Auxinic Herbicide-Resistant and -Susceptible Wild Mustard ( Sinapis arvensis L.) Biotypes: Effect of Auxinic Herbicides on Seedling Growth and Auxin-Binding Activity

Abstract

The effect of the auxinic herbicides dicamba, mecoprop, MCPA, and picloram on seedling development of an auxinic herbicide-resistant and -susceptible biotype of wild mustard ( Sinapis arvensis L.) was studied. When treated with dicamba, MCPA, and picloram differences in the seedling dose-response characteristics were observed between the two biotypes. Conversely, following mecoprop treatment there were no differences between the response of the herbicide susceptible and resistant biotypes. Some characteristics of the resistant and susceptible wild mustard biotype auxin-binding protein (ABP) preparations were examined. Although, both ABP preparations appear to have similar substrate ([ 3H]indole-3-acetic acid ([ 3H]IAA) binding) and time course profiles, Scatchard analysis revealed differences between the biotypes; the resistant biotype lacks a high-affinity population of ABP which is present in the susceptible biotype. The effects of the auxinic herbicides on specific [ 3H]IAA binding to ABP preparations from the resistant and susceptible biotypes were also examined. Mecoprop and MCPA were the most potent inhibitors of [ 3H]IAA binding to the susceptible biotype ABP preparations, followed by dicamba and picloram. The same pattern of inhibition was also observed when the effect of these herbicides on seedling growth was examined in the susceptible biotype. When picloram and dicamba were used to inhibit [ 3H]IAA binding and seedling growth, the susceptible biotype was significantly more sensitive to inhibition than the resistant biotype ABP preparations; however, no differences between the two biotypes were observed following MCPA or mecoprop treatment. Our results suggest the effects of various auxinic herbicides on the binding of [ 3H]IAA to ABP preparations and seedling growth correlate with the effect of these herbicides on whole plants. This data suggests there is a relationship between ABP binding activity and sensitivity to auxinic herbicides and this relationship may explain the mechanism of auxinic herbicide resistance in this wild mustard biotype.