Electron Microscopy of the Potato Leafroll Virus in Leaves of Three Kinds of Host Plants and the Partial Purification of the Virus

Abstract

Ultrathin sections of affected leaves from plants of Physalis floridana Rydb., Datura stramonium L. and Solonum tuberosum L. cultivar Danshaku, infected with potato leafroll virus respectively, were prepared for electron microscopy. Examination of the sections revealed uniform particles, spherical with a diameter of approximately 24mμ, in the phloem tissue of all 3 hosts. These particles were observed within the ground cytoplasm or vacuoles in the phloem companion cells, phloem parenchyma cells and young xylem vessel, but not in the mesophyll cells. In the phloem companion cells of Physalis plant, the particles often occurred in great abundance within the cytoplasm, apparently causing necrosis of the cell. In the vacuoles, the particles generally formed amorphous aggrega tes and sometimes crystalline aggregates. Similar particles were never found in healthy plants. The procedure adopted for partial purification of the virus involved homogenation of infected leaves in 0.5M phosphate buffer pH 6.5, chloroform clarification, and differential centrifugation. The resulting preparations from all 3 hosts were found to contain spherical, probably polyhedral, particles approximately 25mμ in diameter. Infectivity of the preparations was demonstrated not only by injected aphid transmission, but also by mechanical inoculation into test plants by means of needle-prick method. The evidence seems to indicate that these spherical or polyhedral particles with a diameter of approximately 25mμ are the potato leafroll virus.