Molecular basis of Cyperus difformis cross-resistance to ALS-inhibiting herbicides.

Abstract Applications of 64 times higher rates of azimsulfuron and halosulfuron-methyl failed to reduce by 50% growth of 10 putative resistant (R) Cyperus difformis (smallflower umbrella-sedge) populations collected from rice fields located in Northern Greece. However, the growth rate of the susceptible (S) population was reduced by 50% by 1/4 of the recommended rate of both herbicides. The als gene sequencing identified point mutations at Pro-197 position, leading to amino acid substitution by Ala or Ser of the ALS enzyme. The in vitro activity of the ALS enzyme indicated that the I 50 values (herbicide concentration required for 50% reduction of the ALS activity) ranged from 10.7 to 55.5 μM azimsulfuron and from 6.7 to 50.6 μM halosulfuron-methyl, whereas the respective values for the S population were 0.09 and 0.11 μM. These results strongly support that cross-resistance of 10 C. difformis populations was due to point mutations of the als gene, which resulted in a less-sensitive ALS enzyme. This study reports the presence of a 1240 bp intron in the C. difformis als gene with the Pro-197 point mutations near the splice junction.