Camptothecin, a topo isomerase-I inhibitor used in cancer therapy, induces apoptosis in animal cells. In tomato (Lycopersicon esculentum Mill.) suspension cells, camptothecin induces cell death that is accompanied by the characteristic nuclear morphological changes such as chromatin condensation and nuclear and DNA fragmentation that are commonly associated with apoptosis in animal systems. These effects of camptothecin can effectively be blocked by inhibitors of animal caspases, indicating that, in tomato suspension cells, camptothecin induces a form of programmed cell death (PCD) with similarities to animal apoptosis (A.J. De Jong et al. (2000) Planta 211:656-662). Camptothecin induced cell death was employed to study processes involved in plant PCD. Camptothecin induced a transient increase in H2O2 production starting within 2 h of application. Both camptothecin-induced cell death and the release of H2O2 were effectively blocked by application of the calcium-channel blocker lanthanum chloride, the caspase-specific inhibitor Z-Asp-CH2-DCB, or the NADPH oxidase inhibitor diphenyl iodonium, indicating that camptothecin exerts its effect on cell death through a calcium- and caspase-dependent stimulation of NADPH oxidase activity. In addition, we show that ethylene is an essential factor in camptothecin-induced PCD. Inhibition of either ethylene synthesis or ethylene perception by L-alpha-(2-aminoethoxyvinyl)glycine or silver thiosulphate, respectively, blocked camptothecin-induced H2O2 production and PCD. Although, in itself, insufficient to trigger H2O2 production and cell death, exogenous ethylene greatly stimulated camptothecin-induced H2O2 production and cell death. These results show that ethylene is a potentiator of the camptothecin-induced oxidative burst and subsequent PCD in tomato cells. The possible mechanisms by which ethylene stimulates cell death are discussed.

A new system to study programmed cell death in plants is described. Tomato (Lycopersicon esculentum Mill.) suspension cells were induced to undergo programmed cell death by treatment with known inducers of apoptosis in mammalian cells. This chemical-induced cell death was accompanied by the characteristic features of apoptosis in animal cells, such as typical changes in nuclear morphology, the fragmentation of the nucleus and DNA fragmentation. In search of processes involved in plant apoptotic cell death, specific enzyme inhibitors were tested for cell-death-inhibiting activity. Our results showed that proteolysis plays a crucial role in apoptosis in plants. Furthermore, caspase-specific peptide inhibitors were found to be potent inhibitors of the chemical-induced cell death in tomato cells, indicating that, as in animal systems, caspase-like proteases are involved in the apoptotic cell death pathway in plants.

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Detached tobacco leaves were infiltrated with an AT toxin preparation from the foliar pathogen Alternaria alternata tobacco pathotype. The AT toxin preparation caused formation of necrotic lesions within 5 days post-infiltration in a concentration-dependent manner. Cell death was accompanied by increased levels of the stress metabolites hydrogen peroxide, malondialdehyde, free proline and by enhanced total protease activity. Lesion development and the production of stress metabolites were suppressed if the infiltration site was pre-infiltrated with caspase-specific peptide inhibitors (irreversible caspase-1 inhibitor acyl-Tyr-Val-Ala-Aspchloromethylketone (Ac-YVAD-CMK) and the broad range caspase inhibitor benzyoxycarbonyl-Asp-2,6-dichlorobenzoyloxymethylketone (Z-Asp-CH2-DCB)), the serine protease inhibitor Na-p-tosyl-l-lysine chloromethylketone and the polyamine spermine. Extensive accumulation of reactive oxygen species (ROS), as determined by staining with 3-3¢-diaminobenzidine and 2¢,7¢-dichlorofluorescein diacetate, was found in the AT toxin-challenged lesions. The data show that AT toxininduced cell death in tobacco is a type of programmed cell death in which caspase-like proteases and ROS signalling play a prominent role.