viruses and their reproduction

Introduction of viruses.

Viruses are infectious pathogens that are too small to be seen with a light microscope, but despite their small size they can cause chaos. The simplest viruses are composed of a small piece of nucleic acid surrounded by a protein coat. As is the case with other organisms, viruses carry genetic information in their nucleic acid which typically specifies three or more proteins. All viruses are obligate parasites that depend on the cellular machinery of their hosts to reproduce. Viruses are not active outside of their hosts, and this has led some people to suggest that they are not alive. All types of living organisms including animals, plants, fungi, and bacteria are hosts for viruses, but most viruses infect only one type of host. Viruses cause many important plant diseases and are responsible for losses in crop yield and quality in all parts of the world.
The purpose of this chapter is to provide an overview of the fascinating microscopic world of plant viruses and to describe the basic concept of a virus, the structure of virus particles and genomes, virus life cycles, the evolution and diversity of plant viruses, as well as the common manifestations of plant virus diseases and major approaches to managing these diseases. We hope to convey to the reader our grudging admiration for these small pathogens and for their success in manipulating their plant hosts so successfully.

Basic Biology.

Viruses represent not just another group of pathogens, but rather a fundamentally different form of life. Unlike all other living organisms, viruses are non-cellular. In contrast to cells, which multiply by dividing into daughter cells, viruses assemble from pools of their structural components. Mature virus particles are dormant; they come alive and reproduce only inside infected cells. In other words, viruses are obligate parasites that cannot be cultivated using any growth media suitable for bacterial, fungal, plant or animal cell types. All viruses lack protein-synthesizing and energy-producing apparatuses. As a rule, virus particles are immobile outside the infected host; they rely on the aid of other organisms or the environment for their dissemination.

Morphology.

There is a simple structural principle that applies to virtually all viruses in their mature form. Virus particles (virions) are composed of two principal parts, the genome that is made of nucleic acid, and a protective shell that is made of protein. In addition, some virus particles are enveloped by an outer membrane containing lipids and proteins (lipoprotein membrane). The protein shells of plant viruses (capsids) are assembled in accord with one of the two fundamental types of symmetry. The first type of virion is helical (roughly elongated). The elongated viruses come in two major variants, rigid rods and flexuous filaments. In both of these variants, the nucleic acid is highly ordered: it assumes the same helical conformation as the proteinaceous capsid. The second type of virus particle is icosahedral (roughly spherical; the variations of this basic shape include bacilliform virions and twin virions composed of two joined incomplete icosahedra . In the icosahedral virions, the genomic nucleic acid forms a partially ordered ball inside the proteinaceous capsid. The icosahedral and elongated virions alike can self-assemble in a test tube if the nucleic acid and protein subunits are incubated under proper conditions.
Viruses are the smallest among all known organisms. The typical diameter of a spherical plant virus is ~30 nm. The rigid, rod-shaped TMV particle is 300 x 18 nm and consists of an RNA genome of about 6,400 nucleotides encapsidated by 2,130 copies of the TMV coat protein. Some of the filamentous viruses reach the length of ~2000 nm or 2 µm. For comparison, the typical size of a leaf mesophyll cell is ~50 µm.
Reproduction
As in other organisms, the information for virus reproduction is contained within its genome.
Althouh the genetic material for most organisms is double-stranded (ds) DNA, only a minority of plant viruses possess ds DNA genomes. Some of the plant viruses have genomes that are composed of single-stranded (ss) DNA. However, the majority of plant viruses do not use DNA at all. Instead, the genomes of nearly all plant viruses are made of RNA. Most of these genomes are composed of ssRNA that is the same (positive-sense) polarity as the messenger RNAs of the cell. Some of the RNA viruses use ssRNAs of negative polarity, and yet others have genomes made of dsRNA. Due to this enormous variation in the very nature of the genetic material of viruses, the reproductive cycles and life styles of different viruses are often very distinct from each other.
Since plant viruses are obligate, biographic parasites, their life cycles start by penetration of the virion into the cell. Plant viruses are unable to penetrate the plant cuticle and cell wall. It is believed that the virion enters the cytoplasm of the cell passively through wounds caused by mechanical damage to the cuticle and cell wall. The next phase of virus infection is the partial or complete removal of the coat protein shell of the virion in the cytoplasm. Next the cell mediates expression of the viral genome by providing a transcription apparatus (for DNA viruses) and a translation apparatus (for all viruses). The DNA viruses must be transported to the nucleus for transcription in order to gain access to the cell proteins required for the production of messenger RNA from viral DNA. Translation of viral RNA in the cytoplasm produces viral proteins that are required for completion of the virus life cycle.
All viruses must direct the formation of at least three types of proteins: replication proteins that are essential for nucleic acid production, structural proteins that form the protein shell and other components contained in the virions, and movement proteins that mediate virus transport between plant cells .The viral replication proteins combine with cellular proteins to produce a complex of proteins that manufactures multiple copies of the virus genome. These newly made genomes interact with the structural proteins to form new virions.