Amino Acid Biosynthesis Inhibitors

Front 1

What are the most widely used herbicides in the world?

Back 1

Amino Acid Biosynthesis Inhibitors.

Front 2

How do Amino Acid Biosynthesis Inhibitors work?

Back 2

By affecting the plant's amino acid and therefore protein synthesis in plants.

Front 3

What are the symptoms of Amino Acid Biosynthesis Inhibitors?

Back 3

The symptoms vary from type to type.

Front 4

What are amino acids?

Back 4

They are the building blocks of proteins which are critical to the function of all biological organisms.

Front 5

What are proteins?

Back 5

As enzymes, proteins function as catalysts, which directly accelerate biochemical reactions. Some proteins assist with the transport of components throughout the plant, while others serve as nitrogen and energy storage compounds.

Front 6

Why do Amino Acid Biosynthesis Inhibitors have very low animal toxicity?

Back 6

Plants manufacture all essential amino acids, while mammals, including humans, have a limited ability to synthesize amino acids. Therefore, a chemical that inhibits the synthesis of amino acids in plants is toxic to plants, but is less likely to find a target in animals.

Front 7

Although there are several types of herbicides that affect amino acid synthesis, what three are discussed in great detail?

Back 7

Acetolactate Synthase (ALS) inhibitors;

ESPSPS inhibitors; and,

Glutamine Synthetase inhibitors.

Front 8

What are 6 distinguishing characteristics of acetolactate synthase (ALS) inhibitors?

Back 8

They are selective herbicides that can affect both broad-leaved and grass herbicides;

They are soil active;

Their soil persistence varies but can limit rotational options;

They move readily throughout the plant;

Plant death is slow; and,

They are not toxic to mammals.

Front 9

What are the three classes of acetolactate synthase?

Back 9

Sulfonulaminocarbonyltriazolinones;

Sulfonylurea; and,

Imidazolinones.

Front 10

What are the physical and chemical properties of sulfonylureas and imidazolinones?

Back 10

They are weak-acid herbicides with intermediate water solubility. As such they are readily taken up by leaves and roots and transported in both the phloem and xylem;

Soil persistance varies with soil conditions (such as organic matter, pH, soil moisture and temperature).

Front 11

Is there a difference in response to soil pH between sulfonylureas and imidazolinones?

Back 11

Yes, as pH decreases, sulfonylurea persistence decreases, while with imidazolinones, persistence increases as pH decreases.

Front 12

What is acetolactate synthase (ALS)?

Back 12

ALS is a key enzyme in the synthesis of branched-chain amino acids.

It synthesizes two reactions, the condensation of two molecules of pyruvate to form an acetolactate and the condensation of a molecule of pyruvate with of alpha-keytobutyrate to form acetohydroxybutyrate.

Front 13

Where is acetolactate synthase located?

Back 13

It is located in the chloroplasts of green tissue and the plastids of non-green tissue.

It is most sensitive to inhibition in young meristematic regions of plants.

Front 14

How do Amino Acid Biosynthesis Inhibitors work?

Back 14

By binding to and inhibiting ALS. This inhibition leads to reduced amounts of the branched-chain amino acids: leucine, valine and isoleucine.

Plant death is not only due to starvation for these amino acids; following herbicide application, cell division is inhibited and there may be abuild-up of the toxic intermediate alpha-ketobutyrate and a decrease in the phloem translocation.

The initial inhibition apparently causes a cascade of events that eventually kills susceptible plants.

Front 15

What does it mean when it is states that ALS inhibitors, even when foliar-applied, can have soil activity?

Back 15

These herbicides may persist in the soil and affect newly germinating weeds in addition to those weeds that received a foliar application.

Front 16

How are ALS inhibitors translocated within the plant?

Back 16

Foliar-applied herbicide is taken up readily by the plant and translocated in the xylem and phloem. They are also readily taken up from the soil and can move in both the xylem and phloem.

Front 17

What is is the selectivity of Acetolactate Synthase herbicides based on?

Back 17

It is based on the ability of crop plants to rapidly metabolize the ALS-inhibitng herbicides before they become toxic. Weeds are controlled because they metabolize these herbicides at a slower rate.

Front 18

What is one of the reasons for the widespread use of acetolactate synthase herbicides?

Back 18

ALS inhibitors have a broad spectrum of selectivity and flexibility of use.

For example, the herbicide ethametsulfuron methyl (Muster) is selective enough to remove wild mustard from canola crops.

Front 19

How toxic are acetolactate synthase herbicides?

Back 19

Branched-chain amino acids are not synthesized by mammals; therefore, the ALS enzyme is not present in mammals.

Front 20

What are symptoms of plants affected by acetolactate synthase herbicides?

Back 20

The first visible symptom is an inhibition of growth in susceptible species. A few days after application, chlorosis followed by necrosis and terminal bud death are visible. Yellow and purple coloration may appear on leaves and stems. Root growth can also be inhibited.

Plant death is relatively slow.

Front 21

What are some implications for application of acetolactate synthase herbicides? (5 of them)

Back 21

ALS herbicides are active at very low rates;

They are active both as a foliar application and following uptake from the soil;

Selectivity, conferred by differential metabolism allows use with a wide range of crops and target weeds;

Soil activity, persistence in the soil, and extreme sensitivity of some crops impose rotational restrictions; and,

ALS herbicides have a very low acute mammalian toxicity.

Front 22

What are five traits of EPSPS inhibitors?

