Atrazine Resistance in a Velvetleaf ( Abutilon theophrasti ) Biotype Due to Enhanced Glutathione S - Transferase Activity 1

We previously reported that a velvetleaf (Abutilon theophrasti Medic) biotype found in Maryland was resistant to atrazine because of an enhanced capacity to detoxify the herbicide via glutathione conjugation (JW Gronwald, Andersen RN, Yee C [1989J Pestk Blochem Physiol 34: 149-163). The biochemical basis for the enhanced atrazine conjugation capacity in this biotype was examined. Glutathione levels and glutathione Stransferase activity were determined in extracts from the atrazine-resistant biotype and an atrazine-susceptible or "wild-type" velvetleaf biotype. In both biotypes, the highest concentration of glutathione (approximately 500 nanomoles per gram fresh weight) was found in leaf tissue. However, no significant differences were found in glutathione levels in roots, stems, or leaves of either biotype. In both biotypes, the highest concentration of glutathione S-transferase activity measured with 1-chloro-2,4-dinitrobenzene or atrazine as substrate was in leaf tissue. Glutathione S-transferase measured with 1-chloro-2,4-dinitrobenzene as substrate was 40 and 25% greater in leaf and stem tissue, respectively, of the susceptible biotype compared to the resistant biotype. In contrast, glutathione S-transferase activity measured with atrazine as substrate was 4.4and 3.6-fold greater in leaf and stem tissue, respectively, of the resistant biotype. Kinetic analyses of glutathione S-transferase activity in leaf extracts from the resistant and susceptible biotypes were performed with the substrates glutathione, 1-chloro-2,4-dinitrobenzene, and atrazine. There was little or no change in apparent Km values for glutathione, atrazine, or 1-chloro-2,4-dinitrobenzene. However, the Va,, for glutathione and atrazine were approximately 3-fold higher in the resistant biotype than in the susceptible biotype. In contrast, the Vm,, for 1-chloro-2,4-dinitrobenzene was 30% lower in the resistant biotype. Leaf glutathione S-transferase isozymes that exhibit activity with atrazine and 1-chloro-2,4-dinitrobenzene were separated by fast protein liquid (anion-exchange) chromatography. The susceptible biotype had three peaks exhibiting activity with atrazine and the resistant biotype had two. The two peaks of glutathione S-transferase activity with atrazine from the resistant biotype coeluted with two of the peaks from the susceptible biotype, but peak height was threeto fourfold greater in the resistant biotype. In both biotypes, two of the peaks that exhibit glutathione Stransferase activity with atrazine also exhibited activity with 1chloro-2,4-dinitrobenzene, with the peak height being greater in the susceptible biotype. The results indicate that atrazine resistance in the velvetleaf biotype from Maryland is due to enhanced ' Cooperative investigation of the USDA-Agricultural Research Service and the Minnesota Agricultural Experiment Station. Paper No. 18,394, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, MN. 2 Present address: Department ofAgronomy, Oklahoma State University, Stillwater, OK 74078. glutathione S-transferase activity for atrazine in leaf and stem tissue which results in an enhanced capacity to detoxify the herbicide via glutathione conjugation. Approximately 7 years ago, an atrazine-resistant velvetleaf biotype was discovered in a field in Maryland (24). Triazine herbicides, primarily atrazine, had been applied to the field for at least a decade and the field had been in no-till, continuous corn production for 5 years prior to the discovery of resistance. We (15) previously showed that the biotype from Maryland had a 10-fold greater tolerance to atrazine than an atrazine-susceptible or "wild-type" biotype from Minnesota. Resistance was not due to a modification at the herbicidebinding site, the 32-kD quinone-binding protein (15). Rather, the resistant biotype had an enhanced capacity to detoxify atrazine by conjugating it with GSH. Resistance was shown to be controlled by a single nuclear gene exhibiting partial dominance (2). GSTs3 are multifunctional, dimeric proteins that catalyze the nucleophilic attack of the thiolate anion of GSH with electrophilic groups on various substrates (22). These proteins, which are found in mammals (4, 21, 22), insects (3, 14), and plants (25), play a major role in the detoxification of xenobiotics. In plants, GSTs catalyze the detoxification of the striazine (5, 10, 12, 13, 15, 16, 18, 25), chloroacetanilide (8, 10, 23, 25), and thiocarbamate (20, 25) herbicides. It is well established that the tolerance of maize and sorghum to atrazine is due to the high levels of GSH and GST (atrazine) in these species that facilitate the detoxification of the herbicide via GSH conjugation (13, 25, 28). In maize, GST (atrazine) activity is constitutively expressed in leaf and stem tissue (5, 11, 12, 13, 29). Maize roots contain little or no activity (12, 13, 29). Although GST (atrazine) is constitutively expressed in maize leaves, one report suggested that the levels of this enzyme are enhanced by pretreatment with atrazine. Jachetta 3 Abbreviations: GST, glutathione S-transferase; GST (atrazine), glutathione S-transferase activity measured with atrazine as substrate; GS-atrazine, the glutathione conjugate of atrazine; CDNB, l-chloro2,4-dinitrobenzene; PVPP, polyvinylpolypyrrolidone; GST (CDNB), glutathione S-transferase activity measured with CDNB as substrate; Taps, N-tris[hydroxymethyl]methyl-3-amino-propanesulfonic acid; FPLC, fast protein liquid chromatography.