Benzyladenine arrests cell cycle progression in G1 phase in tobacco BY-2 cells preceding induction of cell death

Abstract

Micromolar concentrations of exogenously applied cytokinins can inhibit the growth of tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2) cells and induce cell death. To determine whether the antiproliferative and cell death-inducing mechanisms of cytokinins are linked, we investigated the inhibition of cell cycle transition by the cytokinin benzyladenine using BY-2 cells. Mitotic index and flow cytometric analyses revealed that benzyladenine decreased the rate of cells entering the G2 and M phases a few hours after treatment of unsynchronized cells. Additionally, for cells synchronized in M phase, benzyladenine delayed (at 10 mM) or arrested (at 50 mM) cell cycle progression at G1. Expression patterns of cell cycle-related genes (PCNA and A3-, A1-and B1-type cyclins) also indicated G1 arrest by a 50 mM benzyladenine treatment at the M or G1 phase. Cell cycle arrest was detected prior to the induction of cell death by the treatment. Increase in number of dead cells was observed 16 h after each treatment at M or G1, suggesting that cell death may not be induced when cells reach a specific time point in G1 phase but, rather, in a time-dependent manner following benzyladenine treatment.Key words: Cell cycle, cytokinin, Nicotiana tabacum BY-2 cells, programmed cell death, synchronous culture.Plant Biotechnology 26, 225-235 (2009) Original PaperAbbreviations: DAPI, 4Ј,6-diamidino-2-phenylindole; DMSO, dimethyl sulfoxide; FAM-Asp-Glu-Val-Asp-FMK, fluoromethyl ketone peptide inhibitor of caspase-3/7; FLICA, fluorochrome inhibitor of caspase; G1, gap 1; G2, gap 2; M, mitosis; NO, nitric oxide; PCD, programmed cell death; ROS, reactive oxygen species; S, synthesis. This article can be found at http://www.jspcmb.jp/ Evidence also suggests that cytokinins induce cell death in plants via biochemical pathways involving the activation of caspase-like proteases, production of reactive oxygen species (ROS) or nitric oxide (NO), release of cytochrome c from mitochondria, and upregulation of Bax inhibitor-1 expression and cytosolic Ca 2ϩ fluxes, which are similar to those of animal apoptosis. Therefore, cytokinin-mediated cell death is considered to be a programmed cell death (PCD) with conserved pathways in plants and animals, which is also observed after stress treatments such as heat shock (Vacca et al. 2004(Vacca et al. , 2006 or hydrogen peroxide (Houot et al. 2001). However, the pathways through which cytokinins induce cell death are mostly unknown. Moreover, there remains one problem that whether the antiproliferative mechanism, which seems to occur simultaneously with the cell death, is correlated to the induction of cell death.Recent studies have suggested that the cell cycle regulates the induction of cell death. Oxidative stress mediated by menadione slows DNA replication and delays cellular entry into mitosis by inhibiting the G1 to S phase transition, and a high concentration of menadione induces cell death in tobacco BY-2 cells (Reichheld et al. 1999). Ethylene treatment increases BY-2 cell mortalit...

Should I fight or should I grow now? The role of cytokinins in plant growth and immunity and in the growth–defence trade-off

“…It would be interesting to know whether the activation of these CDK proteins is dependent on the transcriptional regulation of CYCLIN D3 by cytokinins. It is noteworthy that cytokinins have also been implicated in the control of HR-like programmed cell death, through mechanisms that may involve cell cycle control (Suda et al, 2009;Novak et al, 2013).…”