The host-selective toxin victorin is produced by Cochliobolus victoriae, the causal agent of victoria blight of oats. Victorin has been shown to bind to the P protein of the glycine decarboxylase complex (GDC) in mitochondria, and induce defense-related responses such as phytoalexin synthesis, extracellular alkalization and programmed cell death. However, evidence demonstrating that the GDC plays a critical role in the onset of cell death is still lacking, and the role of defense-like responses in the pathogenicity has yet to be elucidated. Here, cytofluorimetric analyses, using the fluorescein (VicFluor) or bovine serum albumin-fluorescein derivative of victorin (VicBSA), demonstrated that victorin-induced cell death occurs before these conjugates traverse the plasma membrane. As with native victorin, VicBSA clearly elicits apoptosis-like cell death, production of phytoalexin, extracellular alkalization, and generation of nitric oxide and reactive oxygen intermediates. These results suggest that the initial recognition of victorin takes place on the cell surface, not in mitochondria, and leads to the activation of a battery of victorin-induced responses. Pharmacological studies showed that extracellular alkalization is the essential regulator for both victorin- and VicBSA-induced cellular responses. We propose a model where victorin may kill the host cell by activating an HR-like response, independent of the binding to the GDC, through ion fluxes across the plasma membrane.

We have developed an oat cell-free apoptosis system to investigate the execution mechanisms of plant apoptosis. Cell extracts derived from oat tissues undergoing toxin (victorin)-induced apoptosis caused nuclear collapse and internucleosomal DNA fragmentation in isolated nuclei. Pharmacological studies revealed that cysteine protease, which is E-64-sensitive but insensitive to caspase-specific inhibitors, is a crucial component in the morphological change of isolated nuclei, and that nuclease and the cysteine protease act cooperatively to induce the apoptotic DNA laddering. Interestingly, this finding is contrasted with those in well-studied animal cell-free systems in which an apoptotic endonuclease is solely responsible for the DNA fragmentation.

These results suggest that RA inhibits bFGF cell proliferation in hRPE cells by decreasing the bFGF receptor number.