (Avena fatua) is a global crop weed and many populations have evolved herbicide resistance. We studied plastidic acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicide resistance in hexaploid wild oat and revealed that resistant individuals can express one, two or three different plastidic ACCase gene resistance mutations (Ile-1781-Leu, Asp-2078-Gly and Cys-2088-Arg). Using ACCase resistance mutations as molecular markers, combined with genetic, molecular and biochemical approaches, we found in individual resistant wild-oat plants that (1) up to three unlinked ACCase gene loci assort independently following Mendelian laws for disomic inheritance, (2) all three of these homoeologous ACCase genes were transcribed, with each able to carry its own mutation and (3) in a hexaploid background, each individual ACCase resistance mutation confers relatively low-level herbicide resistance, in contrast to high-level resistance conferred by the same mutations in unrelated diploid weed species of the Poaceae (grass) family. Low resistance conferred by individual ACCase resistance mutations is likely due to a dilution effect by susceptible ACCase expressed by homoeologs in hexaploid wild oat and/or differential expression of homoeologous ACCase gene copies. Thus, polyploidy in hexaploid wild oat may slow resistance evolution. Evidence of coexisting non-target-site resistance mechanisms among wild-oat populations was also revealed. In all, these results demonstrate that herbicide resistance and its evolution can be more complex in hexaploid wild oat than in unrelated diploid grass weeds. Our data provide a starting point for the daunting task of understanding resistance evolution in polyploids
BACKGROUND: Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene famil... more BACKGROUND: Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target-site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence ⊍-tubulin transcripts. RESULTS: Four ⊍-tubulin isoforms (TUA1, TUA2, TUA3 and TUA4) were identified in L. rigidum. Variations in the number and sequence of transcripts encoding these ⊍-tubulin proteins were found in individuals from the two L. rigidum populations examined. Within and among populations, differences in the 5 0-and 3 0-untranslated regions of cDNA in TUA3 and TUA4 were identified. Furthermore, a novel double mutation, Arg-390-Cys+Asp-442-Glu, in the TUA3 transcript was identified and has the potential to confer dinitroaniline resistance. CONCLUSION: This research reveals the complexity of the ⊍-tubulin gene family in individuals/populations of the crosspollinated weedy species L. rigidum, and highlights the need for better understanding of the molecular architecture of tubulin gene families for detecting resistance point mutations. Although TUA4 is a commonly expressed ⊍-tubulin isoform containing most frequently reported resistance mutations, other mutant tubulin isoforms may also have a role in conferring dinitroaniline resistance.
BACKGROUND: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers ... more BACKGROUND: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers to control Lolium rigidum Gaud. and other weed species in trazine tolerant (TT)-canola fields. A L. rigidum population (R) collected from a TT-canola field from Western Australia showed multiple resistance to PSII, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors. The mechanisms of multiple resistance in this R population were determined. RESULTS: The R population showed a low-level (about 3.0-fold) resistance to the PSII-inhibiting herbicides metribuzin and atrazine. Sequencing of the psbA gene revealed no differences between the R and susceptible (S) sequences. Furthermore, [ 14 C]metribuzin experiments found no significant difference in metribuzin foliar uptake and translocation between the R and S plants. However, [ 14 C]-metribuzin metabolism in R plants was 2.3-fold greater than in S plants. The cytochrome P450 monooxygenase inhibitor piperonyl butoxide (PBO) enhanced plant mortality response to metribuzin and atrazine in both R and S populations. In addition, multiple resistance to ALS and ACCase inhibitors are due to known resistance mutations in ALS and ACCase genes. CONCLUSION: The results demonstrate that enhanced metribuzin metabolism likely involving cytochrome P450 monooxygenase contributes to metribuzin resistance in Lolium rigidum. This is the first report of metabolic resistance to the PSII-inhibiting herbicide metribuzin in Australian Lolium rigidum.
