Studies of Vegetative Compatibility-Incompatibility in Higher Plants. II. A Structural Study of an Incompatible Heterograft between Sedum telephoides (Crassulaceae) and Solanum Pennellii (Solanaceae)

Abstract

An ontogenetic study of a shoot heterograft between Sedum telephoides and Solanum pennellii was performed in order to characterize the cellular events that occur during an incompatible graft union. Adhesion of stock and scion occurred by 21 hr after grafting and was correlated with an accumulation of dictyosomes in cells at the graft interface. The graft incision triggered a wound response in cells of both partners along the graft interface that was characterized by: 1) a reorientation of cytoplasm in cells adjacent to the cut surfaces; 2) deposition of cell wall; 3) starch accumulation; 4) a reduced staining intensity of the cytoplasm, and 5) callus proliferation. While Solanum cells recovered from the effects of wounding at an early and nonlethal stage, Sedum cells bordering the graft interface underwent a lethal cellular senescence characterized by: 1) cell wall suberization; 2) vesiculation of the cytoplasm; 3) degeneration of cellular organelles; 4) loss of membrane integrity, and finally 5) death and collapse of the cell. Thus, cellular necrosis in the Sedum partner characterized the incompatibility response between Sedum and Solanum and resulted in formation of a necrotic layer of collapsed cells and cytoplasmic remnants at the graft interface. This necrotic layer was never ruptured by callus proliferation and consisted of up to 8–10 collapsed cells by 10–14 days after grafting. Vascular redifferentiation did not occur in the callus masses at the graft interface. Sedum abscission occurred by 3–5 wk after grafting, presumably due to desiccation of tissues. Both Sedum and Solanum stocks grafted to wooden applicator sticks in the same manner they grafted to a living scion, indicating that initial adhesion of the graft partners did not involve a recognition event between opposing living cells. The possible roles of toxins that may elicit the incompatibility response and of a direct cellular interaction that may contribute to procambial redifferentiation are discussed.