Institute of Genetics, Academy of Sciences, Chisinu 2002, Moldova

Background and objectives
Specific inclusions, the so-called x-bodies, are known to be induced in tissue cells of tomato flowers infected with the potato virus X, and to be mainly localized in the cytoplasm of different parts of this plant organ. The appearance of such type inclusions can serve as a diagnostic sign of the disease.

Materials and methods
Potato virus X was used as an infection agent. For identification of this disease, tissue samples were fixed using immunocytological and electron microscope methods. Ultrathin sections were examined in an electron microscope EMV-100a. Experiments were performed on tomato flowers (Lycopersicon esculentumcv. Nistru) comprising petals, pistal, stamens and sepals. Plants were growing under conditions of virus pathogenisis.

Results and conclusions
Infected cells contained x-bodies which were subdivided into the following groups. (i) Simple polysome-like structures consisting of thin protein fibrils, on both sides of which ribosome-like particles are located. They may have a linear or horseshoe-shaped structure, or form rings. (ii) Tape-like structures containing no granules of ribosomes and consisting of parallel located membranes or fibrils. They are sometimes thinner in width than viral particles, they have an ungranulated structure, but are located in the form of bundles or irregularly in cytoplasm. (iii) Spirally twisted protein fibrils or membranes situated more-or-less parallel to each other, between which ribosome-like particles are found on ultrathin sections. These can have a strictly concentric arrangement. Ribosomes are observed between their fibrils and membranes, and sometimes these structures can adhere to each other, leading to the formation of electron-dense bodies. (iv) Combined x-bodies consisting of the second and third type of inclusions. They are found at advanced stages of pathogenesis with a high accumulation of the virus in the cell.

The above-mentioned intracellular specific inclusions induced with potato virus X were observed only in the cyptoplasm of infected cells, without surrounding membrane. Thus topographic contact between x-bodies and different cell organelles is likely to exist. In all probability, virus particles are an integral part of all the types of formation.