Extracellular RNase activity in healthy and rust infected wheat leaves

Abstract

Intercellular washing fluids (IWFs) from six spring wheat cultivars were tested for RNase activities. IWFs from all of the cultivars showed relatively high RNA-splitting activity. The activities of the IWFs were raised by 25–55 % after infection with stem or stripe rust in compatible combinations. The incompatible combinations resulted only in a 10–17 % increase in RNase activity. Polyacrylamide gradient gel electrophoresis of the IWFs revealed up to six bands with RNase activity. In IWFs from infected leaves in susceptible combination the activity of bands of about 12 and 16 kD was raised. There were only slight changes in the activity of the higher molecular weight bands, except that the band of about 100 kD in the IWFs from the cultivar Arkas showed a very characteristic increase of activity after infection of the leaves. Moreover, rust infection increased not only the activity but also the pH optimum of the same RNase enzymes. Comparing the RNA-splitting pattern of the IWFs with that of the leaf homogenates in gel, the differences were more pronounced in the case of the infected plants. In homogenates of susceptible leaves after infection not only was the increase in total RNase activity much higher than in the IWFs, but besides the low molecular weight bands, the activities of bands of about 75 kD and of 20–30 kD were strengthened. On the other hand, the much higher specific RNase activity (calculated on protein basis) of the IWFs, as compared to the homogenates, suggests that RNases found in the IWFs are located in the cell wall and in the outer cell membranes. The significance of the extra- and intracellular RNases in rust infection is discussed.