**6. Glutathione S-transferases and evolved herbicide resistance**

Glutathione S-transferases (GSTs) which catalyze the conjugation of glutathione to variety of hydrophobic, electrophilic substrates, are multifunctional enzymes. Glutathione S-transferases (GSTs) have a special role in protecting the plant from oxidative stress (e.g., from reactive oxygen species), thus functioning as protective mechanism [16]. Glutathione S-transferases (GSTs) detoxify several herbicides in some crop and weed species. These enzymes play a role in stress response [148–150]. Glutathione-conjugated herbicides can be sequestered in the vacuole or exuded via root tips [149, 151]. Herbicide-metabolizing GSTs have been purified and characterized from several crops [150, 152]. Some studies such as molecular modeling, mutagenesis studies, and also the resolution of the 3D structure of plant GST (including herbicide-induced GST) provide an understanding of the molecular basis of GST-catalyzed herbicide binding and how single amino acid substitution(s) can improve GST catalytic efficiency and affect substrate specificity for herbicides and xenobiotics [153–155]. Because the Glutathione S-transferases (GSTs) catalyze the conjugation of triazines to glutathione through their high activity, these herbicides are selective for corn. This feature cause to widespread utilize of triazines can elect some weeds with glutathione S-transferases capable to eliminate them. Actually, in some weed species such as *Abutilon theophrasti*, developed GST-intervened triazine herbicide resistance has been observed [49, 156]. More researches demonstrated that enhanced activity of glutathione S-transferases is owing to higher catalytic susceptibility compared with overexpression enzyme or presence of a novel glutathione S-transferases [157]. This shows a conceivable transformation (mutation) in the gene of glutathione S-transferase gene which could better herbicide binding and so glutathione S-transferase catalytic performance. Resistance to atrazine as a singular nuclear gene with sectional predomination is inherited in this biotype [5]. It was demonstrated that in a resistant *Echinochloa phyllopogon* biotype, fenoxaprop-*p*-methyl resistance can be due to glutathione-herbicide conjugation [80]. Investigates with multiple resistant *A. myosuroides* biotypes with increased P450-catalyzed herbicide metabolism also show that they have higher GST activity [149, 158, 159]. Generally, GST enzymes can play both a direct role and an indirect role in evolved herbicide resistance [17].
