Particularly useful for the study of tracheary element differentiation is the Zinnia elegans experimental system. In this system, isolated mesophyll cells nurtured in cultures containing auxin and cytokinin can be made to dedifferentiate and redifferentiate into tracheary elements (Figure 27–23). The tracheary element maturation phase, which involves secondary cell wall formation and programmed cell death, is preceded by a rapid increase in concentrations of brassinosteroids, which are necessary for this
final stage of tracheary element differentiation.