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RIGID RYEGRASS (Lolium rigidum)
with GROUP B/2 resistance: (ALS INHIBITORS)
Inhibition of acetolactate synthase ALS (acetohydroxyacid synthase AHAS)

MUTATION: PROLINE 197 to SERINE

Rigid Ryegrass (Lolium rigidum) is a monocot plant in the poaceae family. A single amino acid substitution from Proline 197 to Serine has led to resistance to ALS inhibitors as indicated in the table below.

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Rigid Ryegrass

Chemistry	Example	Resistance
Imidazolinones	Imazethapyr	Susceptible
Pyrimidinylthiobenzoates	Bispyribac-Na	Not Determined
Sulfonylureas	Chlorsulfuron	Resistant > 10 fold
Triazolopyrimidines	Chloransulam-methyl	Not Determined
Sulfonylaminocarbonyltriazolinone	Flucarbazone-Na	Not Determined

NOTES

REFERENCES

Yu, Q., H. Han and S. B. Powles. (2008). Mutations of the ALS gene endowing resistance to ALS-inhibiting herbicides in Lolium rigidum populations. Pest Management Science , 64 (12) : 1229-1236.

BACKGROUND: In the important grass weed Lolium rigidum (Gaud.), resistance to ALS-inhibiting herbicides has evolved widely in Australia. The authors have previously characterised the biochemical basis of ALS herbicide resistance in a number of L. rigidum biotypes and established that resistance can be due to a resistant ALS and/or enhanced herbicide metabolism. The purpose of this study was to identify specific resistance-endowing ALS gene mutation(s) in four resistant populations and to develop PCR-based molecular markers. RESULTS: Six resistance-conferring ALS mutations were identified: Pro-197-Ala, Pro-197-Arg, Pro-197-Gln, Pro-197-Leu, Pro-197-Ser and Trp-574-Leu. All six mutations were found in one population (WLR1). Each Pro-197 mutation conferred resistance to the sulfonylurea (SU) herbicide sulfometuron, whereas the Trp-574-Leu mutation conferred resistance to both sulfometuron and the imidazolinone (IMS) herbicide imazapyr. A derived cleaved amplified polymorphic sequences (dCAPS) marker was developed for detecting resistance mutations at Pro-197. Furthermore, cleaved amplified polymorphic sequences (CAPS) markers were developed for detecting each of the six mutant resistant alleles. Using these markers, the authors revealed diverse ALS-resistant alleles and genotypes in these populations and related them directly to phenotypic resistance to ALS-inhibiting herbicides. CONCLUSION: This study established the existence of a diversity of ALS gene mutations endowing resistance in L. rigidum populations: 1-6 different mutations were found within single populations. At field herbicide rates, resistance profiles were determined more by the specific mutation than by whether plants were homo- or heterozygous for the mutation.

Kaundun, D. (2007). Mechanisms of resistance to ACCase and ALS inhibitors in Lolium species. Resistance 2007. Rothamsted Research, Harpenden, UK, Abstracts , () : 19.