Glutathione transferases in soybean Glycine max (L.) Merr.
Glutathione transferases, also known as Glutathione S-transferases (GSTs), are a diverse group of enzymes that catalyse the conjugation of the tri-peptide glutathione to a wide range of electrophilic substrates. Their biological function in endogenous metabolism in plants is not well characterised, although their role in herbicide metabolism and herbicide selectivity is well documented. Many herbicides used in soybean. Glycine max (L.) Merr., are selective against weeds due to their rapid detoxification in the crop through conjugation with homoglutathione (γ-glu-cys-β-ala), the predominant free thiol in many legumes. However, an in depth characterisation of the GSTs which can potentially catalyse these reactions in soybean has never been performed. This work describes the biochemical and molecular characterisation of GSTs in soybean with emphasis on the identification of specific isoenzymes involved in herbicide metabolism. GST activity toward the chloroacetanilide herbicides acetochlor and metolachlor, the diphenyl ethers acifluorfen and fomesafen and the sulphonyl urea chlorimuron-ethyl were all detected in crude protein extracts from five-day-old suspension cultured soybean cells. GST activity was also determined in five-day-old soybean seedlings, though this activity was significantly lower than that observed with the cell suspension cultures. Treatment of soybean plants with herbicides and herbicide safeners resulted in increased GST activity toward the model substrate l-chloro-2,4-dinitrobenzene (CDNB), but no change in activity toward herbicide substrates. In both plant and cell cultures GST-catalysed conjugation of the diphenyl ethers acifluorfen and fomesafen was over five-fold greater in the presence of homoglutathione as compared with glutathione. The preferential detoxification of these herbicides in the presence of homoglutathione appeared to be an important determinant of their rapid detoxification in soybean and an important factor in herbicide selectivity. GSTs were purified from five-day-old soybean cell cultures using S-hexylglutathione affinity chromatography and anion-exchange chromatography. A combination of reversed-phase HPLC, SDS-PAGE and MALDI-TOF mass spectrometry of the purified fractions indicated the presence of nine putative GST subunits, each with a molecular mass between 25 and 29 kDa. Soybean GST cDNA clones were obtained using a combination of RT-PCR, utilising degenerate oligonucleotides designed to conserved regions within plant GSTs, and screening of cDNA libraries prepared from soybean plants and cell cultures. This process failed to identify any theta-type GSTs, the class associated with herbicide detoxification in maize. In contrast, seven distinct tau-type GSTs were isolated together with a number of clones showing minor variations in individual sequences. Expression of these cDNAs in Escherichia coli showed the purified recombinant GSTs were active toward a diverse range of substrates, and possessed additional glutathione peroxidase activity. GST activities for each recombinant enzyme varied with substrate and thiol type, with a marked preference for homoglutathione with selected substrates. From the work reported in this study it would appear that the tau-type GSTs of soybean are at least as complex as those previously reported in cereals and have an important role in determining herbicide metabolism and selectivity in this major crop.