Unravelling the resistance mechanism to dicamba in Palmer amaranth (Amaranthus palmeri)

Abstract

Auxin-mimic herbicides (AMHs) have been widely used for more than 70 years, primarily for control of pernicious broadleaf weeds and a few grassy weeds. So far 87 weeds have evolved resistance to AMHs, and it is expected to continue to increase over time. Resistance mechanisms to AMHs are not well understood and most of the reported cases have not been investigated to determine each individual resistance mechanism. Herbicide resistance mechanisms are classified into two main branches. Target-site-resistance (TSR) is when a mutation in herbicide binding site prevents the herbicide from interacting with the enzyme or when overexpression of the herbicide target site results in more enzyme than what the herbicide can inhibit in the plant cell. Non-target-resistance (NTSR) include enhanced herbicide metabolism, reduced absorption, altered translocation, and sequestration. In this current research, resistance to dicamba in a population of Amaranthus palmeri (Palmer amaranth) was investigated. In 2020 a population of the troublesome weed Amaranthus palmeri (Palmer amaranth) from Lauderdale county in Tennessee, USA, with a 12-fold resistance to dicamba was identified. Metabolism of dicamba was evaluated and tested using inhibitors of important enzymes involved in herbicide detoxification (e.g., cytochrome P450 monooxygenase and glutathione S-transferases). There was no difference in dicamba metabolism between the resistant Lauderdale (R_PA) and susceptible Arizona (S_PA) populations. RNA-seq study was conducted to investigate potential mutations in AUX/IAAs, which are transcriptional repressors. They regulate transcription factors like Auxin Response Factors (ARFs), and are also co-receptors of auxin-mimic herbicides, and are involved in the regulation of auxin response genes. A mutation in co-receptors can lead to auxin-mimic herbicide resistance; however, there were no mutations in 18 AUX/IAAs and also in other important proteins such as Transport Inhibitor Response 1 (TIR1), Auxin Binding Protein (APB), and Auxin Signaling F-box (AFB). In addition, auxin-response genes responded similarly or differently to dicamba in treated biotypes. Nevertheless, it is noteworthy that the expression of some AUX/IAAs genes changed after dicamba treatment in sensitive plants but not in resistant plants, especially AUX/IAA29. The results suggest that a physiological response is not primarily involved in the resistance mechanism to dicamba because no significant differences in dicamba metabolism were identified, suggesting that dicamba is broken down to less active metabolite, but at the same rate in both R_PA and S_PA. Additionally, no epinasty was observed in resistant plants, a common response when TSR is involved as a primary mechanism. PIF3/4, which is a key transcription factor involved in regulating plant development and response to light, responded to dicamba treatment in sensitive plants, while not responding in resistant plants after dicamba application. Also, expression of AUX/IAA29 did not respond in resistant plants, which is directly involved in the activation of PIF3/4, a transcription factor involved in auxin perception. We hypothesize that PIF3/4 may be involved in the resistance mechanism to dicamba through auxin signaling and/or regulation. However, this hypothesis should be validated by molecular techniques to confirm it. This dicamba-resistant Palmer amaranth biotype has a novel resistance mechanism that remains to be fully elucidated.