Fundamentals of Plant Virology

R C Matthews

• Publisher: Academic Press
• Year: 2012
• ISBN:
  9780323138499
• Topic: Biowissenschaften
• Subtopic: Botanik

Fundamentals of Plant Virology is an introductory student text covering all of modern plant virology. The author, Dr. R.E.F. Matthews, has written this coursebook based on his classic and comprehensive Plant Virology, Third Edition. Four introductory chapters review properties of viruses and cells and techniques used in their study. Five chapters are devoted to current knowledge of all major plant viruses and related pathogens. Seven chapters describe biological properties such as transmission, host response, disease, ecology, control, classification, and evolution of plant viruses. A historical and future overview concludes the text. Fundamentals of Plant Virology is a carefully designed instructional format for a plant virology course. It is also an invaluable resource for students of plant pathology and plant molecular biology.

• Summarizes knowledge on all aspects of plant virology
• Condenses all essential material from Plant Virology 3/e
• Compares basic properties of cells and viruses
• Outlines principles of gene manipulation technology
• Discusses serological techniques including monoclonal antibodies
• Geared to student level course

1

What are Viruses?

1 Viruses and Cells Compared

1.1 Size

1.2 Composition

1.3 Structure

1.4 Increase in Population Size

1.5 Biochemistry

1.6 Summary

2 Definition of a Virus

Further Reading

About 50 years ago the first viruses were being isolated, and characterized chemically. They were viruses infecting plants. At the time there were many arguments about whether these agents belonged in the living or the nonliving world. This was because, on the one hand, they could be crystallized in the test tube, and on the other, they could organize their own increase in number, when they were present in a suitable host organism. With the application of the techniques and ideas of molecular biology over the past 30 years, it has become quite clear that viruses are part of the living world. In essence, they are very small obligate parasites that contain between one and several hundred genes of their own, which can mutate and evolve as do cellular genes.

1 VIRUSES AND CELLS COMPARED

In this section, it will be useful to consider some properties of viruses infecting other host groups, as well as those of viruses infecting plants.

1.1 SIZE

Most viruses are very much smaller than most cells, and require electron microscopy for their particles to be visualized. Only the pox viruses, which infect animals, can be seen as tiny dots in the light microscope. Relative sizes can be compared using various properties.

1.11 Dimensions

Figure 5.1 provides outline drawings of the different kinds of plant viruses. They vary widely in size and shape, but all are very small in relation to the size of the cells they infect. Thin sections of infected cells (e.g., Fig. 8.3) give some indication of relative sizes, but they are rather misleading. Such illustrations are two dimensional, whereas cells and viruses exist in three dimensions; thus, a more realistic procedure is to compare volumes. A typical tobacco mesophyll cell has a volume of about 2 × 10¹³ nm³; a large plant virus, about 6 × 10⁵ nm³; and a small plant virus, about 2 × 10⁴ nm³. Thus in a typical plant cell containing 10 million particles of a small virus, only about 1% of the volume of the cell would be occupied by virus.

1.1.2 Size and Complexity of Genome

In making comparisons between different kinds of organisms, the most fundamental single character is the size of the genome. Figure 1.1 shows that the genomes of viruses infecting all kinds of hosts span a range of almost three orders of magnitude. The smallest are about the size of a cellular messenger RNA (mRNA). The largest viral genomes are about the same size as those of the simplest cells; thus, there is a very wide range of size and complexity.

Figure 1.1 The range of genome sizes for viruses, prokaryotes, and eukaryotes. The vertical axis gives an approximate indication of the relative number of species within the size range of each group. From Matthews (1981); modified from Hinegardner (1976).

A biologically more meaningful way of comparing genome sizes is to consider their information content, that is to say, the number of genes they contain or the number of proteins they code for. Some examples are given in Table 1.1.

Table 1.1

Relative Sizes of Genomes Measured by the Number of Proteins Coded for

Type of organism	Example	Number of proteins
Higher plant	Average (approximate estimate)	50–100,000
Eubacteria	Escherichia coli	4100
Mycoplasma	—	820
Chlamydiae	Psittacosis	660
Large virus infecting vertebrates	Vaccinia	260
Virus infecting a chlorella-like alga	PBCV1	200–250
Large virus infecting angio sperms	Wound-tumor virus	12
Small virus infecting angio sperms	Tobacco mosaic virus	4
Smallest known virus	Satellite virus of tobacco necrosis virus	1

From this section we can conclude that (1) by several criteria, viruses in general span a very wide range of sizes; (2) viruses infecting angiosperms are confined to the small end of the size spectrum; and (3) plant viruses are indeed very small and genetically simple compared to the cells they infect.

1.2 COMPOSITION

1.2.1 Macromolecules and Structural Components

In cells, the most important molecules are those containing the genetic information—DNA—and the functional molecules synthesized using that information—RNA and proteins. A lipoprotein plasma membrane defines the boundary of the cells. In addition, in eukaryotes, many cell organelles are bounded by, or composed of, lipoprotein membranes, for example, the nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, etc. Most plant cells have an outer wall made up largely of cellulose.

All viruses have their genetic information in the form of nucleic acid. Some use double-stranded DNA (dsDNA), as do cells. Others use single-stranded DNA (ssDNA), dsRNA, or ssRNA. Most known plant viruses uses ssRNA. As well as genomic DNA, all cells competent to divide contain RNAs of various kinds and functions, mainly ribosomal (rRNA), mRNA, and transfer (tRNA). Most virus particles contain only their genomic nucleic acid, which is either DNA or RNA. However, a few DNA viruses contain small amounts of RNA, either host or viral coded, with specific functions. During the replication of most viruses, mRNAs are transcribed from the viral genomic nucleic acid.

The genetic material of viruses is almost always protected by a covering or coat of protein molecules coded for by the virus. Some viruses, especially the larger ones, have an outer lipoprotein membrane as well. Larger viruses may also contain viral-coded enzymes involved in nucleic acid synthesis.

The DNAs and RNAs found in viruses are made up of the same four nucleotides that are found in the corresponding cellular nucleic acids. Viral proteins contain a selection of the same 20 amino acids that are found in cells. The proportions of different amino acids in a typical viral protein are usually quite similar to those found in typical soluble cellular proteins. In viruses with a lipoprotein envelope, the proteins in the membrane are coded for by the virus and may be glycoproteins containing carbohydrate. The lipid is usually borrowed from some cellular source. No viruses contain cellulose or other components found in plant cell walls.

1.2.2 Low-Molecular-Weight Materials

Besides water, cells contain a very large number and variety of low-molecular-weight substances. These are involved in such processes as intermediary metabolism, the production of the amino aci...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• 1: What are Viruses?
• 2: Principal Techniques for the Study of Virus Particle and Genome Structure
• 3: Serological Methods in Plant Virology
• 4: Assay and Purification of Virus Particles
• 5: Virus Structure
• 6: Introduction to the Study of Virus Replication
• 7: Replication of Viruses with ss-Positive Sense RNA Genomes
• 8: Replication of Other Virus Groups and Families
• 9: Small Nucleic Acid Molecules that Cause or Modify Diseases
• 10: Transmission, Movement, and Host Range
• 11: Host Plant Responses to Virus Infection
• 12: Variability
• 13: Relationships between Plant Viruses and Invertebrates
• 14: Ecology
• 15: Economic Importance and Control
• 16: Nomenclature, Classification, Origins, and Evolution
• 17: Future Prospects for Plant Virology
• Appendix: List of Standard Acronyms for Selected Plant Viruses and Viroids
• Bibliography
• Index