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Herbicide-resistant Clearfield™ 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 — Pro 93 Thr, Glu 630 Asp, and Val 669 Met — in four accessions: Ark-4, Ark-9, Poi-1 and Poi-6. The GR 50 values and inhibition of root and shoot growth (%) of these accessions differed. The Glu 630 Asp substitution occurs in the herbicide binding domain B and Val 669 Met 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™ rice to weedy rice will lead to the evolution of ALS-resistant weedy rice populations. |
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