Metabolic cross-resistance to florpyrauxifen-benzyl in barnyardgrass (Echinochloa crus-galli) evolved before the commercialization of Rinskor™

Abstract

Herbicide options for selective control of monocot weeds in rice (Oryza sativa L.) have historically been limited to a few modes of action such as inhibitors of acetolactate synthase (e.g. penoxsulam, imazamox), photosystem II (e.g. propanil), and acetyl-CoA carboxylase (e.g. cyhalofop). Florpyrauxifen-benzyl (RinskorTM) is a synthetic auxin molecule introduced to the US rice herbicide market in 2018, providing broad spectrum weed control (monocots and dicots) including hard-to-control species such as barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], along with post-emergence rice selectivity at very low use rates. Within the same year of its commercialization, field agronomists and academics identified E. crus-galli escapes in some areas where florpyrauxifen-benzyl had been sprayed. Further evaluation under controlled environments confirmed that those plants were able to survive florpyrauxifen-benzyl application at the label rate. Here, we identify the mechanism of resistance to florpyrauxifen-benzyl and penoxsulam in two E. crus-galli populations from Arkansas (AR-27) and Missouri (MO-18). Using high-resolution mass spectrometry, the two resistant biotypes were compared with known susceptible plants regarding their ability to metabolize florpyrauxifen-benzyl, florpyrauxifen-acid, and penoxsulam in planta. We discovered that the resistant plants share a common resistance mechanism to florpyrauxifen-benzyl and penoxsulam, involving hydrolysis of a methoxy group (likely mediated by a cytochrome P450 monooxygenase) followed by glucose conjugation. Given that penoxsulam has been widely used in rice fields for the past decade, these data suggest that some populations of E. crus-galli may have evolved resistance prior to the commercialization of florpyrauxifen-benzyl.

Non–target site mechanisms of cross-resistance evolution to florpyrauxifen-benzyl in 2,4-D–resistant Palmer amaranth (Amaranthus palmeri)

“…The metabolic inhibitors NBD-Cl and malathion are known to reduce the activity of plant endogenous enzymes GST and cytochrome P450s, respectively, that can detoxify xenobiotics such as herbicides. Hence, many studies have employed such inhibitors to evaluate the presence of metabolism-based resistance mechanisms in the resistant weed populations (Chen et al 2020; Fang et al 2019; Oliveira et al 2018; Takano et al 2023). Treatment with NBD-Cl followed by FPB did not affect the sensitivity of the R2 population to FPB.…”

“…These results show that the FPB resistance evolved in the R2 population may be partially related to reduction in herbicide metabolism by reduced activity of cytochrome P450 enzymes. A previous study using E. crus-galli reported that the addition of malathion can keep the residual levels of the active form (FPA) higher in the weed for a longer time (Takano et al 2023). The greater and longer retention of FPA by malathion may have contributed to the restoration of FPB sensitivity in some R3 plants.…”

“…Seeds of R populations were subjected to resistance screening tests with other auxin-mimicking herbicides, such as FPB (30 g ai ha −1 ) and dicamba (560 g ae ha −1 ), registered for control of A. palmeri . The R populations were also used for metabolism inhibition experiments using metabolic inhibitors such as chloro-7-nitrobenzofurazan (NBD-Cl; 270 g ai ha −1 ; glutathione S -transferase [GST] inhibitor) and malathion (2,000 g ai ha −1 ; cytochrome P450 inhibitor) (Chen et al 2020; Fang et al 2019; Oliveira et al 2018; Takano et al 2023). Malathion and NBD-Cl were applied 2 and 48 h before herbicide application, and the number of surviving plants was recorded 28 d after the herbicide application to calculate the mortality.…”

“…Beauv.] (Hwang et al 2021, 2022; Jin et al 2023; Takano et al 2023) and not in A. palmeri . Our previous study confirmed three A. palmeri populations resistant to 2,4-D and evaluated their resistance mechanisms (Hwang et al 2023).…”

Echinochloa in mid-southern U.S. and California rice: What is known and what are the knowledge gaps?

“…Recently, resistance to the newly registered rice herbicide, florpyrauxifen-benzyl, has been reported in barnyardgrass (Hwang et al 2022). The mechanism is nontarget site-based and involves hydrolysis of a methoxy group followed by glucose conjugation (Hwang et al 2022; Takano et al 2023), and it is believed to have evolved under the selection of an older rice herbicide, penoxsulam (Takano et al 2023). Earlier, a similar case was reported in a population of barnyardgrass from China that had been under penoxsulam selection for several years (Fang et al 2019).…”