CHANGES IN MICROTUBULAR ORGANIZATION MARK THE TRANSITION TO ORGANIZED GROWTH DURING ORGANOGENESIS IN PETUNIA-HYBRIDA

We have studied microtubular organization during the induction of organogenesis in callus derived from leaf protoplasts of Petunia hybrida, line PC6. The conditions for regeneration have been determined in detail and it is possible to obtain and compare calli which differ in their competence to regenerate. The main conclusions are: (1) that dividing cells in non-polarized callus have a low relative amount of preprophase bands (PPB-index), regardless of their potential to regenerate; (2) that this low PPB-index is due to the absence of PPB's in preprophase and early prophase; (3) that a 3-5-fold increase in the PPB-index marks the transition to organized growth; (4) that the interphase cytoskeleton is unordered in the non-organized callus and that ordered, helical arrays of interphase microtubules do not appear before polarized division has been restored. © 1990.