REDROOT PIGWEED (Amaranthus retroflexus) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: PROLINE 197 to LEUCINE
Redroot Pigweed (Amaranthus retroflexus) is a dicot plant in the amaranthaceae family. A single amino acid substitution from Proline 197 to Leucine has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Redroot Pigweed

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

REFERENCES

Sibony, M. ; Michel, A. ; Haas, H. U. ; Rubin, B. ; Hurle, K.. 2001. Sulfometuron-resistant Amaranthus retroflexus: cross-resistance and molecular basis for resistance to acetolactate synthase-inhibiting herbicides. Weed Research (Oxford) 41 : 509 - 522.

A biotype of Amaranthus retroflexus is the first weed in Israel to develop resistance to acetolactate synthase (ALS)-inhibiting herbicides. The resistant biotype (Su-R) was collected from Ganot, a site that had been treated for more than 3 consecutive years with sulfometuron-methyl+simazine. On the whole-plant basis, the resistance ratio (ED₅₀ Su-R)/(ED₅₀ Su-S) was 6-127 for sulfonylureas, 4-63 for imidazolinones, 20-35 for triazolopyrimidines and 11 for pyrithiobac-sodium. Similar levels of resistance were found also when the herbicides were applied before emergence. Based on a root elongation bioassay, Su-R was 3240-fold more resistant to sulfometuron-methyl than Su-S. In vitro studies have shown that the Su-R biotype was resistant at the enzyme level to all ALS inhibitors tested. The nucleotide sequences of two amplified regions between the Su-S and the Su-R differed in only one nucleotide. One substitution has occurred in domain A, cytosine by thymine (CCC to CTC) at position 248, that confers an exchange of the amino acid proline in the biotype susceptible to leucine in the Su-R. The proline to leucine change in domain A is the only difference in the amino acid primary structure of the regions sequenced, indicating that it is responsible for the ALS-inhibitor resistance observed..

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

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