Highly efficient plant regeneration and in vitro polyploid induction using hypocotyl explants from diploid mulberry (Morus multicaulis Poir.)

Efficient plant regeneration is essential for successful transformation and in vitro polyploidy induction in mulberry. A high frequency (80%) of plant regeneration from hypocotyls occurred under in vitro conditions in mulberry (Morus multicaulis Poir.). We identified three key factors for enhancing successful regeneration based on earlier work: (1) hypocotyl position, (2) the combination and concentration of growth regulators, and (3) the addition of AgNO3. The highest frequency of shoot regeneration was achieved using hypocotyl segments, which are proximal to apical meristems, and the optimal culture conditions were Murashige and Skoog's (MS) (Murashige and Skoog, 1962) basal medium supplemented with 3.0 mg 1 —1 6-benzylamino purine, 0.3 mg 1 —1 indole-3-acetic acid, 0.1% polyvinypyrrolidone, and 1.0 mg/l silver nitrate (AgNO 3 ) under subdued light at 25±2°C. Treating the shoots with 0.2% colchicine (dipping for 72 h) resulted in a 14% tetraploid frequency, whereas a 20% tetraploid frequency resulted from using a 0.25% colchicine (dripping for 5 d) treatment, as determined by chromosome number counts. The diploid plant chromosome number was 28 (2n=2x=28) and that of tetraploid plants was 56 (2n=4x=56). Regenerated shoots rooted easily in 8—10 d using half-strength basal MS medium with 0.5 mg 1 —1 indole-3-butyric acid and were successfully established in the soil.