Abstract
Polypogon fugax is a common winter weed in China and other Asia countries. We have previously found a P. fugax biotype (R) resistant to acetyl co-enzyme A carboxylase (ACCase) herbicides also cannot be effectively controlled by some acetolactate synthase (ALS) herbicides. This study evaluated the level of resistance to four ALS herbicides (metsulfuron-methyl, chlorsulfuron, monosulfuron, pyribambenz isopropyl) in the R biotype and the associated resistance mechanism. The R biotype exhibited moderate level of resistance to metsulfuron-methyl (6.0-fold) compared with the sensitive biotype (S). Sequence analysis of ALS gene revealed that two ALS genes existed in P. fugax. However, no substitution associated with ALS resistance mechanism were found in ALS genes between the S and R biotypes. The activity of ALS enzyme isolated from the R biotype was inherently higher and less sensitive to metsulfuron-methyl than the S biotype. Glutathione S-transferases (GST) activity was also less sensitive to metsulfuron-methyl in the R than as the S biotypes. Malathion, a cytochrome P450 (CYP) monooxygenase inhibitor, had much greater synergistic effect with metsulfuron-methyl on the R than as the S plants, reducing the ED50 value (herbicide dose to inhibit growth by 50%) of metsulfuron-methyl by 23- and 6-fold, respectively, suggesting that CYP mediated enhanced metabolism might contribute to the resistance to ALS herbicides. These results suggest that metsulfuron-methyl resistance in the R biotype was associated with the up-regulated ALS enzymatic activity and the GST and CYP-mediated enhanced herbicide metabolism.
Highlights
Polypogon fugax, called Asia minor bluegrass, is a common winter weed species distributed across China and other Asian countries
The dose response experiment showed that the R biotype was resistant to metsulfuronmethyl (Figure 1A), with a resistance index (RI) of 6.0, while there were no significant differences between the R and S biotypes in their responses to chlorsulfuron, monosulfuron and pyribambenz isopropyl (Figure 1B–D)
The results showed that the R biotype had moderate resistance to metsulfuron-methyl compared with the S biotype, while no significant resistance to other tested acetolactate synthase (ALS) inhibitors
Summary
Called Asia minor bluegrass, is a common winter weed species distributed across China and other Asian countries. This annual grass has an extended period of emergence from early November to late December. Substitutions in the carboxyltransferase (CT) domain of plastidic ACCase genes are associated with target-site resistance to ACCase herbicides [9,10]. Resistance in this P. fugax biotype is conferred by an Ile-2041-Asn substitution in ACCase CT domain [6]. Alternative herbicidal options are needed to control this ACCase-resistant P. fugax biotype
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