Viruses **Images and lecture material were not entirely created by J. Bond. Some of this material was created by others.**

Viruses

**Images and lecture material were not entirely created by J. Bond. Some of this material was created by others.**

Viruses

The term virus was used in the Middle Ages and comes from the Greek word for poison or Latin meaning slimy, poisonous or malodorous liquid.

Its application in those early days did not have the modern understanding of the word.

By the late 1930s scientists were regularly using the term 'filterable virus' to describe those agents capable of passing filters fine enough to retain bacteria.

Viruses

Virus - a submicroscopic, obligate parasite consisting primarily of protein and nucleic acid.

"Submicroscopic" - implies that the object can not be seen using a light microscope; must use electron microscope.

Viruses share traits with both living and non-living things. Living - can replicate themselves; can infect hostsNon-living - can be crystallized and stored.

Fathers of Plant VirologyIn 1892 D. lvanovski - the passage of the tobacco mosaic agent, now

known as tobacco mosaic virus (TMV), through the bacteria-proof Chamberland filter. – Living cells?

In 1898, M. Beijerinck- concept that viruses were small and infectious. Infectious agent as a “contagium vivum fluidum”

• Discovered that the virus readily passed through a porcelain filter, = smaller than bacteria.

• Observed that the 'agent' could diffuse through agar that retained bacteria, and furthermore, that the virus could not be cultured except in living, growing plants. 

First report, suggesting that 'microbes' need not be cellular.

VirusesFirst published record of a virus was in 1576.Descriptions, paintings of tulips with a broken color pattern, now

known to be caused by Tulip Breaking Virus (TBV). This virus induces in foliage of infected plants beautiful, variegated color patterns.

Such bulbs were extremely expensive, giving rise to "tulipmania" in the 17th century.

Viruses have been identified only recently.1935 - W.M. Stanley purified and crystallized Tobacco Mosaic

Virus (TMV). late 1930's - electron microscope developed, viruses visualized.

Characteristics of Viruses1. Very small requires electron microscope to see them.

2. Virus particles (aka. virions) are composed of: a. Nucleic acid - 5 to 40%, depending on virus.

Nucleic acid typically is RNA, but some have DNA. Nucleic acid can be single- or double-

stranded

b. Protein sheath (aka. capsid) - 60 to 95% protein, depending on virus.

Capsid protects virus from heat, enzymes, UV and other denaturing agents.

Characteristics of Viruses3. Shapes

a. Rigid rods - long, straight rods with nucleic acid core and protein coat. Raw spaghetti.Ex: Tobacco Mosaic Virus (TMV) is 15 x 300 nm.

b. Flexuous rods - long curved rods with nucleic acid core and protein coat. Cooked spaghetti.Ex: Citrus Tristeza Virus is ca 2,000 nm long.

c. Short rods (aka. Rhabdoviruses) - truncated, bullet shaped.Ex: Potato Yellow Dwarf Virus (PYDV) is 75 x 380 nm.

d. Eicosahedrons (20-sided isometric particles). Formerly referred to as spherical viruses, because they appeared spherical in the earliest electron micrographs with poor resolution.Ex: Wound Tumor Virus (WTV) can vary in diameter from 17 to 60 nm

Characteristics of Viruses

4. Virus replication depends on "directions" contained in viral nucleic acid. Viruses code for their own enzymes (replicases) to replicate themselves.

5. Multi-component viruses - complete genetic information is carried in > particles.

Ex: Tobacco Rattle Virus (TRV) - consists of two particles of different lengths.

Can infect but unstable because it lacks the gene for protein coat.

Carries info for protein coat.

Virus is stable only when both are present.

Satellite Viruses & Satellite RNAs Satellite Viruses

Viruses that must always be associated with certain typical viruses (helpers) because they depend on the helper for multiplication and plant infections.

They often reduce the ability of the helper virus to multiply and cause disease, so the satellite viruses act as parasites.

Satellite RNAs - small RNAs found inside virions of certain multi-component viruses. The RNA is not related or could be only partially related to that inside the virions. – They may increase or decrease the severity of viral infections.

Viroids

1. They are small, low molecular weight RNAs that can infect plants, replicate themselves, and cause disease.

2. Naked nucleic acid (no protein coat). However they are stable because of nucleic acid side chains "double over" and block sites of attachment for enzymes and protect against other environmental variables.

3. Much smaller than viruses. RNA contains 250-376 base compared to 4,000-20,000 bases in viruses.

Viroids

4. This is too small to encode for even the smallest replicase! So how do they replicate? Apparently they rely on host somehow, but the method is not understood.

5. Recently discovered in 1971 as the cause for potato spindle tuber disease. Now known to cause ~ 20 diseases.

How do viruses enter plants and cause symptoms?

1. They require wounds - either through vector (insects, nematodes) feeding or mechanical.

2. Once inside host cell, protein coat disassociates and nucleic acid replicates. Virus and nucleic acid spread from cell to cell.

How do viruses enter plants and cause symptoms?

1. They require wounds - wither through vector feeding or mechanical.

2. Once inside host cell, protein coat disassociates and nucleic acid replicates. Virus and nucleic acid spread from cell to cell.

3. Some viruses cause local lesions only, but the majority are systemic.

How do viruses cause symptoms?1. They require wounds - wither through vector feeding or mechanical.

2. Once inside host cell, protein coat disassociates and nucleic acid replicates. Virus and nucleic acid spread from cell to cell.

3. Some viruses cause local lesions only, but the majority are systemic.

4. Viruses produce a variety of symptoms, depending on the specific virus/host combination. Some viruses induce one type of symptom in one host but a different symptom in another host. They usually do not kill their hosts.

