Acetolactate Synthase Acetohydroxyacid SynthaseInhibiting Herbicides

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Acetolactate Synthase/ Acetohydroxyacid
Synthase-Inhibiting Herbicides

• Stefan Meyer
• Herbicide Physiology 470
• HALLOWEEN 2006

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Site of Action 1 Branch Chain Amino acids

• Pyruvate (3C)
• Ketobutyrate(4C)
• AA (5-6 C)

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Site of Action 1 Pathway
Powles Holtum 1994 Shaner and Singh 1997
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Site of Action 3

• Acetolactate Synthase enzyme blocked
• Branched chain AA Valine, Leucine
• ? Protein Synthesis ? Growth
• Reduced photosynthate distribution

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Site of Action 3 Metabolism and Enzymology

• Inhibition of AHAS
• causes the buildup of 2-ketobutyrate and
  2-aminobutyrate
• accumulation causes phytotoxicity false
• Starvation for branched-chain AA
• isoleucine
• Shaner and Singh . 2002

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Herbicide Structure 1

• Herbicide Classes
• Herbicide structure
• Molecule alterations
• Different
• Potency
• Selectivity
• Weed control spectrum
• 50 ALS inhibiting compounds exist
• (Tranel Wright 2002)

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Herbicide Structure 2 5 Classes of ALS
Inhibitors

• imidazolinones (IMI)
• sulfonylureas (SU)
• pyrimidynyloxybenzoates (POB)
• triazolopyrimidines (TP)
• sulfonylamino-carbonyl-triazolinones

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Herbicide Structure 3 Imadazolinone (IMI)

• Imidazolinone ring
• Carboxylic acid
• Back bone
• Imazamethabenz-methyl (Wht/BLY)
• Assert
• Imazethapyr(legume)
• Pursuit, Pursuit Ultra

Carboxylic Acid
backbone
Imi ring
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Herbicide Structure 4Sulfonylureas
Aryl group

• aryl group
• bridge
• Heterocycle
• Sulfosulfuron
• Sundance (Monsanto)
• Thifensulfuron-methyl
• Triumph Plus, Refine Extra (DuPont)
  physiological resistance)
• Ethametsulforun methyl
• Muster (Dupont)
• 2-8g/ha

bridge
Heterocycle
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Herbicide Structure 5 Triazolopyrimidine

• Florasulam (Cereals)
• Frontline (DowAgro)
• Hydroxylation- Metabolically detoxed
• Cereals go to AA
• 5x less toxic
• Soybeans- pyrimidine ring

Pyrimidine
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Herbicide Structure 6 Sulfonylamino-carbonyl-tria
zolinones

• flucarbazone
• Everest

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Absorption and Translocation
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Absorption and Translocation

• Need to inhibit Growing points
• AA occurs there
• Weak acid trapping
• Hydrophilic at high pH
• GER s/t donnan-exclusion
• Energy dependant uptake
• Rapid absorption and translocation

Ion trapping of Imi
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Plant Uptake
Devine
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Foliar Absorption ( of Applied)

Discuss Hot Dry Palliser Triangle vs. Beautiful
Precipitous lush warm Peace
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Root Absorption

• Passive UpTake
• Lipophillic
• pH 5.6 vs 2.2
• Adsorbed/Unavailable under highly acid conditions
• Increase dose
• OM, moisture, clay

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Translocation 1 (Sweet)

• Xylem mobility
• Physiochemical characteristics
• Lipohillicty increases translocation
• Phloem Mobility
• Varies
• Species dependant
• Weed dependant
• Weak Acid Trapping

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Translocation 2 Comparing Imadazolinones
(Shaner Singh. 1997).
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Foliar vs Root translocation
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Symptoms 1
Bottle Brush/Witches Broom
Darkened Veins
Residual Carryover
Chlorotic Leaves
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Symptoms 2

• Roots
• Bottle brush growth
• More sensitive than shoots
• Stems/Leaves
• Inward yellowing from edges to veins
• Purpling-especially veins
• Stunting
• Growth inhibited 1-2 days
• Symptoms 1-2 weeks
• Young leaves die first

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Symptoms 3
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Selectivity 1

• Differential
• Metabolism
• Ie natural tolerance of crops
• Translocation and absorption
• Ie not really
• Inhibition at site of action
• Ie Transgenic corn

Brown Miller.1987
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Selectivity 2
Devine et al, 1993
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Selectivity 3 Sulfonylurea herbicides

• Metabolize
• at what rate
• Into ACID
• Detoxify
• Conjugate
• Devine et al, 1993

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Selectivity 4 Differential Activation

• Imazamethabenz methyl
• Selectively controls wild oats
• Glucose conjuction
• Ethametasulfuron-muster in Canola
• Controls wild mustard-very selective
• Resistance issues
• O-dealkylation
• (Shaner Singh. 1997) 146.

