Determination of the mechanisms of MCPA resistance in Amaranthus powellii

Resistance to 2-methyl-4-chloro-phenoxyacetic acid (MCPA) was recently confirmed in a population of green pigweed (Amaranthus powellii) from Dresden, Ontario, Canada, with a resistance factor of 4.4. Resistance to synthetic auxin herbicides in Amaranthus species has previously been linked to non-target site resistance mechanisms with low-level resistance factors (<10). Based on this information an investigation into the mechanism of resistance to MCPA was conducted in this population of green pigweed. No significant differences in absorption, translocation, and metabolism of 14C-MCPA existed between the resistant and a susceptible population of green pigweed. An RNA-Sequencing experiment to identify differentially expressed genes also confirmed this result. Genes that were differentially expressed in the resistant population were linked to target site modifications. A single nucleotide polymorphism (SNP) conferring a leucine to phenylalanine substitution was identified in auxin response factor (ARF) 9. This mutation may be in the Phox and Bem1p (PB1) domain in ARF9 which facilitates the interaction between ARFs and Aux/IAA repressor proteins. The results demonstrate that the mechanism of resistance to MCPA is not a non-target site mechanism and may be linked to a target site modification. Specifically, a SNP in ARF9 could disrupt the interaction between ARF9 and other Aux/IAAs which could prevent ubiquitination of Aux/IAAs and subsequent lethal action of MCPA.