Characterisation of ALS genes in the polyploid species Schoenoplectus mucronatus and implications for resistance management.

BACKGROUND: The polyploid weed Schoenoplectus mucronatus has evolved target-site resistance to ALS-inhibiting herbicides in Italian rice crops. Molecular and genetic characterization of the resistance mechanism is relevant to the evolution and management of herbicide resistance. The authors aimed a) to study the organization of the target-site loci in two field-selected S. mucronatus populations with different cross-resistance patterns, b) to identify the mutations endowing resistance to ALS inhibitors and determine the role of these mutations by using transgenesis and c) to analyze the implications for the management of the S. mucronatus populations. RESULTS: Two complete ALS genes (ALS1 and ALS2) having an intron and a third partial intronless ALS gene (ALS3) were identified. The presence of multiple ALS genes was confirmed by Southern blot analyses and ALS loci were characterized by examining cytosine methylation. In S. mucronatus leaves we detected the transcripts of ALS1, ALS2 and ALS3. Two mutations endowing resistance (Pro197 to His and Trp574 to Leu) were found in both resistant populations but at different frequencies. Tobacco plants transformed with the two resistant alleles indicated that the Pro197 to His substitution conferred resistance to SU and TP herbicides while the allele with the Trp574 to Leu substitution conferred cross-resistance to SU, TP, IMI and PTB herbicides. CONCLUSION: S. mucronatus has multiple ALS genes characterized by methylated sites that can influence the expression profile. The two mutated alleles proved to be responsible for ALS-resistance. At population level, the resistance pattern depends on the frequency of various resistant genotypes and this influences the efficacy of various ALS-inhibiting herbicides.