Studying in vitro stem cuttings of Solanum tuberosum induced for tuberization and those of a non-tuberizing Solanum species, differences both in morphology and in gene expression were detected. Stolon formation essentially depended on light while tuberization was triggered by the elevated level of sucrose in the medium. Genes involved in starch synthesis were induced by sucrose in both species, however, starch granules were detected only in potato. A new tuber specific cDNA clone, GM7, encoding a putative metallocarboxypeptidase inhibitor and the cDNA of a proline rich cell wall protein with S. brevidens specific expression were isolated by differential screening. Sucrose mediated transcription of the tuber storage proteins like patatin and proteinase inhibitors (Kunitz-type, winI, GM7) failed in S. brevidens.