Symplasmic Isolation Contributes to Somatic Embryo Induction and Development in the Tree Fern Cyathea delgadii Sternb.

: In this report, we describe studies on symplasmic communication and cellular rearrangement during direct somatic embryogenesis (SE) in the tree fern Cyathea delgadii. We analyzed changes in the symplasmic transport of low-molecular weight fluorochromes, such as 8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt (HPTS) and fluorescein (delivered to cells as fluorescein diacetate - FDA), within stipe explants and somatic embryos originating from single epidermal cells and developing during 16 days long culture. Induction of SE is preceded by a restriction in fluorochrome distribution between certain explant cells. Microscopic analysis showed a series of cellular changes like decrease of vacuole size, increase of vacuoles numbers, increased density of cytoplasm and deposition of electron-dense material in cell walls which may be related with embryogenic transition. In somatic embryo, the limited symplasmic communication between its cells was observed first in linear tri-cellular embryo. Further development of the fern embryo was associated with the formation of symplasmic domains corresponding to the four segments of the plant body. Using symplasmic tracers, we provided evidence that the changes in plasmodesmata permeability are corelated with somatic-to-embryogenic transition and somatic embryo development.