Lynn L. Hoefert

Development of infection with beet western yellows virus in the sugarbeet.

Abstract Electron microscopy of successively older leaves of sugarbeet ( Beta vulgaris L.) infected with beet western yellows virus (BWYV) revealed the steps in the infection of leaf tissues by the virus and gave some insight into the events leading to viral multiplication in host cells. The sequence of the infection is interpreted as follows. Virus particles are seen in mature sieve elements before external symptoms develop in in the leaf. Particles then appear in parenchyma cells next to plasmodesmata connecting the parenchyma cells with the mature sieve elements. Invasion of parenchyma cells by the virus is accompanied by development of vesicles containing networks similar to those usually interpreted as nucleic acids. The vesicles are enclosed in endoplasmic reticulum (ER) cisternae, singly or in groups. Some of the vesicle-containing ER fuses with the nuclear envelope so that the vesicles become located in the perinuclear space. After the vesicles and the nuclear envelope become associated, virus particles appear in the nucleus, first next to the nucleolus. As the amount of virus increases, particles are scattered throughout the nucleus. Virus partiples also increase in number in the cytoplasm, presumably by being released from the nucleus. Eventually, the virus-induced vesicles cease to be seen in association with the nucleus. Those remaining in the cytoplasm degenerate. The infected cells also degenerate. Virus multiplication first occurs in cells next to infected sieve elements. Later, cells farther away from the sieve elements also become infected.

Ultrastructure of sugarbeet leaves infected with beet western yellows virus.

An electron microscope study of sugarbeet ( Beta vulgaris L.) leaves infected with beet western yellows virus (BWYV) revealed isometric particles, 24–30 nm in diameter, in phloem and mesophyll cells. Compared to ribosomes the particles were slightly larger, more deeply stained, and sharper in outline. Many particles showed an electron lucent center. The particles are assumed to be the virus. A comparison of leaves of different ages from the same systemically infected plant suggested the following sequence in the spread of the virus. In a given leaf, the particles appear initially in mature sieve elements, then move to adjacent companion and parenchyma cells. From these cells, the virus spreads to phloem and mesophyll cells not in contact with the sieve tubes. The presence of virus particles in plasmodesmata between sieve elements and adjacent nucleate cells, as well as between contiguous parenchyma cells, indicates that complete particles are transported from cell to cell through plasmodesmata. Within the sieve tube, the particles traverse the sieve plates, for they frequently occur in sieve plate pores jointly with the P-protein. In the lumen of the sieve element, the particles occur close to the cell wall, sometimes in association with endoplasmic reticulum. In parenchyma cells (including companion cells), particles are most conspicuous and most numerous in the nuclei. Their close association with the nucleolus suggests that the latter may be involved in viral multiplication. In older infected leaves, particles within the nuclei form crystalline arrays. Within the cytoplasm, particles are commonly located close to the wall and may be aligned along the microtubules.

Fine structure of sperm cells in pollen grains ofBeta

SummaryThe structure of sperm cells in mature trinucleate pollen grains ofBeta vulgaris L. was studied with the electron microscope. The ellipsoidal sperm cell nuclei and cytoplasm are products of mitosis and cytokinesis of the ellipsoidal generative cell. Each sperm cell is separated from the vegetative cytoplasm by two contiguous membranes which enclose its cytoplasm and nucleus. Microtubules present in the sperm cell cytoplasm may be responsible for sperm cell motility.

Particles and associated inclusions in sugarbeet infected with the curly top virus

Abstract Electron microscopy of sugarbeet leaves of different ages infected with curly top virus revealed spherical viruslike particles approximately 16 nm in diameter. The particles tended to form clumps and were sometimes associated with fibrils resembling those formed by nucleic acids. The particles were found in nuclei of phloem parenchyma cells. They appeared to arise in the nucleoplasm with no obvious relation to the nucleolus. Nuclear chromatin disappeared concomitantly with the increase in the amount of virus. The nucleolus apparently disintegrated also. Some views indicated that the nuclear envelope breaks down, but, thus far, particles were not detected in the cytoplasm. When particles first appear in a nucleus, a spherical granular inclusion of unknown identity occurs nearby. An amorphous inclusion body is found in the cytoplasm of cells in which the nuclei contain virus.

Cytology of beet yellows virus infection inTetragonia: II. Vascular elements in infected leaf

SummaryThis paper is the second in the series dealing with the ultrastructure ofTetragonia expansa Murr. infected with the beet yellows virus. It considers the relation of the virus to the conducting cells in the phloem and the xylem. Virus particles occurred in mature sieve elements, their amount increasing as the infected leaf became older. In older leaves some sieve elements were completely blocked with virus. Virus particles were seen in pores of sieve plates, in plasmodesmata interconnecting sieve elements and parenchyma cells, and in those between parenchyma cells. Mature and immature tracheary elements also contained virus particles. Presence of inclusions composed of vesicles and virus in some immature tracheary elements may indicate that virus multiplies in these cells. No vesicles and no virus particles were discovered in immature sieve elements.

