On the origin of dark green tissue in tobacco leaves infected with tobacco mosaic virus.

Abstract In tobacco leaves showing a mosaic pattern caused by tobacco mosaic virus (TMV), yellow green tissue contains large amounts of virus. Dark green tissue contains very low concentrations of virus and appears cytologically normal. Two types of dark green tissue can be distinguished: “true” dark green which remains in this state for the life of the leaf, and “pseudo” dark green in which virus production begins at some late stage in leaf development. True dark green tissue does not appear to be able to support replication of the virus because (1) it is resistant to infection by mechanical means, (2) although the boundary is stable between a dark green area and a yellow green area, the protoplasmic connections between cells containing high and low concentrations of virus appear normal. On the dark green side of such boundaries there is a gradient of decreasing concentration of free virus rods extending laterally for at least several cells into the dark green tissue; such boundaries remain stable for weeks. Dark green areas appear only in leaves which were about 2.4 cm long or smaller at the time they were invaded by virus. The third or fourth leaf above the inoculated leaf is the first to show mosaic under our conditions. In this leaf dark green islands must have arisen, at the time of virus invasion, from clusters of several hundred cells extending through all histological layers of the lamina. Clusters of this size could not have arisen by chance. Therefore, some as yet unidentified diffusible agent must be involved in the formation of these clusters.

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