Symptoms and Electron Microscopy of Ryegrass Mosaic Virus in Different Grass Species

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Abstract
A virus from Agrostis stolonifera (tribe: Aveneae) with filamentous particles about 685 nm long was serologically closely related to, and considered to be a strain of, ryegrass mosaic virus (RMV). This strain caused symptoms in Polypogon monspeliensis (tribe: Aveneae) more readily and in Lolium multiflorum (Italian ryegrass) less readily than did RMV from Lolium and Festuca spp. (tribe: Festuceae). In L. multiflorum and P. monspeliensis the Agrostis isolate induced pinwheel inclusions with associated laminar aggregates and tubes, but Lolium isolates induced pinwheels with laminar aggregates only. The intracellular distribution of the pinwheels differed with the severity of host response. Thus, in plants with mild symptoms most pinwheels were contiguous with the plasmalemma close to plasmodesmata, but in plants with severe symptoms the pinwheels were free in the cytoplasm. Virus particles occurred either randomly or in bundles in the cytoplasm of mesophyll and phloem companion cells. In P. monspeliensis infected with the Agrostis isolate, fibrous inclusions, possibly virus particles, occurred in some nuclei. When symptoms were severe, mitochondria and chloroplasts were amorphous and the latter had many marginal vesicles.