Agronomy 317 - Principles of Weed Science

Herbicide Mode of Action


Amino Acid Biosynthesis Inhibitors

Herbicide Resistance Action Committee (HRAC) Groups - B, G, H

Weed Science Society of America (WSSA) Groups - 2, 9, 10

Amino acids are the basic structural units of protein. Proteins are crucial to virtually all biological processes and function in plants as enzyme, transport, signaling, and energy storage molecules. An amino acid consists of an amino group, a carboxyl group, a hydrogen ion, and a distinctive side chain (R group) bonded to a carbon atom (Fig. 1). All proteins in all species, from bacteria to plants to humans, are constructed from the same set of twenty amino acids. These can be thought of as the protein alphabet.

Figure 1. An amino acid.

Knowledge of at least eight amino acids is important for understanding the effects of amino acid biosynthesis inhibitors. The simplest amino acid is glycine, which has only a hydrogen atom as its side chain (Fig. 2). Three amino acids, valine, leucine, and isoleucine, have large, branched side chains (Fig. 3). Another three amino acids, phenylalanine, tyrosin, and tryptophan, have aromatic side chains (Fig. 4). Glutamate has a an acidic side chain that is nearly always negatively charged at physiological pH and its transformation into glutamine by addition of ammonia is an important reaction for nitrogen assimilation in plants.

Figure 2. The amino acid glycine.

Figure 3. The branched-chain amino acids - valine, leucine, and isoleucine.

Figure 4. The aromatic amino acids - phenylalanine, tyrosin, and tryptophan.

In proteins, the carboxyl group of one amino acid is linked to the amino group of another amino acid by a peptide bond (Fig 5.). Many amino acids are joined together to form a polypeptide. Thus, proteins are polypeptides of many amino acid units linked together. Most naturally occurring polypeptides are between 50 and 2000 amino acid units in length. Polypeptide chains can fold into regular three-dimensional structures. This is what gives proteins their functionality as enzymes or signaling molecules.

Figure 5. Formation of a peptide bond.

Herbicides designated as amino acid biosynthesis inhibitors block enzymatic reactions necessary for the formation of at least one of the twenty amino acids. A reduction in one or more of the amino acids restricts the building of proteins important for continued plant function. The end result is generally a slow death from a lack of the enzymes need to sustain growth. Humans and most other mammals do not have pathways for production of nine of the amino acids. Therefore, chemicals that block the synthetic pathways of these nine are effective as herbicides while posing little health risk to mammals.

Amino acid inhibitor herbicides act at one of three main sites. Several families inhibit acetolactate synthase (ALS), an enzyme important for the production of the branched-chain amino acids, isoleucine, valine, and leucine (Figs. 6 and 7). The comitted step is an irreversible reaction, so blockage of an enyzmatic pathway results in a buildup of the substrate of the comitted step. Glyphosate, a derivative of glycine, blocks the formation of aromatic amino acids by inhibiting enolpyruvyl-shikimate-phosphate synthase (EPSP) an important enzyme in the shikimate pathway (Fig. 8). Glufosinate, and other phosphinic acid herbicides, inhibit the assimilation of nitrogen into plants by binding to the enzyme glutamine synthetase (Fig. 9). Phosphinic acid herbicides work faster than most other amino acid inhibitors because toxic levels of ammonia build up within the plant.

Figure 6. The pathway for production of the branched-chain amino acid, isoleucine.

Figure 7. The pathways for production of the branched-chain amino acids, valine and leucine.

Figure 8. The shikimate pathway produces chorismate, a precursor to synthesis of the aromatic amino acids, phenylalanine, tyrosin, and tryptophan.

Figure 9. The formation of glutamine from glutamate and ammonia (NH4+) is crucial for nitrogen assimilation into plants.

Families

Acetolactate Synthase (ALS) Inhbitors - HRAC Group B, WSSA Group 2

Sulfonylureas

Imidazolinones

Triazolopyrimidines

Enolpyruvyl-shikimate-phosphate (EPSP)Synthase Inhibitors - HRAC Group G, WSSA Group 9

Glycines

Glutamine Synthetase (GS) Inhibitors - HRAC Group H, WSSA Group 10

Phosphinic acids


Sulfonylureas

Site of Action

Inhibit the enzyme acetolactate synthase (ALS). This enzyme catalyzes early steps in the biosynthesis of the three branched-chained amino acids valine, leucine, and isoleucine.

