Distinct Detoxification Mechanisms Confer Resistance to Mesotrione and Atrazine in a Population of Waterhemp      

Abstract

Previous research reported the first case of resistance to mesotrione and other 4-hydroxyphenylpyruvate dioxygenase (HPPD) herbicides in a waterhemp (Amaranthus tuberculatus) population designated MCR (for McLean County mesotrione-and atrazineresistant). Herein, experiments were conducted to determine if target site or nontarget site mechanisms confer mesotrione resistance in MCR. Additionally, the basis for atrazine resistance was investigated in MCR and an atrazine-resistant but mesotrione-sensitive population (ACR for Adams County mesotrione-sensitive but atrazine-resistant). A standard sensitive population (WCS for Wayne County herbicide-sensitive) was also used for comparison. Mesotrione resistance was not due to an alteration in HPPD sequence, HPPD expression, or reduced herbicide absorption. Metabolism studies using whole plants and excised leaves revealed that the time for 50% of absorbed mesotrione to degrade in MCR was significantly shorter than in ACR and WCS, which correlated with previous phenotypic responses to mesotrione and the quantity of the metabolite 4-hydroxy-mesotrione in excised leaves. The cytochrome P450 monooxygenase inhibitors malathion and tetcyclacis significantly reduced mesotrione metabolism in MCR and corn (Zea mays) excised leaves but not in ACR. Furthermore, malathion increased mesotrione activity in MCR seedlings in greenhouse studies. These results indicate that enhanced oxidative metabolism contributes significantly to mesotrione resistance in MCR. Sequence analysis of atrazine-resistant (MCR and ACR) and atrazine-sensitive (WCS) waterhemp populations detected no differences in the psbA gene. The times for 50% of absorbed atrazine to degrade in corn, MCR, and ACR leaves were shorter than in WCS, and a polar metabolite of atrazine was detected in corn, MCR, and ACR that cochromatographed with a synthetic atrazineglutathione conjugate. Thus, elevated rates of metabolism via distinct detoxification mechanisms contribute to mesotrione and atrazine resistance within the MCR population.

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

“…Mechanisms that confer NTSR are becoming more common and problematic in grass and dicot weed populations throughout the world (Powles and Yu, 2010;Délye et al, 2011;Ma et al, 2013;Yu and Powles, 2014), potentially arising from the lack of a significant fitness cost to the plant. Research conducted during the 1970s and 1980s directed at improving the POST activity of atrazine and control of grass weeds in maize (Thompson et al, 1971;Thompson, 1972) involved the use of herbicide synergism (Boydston and Slife, 1986).…”

“…Along these lines, current research in our laboratory is investigating the hypothesis that similar but distinct GSTbased NTSR mechanisms for atrazine occur in these two waterhemp populations (Evans et al, 2013). The NTSR and MHR mechanisms in grass (Reade et al, 2004;Délye et al, 2011;Cummins et al, 2013) and dicot (Patzoldt et al, 2005;Ma et al, 2013;Scarabel et al, 2015) weeds are providing complex new challenges for weed management and crop production by limiting effective herbicide options available (Preston, 2004;Yu and Powles, 2014). In addition to understanding cross-resistance patterns, genetic and biochemical mechanisms, and the molecular basis for NTSR and MHR in weeds, novel management tools are needed to combat herbicide-resistant weeds or overcome the mechanism(s) that confer resistance to both PRE and POST herbicides (Preston, 2004).…”

“…Previous research reported that elevated rates of atrazine metabolism via GST activity contribute to atrazine POST resistance within two waterhemp populations . Th e MCR and ACR are both resistant to atrazine POST, and their responses have been compared with an herbicide-sensitive population, WCS (from Wayne County, Illinois) (Patzoldt et al, 2005;Hausman et al, 2011;Ma et al, 2013).…”

Target-Site Resistances to ALS and PPO Inhibitors Are Linked in Waterhemp (Amaranthus tuberculatus)

“…The resistance to atrazine is likely due to enhanced metabolism (Ma et al 2013), whereas resistances to the ALS and PPO inhibitors are due to target-site changes: Trp574Leu substitution and Gly210 deletion (ΔG210), respectively (Patzoldt et al 2006;Patzoldt and Tranel 2007). Despite the resistances to ALS and PPO inhibitors being attributed to distinct genes (ALS and PPX2), anecdotal observations suggested that these two resistances did not follow independent assortment.…”

“…Many such populations likely have a corresponding number of resistance mechanisms (e.g., Bell et al 2013;Legleiter and Bradley 2008;Schultz et al 2015) and therefore exhibit true multiple resistance. Metabolicbased resistance to herbicides has been reported in waterhemp, however, suggesting that resistance to multiple herbicides in some cases could be due in part to cross resistance (Guo et al 2015;Ma et al 2013).…”