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Viruses are extremely small parasites that have none of the structures characteristic of living cells. Many viruses are little more than a protein-coated and particle-containing DNA or RNA.
The protein coat is specifically adapted for breaching the plasma membranes of host organisms. Viroids are even simpler parasites comprising small, single-stranded molecules of RNA with no protein coat. They are the smallest known agents of infectious disease.
The concept of a virus dates back to the late nineteenth century investigations of the mosaic disease of tobacco by Dmitri Iosifovich Ivanovsky in Russia and Martinus Willem Beijerinck in Holland.
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They found that the agent that caused a mosaic pattern of light and dark green areas on tobacco leaves was smaller than a bacterium, being able to pass through a filter known to exclude bacteria. Subsequently, similarly small agents were shown to cause disease in animals.
Particle Components
As their extremely small size suggests, virus particles do not contain everything needed to reproduce themselves. They can multiply only within the cells of living organisms.
Virus particles, also known as virions, universally contain one or more deoxy ribonucleic acid (DNA) or ribonucleic acid (RNA) chains. Encoded in the sequence of nucleotides of the DNA or RNA is all information needed for virus reproduction.
Virions also contain one or more types of capsid proteins, which protect the nucleic acid from destruction by the environment. Virions of some viruses contain additional coatings, including membranes derived from cell membranes containing distinctive, virus-encoded proteins.
Despite their simplicity and small size, viruses are not the smallest infectious agents of plants. That title belongs to viroids: small, circular RNA molecules with extensive secondary structures.
They do not encode proteins and are not encapsulated. Viruses are classified based on the type of nucleic acid strands contained in virions, their mode of replicating the strandor strands, and the shape of virions.
Viruses may have RNA or DNA as genetic material. These nucleic acids may be single-stranded or double-stranded.
If single-stranded, the instructions for making protein may be on the packaged strand (positive sense), on its complement (negative or anti sense), or on both strands (ambisense). The genome of the virus may be in one nucleic acid strand or distributed among several.
The sizes of plant viral genomes vary from about four kilo bases (kb) to about twenty kb. Exceptions are the phycodna viruses of algae whose DNAs are over three hundred kb. Virions may be roundish in shape (in actuality an intricate geometrical form called an icosahedron), rod-shaped, or filamentous.
Replication and Evolution
Most known plant viruses have positive sense RNA as genetic material. However, examples of negative sense and ambisense virion RNAs are known. Virus-encoded RNA replicase enzymes make strands complementary to the virion strands and then use the complements to make strands for packaging in progeny virions.
The single DNA strands of begomoviral virion DNA are made by host-encoded DNA polymerases. Spanning the RNA and DNA worlds, the virion DNA of plant equivalents of retroviruses is copied into RNA by host enzymes. The RNA is then copied into DNA by a virus-encoded enzyme.
Mistakes in viral replication are so frequent, and replication so prolific, that the population of viral genomes in an infected organism is a collection of many different sequences, called a quasi species.
Quasi species allow viruses to evolve rapidly when novel environments act on them, to select a better adapted variant. The study of bacteriophages, viruses that infect bacteria, has contributed to understanding how plant viruses evolve and function.
Viral genomes are collections of gene modules, each with a different purpose. Viruses also evolve by exchanging modules. Areassortant arises when a novel mixture of RNA or DNA strands is packaged, while a recombinant arises by exchange of only part of one strand.
Therefore, one virus may have multiple origins, one for each module. Relationships that have not been obscured by evolutionary divergence suggest modules themselves have multiple origins.
Viral Gene Function Gene modules are classified into those coding for structural (virion-associated) proteins and those coding for nonstructural proteins. Structural proteins are those that can be found in virus particles, including caps id and envelope proteins and, in some viruses, include replication proteins.
Plant viruses are unique in having a module required for movement of infection from one cell to the next. Indeed, an insect-infecting virus can spread in plants engineered to make a movement protein of a plant virus.
Movement proteins alter plasmodesmatal connections between cells, but how they allow virus movement from cell to cell is still under intensive study. Movement of some viruses may depend on an as-yet-undetermined pathway for intercellular RNA movement.
Viruses also move from the infected leaves to other leaves. This movement follows the phloem pathway used by the plant to transport sucrose, with the infection moving from source leaves to sink leaves. Some viruses require the coat protein for long-distance movement. A few viruses move in the xylem.
Movement of a virus from one plant to another requires a vector (another organism that assists in transmission); numerous insect species, nematodes, and fungi can transmit specific viruses.
Specific viral proteins interact with a host-component to assure transmission specificity. Some viruses, such as tobacco mosaic virus, and viroids are transmitted only mechanically, by contact with animals or farm equipment.
Disease and Control
Viruses first grabbed scientists’ attention because they cause disease. In plants, symptoms associated with virus infection include leaf discoloration, foliar distortion, and fruit blotches. It is now known, however, that many viral infections are unapparent. Specifically what causes symptoms is not known.
Most plants are not susceptible to most viruses. A virus may be unable to replicate in cells of the plant species. The plant may mount a hypersensitive response in which it kills its own cells at the site of infection, to limit the infection.
Overproduction of RNA, such as occurs during RNA virus infection, can lead to induction of an RNA destruction mechanism. Some viruses have evolved suppressors of that defense pathway. Systemic acquired resistance is a pathogen-nonspecific state of resistance induced by infection with any kind of pathogen.
Infection of crop plants by viruses causes large agricultural losses. Control methods have been developed. Culling is the removal of infected plants.
Controlling vectors with pesticides can limit the spread of viral outbreaks. Breeding genes from resistant plant species or varieties into the crop variety is a standard approach. Such resistance may break down as viruses evolve to overcome the new genes.
In cross protection, plants are purposely in oculated with a mild strain of the virus and become resistant to other strains. Cross protection led to the biotechnological pathogen-derived resistance, in which protection comes from a viral DNA element incorporated into the plant chromosome.