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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