Role of glutathione transferases in herbicide detoxification in weeds
Glutathione transferases (GSTs) catalyse the conjugation of the electrophilic herbicides atrazine, metolachlor, alachlor and fluorodifen with the tripeptide glutathione (GSH). Maize (Zea mays L), contains multiple GSTs with differing substrate specificities which confer tolerance to a variety of herbicides. In contrast far less is known regarding the GSTs in competing weed species. In vivo metabolism studies using seedlings of maize and the weeds Panicum miliaceum. Digitaria sanguinalis, Sorghum bicolor. Setaria faberi. Abutilon theophrasti and Echinochloa crus-galli demonstrated that all species were capable of metabolising radiolabelled atrazine to GSH conjugates and the relative rates of metabolism related well to GST activities. Similarly, GST activities toward atrazine, metolachlor and alachlor correlated well with herbicide tolerance, with GSH availability being less important. GST activities towards metolachlor, alachlor and atrazine were highest in young maize plants and decreased with age, whilst GST activities in S.faberi remained unchanged. At 35 days GST activities were similar in the two species and the atrazine selectivity was lost. GSH content decreased with age in both species. Protein purification studies showed that S.faberi contains 4 GST isoenzymes with differing substrate specificities. The major GST was estimated to account for 0.1 % of die total soluble protein in S.faberi. PCR-amplification of a cDNA prepared from mRNA showed that S.faberi contains a GST with 88% identity to GST I from maize at the nucleotide level and 82% identity at the amino acid level. Similarly antibodies raised to maize and wheat GSTs recognised GSTs in S.faberi. It is concluded that GSTs determine the relative tolerance to chloroacetanilides and atrazine in weed seedlings but may be less important in older plants. The GSTs in S.faberi are similar in complexity to those determined in maize but are expressed at lower levels.