WHY ARE QUIESCENT MESOPHYLL PROTOPLASTS FROM NICOTIANA-SYLVESTRIS ABLE TO REENTER INTO THE CELL-CYCLE AND REINITIATE A MITOTIC-ACTIVITY

Mesophyll protoplasts of Nicotiana sylvestris incubated in an adequate culture medium re-enter very rapidly into the cell cycle and divide. The transition G0/G1 is accompanied by a complete reversion of the pro gram of gene expression. The program of the photosynthetic differentia ted mesophyll cell is abolished whereas a new multipartite program of a highly stressed but ready-to-divide cell is established. Some genes encode proteins which structure suggests they may play key roles in th ese events. Most of the induced genes are under multiple controls: str ess and/or development. Stress response and cellular re-organization m ight thus be closely related events that cannot be dissociated. It is probable that the re-entry of a protoplast into the cell cycle, ie the initial step of totipotency, closely depends on the coordinated activ ation of a set of genes that share common regulatory mechanisms.