Light and electron microscopy of red clover vein mosaic virus in pea (Pisum sativum)

Epidermal strips of pea infected with the RK31 isolate of the red clover vein mosaic virus were found to contain many diagnostic crystalline inclusions, recognizable two, but more clearly three weeks after inoculation. They could be easily detected by light microscopy, even at low magnification, when stained with phloxine and methylene blue, but more rapidly with a phase-contrast microscope in unstained preparations in water. They were always found after the onset of external symptoms. With the pea streak strain (P42) they were usually absent. In ultrathin sections made 22 days after inoculation or later, virus distribution and accumulation were comparable to those of other members of the carlavirus group. Sometimes extensive irregular bundles were observed. These were distinct from the crystals seen by light microscopy. Sections of the three-dimensional crystals studied in the electron microscope showed a very regular internal striation with a periodicity of about 11 nm, or were sometimes composed of spherical particles in looser array. When crystals were isolated intact and stained negatively, the majority of the material appeared to consist of spherical or polyhedral particles 11–12 nm in diameter.