Back 22

They are a non-selective herbicide;

They are effective on most broad-leaed and grass weeds and crops;

They translocates readily though plants;

They provide good perennial weed control at higher rates; and,

They have no soil activity because it binds tightly to soil and is inactivated.

Front 23

What is the common name of the EPSPS herbicide?

Back 23

Glyphosate, which is one of the best known herbicides in the world.

It is non-selective, foliar active and perceived as one of the safest herbicides in the world.

Front 24

Glyphosate is a weak acid, but it is unusual in what way?

Back 24

It loses not one but several protons at different pH optimums. As such, it is called a zwitterion.

Front 25

Glyphosate strongly bonds to clay particles. What does this mean?

Back 25

It means that once glyphosate is bound to soil it is unavailable to plants resulting in negligible residual activity and leaching.

Front 26

How does glyphosate (EPSPS) work?

Back 26

Glyphosate binds to and inhibits the enzyme 5-enolpruvylshikimate acid-3-phosphate synthase (EPSPS), an enzyme in the aromatic amino acid biosynthetic pathway.

As in the case of ALS inhibitors, phytotoxicity does not result from starvation for amino acids; inhibition leads to deregulation of carbon flow in the plant and a build-up of the toxic intermediates, shikimate or shilimate-3-phosphate.

Front 27

How is glyphosate (EPSPS) taken up by the plant and translocated?

Back 27

Glyphosate is taken up readily from the foliage, although movement through lipid layers may be slow. Like many herbicides, the formulation of the product will affect the rate of uptake (and thus the minimum amount of time needed before the next rainfall).

Front 28

How does glyphosate affect translocation?

Back 28

Like ALS inhibitors, glyphosate seems to inhibit its own movement in phloem.

Glyphosate moves with the sugars in growing plants. When it is applied as a crop desiccant too early, when seed moisture levels are too high, it can move to seeds as they are forming.

Translocation can be maximized by timing application during a period when perennials are moving sugars downward, after flowering for most plants.

Front 29

Is glyphosate (EPSPS) a selective herbicide?

Back 29

No, plants do not readily degrade glyphosate.

Although glyphosate is non-selective, considerable variability exists in the amount of herbicide necessary to control various weeds. Some weeds are remarkably hard to control with Roundup, like field horsetail or wild buckwheat. This difficulty may be because glyphosate cannot penetrate the waxy coating on the plants.

Front 30

What are the symptoms of plants treated with EPSPS (glyphosate)?

Back 30

Although the symptoms are slow to appear, they are most apparent at the growing points of the plant. Leaves turn yellow, followed by browning.

At sub-lethal rates, there can be some twisting of the plants that might be confused with 2,4-D symptoms.

Front 31

Is glyphosate (EPSPS) toxic?

Back 31

No. Glyphosate binds to an enxyme not present in humans and is non-toxic to use. It has low oral and dermal toxicity. Because glyphosate is formulated as a salt, it can irritate the eyes.

Front 32

What are the five implications for application of glyphosate (EPSPS)?

Back 32

Glyphosate binding to soil inactivates it, allowing seedling that germinate after application to survive;

Glyphosate does not leach from soil;

Glyphosate has very low mammalian toxicity;

Perennial weed control depends on translocation to distal roots and rhizomes; and,

Glyphosate can be inactivated in the spray tank if water containing dirt or organic matter is used.

Front 33

What are three traits of Glutamine Synthetase (glufosinate)?

Back 33

It is a non-selective, contact-type herbicide;

Symptoms include the appearance of grey-white patches that appear rapidly; and,

Poor perennial weed control.

Front 34

Is glufosonate (glutamine synthetase) mobile in the soil?

Back 34

Glufosinate adsorbs only weakly to the soil and can be highly mobile; however, glufosinate breaks down rapidly by soil microbial action and, thus, has limited soil persistence and limited potential for leaching.

As an interesting side note, this herbicide was originally discovered in a bacterium species, but it is now manufactured synthetically.

Front 35

How does glufosinate work?

Back 35

It works by binding to glutamine synthetase (a key enzyme in the pathway that assimilates inorganic nitrogen (ammonia) into useful organic compounds), a binding site normally occupied by glutamine.

The binding is irreversible.

In addition to the assimilation of ammonia, GS recylces amonia produced by other metabolic pathways including degradation of storage and transport protein and photorespiration.

Front 36

What are the symptoms of a plant exposed to glufosinate?

Back 36

A rapid accumulation of ammonia;

A rapid drop in photosynthesis;

Depletion of amino acids, glutamine, glutamate, and others;

Growth inhibitions;

Chlorosis; and,

Plant death.

Front 37

Describe the uptake and translocation of glufosinate.

Back 37

Glufosinate is very water soluble and a weak acid. As such, it translocates well in both the phloem and xylem; however translocation is limited because the rapid activity of the herbicide quickly disrupts photosynthesis and cell integrity.

This herbicide effectively limits its own translocation.

Front 38

Describe the metabolism and selectivity of glufosinate.

Back 38

Although it is a non-selective herbicide, considerable variability in species sensitivity exists.

Glufosinate is metabolized in plants, but not quickly enough avoid toxic consequences.

Front 39

What are the symptoms of a plant sprayed with glufosinate?

Back 39

Chlorosis and wilting usually occur within one to three days of application, followed by necrosis in one week.

Symptom development is more rapid in bright sunlight, high humidity, and moist soil.

Front 40

What are the implications for application of glufosinate? (there are 2)

Back 40

Because glufosinate is highly water soluble, it can be washed from leaves by rain for up to four hours following application; and,

Seedling that emerge following application are unaffected.