The Hypersensitive Reaction in Tobacco Leaf Tissue infiltrated with Pseudomonas pisi 5. Inhibition of RNA Synthesis in Mesophyll Cells

The synthesis of mesophyll cell RNA in tobacco leaf tissue infiltrated with the incompatible bacterium Pseudomonas pisi was investigated by light and electron microscope autoradiography of H3‐uridine uptake. Interpretation of the light microscope quantitative data was complicated by the oscillation in levels of cytoplasmic RNA synthesis that resulted from the physical process of fluid infiltration (seen in the control tissue). Direct comparison with the control showed that the presence of bacteria resulted in a rapid decrease in the synthesis of host cell RNA. This effect was clear within the first 1½h (induction period) of the hypersensitive reaction, where it was particularly marked in the cytoplasm of palisade cells. Electron microscope autoradiography showed that the presence of bacteria did not cause complete cessation of RNA synthesis in mitochondria and chloroplasts. The early inhibition of RNA synthesis preceeded fine structural (degenerative) change, and should be regarded as one of the primary events associated with the induction of host cell necrosis.