The fine structure of local lesions induced by tobacco mosaic virus in tobacco

Abstract

Seven days after inoculation of Samsun NN tobacco ( Nicotiana tabacum L.) with tobacco mosaic virus (TMV), small pieces of leaf tissue were excised from the necrotic lesions, from the narrow band of surrounding tissue, from areas more distant than 2 mm from the lesion, and from uninoculated plants. Thin sections of appropriately prepared mesophyll tissue cut parallel to the leaf surface were examined in the electron microscope. Cells within the mature lesions were collapsed and devoid of vacuoles and of most identifiable membrane-bounded organelles, but contained numerous dispersed thylakoidal elements, ribosomes, and starch grains. Abundant TMV particles were seen in most cells of all lesions examined following fixation in acrolein or acrolein plus glutaraldehyde, but not in lesion tissue fixed in glutaraldehyde alone. Mesophyll cells in a narrow band surrounding the lesion showed structural features suggestive of metabolically active tissues. The changes were most marked in cells nearest the lesion edge but were evident in cells about 10 cell diameters beyond the lesion, where there was a gradation into normal-appearing tissue. Compared to normal mesophyll cells, cells within this zone had smaller vacuoles, more cytoplasm, more ribosomes and membrane-bounded organelles, and a very marked system of extensive, continuous elements of endoplasmic reticulum with which many of the ribosomes were associated. Spherosomes in normal mesophyll cells were always devoid of internal structure, but many of those in the zone peripheral to a lesion contained a single well-ordered crystalline body (minimal mean lattice spacing of 68 A). Compared to chloroplasts in normal mesophyll cells, those in this zone were more ameboid, had more ribosomes, were invested with large starch grains and lipid globules, and were more numerous. An occasional dividing chloroplast was seen in this area. It is suggested that the structural alterations in this band encircling a lesion are reflections of induced physiological changes related to the known capacity of these cells to resist virus encroachment.