Identification of metabolic enzyme genes linked to mesosulfuron-methyl resistance in Bromus japonicus

Bromus japonicus is a very troublesome weed in major winter wheat fields in China and substantially reduces wheat yield. Resistance to acetolactate synthase (ALS)-inhibiting herbicides in B. japonicus has become increasingly prevalent in recent years. While the mechanism of target site resistance (TSR) to ALS-inhibiting herbicides in B. japonicus has been well elucidated, the understanding of non-target site resistance (NTSR) remains limited. In this study, we identified a B. japonicus population (BJ-NTSR-1) which has developed resistance to mesosulfuron-methyl. Compared to the mesosulfuron-methyl-susceptible population (BJ-S), the resistance level of BJ-NTSR-1 was found to be 22.56 times higher. Based on the results of ALS gene sequencing and relative expression analyses, TSR was not detected in the BJ-NTSR-1 population. Additionally, pretreatment with cytochrome P450 (CYP450) and glutathione S-transferase (GST) inhibitors did not reverse the resistance to mesosulfuron-methyl in BJ-NTSR-1 population. RNA-seq and RT-qPCR analyses revealed that, three uridine 5'-diphospho-glucosyl transferase (UGT) genes (UGT76F1, UGT88F5, and UGT85A1), four ATP-binding cassette (ABC) transporter genes (ABCB19s, ABCG1, and ABCB21), and three CYP450 genes (CYP71C1, CYP71C2, and CYP72A15) are significantly upregulated in the BJ-NTSR-1 population. Among these genes, the overexpression of ABCG1 enhanced yeast resistance to mesosulfuron-methyl. These genes are likely involved in mediating NTSR to mesosulfuron-methyl in the BJ-NTSR-1 population. This study presents the first global report that CYP450, UGT, and ABC transporter genes may collectively mediate NTSR to ALS-inhibiting herbicides in Brome species.