Glutathione transferases involved in herbicide detoxification in the leaves of Setaria faberi (giant foxtail)

Setaria faberi (giant foxtail) is a major grass weed of maize in North America and can prove difficult to control using selective herbicides. In grasses, tolerance to the chloro-s-triazine and chloroacetanilide classes of selective herbicides is associated with their rapid detoxification by glutathione conjugation catalysed by glutathione transferases (GSTs, EC 2.5.1.18), We were therefore interested in characterising the GSTs in S. faberi and comparing them with the corresponding enzymes in maize. Four S, faberi GST isoenzymes (SfaGSTs 1 to 4) with activities toward the triazine herbicide atrazine, the chloroacetanilide herbicides metolachlor and alachlor and the diphenyl ether herbicide fluorodifen were purified from the foliage of young plants. These GSTs detoxified herbicides with similar efficiencies to those determined for GST isoenzymes from maize, but their levels of expression in the leaves mere 20-fold lower than those reported in the crop. All SfaGSTs were composed of two subunits and the 28 kDa subunit of the isoenzyme SfaGST1 reacted strongly to an antiserum raised to the maize theta-type GSTZm GSTI-II. SfaGST1 also appeared to be very similar in substrate specificity to the major maize GST ZmGSTI-I, The similarity of SfaGST1 and ZmGSTI subunits was confirmed by RT-PCR using primers specific to ZmGSTI, with a 370 bp DNA amplification product from S. faberi showing 88% identity at the nucleotide level to the corresponding sequence of ZmGSTI, However, SfaGSTs also differed significantly from ZmGSTs. Unlike maize, only one isoenzyme, SfaGST2, detoxified chloroacetanilides. Also, SfaGST3 and SfaGST4 resembled tau-type GSTs from maize in showing high activities toward fluorodifen, but these SfaGSTs were not recognised by an antiserum raised to the maize tau-type GST ZmGSTV-VI. SfaGST4 also differed from the ZmGSTs described to date in showing high activities toward atrazine, Our results demonstrate that while some GSTs are conserved in grass crops and weeds, others are quite different.