Variations in the acetolactate synthase (ALS) gene sequence were determined from 28 populations of corn poppy resistant (R) to tribenuron and from 6 populations susceptible (S) to this herbicide. TheALSgene fragment (634 bp) sequence revealed in R populations five point mutations at the codon Pro197, and among them the substitution of Pro197by Ala was the most common. The sequencing chromatograms revealed that nine R individuals had only the mutantALSgene and were homozygous (RR), 18 R individuals had both the wild type and the mutantALSgene and were heterozygous (RS), whereas one R individual was heterozygous but with two different mutantALSalleles (R1R2). The use of restriction digestion profile analysis to verify the DNA sequence results by detecting the existence of point mutations at the codon 197 managed to distinguish the R and S alleles and confirmed the results obtained from the sequencing chromatograms analysis. The secondary protein structure prediction suggested the formation of novelβ-strands for each of the five mentioned amino acid substitutions that was not present in wild type ALS around the mutant site. These findings support the hypothesis that the substitution of Pro197by Ser, Thr, Ala, Arg, or Leu resulted in altered secondary structure, which stabilizes an ALS tertiary conformation that prevents tribenuron binding and thus confers resistance to this herbicide.

This is the first report of Echinochloa oryzicola cross-resistance to ALS-inhibiting herbicides as a result of Trp574Leu mutation. The cpDNA sequence comparison analysis is a reliable tool for discrimination of conventionally classified E. crus-galli and E. oryzicola accessions.

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These findings indicate that Pro(197) substitution by Ala, Ser, Arg or Thr in corn poppy results in a less sensitive ALS enzyme to sulfonylurea herbicides than to other ALS-inhibiting herbicides. The continued use of sulfonylurea herbicides led to cross-resistance to all ALS-inhibiting herbicides, making their use impossible in corn poppy resistance management programmes.

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