A psbA mutation (Val 219 to Ile) causes resistance to propanil and increased susceptibility to bentazon in Cyperus difformis

BACKGROUND: Propanil-resistant (R) Cyperus difformis populations were recently confirmed in California rice fields. To date, propanil resistance in other weed species has been associated with enhanced aryl acylamidase (AAA)-mediated propanil conversion into 3,4-dichloroaniline. Our objectives were to determine the level of propanil resistance and cross-resistance to other PSII inhibitors in C. difformis lines, and to elucidate the mechanism of propanil resistance. RESULTS: The propanil-R line had a 14-fold propanil resistance and increased resistance to bromoxynil, diuron and metribuzin, but not to atrazine. The R line, however, displayed a fourfold increased susceptibility to bentazon. Interestingly, susceptible (S) plants accumulated more 3,4-dichloroaniline and were more injured by propanil and carbaryl (AAA-inhibitor) applications than R plants, suggesting that propanil metabolism is not the resistance mechanism. psbA gene sequence analysis indicated a valine-219-isoleucine (Val219 Ile) amino acid exchange in the propanil-R chloroplast D1 protein. CONCLUSION: The D1 Val219 Ile modification in C. difformis causes resistance to propanil, diuron, metribuzin and bromoxynil but increased susceptibility to bentazon, suggesting that the Val219 residue participates in binding of these herbicides. This is the first report of a higher plant exhibiting target-site propanil resistance. Tank mixing of bentazon and propanil, where permitted, can control both propanil-R and propanil-S C. difformis and prevent the spread of the resistant phenotype. © 2016 Society of Chemical Industry.