Suspected sulfonylurea (SU)-resistant Schoenoplectus juncoides plants were collected from rice paddy fields at 24 sites in Japan in order to discover the occurrence pattern of target-site substitutions on a nationwide scale and at a local field scale. A genetic analysis of the two acetolactate synthase (ALS) genes, ALS1 and ALS2, of the collected plants confirmed that a single-nucleotide mutation at the Pro197, Asp376 or Trp574 site of either ALS1 or ALS2 existed in each suspected SU-resistant plant. On a nationwide scale, it was shown that the ALS1 mutations and the ALS2 mutations occurred at a similar frequency, that the P197S and the P197L substitutions were found most frequently among all the substitutions, and that the W574L substitutions (known as global resistance to any ALS-inhibiting herbicide) were found at a relatively low frequency but in a geographically wide range. In the local field-scale survey, which was conducted at two sites in Hyogo Prefecture, it was shown that the substitutions were less diverse, compared to on a nationwide scale, probably because the investigation involved a limited number of local fields, and that several substitutions and a susceptible biotype were found in single fields suggesting that a number of collections is required in order to understand the local SU-resistant status of S. juncoides. In addition, this study reported new findings, that of the P197R, P197T and D376E substitutions in S. juncoides.This set of diverse substitutions in a weed species can be used for further research purposes.
An investigation, using herbicidal pot tests in a greenhouse condition, was conducted to determine the whole-plant dose-response relationships to several acetolactate synthase (ALS)-inhibiting herbicides of sulfonylurea (SU)-resistant Schoenoplectus juncoides with various Pro197 mutations in ALS that was collected from Japanese rice paddy fields.All the tested SU-resistant accessions with a Pro197 mutation were highly resistant to two commonly used SU herbicides (imazosulfuron and bensulfuron-methyl), but were much less resistant to another SU herbicide, metsulfuron-methyl, and were substantially not resistant to imazaquin-ammonium. These cross-resistance patterns have been known previously in fragments of S. juncoides and other weed species and were comprehensively confirmed in this study with a whole set of Pro197 mutations.The analyses of resistance levels, based on ED90 values, newly showed that different accessions with a common amino acid substitution in ALS1 showed similar responses to these herbicides (confirmed with four amino acid substitutions), that the rankings of resistance levels that were conferred by various Pro197 mutations in ALS1 differed among the SU herbicides and that the resistance levels of the ALS2-mutated accessions were higher than, lower than or similar to those of the corresponding ALS1-mutated accessions, depending on the compared pair, but the deviation patterns were generally similar among the SU herbicides in each compared pair.The final finding might suggest that the abundance of ALS2 is not as stable as that of ALS1. In addition, as a result of these new findings, together with expected further research, a suggested possibility is that substituting amino acids at Pro197 generally could be estimated by plotting each accession's ED90 values of imazosulfuron and bensulfuron-methyl in a two-dimensional graph.
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