History

These herbicides have broad spectrum weed control and are applied at extremely low rates. They were first marketed during the 1980's, primarily by E.I. DuPont, American Cyanamid Company (bought by BASF in 2000), and CIBA-Geigy (merged into Syngenta AG). The core structure of these herbicides combines the urea and triazine groups, although their primary mode of action is inhibition of amino acid synthesis. Their activity is similar to the imidazolinones and triazolopyrimidines.

Representative Herbicides

Common name Trade name
chlorimuron-ethyl Classic
chlorsulfuron Glean, Corsair
halosulfuron-methyl Permit, Manage
metsulfuron-methyl Ally, Escort, Manor
nicosulfuron Accent
primisulfuron-methyl Beacon
prosulfuron Peak
rimsulfuron component of Basis
sulfometuron-methyl Oust
thifensulfuron-methyl Pinnacle
tribenuron-methyl Express
many others  

Crop Use

Corn - halosulfuron, nicosulfuron, prosulfuron, primisulfuron, rimsulfuron, and thifensulfuron

Soybean - chlorimuron and thifensulfuron

Small grains - chlorsulfuron, metsulfuron, prosulfuron, tribenuron, and thifensulfuron

Turf grass - chlorsulfuron, halosulfuron

Noncropland - chlorsulfuron, metsulfuron, and sulfometuron

Application Target

Foliage and soil. Most are used as a postemergence applications to emerged foliage, but some are used for preplant and preemergence treatments.

Translocation Type

Symplastically systemic.

Weed Spectrum

Very broad spectrum control of both grasses and broadleaves. It is difficult to make generalizations about the weeds controlled by this chemistry because the specific weed species controlled differs greatly among the various active ingredients.

Sulfonylureas typically control a few species very well, but have limited activity on others, so they are often used in combination with each other or other herbicides.

Selectivity

Differences in activity are due to metabolic detoxification mechanisms and/or exclusion of the herbicide from the enzyme (lack of binding) in tolerant species.

Prone to resistance development. Some species develop resistance in as little as three years.

Reaction in Soils and the Environment

Primarily broken down by hydrolysis and microbes, which decreases with increasing pH. Longer carryover in high pH soil, drought, dark, cold, and no-till.

More tightly adsorbed to soil particles and soil OM at low pH. High pH increases activity and longevity. Carryover more of a problem in higher pH soils since acid hydrolysis ceases at high pH levels. Variable pH across a field can greatly affect the ability of a herbicide to persist in the soil.

Half-life in soil - 2 to 40 days depending on the herbicide and soil conditions. The half-life of some SU's can be even greater than 40 days in high pH soils.

There is more than a thousand-fold range in sensitivity to these herbicides. For example, wheat is not affect by chlorsulfuron until soil concetrations are as great as 100 parts per billion (ppb), while sugarbeet is injured by concentrations as low as 0.1 ppb. This creates major carryover concerns in cropping systems that include sensitive species. Although the half-life is relatively short, small amounts of the compounds remain in the soil for extended periods. The rotational restrictions for some crops are as great as 48 months from the date of application of sulfonylurea herbicides.

Symptomology

Slows or stops growth and kills meristems. Plants slowly whither and die. Symptoms take 1-2 weeks to develop.

Gradual chlorosis and necrosis; purpling of leaf veins in broadleaves and leaf sheaths and blades in grasses.

Reduced root system.

Interactions between sulfonylurea herbicides and some organophosphate insectides can result in injury to corn plants.

Drift and spray contaimination are a concern because susceptible plants are very sensitive to these herbicides.

Injury Pictures

From the University of Minnesota

From Purdue University

From the University of Missouri


Imidazolinones

Site of Action

Inhibit the enzyme acetolactate synthase (ALS). This enzyme catalyzes early steps in the biosynthesis of the three branched-chained amino acids valine, leucine, and isoleucine.

History

Developed in the 1980's by American Cyanamid Company (which was purchased by BASF in 2000), these herbicides provide broad-spectrum weed control at extremely low rates. They have activity very similar to the sulfonylureas and triazolopyrimidines.

