Victorin is a host-selective toxin produced by Cochliobolus victoriae the causal agent of victoria blight of oats. Victorin-binding and biochemical analyses have implicated the glycine decarboxylase complex (GDC), a key component of the photorespiratory cycle, as a possible site of action of the toxin. Plant dependence on a functional GDC is greatest under photorespiratory conditions and photorespiratory activity is increased with increasing light and decreased with increasing concentrations of CO 2 . We have found that both low light and increased CO 2 concentrations reduce the extent of victorin-induced symptom development in sensitive oats. These data provide physiological evidence that the GDC is a biologically significant site of action of victorin.