Resistance to quinclorac caused by the enhanced ability to detoxify cyanide and its molecular mechanism in Echinochloa crus-galli var. zelayensis

Quinclorac, an auxin-type herbicide, is widely used to control barnyardgrass and some dicotyledon weeds. Echinochloa crus-galli var. zelayensis, a variety of E. crus-galli (L.) Beauv., is widespread in China and some populations have resistance to quinclorac. E. crus-galli var. zelayensis seeds with varying sensitivity to quinclorac were used in the present study. The expression of the ADP/ATP carrier protein (ANT) gene, which plays an important role in the maintenance of cellular energy balance, dramatically rose in the S biotype after exposure to quinclorac, while no change was found in two R biotypes. The activity of beta-cyanoalanine synthase (beta-CAS), which is the key enzyme for cyanide degradation, was higher in two R biotypes than in the S biotype before and after treatment with quinclorac. One single-nucleotide difference was detected in the EcCAS gene of two R biotypes compared with the S biotype. The nucleotide change, which caused one amino acid substitution, replacing Methionine (Met)-295 with Lysine (Lys)-295 in the two R biotypes, which are same as the rice beta-CAS gene at this position. In addition, EcCAS gene expression was higher in the two R biotypes than in the S biotype. In conclusion, beta-CAS may play a crucial role in the resistance of E. crus-galli var. zelayensis to quinclorac. EcCAS gene mutation and higher gene expression may enhance the activity of beta-CAS to avoid the accumulation of toxic cyanide in resistant populations, thus contributing to the resistance mechanism of E. crus-galli var. zelayensis. to quinclorac.