Electron microscopy of the novel barley yellow streak mosaic virus

Unique particles of barley yellow streak mosaic virus (BYSMV) were detected in diseased barley, wheat, and several species of grass. They appeared to be about 64 nm in width and from 127 nm to an astonishing 4000 nm in length. Individual particles were circular in transverse section. The outermost layer of each particle seemed to be a membrane-like envelope. The internal structure of many particles was bead-like. Some particles had centers that were translucent. The BYSMV particles were distributed throughout the leaf, sheath, root, and own organs of barley. Virus particles were present in all cell types of the epidermis, mesophyll, phloem, and xylem. However, mesophyll cells contained the greatest number of particles. Most BYSMV particles occurred in large clusters of quasi-parallel arrays. Both individual and groups of particles were located within the cavities of ER elements. Ribosomes were attached to some outer surfaces of the ER bounding membrane. BYSMV particles are unique because they do not resemble any in presently classified groups or families of plant viruses: they are, however, similar to those of some unclassified viruses that infect insects.