4.12.97
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ALSase Inhibitors:
Sulfonylureas
General Information
Family first discovered and developed by DuPont. George Levitt was the leader of the
discovery group that made this important discovery originally in 1975.
First commercial herbicide was chlorsulfuron in small grains (1981). First commercial
entry in Iowa was chlorimuron-ethyl in soybeans (1986).
First herbicide family with very high specific activity: low (grams) active ingredient
rates per acre.
Toxic target in plant very facile, changeable between species allowing very different crop
and weed specificity changes with minor changes on core sulfonylurea molecule(s):
herbicides for small grains (wheat, barley, oats, rye), soybeans, maize, flax, rapeseed
(Canola), non-crop. For this reason possible to develop sulfonylurea herbicide for every
cropping and weed control situation. Applied preemergence, postemergence, etc., for
control of many broadleaf annual and perennial weeds, grassy weeds, yellow nutsedge.
Herbicide Examples.
-Chlorsulfuron (Glean) for dicot weed control in several small grain crops (wheat, barley,
oats, rye) and flax. Substitute for 2,4-D. Applied preemergence, postemergence. Control of
problem dicot perennial weeds such as Canada Thistle.
-Chlorimuron-ethyl (Classic) for dicot weed control in soybeans, yellow nutsedge, low
velvetleaf control, postemergence.
-Nicosulfuron (Accent), and Primisulfuron (Beacon) for grassy weed control in maize
postemergence. Below is Randy Thornhill looking over his rapeseed plants, and some corn
seedlings he is testing for corn hybrid cultivar differences in response to primisulfuron:
-Thifensulfuron (Harmony) used in wheat, maize, barley, sorghum for postemergence weed
control of dicots.
-Metsulfuron-methyl (Escort, Ally) used in non-crop, brush control, roadside, rough turf
and as grass suppression with postemergence applications.
-Others: sulfometuron-methyl (Oust) and bensulfuron-methyl (Londax)
Chemistry
Many different herbicide members in this family with different chemical structures
with very different plant selectivities.
-Urea: -NH-C=O-NH-
-Sulfonylurea moiety: R1(-R3)-SO2-NH-C=O-NH-R2
-R3 ortho substitution on ring is key often to dramatic changes in plant selectivity
between different sulfonylurea; ALSase target site of toxic action in plants very
sensitive to small changes at this particular molecular site.
Physiology and Metabolism
Mode of Sulfonylurea Action
Mode of action is inhibition of acetolactate synthase (ALSase) enzyme (also known as
acetohydroxyacid synthase: AHASase). ALSase is key enzyme in branched amino acid (i.e.
valine, leucine, isoleucine) biosynthesis pathway
Branched Amino Acid Biosynthetic Pathway:
-There are apparently 10 amino acid sites subject to mutation and change on the ALSase
molecule that can confer resistance with no loss in function.
-This many sites make the ALSase molecule very facile for different specificities of
herbicide binding and inhibition.
There are other, chemically different herbicide families with the same mode of action
(ALSase inhibition) as the sulfonylureas: imidazolinones, sulfonoanilides,
triazolo-pyrimidines.
Inhibition of ALSase caused very rapid cessation of growth, meristems inhibited in
susceptible species: inhibition of cell division (mitosis) and cell elongation due to lack
of essential amino acids needed for protein systhesis. Little effect on seed germination.
Seedlings often emerge and symptoms develop later.
Mode of Sulfonylurea Lethality
Plants die due to starvation for needed proteins in new growth because they cannot
form these needed proteins without the amino acids valine, leucine, isoleucine.
Uptake and Movement of Sulfonylureas in plants
Most herbicide members of family readily taken up by foliar and root plant parts.
Usually observe ambi-mobile translocation patterns in plants: both apoplastic (xylem) and
symplastic (phloem) patterns of herbicide distribution.
Site of uptake (whether either root or shoot) critical to pattern of movement and
distribution in plant subsequent to uptake. E.g. root uptake influences pattern of
translocation and injury symptoms observed, different that if same herbicide was taken up
primarily by shoot foliar parts.
Rapidly translocate to areas of active growth, e.g. meristems, apices, were growth is
inhibited in susceptible species.
