Metabolism-based Resistance
Metabolism-based resistance does not involve the binding site of the herbicide but instead the herbicide is broken down by biochemical processes that detoxify the herbicide. Most plants have the ability to break down herbicides to some extent but the increased rate of detoxification allows the plant to be resistant to the herbicide. (Learn more about herbicide metabolism)
Three groups of enzymes have been implicated in metabolism-based resistance, the glutathione transferases (GSTs), the aryl acylamidases, and the cytochrome P450 mono-oxygenases. To date, the cytochrome P450 mono-oxygenases are the most common group of enzymes responsible for metabolism based resistance. Weed species with metabolism-based resistance are listed in Table 2.
Table 2. Weeds with resistance based on herbicide metabolism and the enzyme systems responsible.
| Enzyme System | Weed Species | Herbicide |
| Cytochrome P450 | Alopecurus myosuroides | chlorotoluron |
| diclofop | ||
| propaquizafop | ||
| chlorsulfuron | ||
| Avena sterilis | diclofop | |
| Lolium rigidum | diclofop | |
| chlorsulfuron | ||
| chlorotoluron | ||
| metribuzin | ||
| simazine | ||
| chlorotoluron | ||
| Phalaris minor | isoproturon | |
| Sinapsis arvensis | ethametsulfuron | |
| Stellar media | mecoprop | |
| Glutathione transferases | Abutilon theophrasti | atrazine |
| Alopecuris mysouroides | fenoxaprop | |
| Aryl acylamidase | Echinochloa colona | propanil |
| Echinochloa crus-galli | propanil |
Glutathione-S-Transferase
Glutathione-s-transferases(GSTs) belong to a family of enzymes that attach to the tripeptide glutathione through the cysteine residue to electrophilic, hydrophobic compounds. Glutathione-s-transferases are involved in the metabolism of triazine and chloracetanilide herbicides, providing crop tolerance in corn and sorghum. Once glutathione is bound to the herbicide, the herbicide is no longer toxic and may be moved to the vacuole.
There are several populations of Abutilon theophrasti (velvetleaf) resistant to triazine herbicides as a result of increased herbicide metabolism mediated by GSTs. The enzyme has increased catalytic ability. Inheritance studies indicate that the GST-endowed resistance is inherited as a single, partially dominant, nuclear-encoded trait.
Aryl Acylamidase
Aryl acylamidase is an enzyme responsible for the metabolism of propanil in rice, which is the mechanism that provides crop tolerance. In areas of rice production, propanil resistance has been selected in several populations of the weed species, Echinochloa colona and E. crus-galli. Resistance was due to increased metabolism of propanil. There is evidence that aryl acylamidase is expressed at a higher level in resistant populations than in susceptible populations. The resistance can be overcome by adding a carbamate or organophosphate pesticide. These pesticides inhibit aryl acylamidase activity and have been used to manage E. colona resistance in the field. The two pesticides are applied together and resistant plants are not able to metabolize the herbicide quickly enough to prevent injury.
Cytochrome P450 Monooxygenases
The cytochrome P450 monooxygenases are a large family of enzymes involved in many plant metabolic processes. The enzymes catalyze different reactions in plants but the most common reactions with herbicides are hydroxylation and demethylation. After hydroxylation, the molecule will usually be conjugated to sugars. The conjugate has no herbicidal activity and may be exported to the vacuole or can be incorporated into the cell wall. P450s are responsible for the selectivity of some herbicides. A crop will be able to detoxify the herbicide more quickly than the weed. However, there is evidence that increased P450-based metabolism is responsible for resistance in several weed species (Table 2). Since there are many different P450s, it is unlikely that the same P450 is responsible for resistance in different species to different herbicides.
Herbicide resistance in several grass species is due to increased metabolism mediated by increased cytochrome P450 activity.