transport), to toxic glyoxylate accumulation, or to insufficient regeneration of intermediates of the Calvin
cycle [86,87].
Glufosinate is a nonselective, postemergence contact herbicide that is highly effective and rapidly
biodegraded.
RESISTANCE Several approaches have been used to produce plants tolerant to phosphinothricin. An
alfalfa cell line tolerant to PPT with an elevated level of GS was used to transform tobacco. Enhanced GS
expression in tobacco was obtained, allowing sufficient enzyme activity even in the presence of the her-
780 DE LA ROSAFigure 4 Scheme of the biosynthesis of aromatic amino acids. The enzyme enolpyruvyl-shikimate-3-phos-
phate synthetase (EPSPS) is the target site for glyphosate.
Figure 5 Scheme of ammonium incorporation on carbon skeletons for amino acid biosynthesis. Glutamine
synthetase is the target site for the herbicide phosphinothricin and analogues.