Characterization of Low-Level Glyphosate Tolerance in an Abutilon theophrasti Population from a Soybean Field

This study investigated the tolerance level and mechanism of a glyphosate-tolerant Abutilon theophrasti population (T2784) collected from a glyphosate-resistant soybean field. Whole-plant dose-response, EPSPS sequencing, gene expression analyses, EPSPS activity assays, glyphosate absorption and metabolism tests, and a malathion inhibition assay were performed. The GR₅₀ values were 953.0 g a.i. ha^-1 for T2784 and 415.3 g a.i. ha^-1 for the susceptible S2857 population, yielding a tolerance index of 2.29. No amino acid substitutions were detected in the amplified EPSPS region. However, EPSPS genomic copy number was not assessed, and therefore EPSPS amplification cannot be excluded. No significant differences in EPSPS expressions were observed between the two populations. T2784 showed significantly higher constitutive EPSPS activity and a 2.66-fold higher IC₅₀ for glyphosate inhibition than S2857. No differences in glyphosate absorption or in the estimated disappearance of recoverable parent glyphosate were detected. Because specific glyphosate metabolites were not quantified, the estimated metabolism values represent operational estimates based on parent-compound disappearance rather than direct measurements of glyphosate biotransformation. Malathion pretreatment did not significantly alter GR₅₀ values in either population. The lower glyphosate sensitivity of T2784 was associated with higher EPSPS activity and a higher inhibition IC₅₀ measured in vitro. However, the causal basis of the whole-plant response remains unresolved. Further studies are needed to clarify the biochemical basis of elevated EPSPS activity and to directly characterize whole-plant glyphosate translocation and metabolic products. This study provides a basis for further investigating non-target-site processes and supports the use of integrated weed-management strategies in glyphosate-resistant soybean production systems.