Week 12 lecture 2 FA & cellulose biosynth_Slides

BSPM 308 - Ecology and Management of Weeds

Instructors

  • Dr. Cynthia (Cini) Brown (Professor)

  • Lauren Myers (TA)

  • Neha Panwar (TA)

  • Logan Cestone (Assistant TA)

Learning Outcomes

  • Acetyl-CoA Carboxylase (ACCase) Inhibitors

    • Describe how they work.

  • Very Long Chain Fatty Acid (VLCFA) Elongase Inhibitors

    • Explain their mechanism.

  • Fatty Acyl Thioesterase (FAT) Inhibitors

    • Describe their functioning.

  • Cellulose Biosynthesis Inhibitors

    • Describe and contrast how they work.

Group 1 - Acetyl-CoA Carboxylase Inhibitors

  • Main Types:

    • Phenylpyrazoline - "the Den"

    • Cyclohexanediones - "the Dims"

    • Aryloxyphenoxy propionic acids - "the Fops"

Group 1 Herbicides

  • Application:

    • Used post-emergent for control of grass weeds in broadleaf crops (e.g., soybean, barley)

  • Characteristics:

    • Limited residual weed control

    • Grass control in broadleaf crops (e.g., Proso millet with sethoxydim)

Aryloxyphenoxy Propionic Acids - "the Fops"

  • Historical Context:

    • First herbicide: diclofop-methyl (HOELON) in 1979

  • Chemical Properties:

    • High Water Solubility: Acid form (3,000 ppm) vs. Ester form (0.8 ppm)

  • Usage:

    • Breakthrough for wild oat control in small grains, replacing previous chemicals (VLCFA and difenzoquat)

Chirality of Fops

  • Stereoisomers of Quizalofop:

    • R-quizalofop (active) vs. S-quizalofop

    • Originally sold as a racemic mixture (1:1 ratio)

    • Current formulations contain only the active isomer

Cyclohexanediones - "the Dims"

  • Example Herbicides:

    • Clethodim, Sethoxydim, Tralkoxydim

  • Chemical Structure:

    • Cyclohexane ring with specific substituents affecting pKa and Log Kow values

Phenylpyrazoline - "the Den"

  • Key Product:

    • Pinoxaden (Axial) with safener cloquintocet-mexyl

    • Selective control in winter wheat and barley

Application, Absorption, and Translocation

  • Herbicides:

    • FOPs and DIMs are phloem mobile with some xylem mobility

    • Translocate to meristematic areas of root and shoot

  • Mechanism:

    • Aryl-propanoic acids are hydrolyzed to free acids to improve absorption

Symptoms on Grass Weeds

  • Effects of Herbicides:

    • Slow-acting: Growth halts in hours

    • Color changes to yellow, orange, or red; necrosis follows

    • Plant death typically occurs 1-3 weeks after application

Basis of Selectivity in Grass Weeds

  • Differential Sensitivity:

    • Isozymes vary between grasses and broadleaves affecting fatty acid biosynthesis

    • Grasses are susceptible to ACCase inhibitors; broadleaf crops show high tolerance

Metabolism and Selectivity

  • Differential Metabolism Examples:

    • Sethoxydim metabolized rapidly in tolerant species

    • Varies significantly between tolerant and susceptible species

Group 15 Herbicides - VLCFA Elongase Inhibitors

  • VLCFA: Very Long Chain Fatty Acids

  • Characteristics:

    • Low water solubility, high vapor pressure, and pre-plant incorporated application

    • Absorbed primarily by the shoot in the vapor phase and has limited movement into developing seedlings

    • substance with high vapor pressure under normal temperatures are considered volatile

    • Applied as PPI (preplant incorporated; applied into the soil) to avoid volatilization and photodegradation losses, especially in moist soils

Example of S-metolachlor

  • Use History:

    • Prior to Roundup Ready crops, most widely used herbicide

    • Metolachlor was racemic mixture; the increasing ratio of active isomer resulted in massive reduction in pesticide use

    • Rate recommendations based on soil texture and % organic matter

    • Nuetral or uncharged regardless of soil pH

    • S-form more effective

    • Competitive inhibitor; enzyme activity rapidly reduces w S-metalachlor; binds irreversibly to enzyme; less possibility of herbicide resistance development

Target of Group 15 Herbicides

  • VLCFAs:

    • Key components of waxes and plant protective materials

      • Epicuticular & cuticular waxes

    • Interfere w biochemical processes in golgi apparatus; later stage of fatty acid biosynthesis

Inhibition of VLCFA Elongases

  • Example:

    • Chloracetamides as competitive inhibitors, affecting elongase activity

  • Key Points:

    • Only active S-enantiomers play a role in herbicidal activity

    • Soil applied; Absorption by hypocotyl or shoots; plants emerge from soil and die soon after; difficulty emerging

Selective Placement of VLCFA Elongases Inhibitors

  • Application Notes:

    • Soil applied, mainly absorbed by hypocotyls or shoots

    • Can affect seedling emergence

Group 30 - Fatty Acyl Thioesterase

  • Characteristics:

    • Newest herbicide mechanism of action identified

  • Example - Cinmethylin:

    • Analog to natural herbicidal products; inhibits fatty acid biosynthesis

Mechanism of Action of Cinmethylin

  • Research Findings:

    • Binds to acyl-ACP thioesterase, leading to slowed fatty acid synthesis

Selectivity in Wheat

  • Tolerance Mechanism:

    • Wheat metabolizes cinmethylin rapidly, ensuring tolerance against it

    • Important for controlling herbicide-resistant weeds

Cellulose Biosynthesis

  • Process Overview:

    • Cellulose synthases (CESAs) produced in Golgi, guiding cellulose formation at the membrane interface

Inhibitors of Cellulose Biosynthesis

  • Group 1 and Group 2 Examples:

    • Group 1: Isoxaben (low solubility, long residual activity)

    • Group 2: Indaziflam, Dichlobenil (specific applications and properties)

Classifying CBI Herbicides

  • Group Categories:

    • Various herbicides with specific modes of action disrupting cellulose synthesis at different stages.