An Electron Microscope Study of Starch Lesions in Cucumber Cotyledons Infected with Tobacco Mosaic Virus. J. Cohen, Virus Laboratory, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel; G. Loebenstein, Virus Laboratory, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. Phytopathology 65:32-39. Accepted for publication 18 July 1974. DOI: 10.1094/Phyto-65-32.
Starch lesions in cucumber cotyledons became evident under the light microscope 2.0 - 2.5 days after inoculation. At this time, many of the chloroplasts were markedly swollen, without necessarily containing starch grains. At 21 C, lesions reached their final size 5 days after inoculation.
Paradermal sections taken through the palisade layer 5 days after inoculation revealed tobacco mosaic virus (TMV) particles in two types of cells.
In central cells of the lesion, chloroplasts were extremely swollen, and contained large starch grains. Virus particles, scattered or in crystalline arrays, were present in the cytoplasm. At the periphery of the lesion about half the cells, although containing virus, were otherwise normal. The others carried virus and contained chloroplasts with large starch grains. Structures, resembling plasmodesmata, connect these cells with neighboring normal noninfected cells. Observations were similar when lesions were sampled 7-8 days after inoculation.
The finding that in a mature lesion virus-carrying, though otherwise normal-appearing, cells directly border noninfected cells, suggests that in cucumber cotyledons localization of TMV is not due to ultrastructural changes in advance of the infection, or to blocking of plasmodesmata.