RICEFIELD BULRUSH (Schoenoplectus mucronatus (=Scirpus mucronatus)) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to HISTIDINE
Ricefield Bulrush (Schoenoplectus mucronatus (=Scirpus mucronatus)) is a monocot plant in the cyperaceae family. A single amino acid substitution from Proline 197 to Histidine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Ricefield Bulrush

Chemical Family	Example Herbicide	Resistance Level
Imidazolinones	Imazethapyr	Susceptible
Pyrimidinyl benzoates	Bispyribac-Na	Susceptible
Sulfonylureas	Chlorsulfuron	Resistant > 10 fold
Triazolopyrimidine - Type 1	Chloransulam-methyl	Resistant > 10 fold
Triazolinones	Flucarbazone-Na	Not Determined

REFERENCES

Scarabel, L. ; Locascio, A. ; Furini, A. ; Sattin, M. ; Varotto, S.. 2010. Characterisation of ALS genes in the polyploid species Schoenoplectus mucronatus and implications for resistance management. Pest Management Science 66 : 337 - 344.

BACKGROUND: The polyploid weed Schoenoplectus mucronatus (L.) Palla has evolved target-site resistance to ALS-inhibiting herbicides in Italian rice crops. Molecular and genetic characterisation of the resistance mechanism is relevant to the evolution and management of herbicide resistance. The authors aimed (a) to study the organisation 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 analyse 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 characterised by examining cytosine methylation. In S. mucronatus leaves, the transcripts of ALS1, ALS2 and ALS3 were detected. 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: Schoenoplectus mucronatus has multiple ALS genes characterised 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..

This case was entered by Patrick Tranel Email: tranel@illinois.edu

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