BACKGROUND: Palmer amaranth (Amaranthus palmeri) is an economically troublesome, aggressive and damaging weed that has evolved resistance to six herbicide modes of action 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 Kansas, respectively. Sequence analysis of the chloroplastic psbA gene did not reveal any known mutations conferring resistance to PS II inhibitors, including the most common Ser264Gly 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 USA, this is the first report elucidating the underlying mechanism of resistance to atrazine. The non-target-site based metabolic resistance to atrazine mediated by GST 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.
BACKGROUND:Palmer amaranth (Amaranthus palmeri) is an economically troublesome, aggressive and damaging weed that has evolved resistance to six herbicide modes of action 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 Kansas, respectively. Sequence analysis of the chloroplastic psbA gene did not reveal any known mutations conferring resistance to PS II inhibitors, including the most common Ser264Gly 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 USA, this is the first report elucidating the underlying mechanism of resistance to atrazine. The non-target-site based metabolic resistance to atrazine mediated by GST 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.