B. EPSPS Inhibitors
Herbicides that belong to this group are derivatives of amino acids and inhibit the enzyme 5-enolpyruvyl-
shikimate-3P synthetase (EPSPS). This enzyme participates in the synthesis of shikimic acid, which is a
precursor of phenylalanine, tyrosine, and tryptophan (Figure 4). Glyphosate, the isopropylamine salt of
phosphonic acid, is a noncompetitive inhibitor of shikimate-3P. Glyphosate binds effectively and re-
versibly provided the enzyme has previously bound shikimate-3P, thereby forming a “dead-end complex”
that cannot catalyze the reaction producing 5-enolpyruvyl-shikimate-3P [70–72].
The uptake of herbicides derived from amino acids occurs in leaves, and they are distributed by
phloem to all parts of the plant. They are postemergence nonselective but especially useful against most
annual and many perennial weeds. They have little residual soil activity, are not toxic to mammals, and
are toxic to almost all annual plants.
RESISTANCE Salmonella typhimuriumhas a biotype tolerant to glyphosate thanks to a mutation in
the gene aroAthat codes for EPSPS [73,74]. This gene has been inserted by genetic engineering into var-
ious crop plants (e.g., tomato, tobacco, petunia) and its expression has been amplified with satisfactory
results [75–78]. Also, resistance to this herbicide has been broadened by selection of some species of
plants with enhanced EPSPS activity [79]. Weeds resistant to glyphosate with mutations in EPSPS have
not been found, although some crop and weed species have low resistance levels [80].
C. Glutamine Synthetase Inhibitors
Phosphinothricin (PPT) is a new herbicide, an analogue of glutamate, that inhibits the enzyme glutamine
synthetase (GS) [1,2,4,12]. This enzyme participates in the assimilation of ammonium and in the regula-
tion of nitrogen metabolism (Figure 5) [81,82]. PPT was originally discovered as a component of
bialaphos, an antibiotic produced by Streptomycesspecies, and is synthesized chemically as the herbicide
glufosinate (2-amino-methylphosphinyl-butanoic acid) [83]. Upon applying PPT to the plant, a rapid
buildup of intracellular ammonium levels and an associated disruption of chloroplast structure are caused,
resulting in the inhibition of photosynthesis and plant death [84,85].
In addition to the effect on glutamine synthetase, phosphinothricin produces an inhibition of photo-
synthesis that may be due to inhibition of protein synthesis (especially some proteins involved in electron
PHYSIOLOGICAL MECHANISMS OF HERBICIDE ACTIONS 779
Figure 3 Scheme of the biosynthesis of the amino acids isoleucine, leucine, and valine. The enzyme aceto-
lactate synthetase (ALS) is the target site for the sulfonylurea, imidazolinone, and triazolopyrimidine herbicide
classes. (TPP, coenzyme thiamine pyrophosphate)