Updated Dec. 3, 2019.
Several new herbicide-resistant biotypes in the weedy amaranths were announced within the last year. First, Illinois reported two populations of waterhemp resistant to metolachlor and other Group 15 herbicides (Hager, 2019). This was followed by reports of metolachlor and then 2,4-D resistance in Palmer amaranth by Arkansas and Kansas (Peterson et al., 2019), respectively. The Kansas Palmer amaranth population is also resistant to dicamba. Although only Illinois has confirmed the resistance mechanism in these populations is metabolism-based, there is a good likelihood that each of these cases is due to the weeds enhanced ability to degrade the herbicide. This article will review herbicide metabolism by plants and describe why metabolism-based resistance (MBR) poses a bigger threat than target site-based resistance. Less is known about many aspects of MBR than target site-based resistance, and in-depth papers detailing the newest resistance cases have yet to be published. Thus, some of the information in this article is speculation by the author.
The traits that allow resistant biotypes to survive herbicides are classified as either target site or non-target site mechanisms. Nearly all herbicide resistant weeds found in the Midwest prior to the turn of the century possessed altered target sites. Target site-based resistance is usually due to a mutation in the gene coding for the herbicide target site, resulting in a change in the sequence of amino acids of the protein, which in turn prevents binding of the herbicide. In most cases, a substitution of a single amino acid in the protein is responsible for resistance. Altered target sites are the most common resistance mechanism for Group 1, 2, 5, and 14 herbicides.
Glyphosate-resistant waterhemp and Palmer amaranth have a unique target site-based mechanism. Rather than an alteration of the EPSPS protein (target site for glyphosate), resistant pigweeds have multiple copies of the gene for EPSPS. This allows resistant biotypes to produce excess quantities of EPSPS, therefore diluting the toxicity of glyphosate and preventing inactivation of the EPSPS enzyme.
Non-target site resistance involves metabolic processes not related to the target site. These processes prevent the herbicide from reaching the target site at toxic concentrations. Non-target site resistance mechanisms include reduced translocation, isolation of the herbicide in vacuoles, and MBR. Metabolism-based resistance is on the increase in the Midwest, and poses unique threats compared to other resistance mechanisms.
Basics of metabolism-based resistance
Herbicide metabolism is generally described as a three-phase process, although some herbicides don’t require the first step (Figure 1). Phase I involves a slight modification of the herbicide molecule which predisposes it to further modification. Phase II involves combining the modified herbicide with another compound (sugar, glutathione, etc.) that facilitates the final step. Phase III uses transport enzymes to move the herbicide into the cell vacuole (often described as a cell’s garbage can) or outside of the cell in the intracellular space. Movement of the herbicide to these areas isolates the herbicide from the target site. Some herbicides are further degraded in the vacuole.
Both plants and animals use the same enzyme families for these processes, but plants typically have more versions of the enzymes than animals (Table 1). For example, rice has more than 360 genes for cytochrome P450 (Phase I reactions), whereas humans have less than 60. The large number of genes allows an organism to process a wide variety of molecules. Plants probably have more genes for these enzymes than animals since they have less ability to control their exposure to toxins. Animals can modify their diet to reduce the intake of toxins, or physically leave areas with toxins – plants don’t have these options.
The major difference between detoxification in animals and plants occurs in Phase III. Plants store the modified herbicide in vacuoles or cell walls where the toxin is isolated from the target site. Phase II reactions increase the ability of an animal’s kidneys to filter the compound from blood. After removal of the herbicide from blood by the kidneys, the compound can be excreted from the body via urine. This process explains why moving animals that have consumed forage from areas treated with picloram (Tordon, Grazon, etc.) or other persistent Group 4 herbicides (clopyralid, aminopyralid) can result in injury to sensitive crops. Phase I and Phase II reactions allow animals to remove the compounds from the blood system, but in the case of Group 4 herbicides do not eliminate their phytotoxic properties.
Many herbicide formulations include safeners that increase crop tolerance to a herbicide by enhancing herbicide metabolism in the crop. Several Group 15 products (Dual II Magnum, Harness, Surpass, et al.) include safeners that enhance the activity of glutathione S-transferase (Phase II). Accent Q, Balance Flexx, Corvus, and other products include safeners that increase the activity of cytochrome P450 (Phase I).
Why should we be concerned about metabolism-based resistance?
Evolution of herbicide resistance within waterhemp indicates the growing importance of MBR (Table 2). Whereas the resistances (Group 2 and 5) that accompanied the rise of waterhemp to Iowa’s most important weed were target site-based, in the past 10 years new resistance traits in waterhemp are due to MBR (Group 4, 15, and 27).
