WILD RADISH (Raphanus raphanistrum) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to HISTIDINE
Wild Radish (Raphanus raphanistrum) is a dicot plant in the brassicaceae 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.		Wild Radish

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

REFERENCES

Yu Qin ; Zhang XiaoQi ; Abul Hashem ; Walsh, M. J. ; Powles, S. B.. 2003. ALS gene proline (197) mutations confer ALS herbicide resistance in eight separated wild radish (Raphanus raphanistrum) populations. Weed Science 51 : 831 - 838.

The biochemical and molecular basis of resistance to acetolactate synthase (ALS)-inhibiting herbicides was investigated in 8 resistant (R) and 3 susceptible (S) wild radish (Raphanus raphanistrum) populations. In vitro enzyme assays revealed an ALS herbicide-resistant ALS enzyme in all R populations. ALS enzyme extracted from the shoots of all eight R populations was highly resistant to the ALS-inhibiting sulfonylurea herbicide chlorsulfuron (20- to 160-fold) and the triazolopyrimidine herbicide metosulam (10- to 46-fold) and moderately resistant to metsulfuron (3 to 8-fold). There was little or no cross-resistance to the imidazolinone herbicides imazapyr and imazethapyr. The ALS gene fragment covering potential mutation sites in these populations was amplified, sequenced, and compared. All 8 R populations had point mutations in the codon for the proline residue in Domain A. However, the point mutations varied and encoded 4 different amino acid substitutions: histidine, threonine, alanine, and serine. No nucleotide difference in the DNA sequence of Domains C and D resulting in amino acid substitutions was observed between the R and S populations examined. In addition, a 3- to 5-fold higher ALS-specific activity was consistently observed in all R populations compared with S populations, whereas Northern blot analysis detected a similar level of ALS mRNA, suggesting a possible translational-posttranslational regulation of the enzyme. It is concluded that selection pressure from chlorsulfuron on eight separate wild radish populations has resulted in target site mutation at the same proline residue in the ALS gene. Higher ALS activity also may play a role in the resistance level..

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

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