Revised Protocols for High Efficient Transformation and Regeneration of Somatic Embryos of Papaya (Carica papaya L.)

Efforts to improve plant tissue culture and transformation is necessary for the production of transgenic papaya resistant to viral diseases. Induction of somatic embryos in liquid culture reduced the time for papaya genetic transformation. In this study, growth medium containing supplements of abscisic acid (ABA)and 2,4-dichlorophenoxyacetic acid (2,4-D)was optimised to control papaya explants proliferation and development into a proembryogenic mass suitable for somatic embryo induction. Our findings indicated that somatic embryos cultured in half-strength liquid MS medium containing 22.5 ìM 2,4-D and 2.5 ìM ABA yielded higher cell mass (dry-weight basis)than parallel treatments with other combinations of plant growth regulators. Subsequent sub-culture of the somatic embryos resulted in a population of somatic embryos at various stages of development. By repeated sieving of the embryogenic mass, through a stainless steel mesh with pore size of 250, 500 and 710 ìm, it was possible to separate the friable embryogenic calli and thus maintain synchronous suspension cultures. No significant difference was found in transient expression and regeneration of somatic embryos of different sizes after sieving. Rooting of papaya plantlets during acclimatization was the limiting factor for regeneration of papaya. Papaya plantlets rooted in husk infused media with an equal amount of half-strength MS medium showed an increase in the root length (453 cm), the number of root tips (20 tips)and survival rate (90