The concern with MBR is that an alteration in herbicide metabolism providing resistance to one herbicide group may provide cross-resistance to multiple herbicide groups. This is well documented in Australia; widespread MBR has resulted in the loss of the majority of herbicide tools for several weeds and led to the widespread adoption of harvest weed seed control and other alternative weed management strategies.
The complexity and risk of herbicide metabolism can be illustrated with two examples of MBR. Although the first triazine-resistant waterhemp in Iowa had an altered target site, more recently biotypes with MBR have been identified. Nebraska researchers reported that MBR was the predominant resistant biotype in eastern Nebraska (Werle et al 2019). Waterhemp with an altered Group 5 target site is resistant to all Group 5 herbicides (e.g. atrazine, simazine, metribuzin, et al.). Metabolism-based triazine resistance provides resistance to atrazine, simazine and other symmetrical chloro-triazine herbicides, but not metribuzin. Metribuzin is an asymmetrical triazine, and although it attacks the same target site as atrazine, the difference in structure of metribuzin compared to other triazine herbicides prevents enhanced degradation of metribuzin by the enzymes associated with MBR in waterhemp (Figure 2).
2,4-D resistant crops provide another example of the complexity of herbicide metabolism. Resistance to 2,4-D in Enlist® corn, soybean and cotton is due to an enzyme that rapidly metabolizes 2,4-D. The enzyme inserted in 2,4-D resistant corn is also able to metabolize quizalofop and other herbicides in the ‘fop’ family, whereas the enzyme doesn’t metabolize clethodim or other ‘dim’ herbicides. While both ‘fop’ and ‘dim’ herbicides are Group 1 herbicides that bind to ACC-ase, the ‘dim’ herbicides lack the chemical bond targeted by the enzyme responsible for 2,4-D resistance (Figure 3). Thus, Enlist® corn is resistant to 2,4-D and ‘fop’ herbicides, but not ‘dim’ herbicides. The version of the gene used in other 2,4-D resistant crops does not provide cross-resistance to ‘fop’ herbicides.
The metolachlor resistant waterhemp in Illinois and 2,4-D resistant Palmer amaranth in Kansas were found in populations known to be resistant to Group 27 herbicides. Illinois researchers previously reported a Group 27 resistant waterhemp that was also resistant to 2,4-D. Group 27 resistance in waterhemp, and metolachlor and 2,4-D resistance in other weed species have all been linked with increased activity of cytochrome P450. It is possible, if not likely, that the genes responsible for Group 27 resistance in the recently discovered Group 4 and 15 resistant Amaranthus populations are also responsible for the new resistances. It is important to note that not all Group 27 resistant populations have resistance to these other herbicide groups. Dr. Owen’s group screened several populations of Group 27 resistant waterhemp from Iowa and did not find any with 2,4-D resistance.
We have moved into an era where MBR will increase in importance. The potential for MBR to provide cross-resistance to multiple herbicide groups is a threat to our current production system. It is critical to develop herbicide programs that rely on multiple effective herbicide groups and provide full-season weed control, therefore minimizing seed production. However, herbicides alone cannot win this battle. Production systems must be evaluated to determine what alternative strategies can be used to supplement herbicides. While strategies such as increased crop competitiveness via narrow-row spacing or planting cover crops do not provide the ‘big impact’ of herbicides, their contribution to suppressing weeds can make the difference between long-term success or failure in weed management.
Hager, A. 2019. Waterhemp resistance to Group 15 Herbicides. Online. Univ. Illinois. The Bulletin. March 15, 2019.
Heap, I. 2019. International Survey of Herbicide Resistant Weeds. Online. www.weedscience.org.
Peterson, D., M. Jugulam, C. Shyam, and E. Borgato. 2019. Palmer amaranth resistance to 2,4-D and dicamba confirmed in Kansas. Online. KSU. eUpdate. March 1, 2019.
Werle, R., M. Jugulam, G. Kruger, A. Vennapusa, F. Falenco, B. Vieira, and S. Samuelson. 2018. Research Report: Prevalence and mechanism of atrazine resistance in waterhemp from Nebraska. Online. U. Neb-Lincoln. CROPWATCH. Oct. 26, 2018.
Yu, Q. and S. Powles. 2014. Metabolism-based herbicide resistance and cross-resistance in crop weeds: A threat to herbicide sustainability and global crop production. Pl. Phys. 166:1106-1118.
Yuan, J.S., P.J. Tranel and C.N. Stewart, Jr. 2007. Non-target site herbicide resistance: a family business. TRENDS in Pl. Sci. 12:6-13.