Branched-chain Amino Acid Inhibitors
Symptomology
Development Summary
Site of Action Summary
Injury Symptoms
Soil Persistence
Herbicide Families
Herbicides
Symptomology Development Summary
Inhibition of the enzyme acetolactate synthetase (ALS) results in inadequate supplies of the branched chain amino acids (leucine, isoleucine and valine). Failure of enzymes and other proteins containing with these amino acids result in stoppage of shoot growth, yellowing of shoot tips, purple pigment development on dicot leaves and stems of grasses, and interveinal chlorosis. Underground roots are reduced in number and length. Roots of sensitive plants often have a bottle-brush appearance. Crop selectivity is due to differential herbicide degradation or nonsensitive binding site.
The sulfonylureas, imidazolinones,
triazolopyrimidines sufonanilides, and the pyrimidinyl oxybenzoates are four
chemical classes of commercial herbicides that have their site of action at
a single enzyme – either acetolactate synthatse (ALS) or acetohydroxyacid
synthase (AHAS ) – in the branched chain amino acid biosynthesis
pathway. The biosynthetic reactions take place in chloroplast and
produce isoleucine, leucine, and valine. In higher plants, ALS is feedback
regulated by valine and leucine. ALS is a nuclear encoded enzyme, but moves
to the chloroplast using a transit peptide. Once the transit peptide is cleaved,
the enzyme becomes functional. Three domains of the enzyme come together from
the two subunits of the dimer to form a pocket for binding of substrates or
an herbicide molecule. Although not overlapping, substrate and herbicide binding
sites are in proximity. Nearly all of the mutation sites on the enzyme that
confer ALS resistance also cluster in this pocket. Models also suggest some
overlapping between the imidazolinone and sulfonylurea binding sites in the
pocket.
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Initial injury symptoms
may take as long as 7 to 10 days to occur. Initial symptoms include
yellowing of growing points and interveinal areas or grasses and broadleaves.
Advanced symptoms include purple
veins of dicot plants and purpling of grass stems followed by necrosis
of tissues and plant death.
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In general all ALS inhibiting
herbicides degrade in the soil by both chemical hydrolysis and/or microbial
breakdown. Because they are active at such low concentrations, carryover injury
to rotation crops is a major concern with some products. This is particularly
true in areas with diversified cropping systems. Sulfonylurea herbicides have
a positive
charge in high pH soils, which slows the degradation process, and
thus increases the potential for carryover injury to rotational crops. For
this reason soils with a high pH may require as long as 48 months in order
for herbicide degradation to progress beyond plant injury. Since chemical
hydrolysis is a key avenue of degradation, areas receiving marginal precipitation
can also experience injury of rotational crops.
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