EPWS 310 - PLANT PATHOLOGY

PLANT VIRUSES

Reading: Chapter 14

Contents

I Definitions

II Properties of viruses

III Taxonomy

IV Taxonomic criteria

V Composition and Structure

VI Transmission - Virus Vectors

VII Virus Replication

VIII Detection

IX Physiology of virus-infected plants

X Symptoms

XI Control

XII Examples

I Definitions:

A virus is a subcellular, submicroscopic, infectious obligate parasite composted of nucleic acid within a protein coat that uses the host cellular machinery to replicate itself.

A viroid is a low molecular weight covalently closed circular RNA molecule that may cause disease. Viroids are ssRNA, but have a high level of secondary folding. An example is potato spindle tuber viroid.

II Properties of viruses

• Do not divide or multiply on their own.
• Reproduce by inducing the host cell to form more virus.
• Do not consist of cells and therefore have no organelles.
• Cannot penetrate cells on their own - enter through wounds or vectors
• Are not self-motile - depend upon various types of vectors for transmission.
• The virus nucleic acid is encapsidated by a protein. The protein surrounding the nucleic acid is referred to as the coat protein or capsid which is formed by protein subunits.
• The majority of plant viruses are made up of RNA. At least 80 viruses contain DNA.
• 60-95% of the virus is made of protein and the remaining 5-40% is nucleic acid.
• The RNA or DNA is what carries the genetic information.

III Taxonomy:

Plant viruses are named on the basis of the symptoms induced on the first reported host plant.

• All viruses belong to the kingdom Viruses. Some viruses are separated into families, ex.Comoviridae, Rhabdoviridae, Potyviridae. Within the families are what was previously considered virus groups and now is called the virus genus. Examples include Comovirus, Cytorhabdovirus, Potyvirus. Within the genus are the plant viruses, ex. cowpea mosaic virus, lettuce necrotic yellows virus, lettuce mosaic virus.
• Generally abbreviated by first initial of each word of the name, i.e., tobacco mosaic virus = TMV.
• Can become confusing because viruses infect more than one host and viruses may cause more than one symptom on a host, or may cause distinctly different symptoms on two different hosts.

IV Taxonomic criteria: (see handout)

1. type, size and number of nucleic acid units

2. architecture, symmetry, size and number of virus particles

3. transmission

4. replication strategy

5. genome organization

1a) Nucleic acid

• (+) ssRNA : i. e. Tobacco mosaic virus.
• (-) ssRNA: i. e. Tomato spotted wilt virus.
• dsRNA: i. e. Wound tumor virus
• ssDNA: i.e. Beet curly top virus
• dsDNA: i. e. Cauliflower mosaic virus

1b) Genome segments

1. monopartite (genome in one segment)
2. bipartite (genome divided in two segments)
3. multipartite (genome divided in three or more segments)

V Composition and Structure:

1) Particle morphology:

• Rigid rod particles : i.e. Tobacco mosaic virus
• Flexuous or filamentous particle i.e. Potato virus X
• Isometric, spherical or polyhedral particles: Cucumber mosaic virus

Geminated particles: i.e. Tobacco yellow dwarf virus

• Bacilliform: i.e. Broccoli necrotic yellow virus

VI Transmission - Virus Vectors:

1. Mechanical/pruning
2. Vegetative propagation
3. seed
4. pollen
5. fungi

-virus can be found external and/or internal to fungus.

-Example: Big vein of lettuce is vectored by the fungus Olpidium.

6. nematodes

-Transmit 27 known viruses

-Nematode-virus relationships tend to be specific.

-Transmit by feeding - virus carried on the stylet/foregut region.

7. eriophyid mites
8. parasitic higher plants - dodder

-Dodder can transmit viruses through twining stems

- Dodder makes vascular connection with the plant, so virus can move through the phloem.

9. Insects

o types: aphids, leafhoppers, whiteflies, thrips, beetles, mealybugs, treehoppers

§ Homoptera: aphids, leafhoppers and whiteflies. - Most common and important of the insect vectors.

§ Piercing/sucking mouthparts - makes virus transmission more efficient as virus is placed directly into the plant.

§ Virus-vector relationships can be general in which the insect is able to transmit a number of viruses, or may be specific, where the insect only transmits one virus.

• Mode of insect transmission:
• Stylet-borne or non-persistent - the virus is carried on the stylet. It takes only a few seconds for the insect to acquire the virus. When the vector moves to the next healthy, susceptible plant and begins feeding, the virus is transmitted. The virus does not accumulate in the vector and the vector can only transmit the virus for a short period of time.
• Semipersitent or stylet/foregut-Long feeding periods (several minutes to few hours) are required before they accumulate enough virus for transmission. These viruses persist in the vector for a few (1-4) days.
• Persistent-circulative: the virus accumulates internally in the vector and is circulated throughout the insect. In general, it takes several hours for the insect to acquire the virus; however, the virus can be transmitted until all the acquired virus has been transmitted.
• Persistent-Propagative: the virus actually replicates in the insect. Again, it generally takes several hours for the insect to acquire the virus and days before the insect can begin transmitting. The insect transmits virus throughout its life.

