Plant regeneration. Colonies of protoplast-derived calli were freed from alginate layers according to the protocol of Damm and Willmitzer (1988). Callus colonies were transferred separately from each layer to 250 ml jars containing regeneration media, of which three were used: MS medium consisting of MS micro- and macro-elements and vitamins with 20 gl−1 sucrose, free of plant growth regulators (PGRs); R1—consisting of MS micro- and macroelements and vitamins with 0.4 mg l−1 calcium panthothenate, 0.1 mg l−1 gibberelic acid (GA3), 3.0 mg l−1 kinetin, and 30 gl−1 sucrose; R2—consisting of MS micro- and macroelements, and modified vitamin composition; 0.5 mg l−1 nicotinic acid, 0.1 mg l−1 pyridoxine and thiamine, 3.0 mg l−1 glycine, and 20 gl−1 sucrose. The media were adjusted to a pH of 5.7–5.8 and 0.25% (w/v) Phytagel was added (Sigma) prior to autoclaving (20 min at 121°C; 1.4× 104 kg m−2). Developing shoots were transferred to fresh medium every 3–4 wk. Cultures were maintained at 26±2°C with 16 h photoperiod at a light intensity of 55 μmol m−2 s−1.