Phytochrome control of the tms2 gene in transgenic Arabidopsis: a strategy for selecting mutants in the signal transduction pathway.

Introduction of the tms2 gene from Agrobacterium tumefaciens into Arabidopsis thaliana yields transgenic seedlings with a new selectable phenotype: the seedlings are strongly growth inhibited on micromolar concentrations of auxin amide substrates that do not significantly affect wild-type seedlings. The tms2 gene encodes an amidohydrolase that catalyzes the conversion of biologically inactive auxin amides into active auxins, which are toxic to plants at elevated concentrations. In the absence of exogenous substrate, tms2+ transgenic seedlings grow normally and are fertile. When grown on auxin amides, both etiolated and green tms2+ seedlings exhibit a variety of dose-dependent auxin toxicity effects. tms2 mRNA and the encoded amidohydrolase activity are both detectable in transgenic but not in wild-type seedlings, demonstrating that a cognate activity is lacking in wild-type Arabidopsis. Furthermore, when the introduced tms2 gene is fused to the Arabidopsis cab140 promoter, the tms2 RNA and its encoded amidohydrolase activity and, thus, the conditional lethal phenotype can be modulated by phytochrome action. The tms2 gene can, therefore, serve as a regulatable selectable marker in Arabidopsis that should be useful in isolation of trans-regulatory mutants that have lost the imposed regulation of tms2 gene activity.