AFRICAN MUSTARD (Brassica tournefortii) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to ALANINE
African Mustard (Brassica tournefortii) is a dicot plant in the brassicaceae family. A single amino acid substitution from Proline 197 to Alanine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		African Mustard

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

Boutsalis, P. ; Karotam, J. ; Powles, S. B.. 1999. Molecular basis of resistance to acetolactate synthase-inhibiting herbicides in Sisymbrium orientale and Brassica tournefortii. Pesticide Science 55 : 507 - 516.

Three Australian Sisymbrium orientale and one Brassica tournefortii biotypes are resistant to acetolactate synthase (ALS)-inhibiting herbicides due to their possession of an ALS enzyme with decreased sensitivity to these herbicides. Enzyme kinetic studies revealed no interbiotypic differences within species in K_m (pyruvate) (the substrate concentration at which the reaction rate is half maximal) but a greater V_max (the rate when the enzyme is fully saturated with substrate) for two of the resistant S. orientale biotypes over susceptible levels. F₁ hybrids from reciprocal crosses between resistant and susceptible biotypes of S. orientale showed an intermediate response to chlorsulfuron compared to the parental plants. ALS herbicide resistance in S. orientale segregated in a 3:1 (resistant:susceptible) ratio in F₂ plants with a single rate of chlorsulfuron, indicating that resistance is inherited as a single, incompletely dominant nuclear gene. Two regions of the ALS structural gene known to vary in ALS-resistant biotypes were amplified and sequenced. Resistant S. orientale biotypes NS01 and SS03 contained a single nucleotide substitution in Domain B, predicting a Trp (in susceptible) to Leu (in resistant) amino acid change. Two adjacent nucleotide substitutions (CCT to ATT) predicting a Pro (in susceptible) to Ile (in resistant) change in the primary amino acid sequence were identified in Domain A of resistant S. orientale biotype SS01. Likewise, a single nucleotide substitution at the same site in the resistant B. tournefortii biotype predicts a Pro (in susceptible) to Ala (in resistant) substitution. No other interbiotypic nucleotide differences predicted amino acid changes in the sequenced regions, suggesting that the amino acid substitutions reported above are responsible for resistance to ALS-inhibiting herbicides in the respective biotypes..

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

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