INVITRO TUBERIZATION IN TRANSGENIC POTATOES HARBORING BETA-GLUCURONIDASE LINKED TO A PATATIN PROMOTER - EFFECTS OF SUCROSE LEVELS AND PHOTOPERIODS

Sections of axenic potato shoots containing one leaf were found to synchronically produce a microtuber in their single leaf axil 7 days after transfer to inductive medium containing 8% sucrose, kinetin and ancymidol (a gibberellin-biosynthesis inhibitor). Shoots of transgenic Solanum tuberosum harboring a chimeric gene containing a patatin promoter sequence followed by the gene coding for beta-glucuronidase as a reporter protein, were utilized. Microtubers were initiated in inductive medium under either long-day (LD) or short-day (SD) photoperiods. The minimal duration of culture in 8% (w/v) sucrose (followed by culture in 2% (w/v) sucrose) is 1 or 3 days, when the shoots were subsequently maintained (for 6 or 4 days) in LD or SD, respectively. Changes in patatin transcription were evaluated by following beta-glucuronidase activity. Strong increase in beta-glucuronidase activity was observed in tuber intials 5 days after transfer to tuber-induction medium. This increase paralleled the accumulation of radioactivity in the tuber initials of shoots cultured in radioactive sucrose. beta-Glucuronidase activity analyses at various periods after induction and in different tissues of the in vitro cultured shoot-sections as well as the elimination of one of the obligatory induction conditions, suggested that the requirement for high sucrose levels does not represent an energy demand but rather an induction signal. Day-length has apparently a decisive role, after exposure to high-levels of sugar, in maintaining the tuberization signal.