Introduction
Herbicides that inhibit very-long-chain fatty acids (VLCFAs) have been in use for more than 60 yr for broad-spectrum control of weeds in several crops (Senseman Reference Senseman2007). S-ethyl dipropylthiocarbamate (EPTC) was the first VLCFA inhibitor developed by Stauffer Chemical Company in 1958, while the most recent discoveries were pyroxasulfone and fenoxasulfone, in 2014 (Table 1). These herbicides inhibit elongases after the formation of malonyl-Co-A in the fatty acid biosynthesis pathway. The VLCFA-inhibiting herbicides are typically applied on the soil before weed emergence (Jhala et al. Reference Jhala, Malik and Wills2015), and upon plant uptake, the compound is metabolically cleaved to produce an active form of the herbicide via sulfoxide enzyme activity (Sherwani et al. Reference Sherwani, Arif and Khan2015).
Table 1. Registration timeline of selected very-long-chain fatty acid–inhibiting herbicides in the United States according to the U.S. Environmental Protection Agency. a
a See https://www.epa.gov/.
The VLCFA-inhibiting herbicides can be applied postemergence (POST) to several crops and provides overlapping residual activity. It can be used throughout the autumn and early preplant to early POST periods for controlling weeds, including those that are resistant to glyphosate, acetolactate synthase (ALS) inhibitor, acetyl CoA carboxylase (ACCase) inhibitor, and triazines (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). In susceptible grass and broadleaf weeds, the growth of the apical meristem and coleoptile is disrupted after germination, resulting in a failure to emerge. When susceptible monocots do emerge, they appear twisted and malformed, with leaves tightly rolled in a whorl and unable to unroll normally. Broadleaf seedlings may exhibit slightly cupped or crinkled leaves, along with shortened leaf midribs, resulting in a drawstring effect on the leaf tip (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
The VLCFA-inhibiting herbicides are classified as Group 15 herbicides by the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (Mallory-Smith and Retzinger Reference Mallory-Smith and Retzinger2017). These herbicides inhibit fatty acid synthesis in plants, an important component of plant cell membranes, waxes, and cuticles, and they play a crucial role in plant growth and development (Millar and Kunst Reference Millar and Kunst1997; Post-Beittenmiller Reference Post-Beittenmiller1996). The VLCFA-inhibiting herbicides are sometimes referred to as shoot-growth-inhibiting herbicides and are commonly applied to soil to control seedling grasses and certain broadleaf weeds as they emerge, and to suppress some perennial weed species that arise from tubers and rhizomes. The VLCFA-inhibiting herbicides are primarily absorbed by plant roots and are translocated to the shoot and leaf tissues primarily via xylem (Fuerst Reference Fuerst1987), in addition to being absorbed to a lesser extent through the shoots and leaves of the plant.
The objectives of this review were to 1) summarize the chemical families of VLCFA-inhibiting herbicides and their use in the United States; 2) provide an overview of VLCFA biosynthesis in plants, VLCFA profiles in Group 15 herbicide–treated plants, and inhibition of VLCFA elongases by Group 15 herbicides; 3) describe VLCFA-inhibiting herbicide–resistant weeds and their mechanism of resistance; and 4) explore the future of VLCFA-inhibiting herbicides.
Chemical Families of VLCFA-Inhibiting Herbicides
Herbicides that inhibit shoot growth were classified into Group 8 (lipid synthesis-inhibitor) and Group 15 (VLCFA-inhibitor); however, the Group 8 herbicides were later reclassified into Group 15 by the WSSA Herbicide Resistance Action Committee (HRAC) in 2021 (Table 2). Napropamide belonged to the acetamide family of Group 15 in the previous herbicide classification; however, it is classified as a Group 0 herbicide in the revised classification in 2021 (WSSA 2021). The reclassification of VLCFA-inhibiting herbicides reflects the understanding of their site of action (SoA). The VLCFA-inhibiting herbicides are currently represented by eight chemical families: α-chloroacetamides, α-oxyacetamides, α-thioacetamides, azolyl-carboxamides, benzofurans, isoxazolines, oxiranes, and thiocarbamates (Table 2). They are known to inhibit various biochemical pathways in plants, including the biosynthesis of fatty acids and lipids (reducing cuticular wax deposition), proteins (isoprenoids, and flavonoids), and gibberellins (due to kaurene synthesis inhibition) (Fuerst Reference Fuerst1987; WSSA 2021). The chemical families of VLCFA-inhibiting herbicides are briefly discussed below.
Table 2. Very-long-chain fatty acid–inhibiting herbicide chemical families and active ingredients according to the Weed Science Society of America and the Herbicide Resistance Action Committee classification list. a
a See https://wssa.net/wssa/weed/herbicides/ (last modified May 5, 2021).
Benzofurans
Benzofurans and thiocarbamates were chemical families of Group 8 that were reclassified into Group 15. Benzofurans are a family of shoot-inhibiting herbicides that act by inhibiting meristem growth, cell division, and the formation of cuticles, ultimately leading to the retardation of plant growth (Kohler and Branham Reference Kohler and Branham2002; Székács Reference Székács, Mesnage and Zaller2021). Ethofumesate, an active ingredient within this family (Figure 1), is commonly used preemergence (PRE) in the United States to control weeds and POST to selectively control annual grasses and broadleaf weeds in crops that include sugarbeet (Beta vulgaris L.), vegetables, and grass seed production (Abulnaja et al. Reference Abulnaja, Tighe and Harwood1992; Albuquerque et al. Reference Albuquerque, Carrão, Habenschus and Oliveira2018). Moreover, ethofumesate reduces the formation of epicuticular wax on leaves (Abulnaja et al. Reference Abulnaja, Tighe and Harwood1992). It has limited mobility, with little leaching occurring in soils with greater than 1% organic matter because the herbicide remains relatively close to the application site (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
Figure 1. Chemical structure of ethofumesate, which inhibits very-long-chain fatty acids.
Thiocarbamates
Thiocarbamates are commonly applied preplant incorporated (PPI) or POST for residual control of annual grasses such as foxtail (Setaria spp.), barnyardgrass [Echinochloa crus-galli (L) P. Beauv], fall panicum (Panicum dichotomiflorum Michx), johnsongrass [Sorghum halepense (L.) Pers], shattercane (Sorghum bicolor L.), nutsedge (Cyperus spp.), and some broadleaf weeds in crops of wheat, barley, pulses, sugarbeet, corn, soybean, cotton, dry bean (Phaseolus vulgaris L.), and others (Fuerst Reference Fuerst1987; Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). The active ingredients of thiocarbamates are characterized by the presence of a thiocarbamate group [-S-C(=O)-NR2] in their chemical structure (Figure 2), which is responsible for their herbicidal activity (WHO 1988). Some examples of thiocarbamate herbicides include butylate, cycloate, EPTC, molinate, prosulfocarb, thiobencarb, and triallate (Table 2; Figure 2). Thiocarbamates are generally not mobile in soils with high organic matter content, but can be subject to volatilization losses when applied to warm soils and not properly incorporated (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Wilson (Reference Wilson1984) reported accelerated degradation of thiocarbamate herbicides in soil with prior thiocarbamate herbicide exposure due to buildup of microbes that break them down.
Figure 2. Chemical structures of some thiocarbamate herbicides.
α-Chloroacetamides
Chloroacetamide herbicides are widely used to control annual grasses and small-seeded broadleaf weeds in a variety of crops including corn, soybean, peanut (Arachis hypogaea L.), cotton, sugarbeet, and solanaceous vegetables. These herbicides can be applied to crops at various stages of plant growth, such as early preplant, PRE, or early POST to provide residual weed control. The α-chloroacetamide herbicides include acetochlor, alachlor, butachlor, dimethenamid-P, pretilachlor, and metolachlor (Figure 3).