Representative Herbicides

Common name Trade name
imazamox Raptor
imazapic Cadre, Plateau
imazapyr Arsenal, Chopper, Stalker
imazaquin Image, Scepter
imazethapyr Pursuit
others  

Crop Use

Soybeans, other legumes, forests, and industrial sites. Resistant corn hybrids have been developed for use with imidazolinone herbicides.

Imazapic is an important herbicide for postemergence control in native prairie grasses and wildflowers.

Application Target

Soil as preplant and/or preemergence treatments and as a postemergence material to foliage.

Translocation Type

Readily absorbed by shoots and roots.

Symplastically and apoplastically translocated.

Weed Spectrum

Different herbicides in this group control different weeds; mostly affect broadleaf species, but most herbicides in this family also control a limited number of grass species.

Selectivity

Differences in activity are due to metabolic detoxification mechanisms and/or exclusion of the herbicide from the enzyme (lack of binding) in tolerant species.

Prone to resistance development. Resistance is generally associated with an altered binding site in tolerant plants.

Reaction in Soils and the Environment

Long lasting herbicides in the soil. Water and sunlight favor breakdown.

Broken down primarily by microbes with very little degradation under anaerobic conditions. Warm moist soils above pH 6.5 increase microbial breakdown.

Strongly bound to soil organic matter. Dry conditions cause them to be adsorbed to soil particles and wet conditions cause the herbicide to be free for breakdown and plant uptake.

Longer carryover with drought, cold weather and no-till.

Soil mobility is low.

Higher activities at high pH. Unlike sulfonylurea herbicides, there is more carryover in low pH soil and this can create a recropping problem for sensitive crops.

Half life in soil: 20 to 150 days depending on herbicide and soil conditions.

Symptomology

Little or no growth occurs after uptake in susceptible species.

Slows or stops growth and kills meristems. Plants slowly whither and die. Symptoms take 1-2 weeks to develop.

Gradual chlorosis and necrosis; purpling of leaf veins in broadleaves and leaf sheaths and blades in grasses.

Reduced root system.

Drift and spray contaimination are a concern, because susceptible plants are very sensitive to these herbicides.

Injury Pictures

From the University of Minnesota

From Purdue University

From the University of Missouri


Triazolopyrimidines

Site of Action

Inhibit the enzyme acetolactate synthase (ALS). This enzyme catalyzes early steps in the biosynthesis of the three branched-chained amino acids valine, leucine, and isoleucine.

History

These herbicides, introduced in the 1990's by Dow AgroSciences, are closely related to the imidazolinones.

Representative Herbicides

Common name Trade name
cloransulam Firstrate
flumetsulam Python

Crop Use

Flumetsulam is used in corn and soybeans and sold in the U.S. mainly in mixtures with other herbicides.

Cloransulam is used in soybeans, especially for control of weeds in the composite family, including cocklebur, common and giant ragweed, and sunflower.

Application Target

Soil as preplant and/or preemergence treatments and as a postemergence material to foliage.

Translocation Type

Readily absorbed by shoots and roots.

Symplastically translocated.

Weed Spectrum

Active on broadleaf species; little activity on grasses.

Selectivity

Differences in activity are due to metabolic detoxification mechanisms and/or exclusion of the herbicide from the enzyme (lack of binding) in tolerant species.

Prone to resistance development. Resistance is generally associated with an altered binding site in tolerant plants.

Reaction in Soils and the Environment

Half life in soil: cloransulam 8-10 days; flumetsulam: 1-3 months (less at higher pH)

Activity and degradation increases as the soil pH increases.

Primarily degraded by microbes. Degradation increases in soils with high pH because in high pH soils the chemical is not adsorbed and is available for plant uptake and microbial breakdown. All factors that increase microbial activity also increase herbicide degradation.

More carryover in low pH soils. Recropping to sensitive species can be a problem with flumetsulam.

Symptomology

Little or no growth occurs after uptake in susceptible species.

Slows or stops growth and kills meristems. Plants slowly whither and die. Symptoms take 1-2 weeks to develop.

Gradual chlorosis and necrosis; purpling of leaf veins in broadleaves and leaf sheaths and blades in grasses.

