Tracheary element differentiation in Zinnia mesophyll cell cultures

Mechanically isolated mesophyll cells of Zinnia elegans differentiate into tracheary elements (TEs) when cultured in a medium containing adequate auxin and cytokinin. Differentiation in this culture system is relatively synchronous, rapid (occuring within 3 days of cell isolation) and efficient (with up to 65% of the mesophyll cells differentiating into TEs), and does not require prior mitosis. The Zinnia system has been used to investigate (a) cytological and ultrastructural changes occurring during TE differentiation, such as the reorganization of microtubules controlling secondary wall deposition, (b) the influences of calcium and of various plant hormones and antihormones on TE differentiation, and (c) biochemical changes during differentiation, including those occurring during secondary wall deposition, lignification and autolysis. This review summarizes experiments in which the Zinnia system has served as a model for the study of TE differentiation.