Gross morphology and serology as a basis for classification of elongated plant viruses

How Seeing Became Knowing: The Role of the Electron Microscope in Shaping the Modern Definition of Viruses

This paper examines the vital role played by electron microscopy toward the modern definition of viruses, as formulated in the late 1950s. Before the 1930s viruses could neither be visualized by available technologies nor grown in artificial media. As such they were usually identified by their ability to cause diseases in their hosts and defined in such negative terms as "ultramicroscopic" or invisible infectious agents that could not be cultivated outside living cells. The invention of the electron microscope, with magnification and resolution powers several orders of magnitude better than that of optical instruments, opened up possibilities for biological applications. The hitherto invisible viruses lent themselves especially well to investigation with this new instrument. We first offer a historical consideration of the development of the instrument and, more significantly, advances in techniques for preparing and observing specimens that turned the electron microscope into a routine biological tool. We then describe the ways in which the electron microscopic images, or micrographs, functioned as forms of new knowledge about viruses and resulted in a paradigm shift in the very definition of these entities. Micrographs were not mere illustrations since they did the work for the electron microscopists. Drawing extensively on primary publications, we adduce the role of the new instrument in understanding the so-called eclipse phase in virus multiplication and the unexpected spinoffs of data from electron microscopy in naming and classifying viruses. Thus, we show that electron microscopy functioned not only to provide evidence, but also arguments in facilitating a reordering of the world that it brought into the visual realm.

Detection and serological relationships of cymbidium mosaic potexvirus isolates

Twenty-two isolates of Cymbidium mosaic virus (CyMV) were isolated from 35 orchid plants suspected of being infected with CyMV. Among the three methods used for detecting CyMV, immunoelectron microscopy (IEM-1) was shown to be the most sensitive method, being able to detect the virus in 71.43% of suspected CyMV-infected plants while the electron microscopic method and the indexing plant method could detect 51.43 and 42.86%, respectively. Out of 12 symptomless plants investigated, 25% were found by IEM-1 method to be infected with the virus. Purified CyMV were flexuous rods having lengths between 470-490 nm. A few end-to-end aggregates were also observed and the 280 260 absorbance ratios were from 0.884 to 0.929. The yield of CyMV was 31.07 to 44.09 mg per kg of Datura leaves. Antibodies against purified CyMV D2 were produced in rabbits and hens. The antibody titers in the yolk and sera of hens indicated that 0.5 mg of virus per immunization efficiently generated an abundant supply of IgY in the yolk, however 1 mg of virus per immunization gave a stronger immune response in both sera and yolk. The average yields of IgY were 6.5 +/- 0.6 and 9.4 +/- 0.9 mg/ml of yolk in the group that received 0.5 mg and the group that received 1.0 mg of the virus, respectively. Positive ELISA reactions were observed in 18 and 20 of 22 CyMV isolates when detected with rabbit IgG and IgY, respectively, demonstrating that those isolates were serologically related and the ELISA reactions were shown to be stronger with IgY than those with rabbit IgG in most isolates. The degree of reaction between the CyMV isolates, O(2) and O(4), and IgY was less than that of the other isolates. The two isolates, D(6) and Cat(6), gave negative reactions to rabbit IgG. The results of ELISA assays showed that the homologous serological reaction was not consistently stronger than the heterologous one. Twelve isolates out of twenty-two gave stronger reactions than the homologous antigen (CyMV D(2)) when IgY was used as the detecting antibody while nine isolates gave stronger reactions when using rabbit IgG. No reactions were observed with other plant viruses and plant proteins from healthy Datura.

Genetic Diversity and Evolution of Closteroviruses

The family Closteroviridae comprises more than 30 plant viruses with flexuous, filamentous virions and includes representatives with either mono- or bipartite positive-strand ssRNA genomes. Closteroviruses are transmitted semipersistently by insects from three families of Homoptera, in infected plants are associated with phloem tissue, and demonstrate an astonishing genetic diversity that suggests extensive, on-going evolution. Phylogenetic analyses of their replicative genes as well as the conserved HSP70 demonstrate that closteroviruses co-evolved with their insect vectors, resulting in three major lineages, i.e. aphid-, mealybug-, and whitefly-transmitted viruses. Closteroviruses apparently represent an ancient and diverse virus family that may pose threats to agriculture and needs serious attention.

