Aromatic Amino Acid Biosynthesis Inhibitors
Symptomology
Development Summary
Site of Action Summary
Injury Symptoms
Time-lapsed Video 1
Time-lapsed Video 2
Herbicide Families
Herbicides
Symptomology Development Summary
Glyphosate inhibits the
production of three essential amino acids within plants resulting plant death.
Injury symptoms caused by glyphosate develop slowly over a 10 days period.
Chlorosis of meristematic
tissue (1-2 days) and no new growth is followed by necrosis progressing from
new to older tissue (top to bottom). Glyphosate is translocated through
the plant’s phloem allowing it to move to metabolic sinks (buds, storage
organs), making it quite effective for total kill on many annual and perennial
weeds.
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Amino acids, the building
blocks of proteins, are synthesized in plants by complex chemical reactions
driven by specific enzymes. Glyphosate inhibits 5-enolpyruvyl-skimate-3-phosphate
(EPSP) synthase, a specific enzyme in the shikimate pathway,
which stops the production of the three aromatic amino
acids ( tryptophan, phenylalanine, and tyrosine) ultimately leading
to plant death.
EPSP synthase combines a phospho-shikimate molecule with phosphoenolpyruvate
(PEP). Blockage of EPSP synthase in this pathway results in (1) the buildup
of shikimate within the plant and (2) a lack of the aromatic amino acids needed
for production of proteins essential for plant growth. These proteins form
other plant essentials such as hormones and flavenoids. Since the shikimate
pathway is feedback regulated (i.e. accumulation of a pathway’s product
decreases the demand of production), blockage of EPSP synthase by glyphosate
results in increased production of the aromatic amino acid precursors (i.e.
shikimate).
An estimated 20% of all carbon fixed in the leaves passes through the shikimate
pathway and 30% of the plants dry weight comes from the aromatic amino acids.
It is estimated that approximately 16% of the dry weight of glyphosate treated
plants is from shikimate accumulation.
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Injury symptoms are chlorosis
of the growing point on broadleaves and yellowing outward from the whorl of
grasses. Chlorotic symptoms develop in 5 – 15 days depending on plant
susceptibility to the herbicide.
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