Abstract:
Victoria blight of oats is caused by the fungus Cochhobolus victoriae. This fungus is pathogenic due to its ability to produce the host-selective toxin victorin. Previously, a 100-kD protein that binds victorin in vivo only in susceptible genotypes was identified as
the P protein of the glycine decarboxylase complex (GDC). Victorin is a potent in vivo inhibitor of GDC. Leaf slices pretreated with victorin displayed an effective Victorin inhibited the
concentration for 50% inhibition (EC₅₀) of 81 μM for GDA.
glycine-bicarbonate exchange reaction in vitro with an EC₅₀ of 23 μM. We also
identified a 15-kD mitochondrial protein in susceptible and resistant genotypes that
hound victorin. Amino acid sequence analysis indicated this protein is the H protein component of the GDC. Thus, victorin specifically binds to two components of the GDC.
Victorin had no detectable effect on GDC in isolated mitochondria, apparently due to the inability of isolated mitochondria to import victorin. The interaction of victorin with the GDC may be central to victorin's mode of action. Supporting this observation is the
finding that CO₂ gives partial protection against victorin. Elevated CO₂ is known to
ameliorate the effect of GDC inhibition. Victorin treated plants incubated in the light
develop more severe symptoms than dark-incubated plants. Victorin appears to induce a
plant-wide signal transduction cascade, resulting in diverse effects. Victorin induces
specific proteolytic cleavage of the Rubisco large subunit (LSU). Leaf slices incubated
with victorin for 4 hours in the dark accumulate a form of LSU which is cleaved after the N-terminal lysine 14. LSU cleavage in leaf slices is prevented by the protease inhibitors E-64 and calpeptin. LaCl₃ prevents this cleavage of LSU and LaCl₃ also confers complete protection against victorin at the whole plant level. Victorin also causes lipid peroxidation as measured by MDA accumulation. DNA laddering is seen in leaves after
3 hr treatment with toxin. The ethylene inhibitors AOA and STS give significant protection against victorin at the whole plant level, and also prevent LSU cleavage.
