Explant type and stress treatment determine the uni- and multicellular origin of somatic embryos in the tree fern Cyathea delgadii Sternb.

Cyathea delgadii is the first fern species for which somatic embryogenesis (SE) has recently been described. With this discovery, a new experimental model for exploration of SE was found. In this study, the effect of explant type (stipe and internode), length and diameter of the explant-donor frond, and stress treatment (by sucrose and air desiccation) on somatic embryo origin and SE efficiency was studied. In control culture, somatic embryos originated from single cells of stipe explants, whereas those induced on internodes were of multicellular origin. Although the activation of cell divisions was more abundant and the formation of somatic embryos occurred earlier in a culture of internodes than of stipe explants, the morphogenic capacity of internodes was much smaller. On their surface, the groups of competent cells formed protuberances that give rise to only three somatic embryos per internode. In contrast, almost 92% of stipe explants that were excised from the first frond, measuring 10mm in length and less than 300 mu m in diameter, produced an average of 21 somatic embryos. Stipes treated with sucrose were not able to SE. More than fourfold increase in SE efficiency was obtained on internodes by their treatment with 0.4M sucrose for 45min. It was achieved by changing the pathway of somatic embryo differentiation from multicellular to unicellular. These results provide an excellent basis for further research on the mechanism of SE induction associated with a single- and multi-cell proliferation, especially that both types of the embryogenic pathway can occur on the same hormone-free medium.