Callus induction in papaya (Carica papaya L.) and synseed production for low temperature storage and cryopreservation

The mid- to long-term preservation of papaya (Carica papaya L.) would allow for the safeguarding of important germplasm. In this study, soft friable callus (SFC) and hard callus (HC) were induced from the first two true leaves of 10-day-old seedlings containing a midrib derived from the germinated seed of two cultivars (‘Rainbow’ and ‘Sunrise Solo’). Following germination on a Murashige and Skoog (MS) medium that contained 3% sucrose and was free of plant growth regulators (PGRs), sections of the first true leaves from 10-day-old seedlings were exposed to seven published callus or somatic embryogenesis protocols for zygotic embryos, leaves or hypocotyls. Optimal SFC and HC induction was carried out on a half-strength MS medium following the Fitch (1993) or the Ascêncio-Cabral et al. (2008) protocol, respectively. SFC formed shoots that could then convert to plants when transferred to a full-strength MS medium devoid of PGRs. Plantlets 10-cm tall were acclimatised in two steps: first by in vitro acclimatisation in aerated vessels, the Vitron, under CO2-enriched (3000 ppm CO₂), then by the transfer of individually rooted plantlets in Rockwool® blocks to a substrate of soil: pine bark : perlite (1:1:1, v/v/v). SFC and HC were then encapsulated in alginate beads, which were exposed to low temperature storage (LTS) at 10°C and 15°C, and also cryopreserved for 30 days. All encapsulated alginate beads that contained SFC, HC or leaf tissue that had been stored under LTS or cryopreserved were able to regenerate callus when placed on an optimal callus induction medium. Plants derived from the control, LTS and cryopreservation protocols, either from SFC or HC, were successfully acclimatised.