POSSIBLE INVOLVEMENT OF DNA-REPAIR EVENTS IN THE TRANSDIFFERENTIATIONOF MESOPHYLL-CELLS OF ZINNIA-ELEGANS INTO TRACHEARY ELEMENTS

In cultures of isolated mesophyll cells of Zinnia elegans, transdiffer entiation into tracheary elements is induced by a combination of auxin and cytokinin and is blocked by inhibitors of DNA synthesis and poly (ADP-ribose) synthesis. During transdifferentiation, a very low level of synthesis of nuclear DNA was found in some cultured cells by microa utoradiography after pulse-labeling with [H-3]thymidine. Density profi les of nuclear DNA that had been double-labeled in vivo with bromodeox yuridine (BrdU) and [H-3]thymidine indicated that this DNA synthesis w as repair-type synthesis. The sedimentation velocity of nucleoids incr eased during the culture of isolated mesophyll cells and the increase was dependent on phytohormones. This phenomenon may reflect the rejoin ing of DNA strand breaks after repair-type DNA synthesis during transd ifferentiation. Treatment of cells with inhibitors of DNA synthesis or of poly(ADP-ribose) synthesis prevented the increase in the sedimenta tion velocity of nucleoids. The data suggest the involvement of DNA-re pair events in the transdifferentiation of mesophyll cells into trache ary elements.