Leaves ofPhaseolus vulgarisvar. Pinto were inoculated with the U1strain of tobacco mosaic virus TMV (U1) and fully expanded lesions and adjacent healthy tissue were examined in the electron microscope. Emphasis was placed on the band of healthy cells (resistant zone) surrounding the lesion, with the object of detecting the first changes in ultrastructure as healthy tissue graded into the infected area. Cells in the resistant zone were characterized by the appearance of membrane-bound vesicular bodies (paramural bodies) between the plasmalemma and cell wall. Where paramural bodies accumulated, the plasmalemma was withdrawn and intercellular cytoplasmic connections through the plasmodesmata were severed. These changes were found most frequently for a distance of about three cell diameters beyond cells visibly infected at the lesion periphery. It is suggested that these changes in ultrastructure are related to the events of localization. Spread of the virus may be inhibited because of a lack of cytoplasmic connections between cells surrounding the virus-induced lesion.
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