Monochoria vaginalis and M. korsakowii are allotetraploid noxious weeds in rice cultivation. Occurrences of resistance to acetolactate synthase (ALS)-inhibiting herbicides have been reported in these weeds in Japan since the 1990s. The existence of multiple copies of ALS genes in both species has hindered and complicated the detailed study of molecular mechanisms in them. To determine the copy number and full-length of ALS genes in both species, we first amplified partial sequences of ALS genes and separated them by cloning. Five and three distinct sequences were identified in M. vaginalis and M. korsakowii, respectively. RACE and TAIL PCR successfully isolated full-length ALS genes, revealing that one copy of ALS genes in both species is a pseudogene formed by a frameshift mutation. Interestingly, one of the four putative functional ALS genes in M. vaginalis contains an intron in the 3'-untranslated region. Amplification and sequencing of the full-length ALS genes in sensitive and suspected resistant lines revealed a non-synonymous point mutation at codon Pro197, resulting in amino acid substitutions (Leu, Ser, or Ala) well known to endow ALS inhibitor resistance. Importantly, codon Pro197 of the M. korsakowii pseudogene encodes leucine (Leu) both in resistant and sensitive plants, which is also known to confer ALS inhibitor resistance when ALS genes are functional. Dose responses to imazosulfuron of the lines analyzed for ALS genes were in agreement with the existence of the mutations. These results suggest that some caution is needed when diagnosing molecular resistance in M. korsakowii. The information of copy number and full-length sequences will help diagnose ALS resistance and make a basis for the study of the evolution of ALS resistance in Monochoria spp.
Keywords: Acetolactate synthetase; Copy number; Polyploid; Pseudogene; Resistance evolution.
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