Figure 3. Chemical structures of some α-chloroacetamides.
Metolachlor is often used as a residual herbicide, and contains four isomers that can be classified into S- and R-metolachlor isomers (O’Connell et al. Reference O’Connell, Harms and Allen1998). The effectiveness of metolachlor is dependent on the concentration of isomers, with the S-isomers being more effective than the R-isomers (O’Connell et al. Reference O’Connell, Harms and Allen1998; Shaner et al. Reference Shaner, Brunk, Belles, Westra and Nissen2006). The application rate of S-metolachlor is 35% lower than the racemic mixture of metolachlor on an active ingredient basis with residues usually not persisting long enough to affect crops in the following season (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Dimethenamid-P is another example of an α-chloroacetamide herbicide widely used for residual control of annual grasses and certain annual broadleaf weeds in crops such as corn, soybean, dry bean, peanut, and sorghum (Figure 3). It is primarily absorbed by emerging shoots and roots and has a half-life of approximately 20 d (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
Isoxazolines
Pyroxasulfone and fenoxasulfone belong to the isoxazoline chemical family of VLCFA-inhibiting herbicides (Figure 4; WSSA 2021). Pyroxasulfone is a residual herbicide that is effective for controlling grass and broadleaf weeds while providing selectivity in crops such as corn, wheat, and soybean (Tanetani et al. Reference Tanetani, Kaku, Kawai, Fujioka and Shimizu2009). Pyroxasulfone is absorbed primarily by emerging shoots and roots and is translocated acropetally throughout the shoots (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). It is moderately adsorbed to soil and has low leaching potential. It is less mobile in fine and medium-textured soils and more mobile in coarse-textured soils. It is not persistent in soils and has a terrestrial field dissipation half-life of 16 to 26 d. Negligible losses occur due to photodegradation or volatilization, and microbial degradation is a major contributor to field dissipation (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Fenoxasulfone is a PRE herbicide that is effective for controlling annual broadleaf and grass weeds in paddy-field rice (Oryza sativa L.) cultivation (Tanetani et al. Reference Tanetani, Fujioka, Horita, Kaku and Shimizu2011a). It has long residual activity and displays herbicidal activity against Echinochloa spp. And other annual weeds at 150 to 200 g ai ha–1 (Fujinami et al. Reference Fujinami, Takahashi, Tanetani, Ito and Nasu2019; Umetsu and Shirai Reference Umetsu and Shirai2020).
Figure 4. Chemical structures of isoxazolines.
Α-Oxyacetamides
The most common oxyacetamide herbicides are mefenacet and flufenacet. Mefenacet is primarily used for weed control in transplanted rice and is not currently marketed in the United States (Fedtke Reference Fedtke1991; Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Flufenacet has been mixed with a relatively low rate of metribuzin (AXIOM™; Bayer CropScience, St Louis, MO), or with isoxaflutole and thiencarbazone-methyl (TriVolt™; Bayer CropScience), a corn herbicide. Flufenacet controls most annual grasses and certain small-seeded broadleaf weeds. Flufenacet has a short to moderate persistence, with a half-life of 29 d in loamy sand soil with 0.9% organic matter and pH 5.7; and a half-life of 62 d on loamy sand with 0.5% organic matter, and pH 5.6. In most soil textures, oxyacetamides have low to moderate mobility and are primarily degraded by microbial activity (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
Azolyl-Carboxamides
Ipfencarbazone-methyl, fentrazamide, and their related compounds are VLCFA-inhibiting herbicides that belong to the azolyl-carboxamides chemical family. Ipfencarbazone is a PRE and early POST herbicide used in rice production with high efficacy and long residual activity against a broad range of weeds, including annual grasses, sedges, and some broadleaf weeds (Kasahara et al. Reference Kasahara, Matsumoto, Hasegawa, Koyama and Takeuchi2019; Kido et al. Reference Kido, Okita, Okamura, Takeuchi and Morita2016; Umetsu and Shirai Reference Umetsu and Shirai2020). Fentrazamide is a selective herbicide widely used to control Echinochloa spp. And annual sedges in rice. At a rate of 200 to 300 g ai ha−1, it has shown efficacy against Echinochloa spp. Within a wide range of growth stages, from PRE up to the 3-leaf stage of the weed in rice (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Fentrazamide has low mobility in Japanese paddy soils, and is absorbed through the roots and shoots of susceptible plants and dissipates rapidly in water with a half-life of soil metabolism of about 30 d in volcanic soil and 20 d in alluvial soil (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
Α-Thioacetamides
Thioacetamides are a chemical family of VLCFA-inhibiting herbicides, with anilofos and piperophos being examples in this class. Anilofos is a PRE and early POST herbicide used to control annual grasses, sedges, and selected broadleaf weeds in transplanted and direct-seeded rice at a rate of 300 to 450 g ai ha−1. It is taken up through the roots and, to some degree, through the leaves, and has an inhibitory effect on shoot and root growth. In soil, anilofos exhibits typical degradation behavior for phosphoric acid, leading to the formation of chloroaniline and CO2 as end products. This herbicide has a field dissipation DT50 of 30 to 5 d and shows low mobility in soil (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014).
Oxiranes
Tridiphane and indanofan belong to the oxirane family (Figure 5). Tridiphane has a synergistic effect on atrazine for controlling grass species such as giant foxtail (Setaria faberi Herrm.) because it inhibits glutathione S-transferase (GST) and prevents the detoxification of atrazine (Lamoureux and Rusness Reference Lamoureux and Rusness1986). Tridiphane has five chlorines (Figure 5), which can have adverse environmental effects. To overcome this, researchers have developed indanofan, an oxirane herbicide that has less chlorine (Figure 5) and shows relatively better environmental properties (Takahashi et al. Reference Takahashi, Schmalfuß, Ohki, Hosokawa, Tanaka, Sato, Matthes, Böger and Wakabayashi2002). Indanofan at 100 to 150 g ai ha−1 provided complete control of Echinochloa oryzicola (Takahashi et al. Reference Takahashi, Schmalfuß, Ohki, Hosokawa, Tanaka, Sato, Matthes, Böger and Wakabayashi2002).
Figure 5. Chemical structures of the oxirane herbicides tridiphane and indanofan.
Overview of VLCFA Biosynthesis in Plants
The VLCFAs are essential to plant growth, development, and defense. VLCFAs are synthesized from plastid-derived 16 or 18 carbon long-chain fatty acids (saturated and unsaturated) that are elongated to 30 carbons or more through sequential additions of two carbon atoms by an endoplasmic reticulum (ER)-bound, multienzyme acyl-CoA elongase complex using the substrates fatty acyl-CoA and malonyl-CoA (Haslam and Kunst Reference Haslam and Kunst2013). The elongase complex in the ER contains separate enzymes with at least four distinct, sequential functions: condensation, reduction, dehydration, and a second reduction (Böger et al. Reference Böger, Matthes and Schmalfuß2000; Krähmer et al. Reference Krähmer, Dücker, Beffa and Babczinski2019). The condensing enzyme (also called VLCFA synthase or 3-ketoacyl-CoA synthase) catalyzes the first, rate-limiting step of these sequential reactions and determines substrate and tissue specificities of VLCFA elongation (Bach and Faure Reference Bach and Faure2010; Millar and Kunst Reference Millar and Kunst1997). By contrast, the other three enzymes are constitutively expressed, exhibit broad substrate specificity, and are common to all tissues with VLCFA biosynthetic capacity (Krähmer et al. Reference Krähmer, Dücker, Beffa and Babczinski2019).
