Introductory Plant Pathology

Class 4

Plant Viruses

This succinct statement by Dr. Agrios sums up the unique properties of a virus. Notice that it places no requirements on the nucleoprotein entity for reproduction or synthesis. In most cases these qualities are coded for, or directed by the nucleoprotein, but they are not part of the definition of a virus. Also notice that no greater simplicity or complexity are required, though often viruses are more complex than "simple" nucleoproteins. Neither the type of nucleic acid (RNA or DNA), strandedness (double or single), type or number of proteins in the nucleocapsid, nor the type of nucleocapsid (rod or sphere) whether it is encompassed by an envelope or has appendages are included in the definition; though they are definitely important for each virus.

CHARACTERISTICS OF PLANT VIRUSES

Detection

Small - require electron microscope for visualization.

ELISA (Enzyme Linked ImmunoSorbent Assay) is often used to detect.

Morphology

Rods -
Rigid rods - 5 x 300 nm Flexuous rods - 10 - 13 x 480 - 2000 nm
Spheres -
Icosadodecahedron - 60 nm Satellite - 17 nm Membrane encased - 100 nm
Bacilluslike, cylindrical rods
52-75 x 300-800 nm

Monopartite - all particles of the virus are the same size and contain the same nucleic acid.

Multipartite - typically di- or tri- partite - virus population is made of two or three, respectively, sizes of particle with distinct nucleic acid strands encased in each capsid.

Composition and Structure

Each plant virus consists of at least one nucleic acid and one protein. Some viruses also have enzymes, lipids, and carbohydrates.

The nucleic acid makes up 5 to 40 percent of the virus with the remaining 95 to 60 percent being protein. The proportion of nucleic acid to protein is greatest in spherical viruses.

This simplest virus has a genome of approximately 1 to 3 x 106 daltons. This is in comparison with bacteria that have 1.5 x 109 daltons.

Composition and Structure of Viral Protein

The proteins of plant viruses are make of the same amino acids common to plant proteins. There are no unique amino acids distinctive of plant viruses.

The protein shells of plant viruses are called capsids and are composed of repeating subunits. For any given virus the capsid proteins are identical and interchangeable though there is no universality of proteins between two different viruses. In many cases a single protein composes the entire capsid, though more complex viral morphologies may require multiple protein types for their capsids.

An individual virion is composed of a specific number of proteins unique to that virus particle. This is true irrespective of the morphological type of the virion.

Composition and Structure of Viral Nucleic Acid

The majority of plant viruses contain their genomes encoded as single strand RNA (ssRNA). However, double strand RNA (dsRNA), single strand DNA (ssDNA), and double strand DNA (dsDNA) plant viruses also occur. The nucleotide bases that make up plant virus RNA and DNA are the same as those typical to RNA and DNA of their host plants.

Satellite Viruses and Satellite RNAs

Some viruses require helper viruses to infect plants. These helper viruses are termed Satellite Viruses. "Satellite viruses are not related, or are only partially related, to the RNA of the virus; satellite RNAs may increase rordecrease the severity of viral infections."

THE BIOLOGICAL FUNCTION OF VIRAL COMPONENTS : CODING

The function of the protein coat is not limited to encasing the viral genome. The outer surface of the protein is a major determinant in vector transmissibility. Barley Stripe Mosaic Virus contains a small carbohydrate component as a glycoprotein on the surface of its rod shaped capsid. This surface carbohydrate has been shown to be bound by lectins in barley seed and is thought to play a role in seed transmission of this virus.

Infectivity is strictly the province of the nucleic acid. In fact, under experimental conditions the nucleic acid, extracted away from its capsid, may be used to inoculate plants. Redundancy is not a feature of the viral genome. The amount on information necessary to code for the capsid protein is small (474 nucleotides for the 158 amino acids of Tobacco Mosaic Virus) relative to the total viral genome (6400 nucleotides); therefore it is obvious that the genome contains more information that that required to produce nucleocapsid proteins. The TMV genome codes for; nucleocapsid protein, two viral replicase (make more viral genome) enzymes, and a protein that facilitates movement through the plasmodesmata.

There is no direct evidence for coding for disease causing factors and it has been assumed that symptoms that are produced a the result of disrupting host cellular process rather than specific viral metabolites.

VIRUS INFECTION AND VIRUS SYNTHESIS

TRANSLOCATION AND DISTRIBUTION OF VIRUSES IN PLANTS

SYMPTOMS CAUSED BY PLANT VIRUSES

No obvious symptoms	Irrespective of titre, some viruses reproduce within plant cells and not apparent symptoms are observed.
Localized Symptoms
Chlorotic lesions	Yellowing and/or reduction in chloroplast metabolism
Necrotic Lesions	May be an advanced progression of the chlorotic expression or may be result of Hypersensitive Response.
Ringspot	Concentric rings (green, yellow, and/or brown) around a central focus.
Systemic Symptoms (Throughout the plant)
Mosaic patterns	Patterns of increased greening or chlorosis scattered of a leaf in a tile pattern.
Dwarfing	Stunting and dwarfing of plant due to an hormonal imbalance. Often the tissue may appear thicker than normal.
Necrosis	Systemic viral symptoms may lead to death of entire plant.
Distortion	Virus infected plants may twist, curl or other wise be distorted. Thought to be due to abnormal hormonal distribution and/or production.

PHYSIOLOGY OF VIRUS-INFECTED PLANTS

TRANSMISSION OF PLANT VIRUSES

Vegetative propagation

Sap Transmission

Seed Transmission

Pollen Transmission

Insect Transmission

Mite Transmission

Nematode Transmission

Fungus Transmission

Dodder Transmission

PURIFICATION OF PLANT VIRUSES

SEROLOGY OF PLANT VIRUSES

NOMENCLATURE AND CLASSIFICATION OF PLANT VIRUSES

ECONOMIC IMPORTANCE OF PLANT VIRUSES

CONTROL OF PLANT VIRUSES

Students, and others, seeking a greater depth of discussion of Plant Virology are referred to the following Web links. Both are excellent.

• Molecular Plant Virology (University of Arizona; Zhongguo Xiong and Merrit Nelson)

• Introduction to Molecular VIrology (University of Cape Town; Ed Rybicki)

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

jpartridge1@unl.edu Home Page References