An inducible transposon system to terminate the function of a selectable marker in transgenic plants

Since the maize transposon Ac can move to a new location within the genome, it has been used in removing selectable markers in transgenic plants. In this paper, we developed an inducible transposon system to truncate a selectable marker in transgenic plants. In this system, the marker gene was accompanied by the inducible transposon, but one end of the transposon was located in the intron of the marker gene. As an example of a marker gene, we isolated the rice 5-enolpyruvylshikimate-3-phosphate synthase (epsps) and modified it for glyphosate tolerance. The transposon contained Ac transposase, which fused with the promoter of the inducible gene for pathogenesis-related protein 1a (PR-1a). This construct was engineered into an expression vector pCAMBIA1300, harboring a hygromycin-resistant gene. The construct was first transformed into rice calli, and transformed plants were selected on hygromycin-containing medium. The stably transformed calli underwent determination for normal transcripts and tolerance to glyphosate. The results were applied to a rice transformation with the same construct, but using glyphosate as the selective agent. By determining the transformation efficiency, T-DNA copy patterns, we demonstrate that the modified epsps could be a suitable selectable marker to create transgenic rice. Furthermore, after obtaining stable transgenic plants and inducing transposition by salicylic acid, the transposon was excised, the marker gene became truncated, and its expression was terminated. This strategy could be applicable to yield self-stabilizing transposon by locating the transposon's end in the transposase gene's intron.