The VLCFA-inhibiting herbicides were discovered and developed in the 1950s (Hamm Reference Hamm1974), but their mode of action (MoA) remained elusive for decades until it was determined that depletion of VLCFA results in phytotoxicity (reviewed by Böger Reference Böger2003). Condensing enzymes are the SoAs for Group 15 herbicides in plants (Krähmer et al. Reference Krähmer, Dücker, Beffa and Babczinski2019); their discovery and characterization has led to a considerable number of publications during the past 30 yr to describe their structures, sequences, physiological functions, and roles in determining Group 15 herbicide SoAs. The VLCFAs are a diverse group of nonpolar compounds and vary in their degrees of unsaturation, function, and structure. They are found in lipid seed reserves and signaling molecules, and comprise the main components of cellular membranes, suberin, cuticle waxes, and the outer coats of pollen grains or “tryphine” layer (Bach and Faure Reference Bach and Faure2010; Haslem and Kunst 2013; Millar et al. Reference Millar, Clemens, Zachgo, Giblin, Taylor and Kunst1999). Physiological studies found that lipid synthesis and formation of cuticle waxes were inhibited by chloroacetamide herbicides (reviewed by Böger et al. Reference Böger, Matthes and Schmalfuß2000; Fuerst Reference Fuerst1987), and studies by Weisshaar and Böger (Reference Weisshaar and Böger1987) eventually led to the discovery of the SoA of chloroacetamides. The first clue was a lack of plasma membrane formation, and it was concluded that chloroacetamides inhibit elongation of C16 or C18 long-chain fatty acids into VLCFA at low (micromolar) concentrations (Weisshaar and Böger Reference Weisshaar and Böger1987). Plants deficient in VLCFA have unstable cells that eventually deteriorate, which inhibits growth in meristematic areas and, ultimately, the death of sensitive plant seedlings (Böger Reference Böger2003; Matthes et al. Reference Matthes, Schmalfuβ and Böger1998).
Inhibition of VLCFA Elongases by Group 15 Herbicides
The SoA of Group 15 herbicides results from inhibition of the VLCFA synthase, which is encoded by several fatty acid elongase (FAE1)-like genes in the VLCFA elongase complex in the ER (Böger Reference Böger2003). The VLCFA synthase is a condensing enzyme, and relies on a conserved, reactive cysteinyl sulfur in its active site (Eckermann et al. Reference Eckermann, Matthes, Nimtz, Reiser, Lederer and Böger2003; Ghanevati and Jaworski Reference Ghanevati and Jaworski2002) that performs a nucleophilic attack on either the natural substrate (fatty acyl-CoA) or herbicide (Böger et al. Reference Böger, Matthes and Schmalfuß2000; Götz and Böger Reference Götz and Böger2004). Binding of the herbicide at the VLCFA synthase consequently limits the four-step process of elongation of long-chain fatty acids (C16 and C18) to VLCFAs through sequential incorporations of two carbons from the second substrate, malonyl-CoA (Böger Reference Böger2003). The active isomer of racemic metolachlor (S-metolachlor) is a competitive inhibitor (with respect to acyl-CoA) of the first enzymatic step of the VLCFA elongase complex (3-ketoacyl-CoA synthase), resulting in depletion of VLCFAs (Böger Reference Böger2003; Trenkamp et al. Reference Trenkamp, Martin and Tietjen2004). Although the MoA of Group 15 herbicides involves covalent binding, or alkylation, of the conserved cysteine residue in VLCFA condensation enzymes (Eckermann et al. Reference Eckermann, Matthes, Nimtz, Reiser, Lederer and Böger2003; Götz and Böger Reference Götz and Böger2004), some Group 15 herbicides (e.g., chloroacetamides) display irreversible inhibition (alkylation), while other herbicides (e.g., pyroxasulfone and thiocarbamate-sulfoxides) show reversible inhibition (Krähmer et al. Reference Krähmer, Dücker, Beffa and Babczinski2019; Tanetani et al. Reference Tanetani, Fujioka, Kaku and Shimizu2011b). Although the primary SoA and MoA of Group 15 herbicides are now accepted in the literature, it is important to note that many findings reported in earlier research are likely valid but may have resulted from secondary responses triggered by higher herbicide concentrations used in their experiments (reviewed by Böger et al. Reference Böger, Matthes and Schmalfuß2000; Götz and Böger Reference Götz and Böger2004).
Different types of fatty acid elongases (FAEs) [e.g., β-ketoacyl-CoA synthase (KCS)] perform condensation reactions in plants (Ghanevati and Jaworski Reference Ghanevati and Jaworski2002; Haslam and Kunst Reference Haslam and Kunst2020; Millar et al. Reference Millar, Clemens, Zachgo, Giblin, Taylor and Kunst1999), as well as type III polyketide synthases (Eckermann et al. Reference Eckermann, Matthes, Nimtz, Reiser, Lederer and Böger2003), and thus are potential SoAs for Group 15 herbicides in weeds. Plants studied to date typically contain between 10 to 30 KCS-like genes per diploid genome (Guo et al. Reference Guo, Zhang, Sun, Li, Hang and Xu2016; Huai et al. Reference Huai, Xue, Li, Wang, Li, Yan, Chen, Wang, Liu, Kang, Wang, Huang, Jiang, Lei and Liao2020). Additional biochemical research demonstrated that several Group 15 herbicides inhibit the activity of different VLCFA synthases in Arabidopsis thaliana, including enzymes encoded by the genes At5g43760, At104220, At1g25450, KCS1, KCS2, and FAE1 (Trenkamp et al. Reference Trenkamp, Martin and Tietjen2004). Expression of FAEs occurs in different plant tissues and organs (Joubès et al. Reference Joubès, Raffaele, Bourdenx, Garcia, Laroche-Traineau, Moreau, Domergue and Lessire2008), varies in response to stresses (Batsale et al. Reference Batsale, Bahammou, Fouillen, Mongrand, Joubès and Domergue2021), and is regulated at different developmental stages. Substrate specificity (i.e., which fatty acyl-CoAs are elongated) of the entire VLCFA elongase complex is determined by the first step catalyzed by FAEs (Krähmer et al. Reference Krähmer, Dücker, Beffa and Babczinski2019; Millar and Kunst Reference Millar and Kunst1997; Trenkamp et al. Reference Trenkamp, Martin and Tietjen2004). These molecular and biochemical factors likely affect which specific FAEs are inhibited by soil-applied Group 15 herbicides, which primarily inhibit shoot growth of emerging seedlings (Fuerst Reference Fuerst1987). Group 15 herbicides are nonionic, which precludes phloem mobility in seedlings, although limited xylem translocation from root absorption to older leaves is possible (Pillai et al. Reference Pillai, Davis and Truelove1979). Several FAEs are expressed in the epidermal cells of shoot tissues (Batsale et al. Reference Batsale, Bahammou, Fouillen, Mongrand, Joubès and Domergue2021; Joubès et al. Reference Joubès, Raffaele, Bourdenx, Garcia, Laroche-Traineau, Moreau, Domergue and Lessire2008), indicating that inhibition of specific target sites (and their homologs in sensitive weeds) may lead to seedling growth inhibition and death.
