INTERNATIONAL HERBICIDE-RESISTANT WEED DATABASE

WILD RADISH (Raphanus raphanistrum) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase

MUTATION: PROLINE 197 to SERINE

Wild Radish (Raphanus raphanistrum) is a dicot plant in the brassicaceae family. A single amino acid substitution from Proline 197 to Serine 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, Q. ; Han, H. ; Li, M. ; Purba, E. ; Walsh, M. J. ; Powles, S. B.. 2012. Resistance evaluation for herbicide resistance-endowing acetolactate synthase (ALS) gene mutations using Raphanus raphanistrum populations homozygous for specific ALS mutations. Weed Research (Oxford) 52 : 178 - 186.

Acetolactate synthase (ALS)-inhibiting herbicide resistance is common in Raphanus raphanistrum (wild radish) populations across the Western Australian (WA) grain belt. This study investigates the molecular and biochemical basis of ALS herbicide resistance in five R. raphanistrum populations. Five known ALS herbicide resistance-endowing mutations (Pro-197-Ala, Pro-197-Thr, Pro-197-Ser, Asp-376-Glu and Trp-574-Leu) were identified, and their resistance spectrum to ALS-inhibiting herbicides was determined using purified populations individually homozygous for each mutation (except for Pro-197-Ala). Plants homozygous for ALS mutations at Pro-197 were found to be cross-resistant to ALS-inhibiting sulfonylurea (SU) and triazolopyrimidine (TP) herbicides, while plants homozygous for Trp-574-Leu were resistant to SU, TP and imidazolinone (IMI) ALS herbicide classes. The Asp-376-Glu mutation is reported here for the first time in R. raphanistrum populations and characterised at both the whole-plant and enzyme level. Plants homozygous for Asp-376-Glu were highly resistant to SU and TP herbicides, based on LD₅₀ R/S ratios (>130 and 128 respectively) and I₅₀ R/S ratios (170 and >110 respectively). In contrast, these plants were moderately resistant to the IMI imazamox (LD₅₀ R/S ratio of 8, I₅₀ R/S ratio of 3) and imazethapyr (I₅₀ R/S ratio of 8) and susceptible to imazapyr (I₅₀ R/S ratio of 0.76). A novel observation in this study is that resistance of homozygous Glu-376 plants is associated with a remarkable growth reduction in the presence of the ALS herbicides tested, making early resistance diagnosis and management difficult..

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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