Main features of Plant Viruses and the diseases they cause Essay
Viruses are small infectious pathogens, which are too small to be seen under a light microscope. However, despite their size they can cause both mild to severe illnesses. Viruses only activate inside a host cell and can affect all life forms including humans, animals, plants, and even microorganisms like bacteria and archaea. Viruses depend on the cellular system of their host for reproduction. Viruses have three main parts, which are nucleic acid, protein coat and lipid membrane. Nucleic acid is the core of a virion, which holds all the genetic information (DNA and RNA) and helps it multiply. The protein coat or capsid protects the virus by covering it and the capsid is covered by a envelope called lipid membrane. Not all viruses have the capsid and the ones that do not have the capsid known as naked viruses.
Viruses are responsible for causing many plant diseases, leading to losses in crop yield and quality all around the world. Plant virology began in the 19th century when Dutch microbiologists Martinus Beijerinck and Russian researcher Dmitrii Iwanowski were investigating the cause behind a disease in tobacco (Scholthof 2001). They described an unusual agent, which caused mosaic disease in tobacco (Zaitlin 1998). There was one factor of this agent, which set it apart from other disease-causing microorganisms, and the factor was its size. It had a size smaller than all other microbes. This agent was the first virus to be described and was known as the Tobacco mosaic virus (TMV). Since then a large number of viruses have to come light; found in animals, plants, fungi and bacteria. The current estimate of plant viruses is about 4000 of which 1000 accounts for plants. The reason why plant viruses are studied is because of the negative impact they have on crop yield and quality. Now viruses are also known for good reasons like; for their use in biotechnology and biomedicine. For example, viruses are being used to derive proteins of interest from plants (Pogue et al. 2002) and to develop safe and inexpensive vaccines against human and animal viruses Walmsley and Arntzen 2000).
All viruses have the same structural principle in their mature form. Virions (Virus particles) have a genome, which is composed of nucleic acid and a protective layer, composed of protein. However, some viruses have an additional protective layer responsible for protecting the protein layer called lipoprotein membrane, which is composed of lipids and proteins. Capsids in plant viruses are assembled according to two fundamental types of symmetry; helical (roughly elongated) and icosahedral (roughly spherical). Each of these symmetries have two variants. The elongated symmetry comes in two variants, which are rigid rods and flexuous filaments. Both of these variants have highly ordered nucleic acid, which is assumed to have a helical conformation same as proteinaceous capsid. The icosahedral symmetry comes in bacilli form and twin virions that are composed of two joined incomplete icosahedra. The nucleic acid of icosahedral virions is found as a partially ordered ball in the proteinaceous capsid. Under proper incubation conditions and in the presence of nucleic acid and protein subunits, the icosahedral and elongated virions can self-assemble in a test tube.
The typical diameter of spherical plant viruses is about 30nm and the rigid rod-shaped TMV particle is about 300 x 18nm. The TMV particle has an RNA genome of 6400 nucleotides and 2130 copies of protein coat. Some filamentous viruses can reach a length of 2000nm or 2µm.
The reproductive cycle and lifecycle differs within viruses, due to the variations in the genetic material of different viruses. Like all other organisms viruses have their genetic information for reproduction contained within their genome. However, the difference lies within type of and structure of the genome. Not all plant viruses have a genome of double stranded DNA like other organisms. Some of them have a single stranded DNA. Majority of plant viruses do not have a genome consisting of DNA. Instead, most plant viruses have a genome consisting of RNA. Most of the genomes have a single stranded RNA i.e. the same polarity (positive sense) as the mRNA of the cell. Some single stranded RNA viruses use negative sense.
Plant viruses are obligate, bio-trophic parasites and so their life cycle initiates by the penetration of the virion into the cell. Plant viruses do not have the ability of penetrating into the cuticle and cell wall of plants. However, the virion can passively enter the cytoplasm of the cell through wounds caused by mechanical damage to the cell wall and cuticle. Damage can be caused due to weather, equipment, fire, animals, insects and human activities like farming and landscaping. Sometimes plant viral diseases can be transferred from the parent to offspring.
In the next phase of the viral infection, the protein coat of the virion is removed partially or completely in the cytoplasm. The cell provides transcription (for DNA viruses) and translation (for all viruses) apparatus to the viral genome, thus mediating the expression of the viral genome. The DNA viruses need to be transported to the nucleus in order to access the cell’s proteins for the production of mRNA from the viral DNA. On the other hand, the translation of RNA in the cytoplasm generates proteins, necessary for the completion of the virus life cycle.
