Distinct biochemical and molecular mechanisms confer resistance to mesotrione and atrazine in a multiple herbicide-resistant waterhemp population
Waterhemp (Amaranthus tuberculatus) is a troublesome annual weed species in maize (Zea mays) and soybean (Glycine max) production, partly due to multiple mechanisms for herbicide resistance. Previous research reported the first case of resistance to mesotrione and other 4-hydroxyphenylpyruvate dioxygenase (HPPD) herbicides in a waterhemp population designated MCR. Experiments were conducted to determine if target-site or non-target-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), and a herbicide-sensitive population (WCS) was also used for comparison. Mesotrione resistance was not due to an alteration in HPPD sequence, HPPD expression, or reduced herbicide absorption. Metabolism studies showed significantly lower level of mesotrione and higher level of metabolites 4-hydroxy-mesotrione in MCR than ACR and WCS, which correlated with previous phenotypic responses to mesotrione. The P450 inhibitors malathion and tetcyclacis significantly reduced mesotrione metabolism in MCR and corn, but not in ACR. These results indicate that enhanced oxidative metabolism via P450(s) 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 time 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 co-chromatographed with a synthetic atrazine-glutathione conjugate. Thus, elevated rates of metabolism via distinct detoxification mechanisms contribute to mesotrione and atrazine resistance within the MCR population. My next objective was to identify and characterize candidate P450 genes in MCR. One candidate chosen was the maize Nsf1 gene, which encodes a P450 that significantly contributes to multiple-herbicide tolerance. Results of real-time quantitative reverse-transcriptase polymerase chain reaction revealed that a P450 transcript most similar to maize Nsf1 (named ArNsf1) is more highly expressed in meristem tissue of MCR and CHR (another mesotrione-resistant waterhemp population) seedlings (10 cm) compared with each HPPD-sensitive population. Significant differences in expression were not detected when comparing two additional candidate P450s (ArHub and ArSI) among these waterhemp populations. ArNsf1 expression in meristem tissue of MCR seedlings harvested at 4, 6, 8, and 10 cm was significantly higher than in WCS seedlings, but not in 2 cm seedlings or in roots of 10 cm seedlings. Therefore, only ArNsf1 expression correlated with mesotrione postemergence resistance in MCR, and growth-stage results suggest that ArNsf1 expression may be growth-stage dependent.
metabolism, resistance, atrazine, mesotrione, waterhemp
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign