DIFFERENTIAL INDUCTION OF DISTINCT GLUTATHIONE-S-TRANSFERASES OF WHEAT BY XENOBIOTICS AND BY PATHOGEN ATTACK

We have previously characterized a pathogen-induced gene from wheat (Triticum aestivum L.) that was named GstA1 based on sequence similarities with glutathione-S-transferases (GSTs) of maize (R. Dudler, C. Hertig, G. Rebmann, J. Bull, F. Mauch [1991] Mol Plant Microbe Interact 4: 14-18). We have constructed a full-length GstA1 cDNA by combinatorial polymerase chain reaction and demonstrate by functional expression of the cDNA in Escherichia coli that the GstA1-encoded protein has GST activity. An antiserum raised against a GstA1 fusion protein specifically recognized a protein with an apparent molecular mass of 29 kD on immunoblots of extracts from bacteria expressing the GstA1 cDNA and extracts from wheat inoculated with Erysiphe graminis. The GstA1-encoded protein was named GST29. RNA and immunoblot analysis showed that GstA1 was only weakly expressed in control plants and was specifically induced by pathogen attack and by the GST substrate glutathione, but not by various xenobiotics. In contrast, a structurally and antigenically unrelated GST with an apparent molecular mass of 25 kD that was detected with an antiserum raised against GSTs of maize was expressed at a high basal level. This GST25 and an additional immunoreactive protein named GST26 were strongly induced by cadmium and by the herbicides atrazine, paraquat, and alachlor, but not by pathogen attack. Compared with the pathogen-induced GST29, GST25 and GST26 showed a high affinity toward glutathione-agarose and were much more active toward the model substrate 1-chloro-2,4-dinitrobenzene. Thus, wheat contains at least two distinct GST classes that are differentially regulated by xenobiotics and by pathogen attack and whose members have different enzymic properties. GST25 and GST26 appear to have a function in xenobiotic metabolism, whereas GST29 is speculated to fulfill a more specific role in defense reactions against pathogens.