Auxinic herbicide resistance may be modulated at the auxin-binding site inwild mustard (Sinapis arvensis L.): A light scattering study

The effects of IAA and the auxinic herbicides picloram and MCPP on the flas h-induced nucleotide-independent light-scattering signal from picloram-resi stant (R) and- susceptible (S) wild mustard (Sinapis arvensis L.) protoplas ts were studied. We previously demonstrated that an increase in H+ efflux l eads to a decrease in amplitude of the light-scattering signal. In this stu dy, IAA increased H+ efflux (i.e., decreased the signal amplitude) in both R and S protoplasts in direct proportion to the IAA concentration. When the signal amplitudes from picloram-trealed R and S biotype were compared, the y were different. Picloram, which adversely affects the S biotype in vivo a nd in vitro, increased H+ efflux (reduced signal amplitude) in S protoplast s but did not affect H+ efflux in R. MCPP, which adversely affects both bio types iir vivo and in vitro, increased H+ efflux in both biotypes. Preincub ation of S protoplasts with IAA did not alter the effects of picloram or MC PP on H+ efflux (i.e., increased efflux). Conversely, preincubation of R pr otoplasts with IAA reduced MCPP-mediated H+ efflux thereby reducing the adv erse affects of MCPP. These results support our hypothesis that the physiol ogical basis for this auxinic herbicide resistance in the wild mustard biot ype is associated with their altered binding to auxin binding sites. (C) 20 00 Academic Press.