The Cross-Resistance Mechanism of Rapeseed 5N to ALS Inhibitor Herbicides

The high sulfonylurea (SU) resistance germplasm 5N has been widely used in the breeding of rapeseed resistant to herbicides. However, the cross-resistance spectrum and mechanisms of 5N to the other four classes of ALS-inhibitor herbicides remain unclear. The study aims to systematically evaluate the cross-resistance level of 5N to the five classes of ALS inhibitors and elucidate its resistance mechanisms, which will provide a theoretical basis for the scientific utilization of 5N.

The resistance levels and toxicological responses in biomass were detected by exogenous spraying of five kinds of ALS inhibitor herbicides (sulfonylurea-SU, sulfonlyaminocarbonyl-triazolinone-SCT, imidazolinone-IMI, pyrimidyl-benzoate-PB and triazolopyrimidine-TP) at the seedling stage. The sensitivity of the target enzyme and the expression patterns of two target genes were analyzed through in vitro enzyme activity inhibition assays and qRT-PCR. Simultaneously, malondialdehyde content and the activities of multiple antioxidant enzymes were measured to evaluate the oxidative stress response.

5N exhibited the cross-resistance of high level of 16 times the recommended concentration (16×RC) to SU and SCT herbicides, mid-level of 4×RC to IMI herbicides, low level to PB and TP herbicides, which was consistent with the trend in the phytotoxic response of biomass. The mechanism revealed that: (1) Target-site mechanism: The sensitivity of the ALS enzyme in 5N to various herbicides (20-300 µmol·L^-1) was significantly lower than that in N131, and the upregulated magnitude and duration of its ALS expression were notably higher than those in N131; (2) Non-target-site mechanism: After treatment with SU and SCT herbicides, the malondialdehyde content, the activities of superoxide dismutase, catalase, peroxidase, and glutathione reductase, as well as the expression levels of detoxification genes such as BnCYP86A2, BnCYP71B3, BnGST39, and BnGST78 in 5N were significantly higher than those in N131. In contrast, the responses under PB, IMI, and TP treatments were more complex and diverse.

5N exhibits broad-spectrum but differential cross-resistance to ALS inhibitors. This resistance is primarily mediated by both the low sensitivity of the target enzyme and mutations in the target gene, while non-target-site resistance mechanisms such as enhanced detoxification and antioxidant capacity, also have an assistant effect on the resistant phenotype. This study shows that SCT herbicides can serve as a safe alternative to SU herbicides at the seedling stage, and provides technical support for the rational utilization of the cross-resistance trait of 5N in herbicide-resistant breeding.