A random survey involving 260 paddocks was conducted to determine the extent of herbicide resista... more A random survey involving 260 paddocks was conducted to determine the extent of herbicide resistant annual ryegrass (Lolium rigidum Gaud.) in the WA cropping belt. Cropped paddocks were randomly selected from within eight adjoining areas determined by rainfall and latitude. These included Badgingarra in the northwest area through to Lake King in the southeast area and were based on the Agriculture WA crop variety testing areas H2, M2, L2, M3, L3, H4, M4 and L4. Surveying took place prior to harvest in 1998 from a 50 m × 50 m area within each paddock. Ryegrass was easily found in 87% of these paddock sampling areas. A visual assessment of ryegrass density was made and seed was collected from paddocks where adequate ryegrass was present. Samples were stored under shelter but exposed to ambient temperatures over summer. Seed was then germinated on agar before being transplanted into trays containing potting mix. 185 populations were grown outdoors for testing in May-June 1999. Initial resistance testing was performed using the recommended field rate of 375 g a.i. ha-l diclofopmethyl (1L ha-l Hoegrass with non-ionic surfactant). Populations which were found to have greater than 25% of ryegrass plants surviving were classified as resistant. The percentage of paddocks containing a herbicide resistant population was found to vary significantly between agronomic areas. Further work is establishing the extent of resistance to clethodim (Select ®) and the relationship between resistance status and observed ryegrass density. Results will be presented.
One-sentence summary: The plant metabolic enzyme aldo-keto reductase has evolved to metabolize gl... more One-sentence summary: The plant metabolic enzyme aldo-keto reductase has evolved to metabolize glyphosate in a glyphosate-resistant weed species
An overreliance on herbicides in several important grain- and cotton-producing regions of the wor... more An overreliance on herbicides in several important grain- and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. We use the term herbicide-only syndrome (HOS) for this quasi-addiction to herbicides. Growers and their advisers focus on herbicide technology, unaware of or ignoring basic evolutionary principles or the necessary diversity provided by other methods of weed control. Diversity in weed control practices disrupts resistance evolution. Significant challenges exist to implementing diversity, including how to address information so that producers choose to alter existing behaviors (HOS) and take calculated risks by attempting new and more complex strategies. Herbicide resistance management in the long term will require creativity in many sectors...
Making crops and weeds research interesting, understandable, and accessible to all. Colleagues, S... more Making crops and weeds research interesting, understandable, and accessible to all. Colleagues, Since the late 1980's it has been evident to us that herbicide resistance in Lolium populations involves non-target site enhanced rates of herbicide metabolism as a very widespread and important resistance mechanism. Of course, Lolium exhibits multiple resistance mechanisms, embracing non-target site resistance mechanisms and target site mutations. As knowledge and technology developed it was possible to identify target site ALS, ACCase mutations, EPSPS gene mutations in resistant Lolium. However, we have been far less successful in identifying non-target site resistance genes.
HWSC systems that target weed seed production during harvest have been in use in Australian crop ... more HWSC systems that target weed seed production during harvest have been in use in Australian crop production systems for over 30 years. Until recently, though, grower adoption of these systems has been relatively low. It is now apparent with the introduction of a range of new weed seed targeting systems that there is renewed grower interest in the use of this approach to weed control. With the aim of determining the current adoption and use of HWSC systems, 600 crop producers from throughout Australia’s cropping regions were interviewed on their adoption and use of these systems. This survey established that 43% of Australian growers are now routinely using HWSC to target weed seed production during grain harvest. The adoption of narrow-windrow burning (30%) was considerably greater than the other currently available techniques of chaff tramlining (7%), chaff carts (3%), bale-direct system (3%), and the Harrington Seed Destructor (HSD) (<1%). When growers were asked about their fu...
BACKGROUND: The auxinic herbicide 2,4-D amine is known, in vitro, as a cytochrome P450 inducer. T... more BACKGROUND: The auxinic herbicide 2,4-D amine is known, in vitro, as a cytochrome P450 inducer. The current study uses 2,4-D pre-treatment, at the whole plant level, to study mechanism(s) of non-target site based herbicide resistance to the ACCase-inhibiting herbicide diclofop-methyl in Lolium rigidum. RESULTS: The 2,4-D pre-treatment caused up to 10-fold shift in LD50 and GR50 in dose–response to subsequently applied diclofop-methyl in a herbicide susceptible L. rigidum population. Foliar uptake and translocation of 14C-diclofop-methyl did not differ in 2,4-D pre-treated versus untreated plants. HPLC analysis revealed that de-esterification of diclofop-methyl to toxic diclofop acid was similar, but further metabolism of diclofop acid to non-toxic metabolites was significantly (1.8-fold) faster in 2,4-D pre-treated than untreated plants. HPLC profile of major polar metabolites was similar when L. rigidum and diclofop-methyl tolerant wheat were compared, but wheat metabolised diclofo...
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