High levels of the cytokinin BAP induce PCD by accelerating senescence

The cytokinin 6-benzylaminopurine (BAP), if added at high dosage to plants and cultivated cell suspensions of Arabidopsis thaliana and Daucus carota, induce programmed cell death (PCD) by accelerating senescence. The Arabidopsis cells in culture express, at the end of subculturing cycle, when they start senescing, SAG12, a cysteine protease which is typically associated with leaf senescence and not found in other forms of programmed cell death (e.g. hypersensitive response). In Arabidopsis cell suspension cultures, cell death and DNA fragmentation, due to high levels of the cytokinin, is preceded by the early expression of SAG12. The perception of BAP is influenced by the physiological state of the cells: carrot embryos are resistant to the hormonal treatment whereas unorganized growing cells are not. The possibility of using suspension cell cultures as a model system for studying senescence is discussed.