Abstract
In short-term experiments, cytidine-H3 was fed to rusted and healthy areas of wheat leaves. The incorporated activity, presumably residing in ribonucleic acid, was detected by microautoradiographic methods. Most of the label was found to be incorporated in host cell nuclei. Little incorporation occurred in extranuclear structures of host cells, including chloroplasts. Very long autoradiographic exposure times failed to reveal any incorporation into the fungus.Host cells in infected leaf areas contained considerably less label in their nuclei and cytoplasm than those in cells further from the site of infection. This effect of the fungus extended over some distance into uninvaded host tissue, but not beyond 100 μ from the periphery of the mycelium. The decreased cytidine incorporation in the affected host tissue is not caused by possible changes in pool size of endogenous cytidine. The significance of these results for the host–parasite interaction is briefly discussed.