Differential Responses to Preemergence and Postemergence Atrazine in Two Atrazine‐Resistant Waterhemp Populations

Abstract

Waterhemp [Amaranthus tuberculatus (Moq) Sauer] is a difficult‐to‐control dicot weed in the United States. Atrazine [6‐chloro‐N‐ethyl‐N’‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] is commonly used for preemergence (PRE) and postemergence (POST) waterhemp control in maize (Zea mays L.). Previous research reported that atrazine metabolism via glutathione S‐transferase (GST) activity contributes to atrazine POST resistance in two waterhemp populations from Illinois, designated MCR (McLean County, Illinois, resistant) and ACR (Adams County, Illinois, resistant). Objectives were to quantify responses of these populations to atrazine PRE and determine if the combination of a GST inhibitor and atrazine PRE or POST increases their control. Dose‐response analyses indicated MCR was resistant to atrazine PRE relative to ACR or WCS (Wayne County sensitive; herbicide‐sensitive population), despite MCR and ACR exhibiting equivalent levels of atrazine resistance POST. The ACR response to atrazine PRE (LD50) was intermediate compared with MCR and WCS. Seedling survival of ACR was reduced by 4‐chloro‐7‐nitrobenzofurazan (NDB‐Cl; a GST inhibitor) and atrazine PRE more than atrazine PRE alone, but not in MCR. Atrazine following NBD‐Cl applied POST inhibited seedling growth in ACR, but not in MCR. Enhanced atrazine activity with NBD‐Cl further supports rapid metabolism via GSTs as the main atrazine‐resistance mechanism in ACR. GST(s) that metabolize atrazine in MCR may not have been completely inhibited by NBD‐Cl, indicating that similar yet distinct atrazine‐resistance mechanisms exist in MCR compared to ACR. In conclusion, atrazine PRE (with or without NBD‐Cl) still controls ACR when applied at typical field‐use rates in maize, but the length of residual activity may be shorter than in sensitive populations.Core Ideas Two waterhemp populations are resistant to atrazine POST due to increased metabolism by GST enzymes. Since atrazine can also be soil applied, quantifying the levels of resistance in these populations is important for weed management in maize. The MCR population was resistant to atrazine PRE and displayed a higher level of resistance to atrazine PRE than the ACR population. NBD‐Cl, a metabolic inhibitor of GST enzymes, enhanced atrazine activity in the ACR population but not MCR. The length of residual activity of atrazine may be shorter for controlling these populations in maize compared with sensitive populations.

Molecular and physiological characterization of six‐way resistance in an Amaranthus tuberculatus var. rudis biotype from Missouri

“…Although GST-mediated metabolism has been previously reported to confer atrazine resistance in A. tuberculatus, 24,46 no activity of the GST inhibitor NBD-Cl was observed in MO-Ren, suggesting the presence of several GSTs other than those inhibited by NBD-Cl which confer resistance to atrazine. Similar results were reported by Ma et al 35 in the case of the GST-mediated atrazine resistant MCR A. tuberculatus biotype. They showed that NBD-Cl followed by atrazine improved control of the atrazine-resistant ACR (Adams County mesotrione-sensitive but atrazine-resistant) A. tuberculatus biotype but not of the MCR a A max or T max is the maximum absorption or translocation (%), t 50 is the time (in hours) required to achieve 50% of the maximum absorption or translocation, and values in parenthesis are ± standard error of the mean.…”

“…[31][32][33][34] Preliminary experiments showed that application of malathion alone up to 1.5 kg ai ha −1 and NBD-Cl alone at 0.27 kg ai ha −1 had no adverse effect on the A. tuberculatus plant growth, therefore these rates were used in this experiment. Malathion was applied 30 min prior to 2,4-D, atrazine, mesotrione and chlorimuron application, whereas NBD-Cl was applied two days before the atrazine application as described by Ma et al 35 The herbicides and rates evaluated were: 2,4-D at 0.56 kg a.e. ha −1 , atrazine at 1.12 kg ai ha −1 , mesotrione at 0.11 kg ai ha −1 and chlorimuron at 0.13 kg ai ha −1 .…”

Biochemical characterization of metabolism‐based atrazine resistance in Amaranthus tuberculatus and identification of an expressed GST associated with resistance

“…In addition to improved resistance screening methods, the coding sequence of AtuGSTF2 could be utilized to engineer targeted gene knockout strategies such as RNAi directed to a specific Amaranthus GST (Yu and Powles, ) or to synthesize new chemical inhibitors of herbicide‐detoxifying GSTs (Cummins et al ., ; Lamoureux and Rusness, ; Ma et al ., ). RNAi‐based knockdown techniques have been used successfully in insect systems where insecticide‐detoxifying P450s have been targeted (Bautista et al ., ; Zhu et al ., ), thus regaining activity of the insecticide in resistant populations.…”

“…This initial study broadly determined which F 2 lines were sensitive or resistant to atrazine. GR 50 values for the sensitive lines ranged from 25 to 69 g/ha, significantly lower than the maximum field‐use rate of 2.2 kg/ha (Ma et al ., ). However, complete death was never achieved in atrazine‐resistant lines, and estimated GR 20 values for all resistant lines were greater than the field‐use rate.…”

Multiple-and Cross-Resistance of Amaranthus retroflexus to Acetolactate Synthase (ALS) and Photosystem II (PSII) Inhibiting Herbicides in Preemergence

“…In preemergence, resistance to atrazine was also found in A. retroflexus populations in Illinois (USA) (Ma et al, 2016). It is possible that the intensive use of triazine herbicides in the areas where cross-resistance was found (GO3, GO5 and MT13) has led to the selection of A. retroflexus biotypes with mechanisms of resistance only to this group, specifically.…”