Interaction between day length and phytohormones in the control of potato tuberization in the in vitro culture

We studied the interaction of the day length, cytokinins, and gibberellins in the control of tuberization in potato (Solanum tuberosum L, cv. Desire) plants and derived transgenic plants with the inserted PHYB gene from Arabidopsis encoding the synthesis of phytochrome B apoprotein and put under the control of the 35S CaMV promoter. Plantlets were cultured in vitro on hormone-free MS medium containing 5% sucrose and kinetin (1 mg/l) or/and GA (0.5 and 1.0 mg/l), at long day (LD, a 16-h photoperiod), short day (SD, a 10-h photoperiod), or continuous darkness conditions. The content of cytokinins (Ck, zeatin, and zeatin riboside) in various plant organs was determined by the immunoenzyme method, and GA activity was measured in bioassay with dwarf pea. Potato plant transformation with the PHYB gene enhanced substantially tuber initiation inhibition by LD. Kinetin addition to culture medium enhanced tuberization and reduced Ck content in aboveground shoots and Ck redistribution in the favor of underground organs. GA addition to the culture medium suppressed tuberization and induced Ck accumulation in aboveground organs. We concluded that Ck role in tuberization depends on their predominant localization in above- or underground potato organs. The involvement of Ck and GA in the competitive relations between growing tubers and shoots is considered.