Molecular cloning and characterization of a type III glutathione S-transferase from cell suspension cultures of opium poppy treated with a fungal elicitor

Abstract

Glutathione S-transferases (GSTs) catalyze the conjugation of electrophilic compounds to glutathione (GSH), and are involved in the detoxification of xenobiotics in plants. However, little is known about the endogenous substrates of plant GSTs, or their role in normal cellular processes. A cDNA library was constructed with poly(A)+ RNA isolated from cell suspension cultures of opium poppy (Papaver somniferum L.) treated with a fungal elicitor. A putative type III GST fragment was amplified by PCR from the cDNA library using degenerate primers designed from conserved domains found in several plant GSTs. The cDNA library was screened with the PCR product and 3 homologous clones were isolated. Deduced amino acid sequences from these clones displayed extensive homology to other plant type III GSTs, especially within the N-terminal domain. One full-length cDNA, designated pGST2, was expressed in Escherichia coli, and the recombinant enzyme exhibited strong GST activity in bacterial protein extracts, using 1-chloro-2,4-dinitrobenzene as the substrate. Recombinant GST2 activity was inhibited by p-coumaroyl- and feruloyl-CoA esters, and p-coumaric- and ferulic-acid amides of tyramine, but not by free hydroxycinnamic acids or aromatic amines. Although type III GSTs in opium poppy appear to bind select endogenous phenolics, the in vitro conjugation of GSH to these compounds was not detected. Genomic DNA gel blot-hybridization analysis confirmed the existence of a small family of type III GST genes in opium poppy. RNA gel blot-hybridization analysis showed that type III GST transcripts were most abundant in the roots of mature opium poppy plants, and were induced in developing seedlings and in cell suspension cultures treated with a fungal elicitor. Relative transcript levels were generally consistent with the levels of total GST enzyme activity in each tissue.