Cells of five grass and cereal species infected with cocksfoot mottle virus, phleum mottle virus or cocksfoot mild mosaic virus were examined in ultra-thin sections. The intracellular distribution of the virus particles and their effects on cell ultrastructure appeared dependent more on the severity of the host/virus interaction than on the particular virus or host species involved. Particles of all three viruses occurred in the cytoplasm of mesophyll and phloem companion cells. Those of cocksfoot mottle virus were observed occasionally also in the nucleus. Relatively few particles, randomly distributed in the cytoplasm, were present in cells of plants with mild symptoms. In plants with severe symptoms the particles were numerous and often in near- or true-crystalline aggregates. In partially disrupted cells, virus particles occurred occasionally in membrane-bound, sometimes vesiculated, packets in the cell vacuole. These seemed to originate from the tonoplast. In more severely disrupted cells the tonoplast was lost and virus particles were scattered randomly throughout the vacuole.

Ultrastructural effects ranged from an increase in endoplasmic reticulum and vacuolation of the cytoplasm in the cells of plants with mild symptoms, to the total disintegration of all cell organelles in plants with the severest symptoms. Extrusions, which often encircled a volume of cytoplasm developed from the chloroplasts and mitochrondia; membrane-bound vesicles, containing virus particles, appeared in the stroma of chloroplasts and in the mitochondria.


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