VLCFA Profiles in Group 15 Herbicide-Treated Plants
The VLCFA depletion in plant cell cultures and/or plant tissues following Group 15 herbicide treatment is typically measured using VLCFA precursors, including radiolabeled stearic acid (C18:0) or oleic acid (C18:1) as biomarkers for subsequent elongation steps (Böger et al. Reference Böger, Matthes and Schmalfuß2000; Hwang et al. Reference Hwang, Norsworthy, Carvalho-Moore, Barber, Butts and McElroy2023; Matthes et al. Reference Matthes, Schmalfuβ and Böger1998; Tanetani et al. Reference Tanetani, Kaku, Kawai, Fujioka and Shimizu2009). In the green algae (Scenedesmus acutus Meyen), radiolabeled C18:1 was supplied to algal cultures and the levels of three VLCFAs (C22:1, C24:1, and C26:1) were measured by high-performance liquid chromatography analysis (Böger et al. Reference Böger, Matthes and Schmalfuß2000). Treatment of the cultures with 1 µM metazachlor completely inhibited the production of three VLCFAs and several more nonpolar compounds (likely VLCFAs >26C), and biochemically phenocopied the known VLCFA-deficient green algae mutant Mz-1 (Böger et al. Reference Böger, Matthes and Schmalfuß2000; Schmalfuβ et al. Reference Schmalfuβ, Matthes, Mayer and Böger1998). In addition to severe VLCFA depletion, accumulation of C18:1 was also measured, which assisted in pinpointing the elongases as targets for inhibition by metazachlor and other Group 15 herbicides (Böger et al. Reference Böger, Matthes and Schmalfuß2000).
Research with cultured rice cells (Tanetani et al. Reference Tanetani, Kaku, Kawai, Fujioka and Shimizu2009) or rice and other grass plants (Tanetani et al. Reference Tanetani, Fujioka, Kaku and Shimizu2011b) has been aimed at determining the SoA and MoA of pyroxasulfone. VLCFA biosynthesis was severely inhibited by pyroxasulfone at nanomolar concentrations, including specific elongation steps from C18:0 to C20:0 up to C26:0 to C28:0, as well as C18:1 to C20:1 (Tanetani et al. Reference Tanetani, Fujioka, Kaku and Shimizu2011b) with a concomitant accumulation of long-chain fatty acid precursors (e.g., C14:0 and C16:0). In preliminary research on a VLCFA inhibitor-resistant Palmer amaranth, gas chromatography was used to examine the concentration of three representative VLCFAs, namely C22:0, C24:0, and C26:0 (Hwang et al. Reference Hwang, Norsworthy, Carvalho-Moore, Barber, Butts and McElroy2023), which were derived from C18:0 substrate synthesized in plastids. This study showed that C22:0 and C24:0 levels slightly increased in S-metolachlor-treated seedlings in both sensitive and resistant Palmer amaranth populations, while the levels of C26:0 decreased in both populations (Hwang et al. Reference Hwang, Norsworthy, Carvalho-Moore, Barber, Butts and McElroy2023). The inhibition of specific VLCFA synthases and the depletion or accumulation of certain VLCFAs in sensitive and resistant weed populations (monocots and dicots) is an area that warrants further research to investigate possible target-site mechanisms of resistance to VLCFA-inhibiting herbicides (Busi Reference Busi2014).
Use of VLCFA Inhibitor in the United States
Acetochlor and S-metolachlor are the most used VLCFA-inhibiting herbicides in corn and soybean crops, respectively (Figure 6). According to a survey conducted in 2021 by U.S. Department of Agriculture–National Agricultural Statistics Service, the estimated use of VLCFA-inhibiting herbicides such as acetochlor, dimethenamid-P, pyroxasulfone, metolachlor, and S-metolachlor in corn production in the United States was 18,903, 1,311, 54, 3,290, and 12, 248 metric tons, respectively (Figure 6A; USDA-NASS 2021a). The survey estimated that about 3,660, 874, 784, 2,085, and 9,116 metric tons of acetochlor, dimethenamid-P, pyroxasulfone, metolachlor, and S-metolachlor, respectively, were used on soybean crops in the United States in 2020 (Figure 6B; USDA-NASS 2020b). On average, VLCFA-inhibiting herbicides were applied about once a year to corn and soybean (USDA-NASS 2020b, 2021a).
Acetochlor
Acetochlor belongs to the α-chloroacetamides family of VLCFA-inhibitor (WSSA 2021). Acetochlor was initially labeled for PPI or PRE applications to corn to control annual monocots, some small-seeded dicots, and yellow nutsedge (Cyperus esculentus L.) (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). After registration in the United States in 1994, acetochlor use increased significantly (Figure 7A; US-EPA 1994; Wieben Reference Wieben2019). Acetochlor was marketed with expectations that formulations with reduced rate would decrease the use of other corn herbicides such as 2,4-D, alachlor, atrazine, butylate, EPTC, and metolachlor. As a result, after 1994 the use of acetochlor in corn production fields increased gradually (Figure 7A; Capel et al. Reference Capel, Ma, Schroyer, Larson and Gilchrist1995; Hackett et al. Reference Hackett, Gustafson, Moran, Hendley, Van Wesenbeeck, Simmons, Klein, Kronenberg, Fuhrman, Honegger and Hanzas2005; Wieben Reference Wieben2019). During its first year of release in 1994, acetochlor was the fifth-most applied corn herbicide, and with its more extensive use in successive years, it became the third-most applied corn herbicide in the Midwestern United States in 1996 (Clark and Goolsby Reference Clark and Goolsby1999). In 2021, acetochlor was the fourth-most applied herbicide to corn in the United States (USDA-NASS 2022). About one-third (34%) of 37.8 million ha (92.1% of the total planted area) received acetochlor (including multiple active ingredients) at an average rate of 1.59 kg ai ha–1, which totaled 18,903 metric tons in 2021 (Figure 7A; USDA-NASS 2022). The major corn-producing states in the Midwestern United States used acetochlor in the range of 406 to 3,768 metric tons per state, with Iowa (3,768 metric tons), Nebraska (3,014 metric tons), Minnesota (2,878 metric tons), and South Dakota (2,312 metric tons) being the top four with the highest usage in 2021 (Figure 8A; USDA-NASS 2021a).
Figure 7. Estimated use of acetochlor and metolachlor in the United States from 1992 to 2017. A) Acetochlor, B) metolachlor, C) metolachlor and S-metolachlor, and D) S-metolachlor. Adapted from USGS-NAWQA (2022) with pesticide use data from Wieben (Reference Wieben2019).
With the release of a micro-encapsulated formulation of acetochlor (Warrant®), its use has expanded in soybean, cotton, and other crops over the last decade (Figure 7A; Wieben Reference Wieben2019). The slow release of acetochlor from the micro-encapsulated formulation increased crop safety and extended residual weed control (Cahoon et al. Reference Cahoon, York, Jordan, Everman, Seagroves, Braswell and Jennings2015; Jhala et al. Reference Jhala, Malik and Wills2015; Parker et al. Reference Parker, Simmons and Wax2005; Riar et al. Reference Riar, Norsworthy, Johnson, Starkey and Lewis2011). Acetochlor use in soybean production was comparatively less than in corn (98 to 733 metric tons per state; Figure 8). Growers in Illinois used the highest amount of acetochlor on soybean (733 metric tons) followed by Iowa (611 metric tons), Nebraska (476 metric tons), and Minnesota (458 metric tons) (Figure 8B; USDA-NASS 2020b). Almost all of Illinois and Iowa, southern Minnesota, eastern Nebraska, and South Dakota used the high-end rate of >7.57 kg km–2 (USGS 2022; Figure 9).
Figure 9. Acetochlor usage on agricultural land across the United States in 2019 (downloaded and modified from the U.S. Geological Survey by the U.S. Department of the Interior) (USGS 2022).
