Transmission of plant viruses
Encyclopedia
Problems associated with viral transmission
Viruses are known to infect both plant cells and animal cells. Since viruses are obligate intracellular parasites they must develop direct methods of transmissionTransmission (medicine)
In medicine and biology, transmission is the passing of a communicable disease from an infected host individual or group to a conspecific individual or group, regardless of whether the other individual was previously infected...
, between hosts
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
, in order to survive. The mobility of animals increases the mechanisms of viral transmission that have evolved, whereas plants remain immobile and thus viruses must rely on environmental factors to be transmitted between hosts.
Natural transmission between plant hosts
The structural differences between plant and animal cells have resulted in a variety of transmission routes being exploited, enabling the virus to be passed between different host plants. The main difference, from the point of view of a virus, is the cell wallCell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
. This forms a tough barrier
Barrier
A barrier or barricade is a physical structure which blocks or impedes something.Barrier may also refer to:-Physical barriers:* Automatic full barriers, which serve to block roads at railway crossings...
between the intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
components and the extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
environment, which has to be penetrated. These differences, combined with the fact that plants are immobile, have resulted in plant viruses relying on the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
and soil to transmit seeds as well as vectors. Vectors either transmit the virus propagative transmission, which results in an amplification of the virus by replication
Self-replication
Self-replication is any behavior of a dynamical system that yields construction of an identical copy of that dynamical system. Biological cells, given suitable environments, reproduce by cell division. During cell division, DNA is replicated and can be transmitted to offspring during reproduction...
within the cells of the vector, or non-propagative transmission which simply carries the virus between the plants without viral replication. Common vectors include Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, Fungi, Nematodes, arthropods and arachnids.
Furthermore, human intervention, including grafting
Grafting
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together. This vascular joining is called inosculation...
and experimental mechanical damage, physically damages the cell wall, contributes to the array of transmission routes. The virus commonly uses these methods to be passed from one host to another. However, the virus is dependent upon physical damage, generated naturally by the wind and feeding of vectors or by human intervention.
Transmission between plant cells
Viral infections often develop into systemic infections as a means of transmission. The virus often infects many tissuesBiological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
, if not the whole plant, where it can continue to replicate. There are a variety of methods the virus can use to spread throughout the organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
but the most common route utilise the vascular system, otherwise known as the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
and phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
, and the plasmodesmata
Plasmodesmata
Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Species that have plasmodesmata include members of the Charophyceae, Charales and Coleochaetales , as well as all embryophytes, better known...
, which interconnect adjacent cells.
Nevertheless, the rigidity of the cell wall of plant cells, not only poses a problem for transmission of the viruses between different plant hosts, but also acts to prevent viral transmission between cells. Consequently the virus must modify the plasmodesmata as they, themselves, are too large to fit through the small and tightly regulated plant structure. There are again a variety of mechanisms used for this, which have evolved for different viruses. The main mechanisms involve expressing
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
proteins which coat the virus and interact with the structure of the plasmodesmata. The array of proteins expressed by the different viruses may act differently but all achieve a similar goal, passage between adjacent cells.