Cellular interactions during the formation of approach grafts in Sedum telephoides (Crassulaceae)

The objective of this study was to determine the competence of epidermal and subepidermal cells to graft. Approach grafts between cut internodal surfaces of Sedum telephoides were characterized by extensive cellular interdigitation at the graft interface. Callus proliferation, but not tissue interdigitation, occurred when these surfaces were separated by a porous or impermeable barrier. Graft partners adhered to the barrier via the deposition and subsequent polymerization of cell wall materials. The outer walls of cells contacting the barriers were of uniform thickness and lacked plasmodesmata. When a cut surface was grafted to an intact surface, callus proliferation occurred at the cut surface, but the epidermis did not dedifferentiate. Callus masses at cut surfaces grafted successfully in environments of high humidity. However, callus masses covered by a water stress induced periderm did not graft successfully. These results indicate that (i) intact epidermal cells of S. telephoides are incompetent to graft, (ii) the presence of a graft-competent (i.e., cut) surface does not cause epidermal cells to dedifferentiate (i.e., become competent to graft), and (iii) graft competence of compatible, subepidermal surfaces depends on the absence of a periderm, which is facilitated by low moisture stress.