S-Metolachlor
Metolachlor is a member of the α-chloroacetamides family of VLCFA-inhibiting herbicides (WSSA 2021). It is a mixture of R- and S-enantiomers in a 1:1 proportion, with most of the herbicidal activity derived from S-isomers (Moser et al. Reference Moser, Rihs, Sauter and Böhner1983). It was widely used (>25,000 metric tons yr–1 in the period 1992–1996) in the United States before the registration of S-metolachlor in 1997 (Figure 7B; Shaner et al. Reference Shaner, Brunk, Belles, Westra and Nissen2006; Wieben Reference Wieben2019). S-metolachlor was commercialized through an innovation in the manufacturing process when a new catalyst system enabled the selective synthesis of S-metolachlor at a commercial scale (Blaser and Spinder Reference Blaser and Spindler1997). These resolved isomer formulations are listed as S-metolachlor and usually contain about 88% of S-isomers and 12% of R-isomers (Shaner et al. Reference Shaner, Brunk, Belles, Westra and Nissen2006). Because S-isomers are more biologically active than R-isomers, the application rate of S-metolachlor is 65% of the racemic mixture of metolachlor (O’Connell et al. Reference O’Connell, Harms and Allen1998; Shaner et al. Reference Shaner, Brunk, Belles, Westra and Nissen2006). As a result, metolachlor use decreased from about 30,152 metric tons in 1996 to about 166 metric tons in 2002 (Figure 7, B, C, and D; Wieben Reference Wieben2019).
In 2021, S-metolachlor was the fifth-most-applied corn herbicide in the United States (USDA-NASS 2022). About one-fourth (27%) of 37.8 million ha of corn received S-metolachlor at an average rate of 1.3 kg ha−1, totaling 12,248 metric tons (Figure 10A; ). Illinois accounted for more than one-fourth (28%; 3,410 metric tons) of the total S-metolachlor applied to corn crops (Figure 10A; USDA-NASS 2021a). S-metolachlor was the fourth-most-applied herbicide to soybean crops in the United States in 2020 (USDA-NASS 2021b). It was applied to about one-fifth (19%) of 33.6 million ha of soybean (96.3% of the total planted area) at an average rate of 1.47 kg ha–1, which accrues to 9,116 metric tons of S-metolachlor (Figure 10B; USDA-NASS 2020b). Illinois was the largest (15%; 1,388 metric tons) user of S-metolachlor followed by Iowa (942 metric tons), Minnesota (861 metric tons), and Nebraska (691 metric tons) (Figure 10B; USDA-NASS 2020b).
S-metolachlor can be applied PPI, PRE, or POST to peanut and cotton; PPI or PRE to sorghum and safflower (Carthamus tinctorius L.); and PRE to some turfgrass species and vegetable crops (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). It was the third-most-applied herbicide to peanut crops in 2018, with a total of 271 metric tons of S-metolachlor applied to 34% of 0.57 million ha (93% of the total planted area) at an average of 1.50 kg ha–1 (USDA-NASS 2019). S-metolachlor was the third-most-applied herbicide to sorghum in 2019, with 1,089 metric tons applied to 36% of 2.14 million ha at an average of 1.42 kg ha–1 (USDA-NASS 2020a). Among vegetable crops, S-metolachlor was the second-most-applied herbicide on pumpkins (Cucurbita spp.) and snap beans (Phaseolus vulgaris L.) in the United States in 2020 (USDA-NASS 2021c). It was applied to 38% of pumpkin-planted area at an average rate of 1.22 kg ha–1 totaling 11,748 kg, and was applied to 38% of snap bean planted area at an average of 1.31 kg ha–1, totaling 37,875 kg of S-metolachlor (USDA-NASS 2021c).
Alachlor
Alachlor was the first widely accepted PRE herbicide from the α-chloroacetamide family and was commercialized in 1969 by Monsanto (Hamm Reference Hamm1974). It was applied as early preplant, PPI, PRE, or early POST to corn; PPI and/or PRE to sorghum, soybean, peanut (where it can also be applied at cracking), and lima bean (Phaseolus lunatus L.); PPI to dry bean; and directed-PRE to woody ornamentals (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). It was primarily used on corn crops, followed by soybean, although its use progressively declined from the 1990s to the 2010s (Figure 11A; Wieben Reference Wieben2019). Wieben (Reference Wieben2019) reported that more than 21,000 metric tons year–1 of alachlor was applied during the years 1992 to 1995 compared with less than 170 metric tons year–1 during the years 2015 to 2017.
Figure 11. Estimated use of very-long-chain fatty acid (VLCFA)–inhibiting herbicides in the United States from 1992 to 2018. A) Alachlor, B) flufenacet, C) dimethenamid-P, D) napropamide, E) propachlor, and F) pyroxasulfone. Adapted from USGS-NAWQA (2022), with pesticide use data from Wieben (Reference Wieben2019). Napropamide belonged to the acetamide family of Group 15 herbicides in the previous herbicide classification; however, in the revised classification, it is classified as a Group 0 herbicide (WSSA 2021).
Dimethenamid-P
Similar to metolachlor, dimethenamid-P is a member of the α-chloroacetamide family and has two main isomers: R- and S-isomers (Böger et al. Reference Böger, Matthes and Schmalfuß2000; WSSA 2021). Dimethenamid-P is an active S-isomer with greater herbicidal activity than R-isomer (Couderchet et al. Reference Couderchet, Bocion, Chollet, Seckinger and Böger1997). It can be applied as early preplant, PPI, PRE, or early POST to corn and soybean (Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). According to estimates by Wieben (Reference Wieben2019), dimethenamid-P use in corn was <600 metric tons yr–1 in the 2000s, though its use increased progressively in the 2010s, rising by more than 5-fold in 2017 (3,390 metric tons) compared with 2007 (576 metric tons). Of the approximately 3,390 metric tons of dimethenamid-P used in the United States in 2017, more than half (55%; 1,860 metric tons) was used on corn, and one-fourth (25%; 831 metric tons) on soybean (Figure 11C; Wieben Reference Wieben2019). In 2021, corn received slightly less (1,311 metric tons) dimethenamid-P (Figure 11A; USDA-NASS 2021a), whereas soybean received slightly more (874 metric tons) in 2020 (Figure 11B; USDA-NASS 2020b).
Flufenacet
Flufenacet belongs to the α-oxyacetamide family of VLCFA inhibitors (WSSA 2021). It may be used as surface preplant, PPI, and/or PRE on many crops such as corn, cotton, peanut, potato (Solanum tuberosum L.), soybean, sunflower (Helianthus annuus L.), and wheat (Shaner Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Flufenacet is not sold as a single active ingredient but is available as a premix for use on corn and soybean (Shaner Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). Use of flufenacet on corn has decreased in the last decade (<40 metric tons yr–1 from 2011 to 2015) compared with the 2000s (>250 metric tons yr–1 from 2001 to 2005) (Figure 11B; Wieben Reference Wieben2019).
Propachlor and Pyroxasulfone
Among VLCFA-inhibiting herbicides, propachlor was commercialized along with CDAA (N,N-diallyl-2-chloroacetamide) by Monsanto in 1965 (Table 1; Heydens et al. Reference Heydens, Lamb, Wilson and Krieger2010; Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014). The granular formulation of propachlor used to cause skin irritation (eye and nose), though less than CDAA (Hamm Reference Hamm1974; Pike et al. Reference Pike, McGlamery and Knake1991), leading propachlor to replace CDAA. However, alachlor took over the market after the 1970s, as it was less irritating than propachlor while offering a similar level of weed control (Pike et al. Reference Pike, McGlamery and Knake1991). Propachlor was applied PRE to corn and sorghum (Shaner Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014), and based on propachlor usage data from 1987 to 1996, an average of about 959 metric tons of propachlor was used annually, with 75% applied to sorghum, and 24% to corn (US-EPA 1998). In 1998, Monsanto voluntarily discontinued production of propachlor (Gómez-Ramírez and García-Fernández Reference Gómez-Ramírez, García-Fernández and Wexler2014) and its use has since decreased (Figure 11E; Wieben Reference Wieben2019).