Reduced root system.

Injury Pictures

From Purdue University

From the University of Missouri


Glycines

Site of Action

Inhibit the enzyme (EPSP synthase) that synthesizes aromatic amino acids (tyrosine, tryptophan, and phenylalanine).

History

These compounds are similar in structure to the amino acid glycine, which is essential for protein production. Glyphosate was developed by Monsanto in 1972 and released in the late 1970's. It probably has the largest sales of any pesticide worldwide.

Representative Herbicide

Common name Trade name
glyphosate Roundup, many others

There are several different salt formulations of the basic glyphosate molecule:

Roundup - glyphosate isopropylamine

Touchdown 5 - glyphosate trimesium (sulfosate)

Touchdown IQ - diammonium glyphosate

Crop Use

Burndown of all plants in an area, especially before crop planting; noncropland areas; spot treatments; wipe-on treatment for control of tall weeds in short crops; and directed postemergence application under tall crops

An alternative EPSP enzyme has been inserted into cultivars of several crops (most notably soybean and corn) to give them glyphosate resistance.

Application Target

Foliage

Translocation Type

Readily absorbed by leaves and easily translocated to all areas of the plant symplastically.

Translocation to underground structures prevents regrowth of perennials from propagules.

Weed Spectrum

Provide very broad-spectrum control of many annual and perennial plant species.

Selectivity

Nonselective herbicides.

A few plants have been found that have genetic resistance.

Reaction in Soils and the Environment

No soil activity; rapidly and tightly held to soil particles. Crops can be seeded directly into treated areas.

Considered as very safe chemicals for the environment.

Half-life in the soil ~ 60 days.

Symptomology

Slow development of symptoms. Visible symptoms occur 2-10 days after application.

Growth stops immediately, chlorosis develops (new leaves first), and then necrosis and death within 10 to 14 days.

Drift and spray contaimination are a concern, because susceptible plants are very sensitive to these herbicides.

Injury Pictures

From Purdue University

From the University of Missouri


Phosphinic Acids

Site of Action

Inhibits glutamine (an amino acid) synthetase, an enzyme essential to assimilation of ammonia into organic nitrogen. Inhibition of this enzyme causes a phytotoxic buildup of ammonia and reduced production of amino acids in the plant. Plant damage most likely occurs from a combination of cell membrane destruction by ammonia and slowing of metabolism from amino acid deficiencies.

History

Glufosinate was released by Hoechst AG (currently Aventis) in 1981.

Representative Herbicide

Common name Trade Name
glufosinate-ammonium Liberty, Finale, others

Crop Use

Postemergence control in genetically-altered crops (mainly corn).

Nonselective control of emerged weeds in noncrop areas.

Postemergence directed sprays in grapes, apples, and tree nuts.

Application Target

Foliage

Translocation Type

Readily absorbed by leaves and symplastically translocated. Movement within the plant is limited due to the destructiveness of ammonia on plant cells. More contact type injury than other amino acid inhibitors.

Thorough spray coverage of small weed is needed for best control. Large weeds often regrow from axillary buds.

Weed Spectrum

Controls a broad spectrum of annual broadleaf and grass weed species.

Glufosinate can be used on perennials, but it mainly kills top growth and they generally regrow from underground structures.

Selectivity

Nearly nonselective.

Transgenic resistance in some field crops (most notably corn). A gene that codes a phosphinothricin acetyl transferase enzyme has been isolated from a soil bacterium and is inserted into the crop plants. This enzyme converts glufosinate to an acetylated, nontoxic metabolite.

Reaction in Soils and the Environment

No soil activity - rapidly degraded by microbes.

Considered as a very safe chemical for the environment, has a half-life in the soil of about 7 days.

Symptomology

Growth ceases immediately, chlorosis develops, and then necrosis and death. Chlorosis and wilting usually occur within 3-5 days followed by necrosis within 1-2 weeks.

Injury symptons often mimic contact type herbicides, but they take longer to develop when compared with other nontranslocted herbicides.

Injury Pictures

From the University of Missouri


Agronomy 317 - Principles of Weed Science
Authored by Dr. Lance R. Gibson
Copyright 2001 Iowa State University. All rights reserved.
Revised: July 23, 2004 .