1. Parasitic higher plants:

VII Virus Replication

A. Infectious virus RNA acts like mRNA, + sense RNA

2. Viral RNA polymerase copies + sense RNA into complementary strand
3. Complementary strands, - strand of RNA acts as a template for replication of + strand RNA
4. + strand RNA serves as messenger RNA for translation of viral proteins, including capsid protein.
5. Capsid proteins polymerizes on the viral (+) RNA -> virion produced capsid - protein coat.

VIII Detection

Serology

Microscopy

Transmission, mechanical or vector

Nucleic acid analysis

IX Physiology of virus - infected plants:

1. Decrease in photosynthesis.
2. Decrease the production of some growth regulators: gibberellin and auxin.
3. Increase the production of other growth regulators: ethylene and ABA (Absisic Acid).
4. Respiration is generally increased immediately after infection as the plant works to replicate the virus. Respiration may then remain high, return to normal or may decrease.

X Symptoms:

-Symptoms produced by viruses may be difficult to distinguish between symptoms caused by other organisms, nutrient deficiencies, chemical toxicities, etc.

-Occur on the entire plant, on leaves, stems, fruit, and roots.

-Reduced growth - stunting, dwarfing

-Necrotic or chlorotic spots (local lesions)

-Mosaics - yellow and green mottling of foliage, occur in what can appear to be a pattern or may appear as streaks.

-Ringspots - chlorotic rings on leaves, fruit, stems.

-Vein clearing - the veins turn yellow or white while the rest of the leaves stay green. May be the first symptom which occurs and as the infection ages, the vein clearing turns into a mosaic type symptom or general chlorosis.

-Leaf and stem distortion - curling, rolling, twisting, rippling, small leaves, bushiness at terminals, enations.

-Fruit distortion - bumps, deformation, streaks.

-Wilt

-Tumors

-Stem pitting - cankers

-Viruses can infect locally (local lesions) or may be systemic (move throughout the vascular system in the plant).

-Viruses can infect plants causing visible symptoms. In this situation the virus is called a latent virus and the host is known as a symptomless carrier. In addition, plants which usually develop symptoms may be temporarily symptomless under certain environmental conditions. In this case, the symptoms are masked.

XI Control:

• Plant resistant varieties
• Control insect vectors
• Control weed hosts
• Sanitation - remove diseased plants
• Alter planting date
• Clean stock or seed

XII Examples:

Virus- host-vector

• Alfalfa Mosaic alfamovirus - aphids.
• Cucumber Mosaic cucumovirus. - aphids, seed-borne, wide host range
• Zucchini yellow mosaic potyvirus - aphids, narrow host range
• Beet curly top geminivirus. - beet leafhopper, wide host range
• Lettuce mosaic potyvirus - aphids, seed borne.
• Pepper Mottle potyvirus. - pepper - aphids
• Tomato spotted Wilt tospovirus. - pepper - thrips.
• Barley yellow dwarf luteovirus - aphids, persistent-circulative
• Beet yellows cloterovirus - aphids
• Citrus tristeza closterovirus - aphids
• Tobacco mosaic tobamovirus

- Alfalfa Mosaic Virus (AMV) on pepper - Calico virus:

• World-wide distribution
• legumes, solanaceous, some woody hosts
• Aphid vectors - ~ 14 spp. Can act as vectors - non-persistent, stylet borne.
• Seed - borne estimate 10 - 50% alfalfa seed contamination
• Symptoms - white blotching, stunting.
• Control - avoid alfalfa.

Cucumber Mosaic Virus (CMV):

• v. Wide host range.
• Leaves mottled.
• distorts fruit
• overwinters in perennial weeds usually in the roots.
• Moves into new spring foliage for pick up by aphids.
• Insecticides + resistant varieties.

- Beet Curly-Top Virus (BCTV) on peppers:

• Wide host range: tomatoes, beans. Tumbleweeds, mustards.
• Vectored by Beet leaf hopper.
• Plant is usually very stiff - due to phloem damage.
• Control - remove weeds, diseased plant.:

-Pepper Mottle Virus. - aphid.:

• Mis-shappen leaves.
• Puckered but not as chlorotic as AMV
• late summer
• Overwinter in Datura.

-Tomato Spotted Wilt Virus - Thrips:

• temperate/sub-tropical
• Wide host range.
• chlorosis/necrosis/stunting
• ring spots
• overwinter in perennial weeds.

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