Ex: mosaic, deformed plant parts, vein necrosis, ringspots, yellowing, dwarfing, stunting, and rarely death.

Diagnosis of Plant Viruses1. Symptoms are NOT reliable.

2. Host reaction - using different hosts that are known to react differently based on the virus.

3. Serology - using antiserum prepared against a particular virus in tests such as ELISA and immunodiffusion.

ELISA

Agar Immunodiffusion

Diagnosis of Plant Viruses1. Symptoms are NOT reliable.2. Host reaction - using different hosts that are known to react differently based

on the virus.3. Serology - using antiserum prepared against a particular virus in tests such as

ELISA and immunodiffusion.

4. Microscopy - usually electron microscopes, although viral inclusion bodies can be seen using a light microscope. Some viruses aggregate into large masses in cells, termed inclusion bodies. These can be very different in shape (i.e., pinwheels) and appearance (crystalline or amorphous)

5. Detection of nucleic acids using electrophoresis or molecular hybridization.

1. Localized – symptoms occur at the point of inoculation only; do not spread. Local lesions.

2. Systemic- symptoms are not limited to point of inoculation. Chlorosis, stunting, etc.

Aspects of Symptom Expression

1. Localized – symptoms occur at the point of inoculation only; do not spread. Local lesions.

2. Systemic- symptoms are not limited to point of inoculation. Chlorosis, stunting, etc.

3. Specific virus/host interactions for symptoms expression.

4. Symptomless carriers – virus is present in a host but symptoms do not occur.

Aspects of Symptom Expression

Aspects of Symptom Expression5. Masked symptoms – virus is present but symptoms are

expressed only in response to some conditions exclusive of virus.

Ex. Temperature A = no symptoms; Temp. B = symptoms

6. Symptom synergy- plant is infected by more than one virus, and resulting symptom combination is more severe than symptoms of each virus alone.

7. Cross Protection – inoculation with a mild strain of a virus provided resistance to a more sever strain.

Ex. Citrus – mild strain of citrus tristeza virus

Tomato - mild strain of tobacco mosaic virus

Problems with cross protection

Mild strains are not available for all viruses

May not be effective against all strains

Labor intensive

Danger of mutations so that mild strain becomes severe.

Virus Dispersal

1. Insects- account for the largest part of virus dispersal.

– Aphids, leafhoppers, planthoppers, beetles, thrips, whiteflies, etc.

a. Insect species vary in the number of viruses they can carry. Viruses vary in the number of insect species that can vector them.

b. Dispersal depends on: i. Amount of virus in plant (aka. Titer)ii. Insect mobility iii. Relationship between insect and virus. Insects- account for the

largest part of virus dispersal.

Insect VectorsInsect VectorsAphids

Whiteflies

Insect VectorsInsect VectorsThrips

Leafhopper Plant Hoppers

Relationships 1) Non-persistent – virus is simply a contaminant on the mouthparts of insect.

It is transmitted immediately and for a fairly short time. Insect vectors = aphids, beetles, etc.

2) Persistent – insect can transmit the virus for a long time after acquisition. a. Non-propagative-does not multiply in vector.

b. Propagative – virus multiplies in the insect vector

i. Circulative – virus acquired by the insect, enters gut in plant sap, enters hemolymph of insect, travels to the salivary glands, multiplies and reinoculated into plants at subsequent feedings. ** Most propagative are circulative.

ii. Transovarial – Similar to cirulative except that the virus also passes into ovaries and enters insect eggs. Egg viability is decreased but vector is infective from hatch. Leafhoppers, planthoppers

Tobacco Mosaic Virus• Non – persistent• Tobacco, tomato, and other solanaceous plants

Barley Yellow Dwarf• Hosts - barley,wheat, maize, rice and other grasses• Persistent, circulative virus

Diagram of an aphid vector feeding on a plant host showing the internal route of the viruses that cause BYD.

Relationships

A necessary latent period between acquisition and transmission ability is required usually several hours.

Often with propagative viruses a substantial period of feeding is required before the viruses is acquired as well as transmitted.

Virus Dispersal1. Insects

2. Nematodes – Microscopic, nonsegmented wormsa. NEPO Viruses (Nematode transmitted Polyhedral viruses)

i. Examples vectors = Longidorus, Xiphenemaii. Example viruses = tomato black ring virus, cherry leaf roll

virus1. NETU Viruses (Nematode transmitted Tubular viruses)

a. Examples vectors = Trichodorus, Paratrichodorusb. Example viruses = tobacco rattle virus, pea early browning

virusa. Nematodes acquire virus when feeding on roots of infected

plants. b. Both adults and juveniles can transmit virus.c. Transmitted not persistently.

Nematode Vectors

Xiphinema indexXiphinema spp.

Longidorus spp.

Virus Dispersal1. Insects

2. Nematodes

3. Soilborne fungi – Primarily Plasmodiophoromycetes and Chytridiomycetes. Viruses are transmitted through zoospores.

a. Olpidium brassicae transmits: tobacco necrosis and lettuce big vein virus

b. Polymyxa graminis transmits: soilborne wheat mosaic virus and barley yellow mosaic virus

4. Seed

a. Fewer than 100 viruses are transmitted through seed.

b. Generally, < 10% of seed is infected by virus. Exception – tobacco ringspot virus on soybean, in which seed transmission is 100%.

Virus Dispersal1. Insects

2. Nematodes

3. Soilborne fungi

4. Seed

5. Vegetative Plant Parts – if parent is infected, cutting is infected

6. Parasitic Seed Plants – some viruses transmitted only through dodder.

7. Mechanical

1. Cutting and harvesting equipment

2. Simple touch of hand or clothing.

3. Carborundum – a powdered abrasive used to wound leaf surface before virus inoculation.