Pyroxasulfone belongs to the isoxazoline family (WSSA 2021). Among VLCFA inhibitors, it is the most recently commercialized PRE herbicide, registered by Kumiai Chemical Industry Co., Ltd in 2014 (Table 1). Pyroxasulfone can be applied from autumn, early preplant, to early POST during the growing season and provides residual control of troublesome weeds that are resistant to ACCase inhibitors, ALS inhibitors, glyphosate, and triazines (Grey et al. Reference Grey, Cutts, Newsome and Newell2013; Kaur et al. Reference Kaur, Bhullar and Kaur2019; Shaner et al. Reference Shaner, Jachetta, Senseman, Burke, Hanson, Jugulam, Tan, Reynolds, Strek, McAllister, Green, Glenn, Turner and Pawlak2014; Umetsu and Shirai Reference Umetsu and Shirai2020). Pyroxasulfone has a relatively lower use rate compared to chloroacetamides (Zollinger Reference Zollinger2011). It provides longer residual activity against troublesome broadleaf weeds such as Palmer amaranth and waterhemp (Nakatani et al. Reference Nakatani, Yamaji, Honda and Uchida2016). In 2013, approximately 65 metric tons of pyroxasulfone was applied in the United States, and subsequently >250 metric tons yr–1 from 2014 to 2017 (Figure 6; Wieben Reference Wieben2019). In 2021, about 54 metric tons of pyroxasulfone were applied to corn (Figure 11F; USDA-NASS 2021a), whereas in 2020, 784 metric tons were applied to soybean (Figure 11F; USDA-NASS 2020b).
VLCFA-Inhibiting Herbicide-Resistant Weeds
The evolution of weeds that are resistant to VLCFA inhibitors is relatively low compared to ALS or Photosystem II (PS II) inhibitors (Heap Reference Heap2023). Several factors might have contributed to the low incidence of weeds that are resistant to VLCFA inhibitors, including possible low frequency of mutations in condensing enzymes in the long-chain fatty acid synthesis (Tanetani et al. Reference Tanetani, Kaku, Kawai, Fujioka and Shimizu2009; Trenkamp et al. Reference Trenkamp, Martin and Tietjen2004), genetic redundancy of fatty acid elongases (Brabham et al. Reference Brabham, Norsworthy, Houston, Varanasi and Barber2019), and the predominant use of these herbicides for PRE application followed by a POST application of other herbicides, making it difficult for surviving weeds to grow to maturity and produce seeds (Busi et al. Reference Busi, Gaines, Vila-Aiub and Powles2014). As of November 2023, resistance to the VLCFA inhibitor has been documented in 13 weed species (Heap Reference Heap2023; Table 3). The VLCFA inhibitor resistance has been found in 11 monocot weeds and two dicot weeds, Palmer amaranth and waterhemp (Heap Reference Heap2023; Table 3), the two most problematic weeds of many cropping systems in the United States (WSSA 2017).
Table 3. Weeds that are resistant to very-long-chain fatty acid–inhibiting herbicides worldwide.
Mechanism of Resistance to VLCFA-Inhibiting Herbicides in Weed Species
Blackgrass
Resistance to multiple herbicides is common in several blackgrass (Alopecurus myosuroides Huds.) populations throughout Europe. The first case of a flufenacet-resistant blackgrass was reported in 2007 in Germany and later in 2011 in Sweden (Heap Reference Heap2023). The enhanced activity of GSTs was attributed to resistance in several blackgrass populations (Dücker et al. Reference Dücker, Parcharidou and Beffa2020). The use of GST inhibitor (e.g., tridiphane) slowed degradation of flufenacet in sensitive and resistant blackgrass populations (Dücker et al. Reference Dücker, Zöllner, Parcharidou, Ries, Lorentz and Beffa2019b). The RNA sequencing indicated an increased expression of six GSTs and nine transcription factors, as well as a keto-acyl-CoA reductase, which is known to be involved in the biosynthesis of VLCFA inhibitor (Dücker et al. Reference Dücker, Parcharidou and Beffa2020).
Echinochloa Species
Barnyardgrass [Echinochloa Crus-galli (L.) Beauv] is a dominant weed in direct-seeded rice (Chauhan et al. Reference Chauhan, Singh, Kumar and Johnson2011), and early watergrass [Echinochloa oryzoides (Ard.) Fritsch] and late watergrass (Echinochloa phyllopogon) are commonly found in lowland rice cropping systems (Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000). Butachlor and triallate are widely used in rice fields for weed management, including for control of Echinochloa spp. Butachlor-resistant barnyardgrass was first reported in China in 1993 (Heap Reference Heap2023), and now several populations in Asia are resistant to this herbicide (Juliano et al. Reference Juliano, Casimero and Llewellyn2010). Early watergrass and late watergrass populations with 5-fold to 22-fold resistance to molinate and thiobencarb were reported (Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000), and these populations also exhibit resistance to inhibitors of ACCase, ALS, and PS II as well as synthetic auxins (Fischer et al. Reference Fischer, Ateh, Bayer and Hill2000; Busi Reference Busi2014). Physiological and biochemical analyses have revealed indirect evidence of metabolism of thiobencarb via cytochrome P450 (CYP) activity in late watergrass (Yun et al. Reference Yun, Yogo, Miura, Yamasue and Fischer2005). Studies involving transgenic Arabidopsis thaliana expressing the CYP81A12/21 gene indicated that CYP provides cross-resistance to ALS and ACCase inhibitors, but not to thiocarbamate (thiobencarb) (Dimaano et al. Reference Dimaano, Tominaga and Iwakami2022), suggesting that resistance to thiobencarb may involve different cluster of CYPs (Dimaano et al. Reference Dimaano, Tominaga and Iwakami2022).
Palmer Amaranth
Resistance to VLCFA-inhibiting herbicides in a Palmer amaranth biotype was first reported in Arkansas, where a 3-fold to 29-fold resistance to S-metolachlor was found (Rangani et al. Reference Rangani, Noguera, Salas-Perez, Benedetti and Roma-Burgos2021). Additional accessions of Palmer amaranth from Arkansas and Mississippi were found to have 2-fold to 7-fold resistance to S-metolachlor, while the progenies generated from resistant plants were 9.2 times less sensitive than the susceptible population (Kouame et al. Reference Kouame, Bertucci, Savin, Bararpour, Steckel, Butts and Roma-Burgos2022). In another Palmer amaranth population from Arkansas, an 8-fold resistance to S-metolachlor was reported, with a low level (2.3-fold to 3.6-fold) of cross-resistance to other Group 15 herbicides (Brabham et al. Reference Brabham, Norsworthy, Houston, Varanasi and Barber2019).
