Graduate Thesis Or Dissertation
 

Evidence for the involvement of a mitochondrial permeability transistion in a victorin-Induced cell death

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  • Production of the host selective toxin victorin is causal to pathogenesis of Cochliobolus victoriae on oats. The dominant Vb gene confers oat sensitivity to victorin, and is genetically inseparable from Pc2, which confers resistance to Puccinia coronata f. sp. avena. Victorin induces apoptotic-like cell death, and cell death is a component of resistance to P. coronata. Thus, the purpose of my research was to characterize victorin-induced cell death to provide insight into both disease susceptibility and resistance. In animals, the mitochondrial permeability transition (MPT), which results in a collapse of mitochondrial transmembrane potential (ΔΨₘ), is a central regulator of apoptosis. Victorin binds in a genotype-specific manner in vivo to the mitochondrial matrix-localized P-protein. Because isolated mitochondria are impermeable to victorin and victorin induces apoptotic-like death, we hypothesized that a IVIPT accounts for in vivo binding to the P-protein and is involved in victorin-induced death. Isolated oat mitochondria were demonstrated to undergo Ca²⁺-mediated high-amplitude swelling, which exhibits size-exclusion, and can result in the release of cytochrome c. These characteristics are consistent with a MPT. The oat MPT did result in victorin binding to the P-protein. In vivo, victorin induced a collapse of ΔΨₘ. The collapse of AWm was preceded by a loss of mitochondrial motility that was likely due to an increase in cytosolic Ca²⁺. Collapse of ΔΨₘ preceded cell shrinkage, which occurred without the loss of tonoplast or plasma membrane integrity. Cell shrinkage occurred concomitantly with other markers of cell death. After cell shrinkage, victorin entered the cell and accumulated in mitochondria. In cells induced to shrink by 5 mM AOA, victorin entered the cytosol but not the mitochondria. Considering that 1) the oat MPT in vitro allows victorin access to the matrix; 2) victorin in vivo induces a collapse in ΔΨₘ indicative of MPT, and binding to the Pprotein; and 3) cell shrinkage alone does not allow victorin access to the matrix, we conclude that victorin induces a MPT in vivo. The timing of the victorin-induced MPT is poised to be a key regulator of PCD, and the retention of membrane integrity after cell shrinkage likely influences the local host-pathogen environment.
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