It is necessary for all viruses to produce at least three types of proteins. These include; replication proteins, needed for producing nucleic acid, structural proteins involved in the formation of protein shells and other components of the virion and other movement proteins involved in transporting virus between plant cells. The replication protein combine with cellular proteins to form a complex of proteins, which produces multiple copies of the virus genome. The newly formed genomes then interact with structural proteins, thus giving rise to new virions.
Small channels called plasmodesmata, which form connections between cells, carry out the transport of virions or the viral genomes to the neighbouring cells. Most viruses create movement proteins that change the plasmodesmata channels and facilitate the transport to neighbouring cells. Cell to cell movement can be slow and can take one or more hours for a virus to multiply in one cell and then move to the next. Therefore, in order to successfully colonize an entire plant; it needs to enter the plant’s vascular system. The life cycle of virus is completed only after transmission from one plant to another.
Only a few viruses like TMV have the ability to survive for decades in the environment and through passive mechanical transmission from plant to plant (Ford and Evans 2003). Plant viruses are mostly transmitted from an infected to healthy plant by vectors. The major types of vectors involved in plant virus transmission include nematodes, Plant-feeding arthropods and plant parasitic fungi (Walkey 1991). Aphids and whiteflies transmit the largest number of virus species. It only takes seconds, hours or days for the transmission to occur. Sometimes transmission can occur through vegetative propagation of infected plants. For example through grafting or tubers or vertically through pollens or seeds of an infected plant. Such type of a transmission is important for the virus during winter.
Virus causes different types of diseases in plants. The first plant virus discovered (tobacco mosaic virus) is responsible for causing diseases in the members of the nightshade or solanaceae family, which include tomato, potato, tobacco, pepper, eggplant, cucumber and petunia. It spreads through a damaged cell wall caused by insects or physical damage.
Cucumber mosaic virus is responsible for causing infections in cucumber, tomato, papper, squash, celery, melons, beet, spinach and other plants. It is spread by aphids, which cause physical damage to the plants, thus allowing the entry of virus via the wind, splashing or dripping sap. It causes twisting of young leaves and stunts the growth of the entire plant, thus reducing the fruit or leaf production. The barley yellow dwarf virus is responsible for infecting several grains and staple crops, including wheat. Aphids primarily spread the virus. The virus causes discoloration of leaves and the tips of the plants, which reduces photosynthesis, stunts growth and decreases production of seed grains. The bud blight virus infects soybeans, a staple crop. It causes the stem to bend at the top and the buds to turn brown and drop off the plant. Nematodes spread this virus. The sugarcane mosaic virus discolors leaves of the sugarcane plant, restricting its ability to feed itself through photosynthesis and grow. It stunts the growth of young plants. Aphids and infected seed spread the virus. The cauliflower mosaic virus infects members of the brassica, or mustard, family, which includes cabbage, brussels sprouts, cauliflower, broccoli and rape seed. It causes a mosaic or mottle on the leaves, which stunts growth. Aphids and mechanical exposure spread the virus. The lettuce mosaic virus mottles the leaves of almost all types of lettuce, stunting its growth and eliminating its market appeal. Aphids and infected seeds spread the virus. The maise mosaic virus causes yellow spots and stripes on the leaves of corn, stunting its growth. Leafhoppers spread the virus. The peanut stunt virus causes discoloration and distortion of the leaves of peanuts and some other rhizomes, stunting their growth. Aphids and sap spread the virus.
Many plant diseases are also caused by viriods. Viriods are infectious RNAs responsible for causing a number of economically important plant diseases. Like plant viruses they possess a RNA genome. However, they differ from plant viruses in two ways. Firstly, they contain naked RNA i.e without a protein coat and secondly they do not produce proteins when they infect plant cells although they are made of RNA. Viroids are spread to other plants through vegetative propagation, mechanical contamination, pollen and seed.
Plant viruses and viroids are diverse and unusual groups of plant pathogens that infect and cause disease in many crop plants. Because these pathogens depend on the normal cellular machinery of their plant host for reproduction, it is difficult to eliminate them without damaging the host plant. Therefore, most management strategies for diseases caused by plant viruses and viroids are directed at preventing infection of the plant.