A potato tuber-expressed mRNA with homology to steroid dehydrogenases affects gibberellin levels and plant development

Using cDNA-AFLP RNA fingerprinting throughout potato tuber development, we have isolated a transcript-derived fragment (TDF511) with strong homology t o plant steroid dehydrogenases. During in vitro tuberization, the abundance profile of the TDF shows close correlation to the process of tuber formati on. However, when tuberization is inhibited by the addition of gibberellins (GAs) to the growth medium, the appearance of TDF511 in the fingerprint is delayed, then steadily increases in intensity during later stages of devel opment. TDF511 was used to isolate the corresponding cDNA (CB12). The DNA a nd deduced amino-acid sequences of the cDNA show high homology to a fruit-r ipening gene from tomato, a series of steroid dehydrogenases, and the maize Ts2 gene. A section of the cDNA was cloned in antisense orientation behind a 35S CaMV promoter and transformed into potato. Transgenic plants express ing the antisense gene showed significantly earlier emergence, an increase in height, and longer tuber shape. In vitro tuberization experiments reveal extended stolen lengths in comparison to the controls. The analysis of end ogenous GA levels showed that the transgenic antisense plants have elevated levels of biologically active GAs and their respective precursors. We prop ose that this gene plays a role in the metabolism of plant-growth substance s important for tuber life cycle and plant development.