Isolation of mutations conferring increased glyphosate resistance in spring wheat

A mutation breeding approach was used to explore the feasibility of isolating glyphosateresistant (GR) wheat (Triticum aestivum L.) lines. Although transgenic GR wheat cultivars were developed, they were never introduced due to lack of consumer acceptance and concern over management of volunteer wheat in rotation. Large-scale screening experiments recovered ethyl methanesulfonate mutants able to resist 360 to 480 g acid equivalent (ae) ha^-1 glyphosate in four spring wheat cultivars, ‘Hollis’, ‘Louise’, ‘Macon’, and ‘Tara2002', indicating that it is possible to recover resistance in a wide range of genetic backgrounds (glyphosate is typically applied at 840 g ae ha^-1 in transgenic crops). Glyphosate rates of 420 to 530 g ae ha^-1 were sufficient to kill the susceptible wild-type parents. Seven GR mutants were characterized: GRH9- 5, GRH9-8, GRL1, GRL33, GRL65, GRM14, and GRT20. Glyphosate resistance was examined at the whole-plant level in dose-response experiments. Three mutant lines-GRL33, GRH9-5, and GRT20-exhibited resistance based on a significant increase in the dose required to retard growth compared with the corresponding susceptible wild type. According to F2 segregation analysis, GRL1, GRL65, and GRT20 segregated as a single dominant gene, whereas GRL33, GRH9-5, and GRH9-8 appeared to be either a single semidominant or polygenic trait. Although GRL1 was associated with an amino acid substitution (L239F) in TaEPSPS-7D1, no nucleotide changes were observed in the coding regions of wheat 5-enolpyruvylshikimate- 3-phosphate synthase (EPSPS) gene in GRL33 and GRH9-8. Results suggest that glyphosate resistance can result from multiple genetic mechanisms in wheat.