The role of peroxidase isozymes in resistance to wheat stem rust disease

In common with other disease situations, rust-resistant wheat leaves show a large increase in peroxidase activity during infection. Peroxidase isozymes from healthy or infected lines of wheat (Triticum aestivum L.) near isogenic for resistance and susceptibility to race 56 of Puccinia graminis tritici were separated by gel electrophoresis and the activity of each was estimated by photometric scanning. In order to ensure that the activity of isozymes observed on gels reflected the changes found in peroxidase enzymes assayed spectrophotometrically in extracts, a study was made of extraction procedures, substrates, and reaction conditions for both types of enzyme measurements. Of the 14 isozymes detected in both healthy and infected leaves, increases in only 1 (isozyme 9) were associated consistently with the development of resistant disease reaction at 20 C. Additional evidence was obtained to show that this isozyme can account for the increased peroxidase activity observed in extracts from resistant plants. When plants with high induced peroxidase activity due to resistance at 20 C were treated with ethylene or transferred to 25 C, they reverted to complete susceptibility. However, the disease-induced activity of isozyme 9 did not fall. The data suggest that, in this case, the association of peroxidase with resistance was a consequence of, not a determinant in, resistance.