Treatment with a known GST inhibitor (NBD-Cl) indirectly implied the involvement of GSTs in metabolizing S-metolachlor in Palmer amaranth plants (Rangani et al. Reference Rangani, Noguera, Salas-Perez, Benedetti and Roma-Burgos2021). That research (Rangani et al. Reference Rangani, Noguera, Salas-Perez, Benedetti and Roma-Burgos2021) suggested that the inactivation of herbicide by enhanced activity of GSTs was predominant in roots compared with leaves. Additionally, two GST genes, ApGSTU19 and ApGSTF8, were constitutively upregulated in the roots of resistant plants, and the expression of these genes increased in response to S-metolachlor. Gene expression analyses revealed the upregulation of GST gene clusters ApGSTU19, ApGSTF8, ApGSTF2, and ApGSTF2like, which appear to be responsible for increased GST activity in resistant plants (Rangani et al. Reference Rangani, Noguera, Salas-Perez, Benedetti and Roma-Burgos2021). Thus, it is likely that the resistance to S-metolachlor in Palmer amaranth is bestowed because of elevated constitutive and induced expression of GST genes (Rangani et al. Reference Rangani, Noguera, Salas-Perez, Benedetti and Roma-Burgos2021). Similarly, in another Palmer amaranth population from Arkansas, using NBD-Cl (4-chloro-7-nitrobenzofurazan), Brabham et al. (Reference Brabham, Norsworthy, Houston, Varanasi and Barber2019) suggested the possible role of GST in S-metolachlor metabolism and, thereby resistance.
Although the studies described above provide reason to believe that GSTs are indirectly involved in metabolizing S-metolachlor in Palmer amaranth, recent studies suggest that CYPs contribute to S-metolachlor resistance in Palmer amaranth (Concepcion et al. Reference Concepcion, Brown, Morris, Hutchings, Kaundun and Riechers2023). Rapid metabolism of herbicidally active S-metolachlor to polar compounds (including putative glutathione, dipeptide, and cysteine conjugates) was noted to have occurred, but some metabolites did not match the migration distances of the glutathione-derived conjugates, implying that some S-metolachlor metabolites may arise from Phase I oxidation and Phase II glucose conjugation via CYP activity in resistant Palmer amaranth seedlings (Concepcion et al. Reference Concepcion, Brown, Morris, Hutchings, Kaundun and Riechers2023). Those results suggest that CYPs are equally involved in S-metolachlor metabolism as GSTs.
Rigid Ryegrass
Resistance to triallate and metolachlor was reported in a rigid ryegrass (Lolium rigidum Gaud.) population from Australia (Heap Reference Heap2023). Resistance to pyroxasulfone and other chloroacetamides was found later in rigid ryegrass (Burnet et al. Reference Burnet, Barr and Powles1994; Busi et al. Reference Busi, Gaines, Walsh and Powles2012, Reference Busi, Gaines, Vila-Aiub and Powles2014). The pyroxasulfone-resistant population was also cross-resistant to prosulfocarb and triallate (Busi and Powles Reference Busi and Powles2013, Reference Busi and Powles2016). The rigid ryegrass population exhibited 6-fold to 31-fold resistance to S-metolachlor and was cross-resistant to alachlor and propachlor (Burnet et al. Reference Burnet, Barr and Powles1994; Brunton et al. Reference Brunton, Boutsalis, Gill and Preston2019). Importantly, resistance levels of 17- to 44-fold, 6- to 45-fold, and 4- to 8-fold to triallate, prosulfocarb, and pyroxasulfone, respectively, were reported in this species (Brunton et al. Reference Brunton, Boutsalis, Gill and Preston2019, Reference Brunton, Boutsalis, Gill and Preston2020). Rigid ryegrass biotypes with greater than 9-fold resistance to triallate, prosulfocarb, ETPC, thiobencarb, and trifluralin have also been found (Brunton et al. Reference Brunton, Boutsalis, Gill and Preston2018).
Rapid metabolism of chloroacetamides (S-metolachlor) (Burnet et al. Reference Burnet, Barr and Powles1994) and pyroxasulfone (Tanetani et al. Reference Tanetani, Ikeda, Kaku, Shimizu and Matsumoto2013) possibly mediated by CYP activity may confer resistance in rigid ryegrass (Burnet et al. Reference Burnet, Barr and Powles1994; Tanetani et al. Reference Tanetani, Ikeda, Kaku, Shimizu and Matsumoto2013); however, direct evidence of CYPs in metabolizing these herbicides is elusive in this weed species (Busi Reference Busi2014). Nonetheless, in another pyroxasulfone-resistant rigid ryegrass population, rapid metabolism of [14C] pyroxasulfone (>88%) via a glutathione conjugation mediated by GST was found (Busi et al. Reference Busi, Porri, Gaines and Powles2018). Additionally, pyroxasulfone-resistant rigid ryegrass showed a significant increase in constitutive expression of tau classes of GST genes (GST-1 and GST-2) compared with susceptible plants (Busi et al. Reference Busi, Porri, Gaines and Powles2018). Similarly, conjugation of flufenacet via GST activity was found in a rigid ryegrass population from Europe (Dücker et al. Reference Dücker, Zöllner, Lümmen, Ries, Collavo and Beffa2019a). The use of phorate showed synergism with enhanced activity of pyroxasulfone with reduced resistance (Busi and Powles Reference Busi and Powles2016). Conversely, phorate antagonized thiocarbamate herbicides (prosulfocarb and triallate) with an increased activity of thiocarbamates via bioactivation (Fuerst Reference Fuerst1987). Busi et al. (Reference Busi, Gaines, Vila-Aiub and Powles2014) demonstrated that the resistance to pyroxasulfone, prosulfocarb, and triallate is conferred by one or more semidominant alleles.
Waterhemp
Two populations of waterhemp from Illinois were reported to be resistant to VLCFA-inhibiting herbicides, including S-metolachlor (Evans et al. Reference Evans, Strom, Riechers, Davis, Tranel and Hager2019; Strom et al. Reference Strom, Gonzini, Mitsdarfer, Davis, Riechers and Hager2019). These populations were also resistant to ALS-, HPPD-, and PS II-inhibitor (Heap Reference Heap2023). The resistant plants degraded S-metolachlor rapidly than sensitive plants, and similar to corn at 2–24 h after treatment (HAT) (Strom et al. Reference Strom, Hager, Seiter, Davis and Riechers2020). The use of GST and CYP-inhibitor decreased the amount of S-metolachlor metabolized by the resistant plants at 4 HAT but not in sensitive waterhemp or naturally tolerant corn plants (Strom et al. Reference Strom, Gonzini, Mitsdarfer, Davis, Riechers and Hager2019). Interestingly, thin-layer chromatography (TLC) analysis of radiolabeled S-metolachlor indicated that resistant plants formed metabolites that were not present in sensitive waterhemp or corn (Strom et al. Reference Strom, Gonzini, Mitsdarfer, Davis, Riechers and Hager2019). It was revealed that conjugates of O-demethylated S-metolachlor were abundant in resistant plants compared to susceptible waterhemp plants. The resistant waterhemp had a greater ability to oxidize S-metolachlor through O-demethylation than susceptible waterhemp or corn, and this oxidation reaction appears to be the predominant resistance mechanism in waterhemp (Storm et al. 2021). Importantly, the microsomal stability assays indicate greater than 20-fold activity of CYPs in resistant waterhemp compared to GST (∼2-fold) implying Phase I and Phase II metabolism of S-metolachlor, likely mediated by CYPs (Strom et al. Reference Strom, Hager, Concepcion, Seiter, Davis, Morris, Kaundun and Riechers2021). Thus, results indicate that S-metolachlor-resistant waterhemp possibly uses a different or additional pathway to detoxify S-metolachlor than corn (Strom et al. Reference Strom, Gonzini, Mitsdarfer, Davis, Riechers and Hager2019; Reference Strom, Hager, Concepcion, Seiter, Davis, Morris, Kaundun and Riechers2021).
