Association of Mycoplasmalike Organisms (MLOs) with Mild Type of Hydrangea virescence: A Study with 60–1000 nm Thick Sections

The study of the agent associated with the mild type of Hydrangea virescence in France involved three steps, with the aid of transmission electron microscope (TEM). In the first step, we observed the presence of polymorphic procaryotes in thephloem sieve tubes of diseased plants and their absence from corresponding healthy plant parts. The procaryotes were detected in the areas suspected in 1000 nm thick sections stained with thionin-acridine orange. In the next step, the ultrastructure of their unit membrane was studied at magnifications higher than 100 000. The two osmiophilic layers of the membrane were 6 nm distant and no preliminary parietal shape was detected. These observations on ultrathin (60 nm) sections allowed us to classify the, particles into the class “Mollicutes”. The third step involving the examination of 350–1000 nm thick sections revealed the absence of spiral forms. The TEM observations are consistent with the hypothesis that the agents associated with the mild type of Hydrangea virescence observed in France should be included within the MLO group. A method specially adjusted to the fixation of MLO inside sieve tubes has been mentioned.