Ultrastructure of Tobacco Ringspot Virus‐Induced Local Lesions in Lima Bean Leaves

Abstract

AbstractTobacco ringspot virus (TRSV), a nepovirus, induced unusually large, expanding lesions in the primary leaves of lima bean (Phaseolus lunatus cv. Nemagreen). During the course of lesion development, two phases were identified on the basis of ultrastructural observations. In the primary phase, cytological changes followed a pattern that was characteristic of compatible or systemic nepovirus infections. Such changes included the formation of large, membranous inclusion bodies in the cytoplasm of mesophyll parenchyma and the accumulation of phytoferritin in the stroma of chloroplasts. The cells maintained their structural integrity and appeared metabolically active. The secondary phase was degradative in nature. Ultrastructural changes observed during this phase were representative of those that occur in a virus‐induced incompatible or hypersensitive (necrotic) reaction. Such changes included rounding of chloroplasts, vesiculation and disintegration of the plasma membrane and tonoplast, degeneration of cell organelles, and generalized plasmolysis. Subsequently, the cell walls collapsed, developing folds and loops around the shrunken cytosol. The collective result of these two ultrastructurally distinctive phases was the production of large necrotic lesions in the inoculated leaf, but no systemic viral spread.