Wild Oat
The first case of triallate resistance was reported in wild oat (Avena fatua L.) in 1989 in Canada (Heap Reference Heap2023). The triallate-resistant wild oat exhibits cross-resistance to difenzoquat (O’Donovan et al. Reference O’Donovan, Sharma, Harker, Maurice, Baig and Blackshaw1994). While the precise mechanism of resistance was unknown, Rashid et al. (Reference Rashid, O’Donovan, Khan, Blackshaw, Harker and Pharis1998) proposed that elevated levels of endogenous gibberellins may contribute to the resistance. Beckie and Jana (Reference Beckie and Jana2000) reported a case of resistance to triallate in wild oat in a field with a 10- to 15-yr history of triallate use. Interestingly, triallate resistance is often associated with resistance to pyroxasulfone and sulfentrazone without use history (Mangin et al. Reference Mangin, Hall and Beckie2016). Wild oat populations that are resistant to these herbicides were found in several states in the United States in the 1990s (Heap Reference Heap2023). Some populations exhibited differential metabolism of triallate (Kern et al. Reference Kern, Colliver, Maxwell, Fay and Dyer1996), while others had reduced sulfoxidation (i.e., the conversion of triallate to the active triallate sulfoxide) (Kern et al. Reference Kern, Jackson and Dyer1997). Kern et al. (Reference Kern, Myers, Jasieniuk, Murray, Maxwell and Dyer2002) demonstrated that two recessive genes endow triallate resistance in wild oat. Such inheritance can delay the incidence of resistance, as the recessive traits tend to spread slower than the dominant or semidominant traits.
Although resistance to VLCFA inhibitors in weeds is slow to evolve, more cases of weed species becoming resistant to this group of herbicides have been recently documented (Heap Reference Heap2023). The predominance of GST-mediated metabolism of these herbicides was present in the resistant populations, which were found to be resistant to other herbicides such as inhibitors of ACCase, ALS, HPPD, and PS II. Best management practices should be adopted to protect this chemistry for sustainable weed management because of their widespread use in row, vegetable, and fruit crops.
The Future of VLCFA-Inhibiting Herbicides
The isoxazolines, including pyroxasulfone and fenoxasulfone, discovered by Kumiai Chemical Industry Inc., were the latest subclass of VLCFA-inhibiting herbicides commercialized in 2014 (Table 1). Pyroxasulfone has experienced substantial growth in the marketplace for use on cereal grains, soybeans, corn, cotton, and some vegetable crops, whereas fenoxasulfone is labeled for control of Echinochloa spp. and other annual weeds in rice crops (Fujinami et al. Reference Fujinami, Takahashi, Tanetani, Ito and Nasu2019). The favorable environmental attributes of isoxazolines as low-use-rate VLCFA-inhibiting herbicides with relatively fewer cases of resistant weeds has prompted discovery research efforts in the last two decades. Related pyrimidinone-substituted isoxazolines (Figure 12A) were investigated by DuPont Chemicals Company, with field trials on corn, soybean, and cereals (Smith et al. Reference Smith, Selby, Stevenson, Clark and Taggi2009). Researchers at the Chinese Academy of Agricultural Sciences reported a series of potent N-trifluoroethylpyrazole-substituted isoxazolines that had encouraging weed control activity along with corn selectivity in the greenhouse and field studies at rates lower than that of metolachlor (Ma et al. Reference Ma, Li, Zhao, Zhang, Xie, Mei and Ning2010) (Figure 12B). Researchers at Dongguan HEC Pesticides R&D Company in China published a family of benzisoxazole-substituted isoxazolines (Figure 12C) that was competitive with pyroxasulfone to control some weed species, with improved crop safety in rapeseed, rice, and wheat crops (Lin et al. Reference Lin, Li, Hu, Chi, Zeng and Xu2021). Substantial research efforts have been extended in search of bioisosteres of isoxazolines. Replacement of the isoxazoline ring by a pyridine-N-oxide to compounds such as sulfoxide (Figure 13A) was disclosed in a patent application by DuPont Chemicals as a highly active herbicide (Selby et al. Reference Selby, Smith and Stevenson2008). Syngenta Corporation has disclosed sulfoxides (Figure 13B) in which thiazole replaced the isoxazoline ring (Elliott et al. Reference Elliott, Hughes and Plant2006). A published world patent application from OAT Agrio Company (Japan) in 2018 disclosed benzylsulfonylthiazoles (Figure 13C) in which thiazole replaced the isoxazoline ring for controlling barnyardgrass, Monochoria vaginalis, and Scirpus juncoides at 200 g ai ha–1 in paddy rice (Fukunaga et al. Reference Fukunaga, Sumitomo, Sumiyoshi, Noyama, Shirai, Norimura and Matsuzaki2018).
Figure 12. (A) Pyrimidinone-substituted isoxazolines investigated by DuPont Chemicals Company, (B) potent N-trifluoroethylpyrazole-substituted isoxazolines with encouraging herbicidal activity and corn selectivity at the Chinese Academy of Agricultural Sciences, and (C) a family of benzisoxazole-substituted isoxazolines at Dongguan HEC Pesticide R&D Company, China.
Figure 13. Different bioisosteres of the isoxazolines (A) pyridine-N-oxide substituted sulfoxides discovered by DuPont Chemicals Company, (B) thiazole substitutes sulfoxides discovered by Syngenta Corporation, and (C) benzylysulfonylthiazoles discovered by OAT Agrio (Japan).
A new direction in VLCFA-inhibiting herbicide research has been undertaken with a renewed interest in haloalkylsulfonanilide herbicides. Although they were discovered decades ago, the haloalkylsulfonanilides mefluidide and perfluidone, had an unknown SoA, though evidence was ultimately reported in 2012 to support the KAS (3-ketoacyl-ACP synthase) enzyme involved in VLCFA biosynthesis (Tresch et al. Reference Tresch, Heilmann, Christiansen, Looser and Grossmann2012). In conjunction with this finding, Nissan Chemical Industries began to explore an alternative haloalkylsulfonanilide chemotype, starting with a patent in 2010 (Kudou et al. Reference Kudou, Tanima, Masuzawa and Yano2010). Nissan’s continued work in this area eventually led to the commercial development of rice herbicide dimesulfazet, an inhibitor of VLCFA biosynthesis. Recently, Nissan introduced dimesulfazet based on symptomology and endogenous fatty acid analysis (Takamasa et al. Reference Takamasa, Mama, Masato, Daisuke and Yoshihiko2023; Figure 14). Dimesulfazet is being developed to control weeds of rice in Japan such as Echinochloa oryzicola, Schoenoplectiella juncoides (Roxb) Lye, and other difficult-to-control perennial species of the Cyperaceae family such as kuro-guwai (Eleocharis kuroguwai Ohwi) and sea clubrush [Bolboschoenus maritimus (L.) Palla] (Takamasa et al. Reference Takamasa, Mama, Masato, Daisuke and Yoshihiko2023; Uetsu and Shirai 2020). Although the intensity of VLCFA-directed research efforts has decreased somewhat over the past decade as evidenced by the declining number of patent applications, this biochemical pathway remains a viable mechanistic target for herbicide discovery, especially in the search for new Group 15 chemotypes.
Figure 14. Dimesulfazet was discovered by Nissan Chemical Industries based on symptomology and endogenous fatty acid analysis (Takamasa et al. Reference Takamasa, Mama, Masato, Daisuke and Yoshihiko2023).
Acknowledgments
This research received no specific grant from any funding agency, commercial or not-for-profit sectors. No competing interests have been declared.
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