The in vivo behaviour of twenty-four strains of alfalfa mosaic virus

Abstract

The in vivo behaviour of 24 strains of alfalfa mosaic virus (AMV) in cells of tobacco leaves was investigated by electron microscopy of thin sections. In the cytoplasm the particles of 13 of the strains packed in four distinct aggregation forms which are described in detail. These aggregation forms appear to be strain specific. The particles of the other 11 strains were not found to aggregate in the cytoplasm of tobacco. In an attempt to find how two of the types of cytoplasmic aggregates of AMV particles were formed, samples were taken at regular intervals from plants infected with Caldy and 15 64 strains. Aggregates were found in Caldy infected plants from the earliest stage (2 days) at which virus particles were detected. Particles were found in 15 64 infected plants 2 days after inoculation and aggregates, first detected in the third day sample, became common by the fifth day; but the cause of the aggregates was not detected. Possible mechanisms of aggregation formation are discussed. Aggregates which resembled those formed by virus particles were found in nuclei of tobacco plants infected with 15 64 , AMV 425, and VRU strains. AMV-like particles were extracted from nuclei from plants infected with 15 64 strain. Aggregates of long rod-shaped particles were found in the nuclei of plants infected with three strains. Those associated with 15 64 and ATCC 91 strains were in loose aggregates whereas those associated with Caldy strain were in large crystalline aggregates. Both types of aggregate of long tubular particles were usually found near the nucleoli. The long rod-shaped particles extracted from nuclei from 15 64 and Caldy infected tobacco were indistinguishable in the electron microscope.