Characteristics of the Inhibitory Effect of 5-Fluorodeoxyuridine on Cytodifferentiation into Tracheary Elements of Isolated Mesophyll Cells of Zinnia elegans

The inhibitory effect of 5-fluorodeoxyuridine (FdU) on the differentiationinto tracheary elements was characterized in isolated mesophyllcells of Zinnia elegans. Both thymidine and uridine counteracted the inhibitory effectof FdU on the differentiation into tracheary elements, whileonly thymidine was effective in counteracting the effect ofFdU on cell division. Higher concentrations of thymidine wereneeded for the restoration of the differentiation that was blockedby FdU than for the restoration of cell division. These resultssuggest that FdU prevents the differentiation via a mechanismthat is different from the inhibition of thymidylate (dTMP)synthase by fluorodeoxyuridine monophosphate (FdUMP), derivedfrom FdU, to which the blockage of cell division by FdU shouldbe attributable. The differentiation into tracheary elements was prevented whenFdU was added earlier than the 36th hour of culture, and thymidineovercame the inhibitory effect of FdU only when added withinthe first 4 h of culture. Pretreatment with FdU before applicationof 6-benzyladenine (BA) and 1-naphthaleneacetic acid (NAA),which are essential for the formation of tracheary elements,also inhibited the differentiation. Thus, the aspect of thedifferentiation that is the target of inhibition by FdU appearsto occur between the 4th hour and the 36th hour of culture andto begin even in the absence of exogenous plant growth regulators. (Received April 3, 1989; Accepted October 27, 1989)