2.1. Transgenic Plants Conferring Tolerance to Herbicides
The most studied group of plant enzymes involved in herbicide metabolism is by far the
GSTs, which catalyze the conjugation of the major cellular thiol, GSH, to an electrophilic site
of lipophilic substrates. The catalytic mechanism involves the addition of the GSH thiolate
anion to the substrate or release of a leaving group (e.g. a halogen or phenolic moiety) from
the substrate. GSTs are involved in the detoxification of herbicides, a highly significant
function with potential use in agriculture and industry. The first report of GST isoenzymes
being involved in detoxification of herbicides was published more than 40 years ago [18], but
still the corresponding literature is limited. Edwards and Owen (1986) have described the
tolerance conferred by GSTs against the herbicides s-triazine, chloroacetanilide and
thiocarbamide in plants [19]. Nevertheless, the role of GSTs in the protection from
xenobiotics was proved much later, when expression of the maize GSTIV in tobacco provided
protection from metolachlor [20].
Maize GST-27, which belongs to the theta class [21], was introduced into wheat. The
transgenic plants became resistant to herbicides belonging to the chloroacetanilide group
(alachlor and dimethenamid) and the thiocarbamate herbicide EPTC. In contrast, GST-27 did
not provide any tolerance against atrazine or oxyfluorodifen [22]. Although the research of
Milligan et al aimed at the development of selectable markers, it showed the potential of
GSTs for developing herbicide resistant plants.