RED RICE (Oryza sativa var. sylvatica) with GROUP B/2 resistance: (INHIBITION OF ACETOLACTATE SYNTHASE ) Inhibition of Acetolactate Synthase
MUTATION: GLYCINE 654 to GLUTAMATE
Red Rice (Oryza sativa var. sylvatica) is a monocot plant in the poaceae family. A single amino acid substitution from Glycine 654 to Glutamate has led to resistance to Inhibition of Acetolactate Synthase as indicated in the table below.		Red Rice

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

NOTE

Both Gly654Glu and Val669Met substitutions were identified, but research by Shivrain et al. 2010 suggests the latter substitution confers little or no resistance. 

REFERENCES

Shivrain, V. K. ; Burgos, N. R. ; Sales, M. A. ; Kuk, Y. I.. 2010. Polymorphisms in the ALS gene of weedy rice (Oryza sativa L.) accessions with differential tolerance to imazethapyr. Crop Protection 29 : 336 - 341.

Herbicide-resistant Clearfield^TM rice technology allows the use of ALS inhibitors to control weedy rice. Weedy rice plants have differential tolerance to imazethapyr, perhaps due to ALS polymorphisms. We aimed to assess ALS polymorphisms in weedy rice accessions from Arkansas, USA, with differential tolerance to imazethapyr in seedling growth bioassays. Six base changes were identified in the ALS of 14 weedy rice accessions. Three of these nucleotide changes resulted in amino acid substitutions - Pro⁹³Thr, Glu⁶³⁰Asp, and Val⁶⁶⁹Met - in four accessions: Ark-4, Ark-9, Poi-1 and Poi-6. The GR₅₀ values and inhibition of root and shoot growth (%) of these accessions differed. The Glu⁶³⁰Asp substitution occurs in the herbicide binding domain B and Val⁶⁶⁹Met occurs at the C-terminal tail where the co-factor binds. Variability in weedy rice ALS exists, but polymorphism patterns did not relate to tolerance levels. The observed mutations presented the possibility that sustained selection pressure will eventually lead to selection of a herbicide-tolerant individual that will be the progenitor of a resistant population. Concomitantly, pollen-mediated gene flow from Clearfield^TM rice to weedy rice will lead to the evolution of ALS-resistant weedy rice populations..

Kuk, Y. I. ; Burgos, N. R. ; Shivrain, V. K.. 2008. Natural tolerance to imazethapyr in red rice (Oryza sativa). Weed Science 56 : 1 - 11.

Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas, USA, were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (Clearfield) rice cultivars (CL121, CL161 and CL-XL8) and conventional rice cultivars (Bengal, Dongjin, Drew and Wells). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively, than the susceptible standard. The imazethapyr-tolerant red rice and Clearfield cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and Clearfield rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before Clearfield rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure..

Sales, M. A. ; Shivrain, V. K. ; Burgos, N. R. ; Kuk, Y. I.. 2008. Amino acid substitutions in the acetolactate synthase gene of red rice (Oryza sativa) confer resistance to imazethapyr. Weed Science 56 : 485 - 489.

Two red rice accessions from Arkansas have been found to be resistant to the labelled rate of imazethapyr, which is used to control red rice in Clearfield^TM rice. Full-length amplification of the acetolactate synthase (ALS) gene in imazethapyr-resistant red rice revealed a coding sequence of 1,935 base pairs, which is the same as that of the cultivated rice. Coding sequences were generated from four red rice accessions collected from different geographical regions in Arkansas, consisting of accessions that were either resistant or susceptible to imazethapyr. Nucleotide sequence alignments identified six base polymorphisms, three of which resulted in amino acid substitutions in the ALS gene. One amino acid substitution, Gly⁶⁵⁴Glu, involves a residue required for imazethapyr binding to the ALS. The other substitution, Val⁶⁶⁹Met, implies conformational changes in the ALS structure that enhances binding of thiamine diphosphate, an ALS cofactor. These novel amino acid substitutions first reported for ALS-resistant red rice accessions support the hypothesis that ALS-resistant red rice can evolve with sustained herbicide selection pressure. Thus, it behooves growers to integrate the Cleafield rice technology with other tools to achieve a successful, long-term weed management program..

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

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