Rapid detoxification via glutathione S‐transferase (GST) conjugation confers a high level of atrazine resistance in Palmer amaranth (Amaranthus palmeri)

BACKGROUND Palmer amaranth (Amaranthus palmeri) is an economically troublesome, aggressive and damaging weed that has evolved resistance to six herbicide modes of action (MOA) including photosystem II (PS II)-inhibitors, such as atrazine. The objective of this study was to investigate the mechanism and inheritance of atrazine resistance in Palmer amaranth. RESULTS A population of Palmer amaranth from Kansas (KSR) had a high level (160–198 fold more; SE ± 21–26) of resistance to atrazine compared to the two known susceptible populations MSS and KSS, from Mississippi and KS, respectively. Sequence analysis of the chloroplastic psbA gene did not reveal any known mutations conferring resistance to PS II-inhibitors, including the most common serine264glycine substitution for triazine resistance. However, the KSR plants rapidly conjugated atrazine at least 24 times faster than MSS via glutathione S-transferase (GST) activity. Furthermore, genetic analyses of progeny generated from reciprocal crosses of KSR and MSS demonstrate that atrazine resistance in Palmer amaranth is a nuclear trait. CONCLUSION Although triazine resistance in Palmer amaranth was reported more than 20 years ago in the US, this is the first report elucidating the underlying mechanism of resistance to atrazine. The non-target-site based metabolic resistance to atrazine mediated by GSTs activity may predispose the Palmer amaranth populations to have resistance to other herbicide families, and the nuclear inheritance of the trait in this dioecious species further exacerbates the propensity for its rapid spread.