Comparative Study of Accumulation of Two Potato Virus X S trains Differing in Virulence, Hydrolase Activity and Morphological Abnormalities in D atura stramonium L . Leaves

In young systemically infected leaves of D atura stramonium L ., a severe strain of Potato virus X ( PVX ) accumulated to a lower degree than a mild strain. Infected leaves had increased protease and RN ase activities in comparison with those of healthy controls. The highest hydrolase activities were found in leaves infected with the severe strain. Negative‐staining electron microscopy of dips from the infected leaves indicated that PVX virions underwent destructive changes, which resulted in the appearance of abnormal (swollen and ‘thin’) particles. Immuno‐electron microscopic assays showed that thin PVX particles, in contrast to those of normal diameter, lost the ability to bind with specific antiserum. The relative number of thin virions in leaves infected with the severe PVX strain was considerably higher than in leaves infected with the mild strain. This shows that a correlation exists between increased protease activity and intracellular destruction of virions. In abnormal virions, the viral RNA appears to be available for RN ase attack. Therefore, it seems that high RN ase activity together with increased generation of abnormal virions in the leaves infected with the severe strain promote inactivation of the viral RNA with RN ase. We suppose that the enhanced hydrolase activities in the leaves infected with severe PVX strain, on the one hand, limit viral accumulation and thus play a defensive role and, on the other hand, cause considerable intracellular pathological changes resulting in severe symptoms.