Polychromatic Stains for Thin Sections of Beta Embedded in Epoxy Resin

Thin sections of leaves and anthers of Beta vulgaris L., fixed in glutaraldehyde-OsO4 and embedded in epoxy resin, were stained with different stains at pH ranges from 5 to 9 at 50 C to select those that provided polychromatic staining of suitable intensity. The thionin derivatives, Azure B, Toluidine Blue O, and polychrome Methylene Blue provided adequate staining, as did the commercially prepared stain Paragon PS 1301. Azure B stain was superior for sugar beet 0.5μ monitor sections: cytoplasm appeared grey; nuclei, blue-gray; nucleoli, blue; chloroplasts, blue-green; primary walls, blue; and secondary walls, light blue. Choice of one of the stains mentioned probably would depend upon the plant material under study.

Ultrastructure of tapetal cell ontogeny inBeta

SummaryTapetal cell development and degeneration in anthers ofBeta vulgaris L. were studied with the electron microscope. Tapetal cells become differentiated from sporogenous cells early in anther ontogeny. The tapetal nuclei divide mitotically; binucleate tapetal cells contain relatively little endoplasmic reticulum and otherwise resemble meristematic cells of higher plants. There follows an increase in endoplasmic reticulum and by the time the sporogenous tissue has entered meiotic prophase, the tapetal cells have differentiated the usual characteristics of secretory cells. Degenerative changes begin to appear in tapetal cells after meiosis of the sporogenous tissue. Such changes include loss of inner tangential and anticlinal walls, degeneration of tapetal nuclear envelopes, disruption of the plasmalemma, and changes in the cytoplasmic organelles. Coated tubules are associated with tapetal nucleoli during degenerative stages and the tubules persist after tapetal nuclei have degenerated. Tapetal cell cytoplasm disappears completely by the stage of microspore mitosis.

Electron microscopy of Beta leaves infected with Beet yellow stunt virus.

Abstract Beta vulgaris L. leaves infected with beet yellow stunt virus contain flexuous rods resembling the particles of beet yellows virus. The rods usually occur in the cytoplasm and form aggregates of various sizes. They tend to be arranged parallel to one another in the aggregates and may also form parallel layers within the larger aggregates. Thus far, the rods were seen only within the phloem, in parenchyma cells associated with sieve elements, and in mature sieve elements. The rods are assumed to be the virus particles. Degenerative changes were observed in chloroplasts of mesophyll and phloem parenchyma and in plastids of sieve elements. The presumed virus particles were absent in these cells. Some phloem parenchyma cells with virus either present or absent underwent complete breakdown.

Cytology of beet yellows virus infection in Tetragonia. 3. Conformations of virus in infected cells.

SummaryThis is the third paper of the series dealing with beet yellows virus infection ofTetragonia expansa Murr. It concerns the different kinds of aggregates of virus and the state of the virus particles in the different cells. In vascular parenchyma cells, the aggregates of virus are variable but are consistently intermingled with host cell components. In the sieve elements, the virus may fill the cell lumen solidly either without obvious order or in stacks of layers each as wide as the particle is long. The virus particles appear to be commonly disorganizing in parenchyma cells with degenerating protoplasts and in sieve elements solidly packed with virus. The factors possibly determining the conformation of viruses in plant cells and the terminological problems regarding designations of aggregates of virus particles and other products appearing in infected cells are discussed.

Ultrastructural effects of lettuce infectious yellows virus in Lactuca sativa L.

Lettuce Infectious Yellows Virus (LIYV) is a whitefly-transmitted virus with characteristics similar to the Closterovirus group. We describe an ultrastructural study of the development of LIYV disease in young lettuce ( Lactuca sativa ) leaves at 5, 12, 14, and 35 days after inoculation. LIYV particles are long, flexuous rods found initially in leaf vascular parenchyma cells and sieve elements. The development of virions in the parenchyma cell is preceded by the appearance of vesicles. These vesicles contain fine fibrils and occur in tightly packed arrays. At later stages after inoculation, virions appear in spherical sites, termed “viroplasm” regions, in the cell cytoplasm. Finally, the virions become aggregated into inclusion bodies which often comprise a major volume of the parenchyma cytoplasm. Electron-dense deposits of material of unknown composition are associated with the infection and appear along the plasma membrane.