A method for synchronized in vitro tuber induction in a Hungarian cultivar of Solanum tuberosum designated "Keszthelyi 855" has been developed. It was shown that in this system tuberization and stolon elongation primarily depend on the level of sucrose in the medium. The cytokinin, 6-bensylaminopurine (BAP), also enhances the efficiency of tuber formation, however, only at sucrose concentration above 4% (w/v). The synchronized plant culture provided starting material for isolation of genes specifically expressed in tuberizing Solanum species during the early stage of tuber development. In comparison with the non-tuberizing Solanum brevidens, three types of specific transcripts have been obtained by differential screening. Based on DNA sequence analysis the genes isolated code for the major tuber proteins, patatin and proteinase inhibitors.