In vivo significance of tropinone reductases I and II in Solanum tuberosum

Tropinone constitutes an important metabolite at the branching point of tropane alkaloid biosynthesis. Either tropinone is reduced to tropine by tropinone reductase I (TRI) leading to the formation of tropane alkaloids, or tropinone reductase II (TRII) forms the stereoisomer pseudotropine, precursor of calystegines. Potato (Solanum tuberosum) contains tropinone, accumulates calystegines but no tropane alkaloids. Nevertheless, the potato genome contains coding sequences for both kinds of tropinone reducing enzymes.

Fig. 1

The biological function of the S. tuberosum TRI (StTRI) is unknown. Although no tropine and derivatives could be detected, StTRI is expressed in some potato tissues. To elucidate the biological significance of this StTRI, RNAi and overexpressing potato plants were generated by agrobacteria transformation. sttrI knockdown and overexpression were evaluated by transcript analysis. Overexpression was confirmed by tropinone feeding and detection of enzyme activity. Furthermore, metabolite profiling analysis will help to gain insights into the biological function of the StTRI enzyme.

Concurring with the calystegine accumulation, a gene coding for tropinone reductase II could be isolated from Solanum tuberosum before. The encoded enzyme StTRII is able to reduce tropinone to pseudotropine in vitro but its functional role in potato secondary metabolism was never proven. The in vivo significance will be studied in parallel to StTRI by RNAi of sttrII and analysis of calystegine content in calystegine accumulating plant tissue e.g. tuber sprouts.