Cell wall differentiation during early somatic embryogenesis in plants. II. Ultrastructural study and pectin immunolocalization on chicory embryos

In Cichorium hybrid clone 474 (C. intybus L. var. sativum x C. endivia L. v ar. latifolia), direct somatic embryogenesis was induced from roots. Using transmission electron microscopy, we followed the ultrastructural changes o f the outer cell wall in relation to embryo developmental stage. During the transition from an embryogenic cell to a somatic embryo, the differentiati on of the outer cell wall involved both deposition and rearrangement proces ses. During the first divisions, the cell wall of few-celled embryos still enclosed in the root tissue appeared as a large amorphous layer of cellulos e, thicker than the cell walls of the root cortex cells. When the proembryo emerged from the root cells, the outer wall surface exhibited a fibrillar material designated as the supraembryonic network. As this network disappea red, the outer cell wall changed organization, and two domains were disting uished. At the torpedo stage, the outer cell wall was more compact without any gaps and the protoderm was differentiated. Immunolocalization of an epi tope recognised by JIM5 antibody revealed the unesterified nature of the su praembryonic network. Such pectins were also located at the outer third of the outer cell wall of protodermal cells as well as in the intercellular sp aces. Highly methylesterified pectins recognized by JIM7 antibodies were sl ightly present in the cell walls during the embryogenesis process. The diff erent stages of the outer cell wall differentiation as well as the developm ent of the transient supraembryonic network are described, and its possible roles in somatic embryogenesis are proposed.