Tobacco etch virus leader sequence enhances inducible indoleacetic acid-lysine synthetase activity in transgenic plants

Indole-3-acetic acid (IAA) is one of several plant hormones that interact to coordinate growth and differentiation in higher plants. IAA has been identified in all species investigated and is found not only in its free form but also as sugar ester, peptide and amino acid conjugates. Plant processes ranging from the induction of cell division and expansion to the maintenance of apical dominance are dependent on free IAA levels. The objective of this study was to construct and characterize transgenic plants that contain the tetracycline (Tc)-specific inducible system to control the expression of the IAA-lysine synthetase gene (iaaL). The enzyme activity (IAA-L) encoded by iaaL conjugates free IAA with lysine to produce a foreign inactive conjugate IAA-lysine (IAA-Lys). Characterization of independent tobacco (Nicotiana tabacum cv. Wisconsin 38) transformants identified inducible plants completely repressed in the absence of tetracycline and derepressed to high IAA-L levels in the presence of inducer. The incorporation of the tobacco etch virus non-translated leader sequence (NTR) in context of the tetracycline-regulatable CaMV 35S triple-op promoter enhanced IAA-L activity 7-fold. Unlike tetracycline, the tetracycline analog anhydrotetracycline (ATc) did not inhibit the growth of callus at concentrations up to 50 μg·mL–1 on solid medium. In the selected plants, IAA-Lys accumulation is dose-dependent upon the presence of the inducer molecule, and IAA-L activity in the derepressed state is similar to a plant that constitutively expresses iaaL at high levels from the native CaMV 35S promoter.