Picloram resistance in transgenic tobacco expressing an anti-picloram scFv antibody is due to reduced translocation

Picloram resistance exhibited by transgenic tobacco (Nicotiana tabacum) plants expressing an anti-picloram single-chain variable fragment (scFv) antibody was investigated through the study of homozygous lines expressing the antibody. Dose-response bioassays, using foliar application of picloram, showed that these homozygous transgenic plants were resistant to at least 5 g of ai ha(-1) picloram and grew normally to produce seed, whereas wild-type plants did not survive. Although these lines had improved resistance compared with those previously reported, significant improvements are still required to achieve field-level resistance. Uptake and translocation studies demonstrated that [C-14] picloram translocation from treated leaves to the apical meristem was reduced in transgenic versus wild-type plants. The presence of [C-14] picloram, visualized by autoradiography and quantified by liquid scintillation spectrometry, demonstrated the distribution of more picloram in the treated leaf and less in the apical meristem of transgenic plants when compared to wild-type plants. No differences between transgenic and wild-type plants were found in the distribution of [C-14] clopyralid, a herbicide with structural similarity to picloram as well as the same mechanism of action. No differences were found in the metabolism of [C-14] picloram. Taken together, these results suggest that reduced translocation to the site of action is a major mechanism responsible for picloram resistance in tobacco plants expressing this anti-picloram antibody.