Principles of molecular organization, expression, and evolution of closteroviruses: over the barriers

This chapter focuses on the molecular organization, evolution, and expression of closterovirus genomes, as well as on their unique particle structure. The closterovirus group combines several positive-strand RNA viruses with very flexuous filamentous particles, of which beet yellows virus (BYV) is the type virus. Closteroviruses are distinct from other RNA viruses of plants in some important phenomenological aspects. They have genomes of up to 20 kilobases (kb), a value comparable only to those of the animal coronaviruses and toroviruses, which have the largest RNA genomes of all positive-strand RNA viruses. The existence of such genomes having a coding capacity several times that of an average RNA virus genome raises questions as to the trend whereby the long genomes have evolved and the possible novel functions they have acquired. The dramatic increase in the closterovirus genome coding capacity may be linked to the distinct ecological niche they occupy. Thus, closteroviruses are the only elongated plant viruses known so far to cause phloem-limited infections in plants and to persist in their insect vectors for many hours, in contrast to only minutes.

The coat protein of white clover mosaic potexvirus has a role in facilitating cell-to-cell transport in plants

Functions of the coat protein of white clover mosaic potexvirus (WCIMV) were investigated using C-terminal deletion mutants. Whereas plants inoculated with RNA transcripts of a full-length wild-type clone of WCIMV produced typical infections, plants inoculated with transcripts of each mutant did not produce symptoms, and viral RNA species were not detected by Northern analysis. The mutants were able to replicate in protoplasts, although, relative to the wild-type RNA profile, the level of genomic RNA, but not subgenomic RNA, was reduced. These results indicate a role for the coat protein in efficient cell-to-cell transport in plants. Virus-like particles were detected in protoplast extracts inoculated with transcripts of a mutant in which the coat protein was truncated by 31 amino acids. This result suggests that the lack of detectable transport in plants was not due solely to a failure of the mutants to form virus particles. Possible roles for the coat protein in transport and replication are discussed. A 6-kDa open reading frame, internal to the coat protein gene, was shown by mutational analysis not to be essential for replication or transport.

Potyviruses, chaos or order?

At first potyviruses were easily distinguished by biological and serological properties because only a few were known and information on their host ranges was limited. The first evidence of serological cross reaction between two of these viruses was reported in 1951 and was further corroborated for three obviously distinct members of the group in 1960. In 1968 attention was drawn to the fact that some legume and non-legume potyviruses have much wider host ranges than previously known and that within the potyvirus group there is as much biological variation within viruses and overlap between viruses as there is in serology. The concept of continuity within the group was soon supported by others and became known as the "continuum hypothesis." Results with highly sensitive serological methods using polyclonal antisera were conflicting, and nucleic acid hybridization techniques did not unambiguously discriminate between potyviruses. Recent results, obtained with antibodies directed toward epitopes located in the N-termini of the coat proteins of potyviruses, suggest that there are ways to more definitely group strains of one potyvirus and distinguish them from other potyviruses. However, there are exceptions to this rule, as in the case of bean yellow mosaic virus and clover yellow vein virus which are clearly distinct in host range, inclusion bodies, and migration velocity of coat protein, but which still react with antibodies to the N-terminal epitopes of one virus. So the question remains of whether coat-protein properties, especially the serological reactivity of N-termini, which do not alter overall virus integrity when lost, sufficiently represent the genome of a pathogenic virus entity as a single criterion for classification.

Sequence data as the major criterion for potyvirus classification

Recent knowledge of the structure of the potyvirus particle and its components appears to have resolved what was thought to be an intractable problem of plant virology. This review describes how coat-protein and gene sequence data can be used to provide an hierarchical classification of potyviruses. This classification puts the aphid and non-aphid-transmitted potyviruses into a single family, divides this family into four genera that correspond to the four modes of vector transmission, discriminates distinct potyvirus species from strains, and provides a basis for the formation of subgroups composed of closely related species within a genus.

The general properties of potyviruses

The criteria used during the past three decades for including viruses in the potyvirus group are briefly discussed and evaluated. The biological and physico-chemical properties of the viruses transmitted by aphids, mites, whiteflies, or the fungus Polymyxa graminis are reviewed, and the taxonomic value of their molecular properties in regrouping the viruses into four groups or genera within the family Potyviridae is discussed.

A critical examination of the utilisation of serum coated grids to increase particle numbers for length determination of rod-shaped plant viruses

In this report we examined the use of immuno-electron microscopy (IEM) as a method of obtaining a higher density of particles on a grid for measurement purposes. We examined the use for rod-shaped plant viruses and found that IEM does change the observed population of particles by producing a much higher proportion of small particles than is seen under conditions of negatively stained sap preparations (e.g. leaf-dip techniques). The results clearly indicate that the technique of IEM is not suitable as a means of increasing particle numbers for length distribution determinations.