VIEW ARTICLE

The Ultrastructure of Local Lesions Induced by Potato Virus X: a Sequence of Cytological Events in the Course of Infection. A. V. Allison, Department of Plant Pathology, University of California, Davis 95616; T. A. Shalla, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 64:784-793. Accepted for publication 20 December 1973. DOI: 10.1094/Phyto-64-784.

Expanding necrotic lesions induced by potato virus X (PVX) in leaves of Gomphrena globosa were studied by fluorescence and electron microscopy. When lesions and their marginal tissues were sampled radially, it was possible to determine the sequence in which some infection products were formed. Amorphous inclusion body components, typical of those induced by PVX in systemic host plants, were observed only in cells close to the necrotic center of the lesions, while virus particles were found at greater radial distances in the cytoplasm and plasmodesmata. Abnormal deposits of callose were distributed, primarily in pit areas, to greater radial distances than either inclusion body components or virus particles in the cytoplasm. Complex plasmodesmata and membrane-bound bundles of virus particles embedded in callose outside the protoplast, paralleled the distribution of heavy callose deposits. The extraprotoplasmic sacs of virus in adjacent cells were often connected by plasmodesmata. Since inclusion body components were formed relatively late in the course of infection, it is concluded that they do not function as virus factories. It is proposed that callose deposition and exclusion of virus particles from the protoplast may constitute a mechanism for limiting the spread of virus in hypersensitive tissue.