CHANGES IN 2 BARLEY LEAF RIBONUCLEASE FRACTIONS DURING INFECTION BY THE POWDERY MILDEW FUNGUS

We have isolated two ribonuclease (RNase) fractions from barley leaves (Hordeum vulgare L.) of variety Prior which is susceptible to race 3 of the powdery mildew fungus (Erysiphe graminis f. sp. hordei). These are termed soluble RNase representing the major RNase fraction of these leaves and pH 5 insoluble RNase which is quantitatively a minor RNase component and is apparently associated with ribosomes. These two enzymes are quite different from each other with respect to the effects of various reagents and in their substrate preference as determined by the relative rates of hydrolysis of polyadenylic, polyuridylic and polycytidylic acids. Both enzymes exhibit a predominantly endonucleolytic mode of action although the patterns of yeast RNA hydrolysis by the two enzymes are different. These RNases undergo remarkable changes in their substrate specificity during the infection of barley leaves by the powdery mildew fungus. The changes in the two enzymes are detectable at different stages of disease development. The catalytic properties of the pH 5 insoluble RNase from inoculated barley leaves at the early and later stages of disease development are quite different from those of the enzyme from healthy leaves and the mildew fungus. These changes in the properties of the pH 5 insoluble RNase in barley leaves represent one of the earliest detectable biochemical symptoms of mildew infection. © 1979.