Callus Formation and Organogenesis from Cultured Leaf Segments of Crassula argentea: Cytokinin-Induced Developmental Pattern Changes

Culturing leaf segments on agar medium with a final concentration of 10–4m isopentenyl adenine inhibits regeneration of plantlets, but stimulates some segments to produce regions of dense callus. The callus is initiated from the basipetal end of a cut vascular bundle, and is therefore the positional equivalent of root initiation during normal regeneration. Once callus has emerged, it forms a sphere of tissue attached to the leaf segment. The cells on the surface of the sphere are smaller than the cells toward the center. Some of these larger cells contain tannins or large starch grains. Shoots are sometimes initiated from organized meristematic regions of surface cells. Roots form endogenously from callus after shoot formation. This regeneration sequence, initially induced by cytokinin treatment, is strikingly dissimilar to the normal pattern, and provides an excellent example of a hormone-induced alteration to a developmental pattern.