Basis of Selectivity between Plant Species
Selectivity arises from both metabolic and binding site alteration mechanisms that
confer sulfonylurea resistance to plants.
Metabolic Selectivity Mechanisms: different metabolic detoxification mechanisms occur with
different herbicide members, in different plant species.
-Chlorsulfuron in wheat and barley, nightshades (Solanum spp.), flax and tolerant dicot
species
-Chlorimuron-ethyl in resistant soybeans:
-Thifensulfuron in wheat and soybeans
Resistant Weeds
One of the first cases of weeds resistant to ALSase herbicides were found in
Australia: chlorsulfuron resistant perennial ryegrass (Lolium rigidum) were found
in New South Wales in 1987 in fields with a history of continuous wheat cropping. These
weeds also were found to have a high degree of resistance to other herbicide families. The
mechanisms of resistance appears to be metabolic and alterations in the ALSase target
site, depending on the weed population
Crop ALSase Binding Site Selectivity Mechanisms.
-Target site resistant mutants easy to select for in cell culture and other mutant
screening techniques: ALSase resistant soybeans, imidazolinone resistant corn with high
degree of cross resistance to sulfonylureas
-Cross resistance between sulfonylurea and imidazolinone herbicides is often quite similar
in a species, but differences do exist
Weeds.
-Several U.S. weed species resistant (1000X increase) to chlorsulfuron were found in 1987
in small grain fields, mostly ones with a history of continuous wheat cropping
-Prickley lettuce (Lactuca serriola) found first in research plots in Idaho in
1988.
-Chlorsulfuron resistant Kochia (Kochia scoparia) found in 1988 in Kansas, North
Dakota and other states.
-Resistant Russian thistle
Weed Mechanism of ALSase target site resistance.
-Subsequent research indicated the resistance mechanism is not metabolic (R=S in terms of
uptake, translocation, metabolism), but due to an alteration in the ALSase molecule
preventing sulfonylurea herbicide binding and toxicity from occurring
-Resistance is conferred by variants (mutants) of a single gene encoding the ALSase
molecule
-Resistant variants occur at about 1 X 10-6 (1 in a million) in natural populations of
several weed species. Continuous use of sulfonylurea herbicides, especially those with
long soil residuals, will select out the susceptible members and leaving only resistant
types in as little as 3 years
Fate of Sulfonylureas in the Environment
Soil
Wide range of persistence in the soil environment in family: some very short-lived, some
very persistent in soil.
Both microbial and chemical mechanisms are involved in the degradation of sulfonylurea
herbicides in the soil. Each individual herbicide has a different reliance on
microbes/hydrolysis for its degradation in soils.
Example: chlorimuron:
-Because maize is very sensitive to residues of chlorimuron remaining in the soil after a
soybean crop, this chemical hydrolysis mechanism is very important in assessing carryover
injury.
-Its breakdown in Iowa soils is due primarily to the soil pH, secondarily to soil
temperatures, the #3 factor is soil water availability and the #4 factor is the organic
matter content
Sulfonylurea herbicides such as chlorsulfuron and chlorimuron ethyl are very persistent in
Iowa soils, especially those above pH 6.8. Degradation or chlorimuron is not accomplished
to any significant extent by light, photolysis.
Air
Sulfonylurea herbicides have, in general, low volatility and as such aerial drift
off-target is not a significant environmental threat.
Water
Due to the leachability, and very long persistence in the soil, of chlorimuron at
higher pH's, it has the potential to move into groundwater supplies over time. At the
present time it has not been observed in any groundwater source.
Animal Toxicology
Sulfonylureas herbicides have relatively low toxicities to animals, including humans.
They are apparently one of the safer herbicides to use from this perspective.
Plant Injury Symptomology of Sulfonylureas in Plants
Chlorosis, necrosis, terminal bud and meristem death (below). Stunting, vein
discoloration. The first symptom is cessation of growth, stunting. Then chlorosis slowly
develops.
(Why does this picture look so darn familiar?)
The pattern of injury is a function of the site of uptake: root uptake affects the older
leaves first with preemergence applications (below):
Compared to shoot uptake with the new, young, leaves chlorotic with postemergence
applications (below):
Annual susceptible species turn yellow within 3-5 days and death of the apical
meristem follows. Often a purple coloration associated with the veins is observed in this
process (below):
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