Tissue from symptom-bearing young leaves of plants systemically infected with citrus exocortis viroid (CEV) was investigated with the electron microscope. It was found that, in contrast to a previous report, the appearance of paramural bodies or plasmalemmasomes cannot be considered to be the primary c.p.e. in because these structures are also present at the same frequency in healthy control plants. The plasmalemmasomes in healthy tissue, characterized by vesicular or tubular internal structures, are found at different developmental stages and in the process of spreading between the plasmalemma and cell wall of developing young cells. The viroid-specific cytopathic changes are alterations of the plasmalemmasomes themselves, which may make it difficult to relate them to the corresponding normal structures. The malformed plasmalemmasomes were consistently found in association with the extremely distorted cell walls characteristic of viroid-infected tissue; spreading of plasmalemmasomes was not observed. These findings indicate an intimate functional relationship between plasmalemmasomes and cell wall formation which is disturbed as a consequence of viroid infection.


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