Red rice (Oryza sativa) cross-resistance to imidazolinone herbicides used in resistant rice cultivars grown in northern Greece

A putative resistant red rice (Oryza sativa) accession, an imidazolinone-resistant rice cultivar (Clearfield), a susceptible red rice accession and a susceptible rice cultivar were evaluated for cross-resistance to imazamox and imazethapyr in a whole-plant response experiment and seed bioassay. Additionally, a 210-bp fragment of the ALS gene was sequenced to identify mutations responsible for resistance. Also, a 574 bp of the ALS gene was sequenced and PCR for detection of the ‘Clearfield allele’ was conducted by the Andalusian Institute of Agricultural Research and Training (IFAPA, Spain). In the whole-plant response experiment, the putative resistant red rice was >23 and >21 times more resistant to imazamox and imazethapyr than the susceptible accession, respectively, whereas the respective resistance factor values based on seed bioassay were 86.4 and 141.7. Also, the respective resistance factor values for the Clearfield rice cultivar were similar with those calculated for putative resistant red rice. Additionally, the sequence of the 210 bp ALS gene fragment from the putative resistant red rice and Clearfield rice cultivar revealed the same amino acid substitution of Ser653Asn in both alleles (homozygous). Furthermore, the sequence of a 574 bp ALS gene fragment and the PCR for detection of the ‘Clearfield allele’ confirmed that the putative resistant red rice is homozygous mutant for the Ser653Asn mutation and provided additional evidence that its genetic background matches that of Clearfield rice. These findings indicate clearly that the insufficient control of the putative resistant red rice with imazamox was due to target-site resistance and particularly due to a point mutation at the Ser653 codon, which is positively identified as having been derived from the Clearfield rice cultivar.

Nucleotide alignment of the ALS gene fragment [codons 649 to 658 numbered after Arabidopsis thaliana ALS gene sequence (accession number X51514)] indicating point mutations at the codon 653 in the putative resistant red rice accession, Clearfield rice cultivar and mutant-resistant A. thaliana. Dots represent similar nucleotides to the first line.Figure optionsDownload as PowerPoint slideHighlights
► A red rice accession was 21–23 times more resistant to imazamox and imazethapyr than the S accession.
► ALS sequence revealed Ser653Asn substitution in resistant red rice and Clearfield rice.
► SNP analysis confirmed the transfer of the Clearfield mutant to the red rice resistant type identified.