Over the past decades hundreds of weed species have evolved resistance to numerous herbicide modes of action, reducing the number of herbicides available for successful chemical weed control in several agricultural cropping systems. Among the most troublesome weed species in the U.S. is Palmer amaranth (Amaranthus palmeri), which has already evolved resistance to five different modes of action including 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitors. The mechanism of resistance to HPPD inhibitors in one population of this species has previously been confirmed to be enhanced metabolism. Metabolite studies with the HPPD-inhibitor tembotrione revealed that resistant and susceptible plants form the same metabolites. However, resistant plants are able to hydroxylate tembotrione faster than susceptible plants, a process typically catalyzed by cytochrome P450 monooxygenase enzymes. Therefore, an RNA-Sequencing experiment focused on the identification of differentially expressed CYP genes using F2 individuals from three separate crosses sampled before, six hours, and twelve hours after treatment with tembotrione. The experiment identified two genes with the putative annotations CYP72A219 and CYP81E8 as candidate genes for metabolic resistance in A. palmeri. The contigs were constitutively upregulated in resistant individuals and their expression increased after treatment in both resistant and susceptible individuals. The expression patterns varied between the three different crosses. Several glucosyltransferases, a multicopper oxidase, and a glutathione-S-transferase were additional metabolic genes identified to be constitutively upregulated in resistant plants. Furthermore, eight CYP genes were identified as upregulated after treatment with tembotrione in either susceptible plants, resistant plants, or both. Further research aims to validate the involvement of the candidate genes in HPPD-inhibitor resistance in A. palmeri. Gaining knowledge about the connection between specific CYPs and herbicides they confer resistance to is crucial to predict compound liabilities, improve the design of herbicides, and allow for better diagnostics and management of metabolic resistance in the field.

Identification of genes involved in metabolism-based tembotrione resistance in Palmer amaranth (Amaranthus palmeri)

256th National Meeting (2018)

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