Electron Microscopy of Developmental Stages of Northern Cereal Mosaic Virus in Wheat Plant Cells

Abstract

Northern cereal mosaic virus (NCMV) initially infects phloem cells of the wheat plant. It spreads to mesophyll cells adjacent to the phloem and finally infects all cell types including phloem parenchyma cells, companion cells, sieve elements, developing xylem cells, mesophyll cells, guard cells and epidermal cells. NCMV was usually found accumulated within cisternae of the endoplasmic reticulum (ER) system in the cytoplasm. In some instances, the presence of NCMV particles within the nucleoplasm and perinuclear spaces was observed. A few particles of inner nucleocapsides (‘immature’ NCMV) were also seen within the cytoplasm. Budding profiles were rarely observed. NCMV seems to be enveloped by membranes almost simultaneously as inner nucleocapsides are formed. Initial appearance of NCMV in the cytoplasm was always associated with the formation of a viroplasm like inclusion. This viroplasm-like area (VpA) appeared as an electron dense matrix of granular or fibrous materials, which formed and distributed throughout the cytoplasm and sometimes occupied a large portion of it. At an early stage of infection, virus particles frequently appeared within the ‘non-denatured’ cytoplasm rather than at the periphery of the VpA. A few ‘immature’ particles were only occasionally observed within the VpA, while mature particles of NCMV were never seen scattered in the VpA. However, mature particles in aggregates were rarely found enclosed by the VpA. A second membraneous inclusion was also found in the cytoplasm of infected cells which exhibited an ellipsoidal structure and virus particles were frequently localized at the periphery of this inclusion. In infected mesophyll cells, membranes which protruded into central vacuoles were occasionally observed. Many ‘naked’ particles of NCMV were seen arranged in parallel between the membranes. Abnormal proliferation of Golgi vesicles (about 50-200nm in diameter) with an electron dense core were seen in the cytoplasm of young developing wheat leaves infected with NCMV, but disappeared as leaf stage progressed. The relation of these Golgi vesicles to stunting of wheat plant caused by NCMV infection is discussed.