Cytoplasmic inclusions in cells infected with isolates of L and I serogroups of tomato spotted wilt virus

A comparative analysis of the cytopathology of Nicotiana benthamiana infected with representative isolates of two serogroups of the tospovirus genus of the Bunyaviridae (formerly tomato spotted wilt virus) revealed differences in the accumulation of cytoplasmic inclusions. Transmission electron microscopy of a common isolate (L) of tomato spotted wilt virus (TSWV) from tobacco, TSWV-Lgt, revealed electron-dense areas (viroplasm), single quasispherical particles, and cisternae-bound clusters of mature virions. In addition, filaments arranged in loose bundles were found in a common isolate from lettuce, TSWV-Llet. Tissue infected with an isolate that has a serologically distinct nucleocapsid protein, TSWV-I, showed three differences in cytopathology. TSWV-Igg, from gloxinia, rarely contained particles and had no cisternae-bound mature virions. Filaments were uniquely arranged in layered, paracrystalline arrays. TSWV-Igg also had two forms of electron-dense areas: viroplasm and an area denser and not associated with membranes that we have labeled an amorphous inclusion. Immunogold labeling of tissue infected with TSWV isolates, with antiserum to whole virus, demonstrated that the electron-dense areas previously designated as viroplasm were viral protein. However, viral antibodies did not label paracrystalline arrays of filaments or the nonmembrane-bound amorphous inclusions. The arrangement of filaments and possible amorphous inclusions should be noted as distinguishing characteristics of the I serotype