Abstract
Natural tolerance in hexaploid bread wheat (Triticum aestivum L.) to synthetic auxin herbicides is primarily due to rapid metabolic detoxification, but genes encoding these herbicide-detoxifying enzymes have yet to be identified. Herbicide safeners are commonly applied in wheat to achieve herbicide tolerance by inducing the expression and activity of herbicide-detoxifying enzymes. While safeners have been utilized for decades, knowledge of mechanisms that induce gene expression is limited. Our objective was to identify wheat chromosomes possessing genes that endow natural or safener-induced tolerance to halauxifen-methyl (HM), a postemergence (POST) wheat-selective synthetic auxin herbicide, using alien substitution (the S genome of Aegilops searsii) and aneuploid lines. Two POST rates of HM were applied to seedlings with 1-2 leaves (Zadoks stages 11-12), and the highest HM rate was also applied with the safener cloquintocet-mexyl (CM). Wheat chromosomes possessing genes associated only with natural HM tolerance were identified because Ae. searsii is HM-sensitive but CM-responsive. Lines with substitutions for 5A and 5B displayed sensitivity to HM, and experiments with nullisomic-tetrasomic (NT) lines further indicated major genes associated with HM tolerance are present on 5A and 5B chromosomes. However, the genes on 5A appear to play a larger role because lines lacking 5A chromosomes displayed more sensitivity than lines lacking 5B. Overall, these results can be utilized to guide future transcriptome analyses to identify candidate genes that confer HM tolerance in wheat.
Highlights
Natural tolerance in hexaploid bread wheat (Triticum aestivum L.) to synthetic auxin herbicides is primarily due to rapid metabolic detoxification, but genes encoding these herbicide-detoxifying enzymes have yet to be identified
Results for Ae. searsii indicate it lacks natural HM tolerance, meaning that substitutions from this species should not increase tolerance in ‘Chinese Spring’ and allow for detection of wheat chromosomes possessing genes that govern natural HM tolerance. These results indicate that Ae. searsii possesses CM-responsive genes whose expression enhances HM metabolism, resulting in a significant reduction in injury compared to HM alone
Reductions in HM tolerance were quantified in wheat when chromosomes 5A and 5B were not present in either alien substitution lines with the S genome or in NT lines, indicating these chromosomes contain genes necessary for HM tolerance
Summary
Natural tolerance in hexaploid bread wheat (Triticum aestivum L.) to synthetic auxin herbicides is primarily due to rapid metabolic detoxification, but genes encoding these herbicide-detoxifying enzymes have yet to be identified. Our objective was to identify wheat chromosomes possessing genes that endow natural or safener-induced tolerance to halauxifen-methyl (HM), a postemergence (POST) wheat-selective synthetic auxin herbicide, using alien substitution (the S genome of Aegilops searsii) and aneuploid lines. Current applications of aneuploid and alien substitution lines include identifying chromosomes that possess genes of interest as well as analyzing homoeologous gene expression patterns and regulation[7] These genetic resources have allowed researchers to study a myriad of genes related to grain nutrient content, biotic and abiotic stresses, and defense compound and phytohormone biosynthesis[8,9,10,11,12,13,14,15,16]. While safeners have been prevalent in agriculture for decades and their phenotypic and metabolic effects are well documented, knowledge of safener regulation of corresponding genes or signaling pathways is still limited[33,36]
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