Correlation between Auxin Resistance and the Lack of a Membrane-Bound Auxin Binding Protein and a Root-Specific Peroxidase in Nicotiana tabacum.

The levels of a membrane-bound auxin binding protein (MABP) and a root-specific peroxidase (RSP) were studied in several tobacco (Nicotiana tabacum L.) cell lines including an auxin-resistant variant. Groups of cell lines were distinguished which behaved differentially with respect to MABP and RSP depending on the hormonal composition of the medium. In cell lines in which there existed a correlation between the presence or absence of MABP and that of RSP both phenotypes were expressed if kinetin (1-2 micromolar) was supplied. In contrast, neither MABP nor RSP could be detected under any hormonal conditions tested in the auxin-resistant variant which retains the ability to differentiate shoots but lacks the ability to differentiate roots. About an eightfold increase in the concentration of MABP and a dramatic increase in the activity of RSP occurred in a transformant by a mutant strain of Agrobacterium tumefaciens lacking an intact cytokinin gene when it was grown on medium containing 1 to 2 micromolar kinetin. A correlation between auxin resistance and the lack of MABP and RSP suggests that MABP might be involved in auxin-mediated root differentiation in tobacco.