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What do the Schweitzer, Stefan and Sam have in
common?
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Broad Leave Herbicide Rotation
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Grass Herbicide Rotation
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Resistance

• 95 species
• 48 dicot
• 22 monocot
• Kochia
• First resistance (Christopher et al, 1992)
• Largest number of biotypes
• (Degiorgio King. 1998)

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Target-site Based Resistance 1
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Target-site Based Resistance 2

• Point mutation( Tranel Wright. 2002)
• AA changes- partially dominant- gene passed onto
  offspring through pollen/seed
• Modified herbicide binding site
• L. rigidum
• Herbicide-insensitive ALS (Christopher et
  al.,1992)
• Increased metabolism

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Target-site Based Resistance 3
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Metabolism Based Resistance

• Increased herbicide metabolism (TranelWright
  2002)
• Detoxification
• Cytochrome P450
• Nazi-Soviet Pact
• Relationship with ACCase and Photosystem ll

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Cross-Resistance

• Cross-Resistance first reported in Rye Grass
  (Tranel Wright. 2002)
• Three types of cross resistance
• SU , TP
• IMI,PTB
• SU,IMI,TB PTB broad cross resistance

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Physiological Resistance in Amaranthus powellii

• 6 resistant R , 6 susceptible S
• R Decreased competitiveness
• R Smaller and distorted
• R comparable roots and stems construction
• R four times less root developed
• R 58 less leave leaf area
• R 67 less biomass
• S outperformed R population by 10-15 times

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Resistant Susceptible
Root stem
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Physiological Resistance in Amaranthus powellii 2

• The genetic mutation of Trp574 Leu
  acetohydroxyacid synthase (AHAS) greatly reduces
  its capacity to grow and develop
• reduced fitness when placed under non-ALS
  inhibiting conditions
• (Tardif et al., 2006)

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Resistant Management Procedures

• Different modes of action
• Mechanical management
• Cultural
• Crop rotation
• Do not let go to seed

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Herbicide Resistant Crops 1

• IMI-Rice (Webster Masson. 2001)
• Soybeans- Less
• Imazethapyr
• Red vs domestic Oryza sativa
• Mechanism unknown

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Herbicide Resistant Crops

• Tobacco, Corn, Soybean, Canola, Sugar beet, Rice,
  Wheat,
• Somatic cell divisions
• Mutation breeding
• Plant transformation
• Interspecific crossing
• No residue issues

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Herbicide - Soil Microbe Interactions

• Atrazine- degrading bacteria pseudomonas
• Reduced activity of ALS enzyme by 50-70
• Growth was not affected
• Inhibitory effect on soil microorganisms
• May become desensitized
• Potera et al., 2001

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Soil Life 1
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Soil Life 2

• In the cationic state it is strongly bound to the
  soil
• Undergoes hydrolysis in acid env but not above
  pKa
• In the anionic state it is strongly repelled by
  the soil
• Very leachable
• Half life 40 days
• Susceptible species 3-4 year
• Kd pH

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Soil Life 3 Persistence and Breakdown

• 1-6 weeks
• microbial degradation
• bridge hydrolysis
• gtacid environment
• Sulfonamide nitrogen

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Soil Life 4

• Imidazolinones have tendency to persist in
  acidic soils, whereas sulfonylurea persistence is
  extended in soils which are more basic (Moyer et
  al, 2000).

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Degradation

• Sulfonylurea
• Microbial and hydrolysis
• Imidazolinone
• Only microbial therefore less pH dependant

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Environmental Interactions

• Low mammalian toxicity
• Mammals dont have ALS enzyme
• What about reptiles
• Ld 50
• High potency

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Site Characteristics
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Residual Group 2 Herbicide Usage Weed Survey
Data 1996 2003 (Johnson et al)
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Smart Grower Does What

• Field bioassay
• Understand residual issues
• Rotate herbicide groups
• Use GP-2 herbicides as part of an integrated agro
  management system

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