Cell cycle dependence of elicitor-induced signal transduction in tobacco BY-2 cells

The molecular links between the cell cycle and defense responses in plants are largely unknown. Using synchronized tobacco BY-2 cells, we analyzed the cell cycle dependence of elicitor-induced defense responses. In synchronized cultured apoaequorin-expressing cells, the increase in cytosolic free Ca2+ induced by a proteinaceous elicitor, cryptogein, was greatly suppressed during the G(2) and M phases in comparison with G(1) or S phases. Treatment with cryptogein during the G(1) or S phases also induced biphasic (rapid/transient and slow/prolonged) responses in activation of mitogen-activated protein kinases (MAPKs) and production of reactive oxygen species (ROS). In contrast, elicitor treatment during the G(2) or M phases induced only a rapid and transient phase of MAPK activation and ROS production. Their slow and prolonged phases as well as expression of defense-related genes, cell cycle arrest and cell death were induced only after the cell cycle progressed to the G(1) phase; removal of the elicitor before the start of the G(1) phase inhibited these responses. These results suggest that although cryptogein recognition occurred at all phases of the cell cycle, the recognition during the S or G(1) phases, but not at the G(2) or M phases, induces the prolonged activation of MAPKs and the prolonged production of ROS, followed by cell cycle arrest, accumulation of defense-related gene transcripts and cell death. Elicitor signal transduction depends on the cell cycle and is regulated differently at each phase.