Geographical Distribution and Mechanism of ALS Inhibitor Herbicide Resistant Echinochloa spp. in Korea

Abstract

Echinochloa spp. is one of the most problematic weed in rice cultivation in Korea, and recent confirmation of herbicide resistance (HR) in Echinochloa species has made this grass weed more problematic and difficult to manage. Thus, this study was conducted to investigate the geographical distribution of ALS and ACCase inhibitor resistant E. crus-galli and E. oryzicola in Korea and to clarify the mechanism of the resistance. 121 accessions for E. crus-galli and 113 accessions for E. oryzicola were collected nationwide in Korean rice fields. To evaluate the level of their HR,penoxsulam (PNX) as the ALS inhibitor was applied at a range of doses (0, 3.75, 7.5, 15, 30, and 60 g a.i. ha-1) and cyhalofop-butyl (CHB) as the ACCase inhibitor was treated at a range of doses (0, 31.25, 62.5, 125, 250 and 500 g a.i. ha-1) to two Echinochloa species. When compared with a susceptible reference, every E. crus-galli accessions was susceptible to PNX and CHB. In E. oryzicola, however, the whole plant assay results have revealed that 9 and 7 accessions in PNX and 9 and 6 accessions in CHB as the high (>4.5 in R/S ratio) and medium (3.0<R/S ratio<4.5) resistance, respectively. Especially, 10 accessions showed multiple resistance to both herbicides, PNX and CHB, suggesting that major mechanism of HR in E. oryzicola might be nontarget site resistance (NTSR). Geographical distribution of PNX and CHB resistant accessions were evenly distributed in 4 region of Korea. Among the 17 accessions with showing R/S ratio greater than3.0, in vitro ALS assay results showed no difference in ALS activity and whole plant assay with malathion (cytochrome P450 monooxyganases (P450s) inhibitor) resulted in increased sensitivity to PNX. Both results suggested that the mechanism of HR could be not target-site resistance, but NTSR by P450s. As the results of RNA expression in leaves of 7 HR E. oryzicola after penoxsulam treatment, EcGST2, CYP71C42, and P450K14 gene was differently expressed. Both EcGST2 and CYP71C42 were up-regulated in Seosan, Gimje, SNU-E-08.074, and SNU-E-08.080, and both EcGST2 and P450K14 were up-regulated in SNU-E-08.236 at 24 HAT. This suggests that EcGST2 and CYP71C42 contribute to PNX resistance in 4 accessions, Seosan, Gimje, SNU-E-08.074, and SNU-E-08.080, while EcGST2 and P450K14 contribute to PNX resistance only in SNU-E-08.236. Two other PNX resistant accessions, Iksan and SNU-E-08.253, remain still unclear in their mechanism of herbicide resistance. In conclusion, our findings suggest that herbicide resistance in E. oryzicola is diverse and evolved independently and two different family genes are related in HR involving herbicide metabolism in E. oryzicola.