Characteristics of Viruses

Acellular Nature: Viruses are non-cellular entities, unlike bacteria, fungi, or other pathogens.

1. Sub-Microscopic and Intracellular: They are too small to be seen with a light microscope and can replicate only inside living host cells.
2. Lack of Lipid Membrane System and Energy Production: Viruses do not possess cellular machinery for energy metabolism.
3. Dependence on Host Machinery: They rely entirely on the host’s cellular machinery for replication and protein synthesis.

Structure of Viruses

1. Virion

• Definition: The complete, infectious virus particle is called a virion.
• Components:
  • Nucleoid: The nucleic acid genome (RNA or DNA, but never both).
  • Capsid: A protective protein coat.
  • Nucleocapsid: The combined structure of the nucleoid and capsid.
  • Envelope (in some viruses): A lipid layer derived from the host, containing viral proteins.
  • Virus Core (optional): An additional protein layer between the nucleoid and capsid in some viruses.
  • Accessory Components: Some viruses have lipids, carbohydrates, or enzymes.

2. Nucleoid (Nucleic Acid)

• Location: Inside the capsid.
• Type: Only one type of nucleic acid, RNA (common in plant viruses) or DNA.
• Function:
  • Encodes genetic information for virus replication and assembly.
  • Serves as the infectious component.
• Examples:
  • RNA Viruses: Most plant viruses, e.g., Tobacco mosaic virus.
  • DNA Viruses: Cauliflower mosaic virus.

3. Capsid (Protein Coat)

• Structure: Composed of protein subunits called capsomeres.
• Functions:
  • Protects the nucleic acid from external damage.
  • Facilitates viral entry into host cells.
  • Interacts with vectors for specific transmission.
  • Provides antigenic properties and determines the virus’s shape.

Morphology of Viruses

Viruses exhibit a variety of shapes and sizes:

1. Elongated (Rod or Thread-like)

• Rigid Rods: E.g., Tobacco mosaic virus (15 x 300 nm).
• Flexuous Threads: Long, flexible filaments (1–10 nm wide, 480–2000 nm long).

2. Cylindrical (Bacillus-like)

• Short, rod-shaped, e.g., Rhabdoviruses (52–75 x 300–380 nm).

3. Spherical (Isometric or Polyhedral)

• Polyhedral shapes, e.g., Tobacco necrosis satellite virus (17 nm diameter) and Wound tumor virus (60 nm diameter).

4. Multi-Component Viruses

• Contain multiple genome segments packaged in separate capsids.
  • Bipartite: E.g., Tobacco rattle virus (two rods of different lengths: 195 x 25 nm and 43 x 25 nm).
  • Multipartite: E.g., Alfalfa mosaic virus (four different components).

Composition and Structure

1. Viral Proteins

• Composed of amino acids.
• Encoded by the nucleotide sequence of the viral genome.
• Example: Protein subunit of Tobacco mosaic virus contains 158 amino acids and has a mass of 17,600 daltons.

2. Viral Nucleic Acids

• Type: RNA (most plant viruses) or DNA.
• Structure:
  • Long chains of nucleotides.
  • Each nucleotide comprises:
    • A base (adenine, guanine, cytosine, uracil in RNA, or thymine in DNA).
    • A sugar (ribose in RNA, deoxyribose in DNA).
    • A phosphate group.
• Genetic Role:
  • Directs protein synthesis.
  • Facilitates replication and virion assembly.

Virus Infection and Synthesis

1. Entry into Cells:
   • Through wounds (mechanical or vector-made) or via infected pollen grains into ovules.
2. RNA Virus Replication:
   • Uncoating of the viral RNA.
   • Induction of host cell to produce viral RNA polymerase.
   • Formation of complementary RNA strands.
   • Production of double-stranded RNA that separates to yield viral RNA.
3. DNA Virus Replication:
   • Viral dsDNA transforms into a minichromosome in the nucleus.
   • Formation of single-stranded RNAs.
   • Synthesis of virus-coded proteins and reverse transcription in the cytoplasm.

Transmission of Viruses

1. Vegetative Propagation: From infected plant parts; examples include potato mosaic and leaf roll.
2. Mechanical Transmission:
   • Infected sap through tools, insects, or human contact.
   • Common in TMV, Potato virus X.
3. Seed Transmission:
   • Internal transmission via infected ovules or pollen.
   • Examples: Muskmelon mosaic virus, Tobacco ring spot virus.
4. Pollen Transmission: Example: Sour cherry infected with Prunus necrotic ring spot virus.
5. Insect Transmission:
   • Major vectors: Aphids, leafhoppers, whiteflies, mealybugs, thrips, beetles.
   • Types:
     • Stylet-borne: Non-persistent, e.g., Cucumber mosaic virus.
     • Circulative: Persistent; propagate within the insect body, e.g., Tomato spotted wilt virus.
     • Transovarial: Transmitted across insect generations.
   • Examples of Insects and Viruses:
     • Green peach aphid: Potato virus Y.
     • Leafhoppers: Rice stunt virus.
     • Whiteflies: Papaya leaf curl virus.
6. Mite Transmission:Examples: Wheat streak mosaic virus.
7. Fungal Transmission:
   • Classes involved: Chytridiomycetes, Plasmodiophoromycetes, Oomycetes.
   • Example s: Lettuce big vein virus, Wheat soil-borne mosaic virus.
8. Nematode Transmission: NEPO and NETU viruses.Examples: Grapevine fan leaf virus.
9. Dodder Transmission: Parasitic bridge between plants. Examples: Cucumber mosaic virus, Tobacco mosaic virus.

Classification of Plant Viruses

The classification system includes orders, families, genera, and species. The major basis for classification includes genome type (RNA/DNA), strandedness, and replication method.

Major Groups of Plant Viruses: Plant viruses are divided into five major groups:

1. Single-Stranded Positive-Sense RNA [(+) RNA] Viruses

Characteristics:

• Genome: Single-stranded RNA (+)
• Largest group of plant viruses.

Examples:

1. Order: Nidovirales
2. Families:
   • Bromoviridae:
     • Genera: Bromovirus (Brome mosaic virus), Cucumovirus (Cucumber mosaic virus).
   • Closteroviridae:
     • Genera: Closterovirus (Beet yellows virus), Ampelovirus (Grapevine leafroll-associated virus).
   • Potyviridae:
     • Largest family with genera like Potyvirus (Potato virus Y), Ipomovirus (Sweet potato mild mottle virus).
   • Luteoviridae:
     • Genera: Luteovirus (Barley yellow dwarf virus), Polerovirus (Potato leafroll virus).

A. Single-Stranded Negative-Sense RNA [(−) RNA] Viruses

Characteristics:

• Genome: Single-stranded RNA (−).
• Only enveloped plant viruses fall into this group.

Examples:

1. Order: Mononegavirales
2. Families:
   • Rhabdoviridae:
     • Genera: Cytorhabdovirus (Lettuce necrotic yellows virus), Nucleorhabdovirus (Potato yellow dwarf virus).
   • Bunyaviridae:
     • Genera: Tospovirus (Tomato spotted wilt virus, Groundnut bud necrosis virus).

III. Double-Stranded RNA (dsRNA) Viruses

Characteristics:

• Genome: Double-stranded RNA.

Examples:

• Families:
  • Reoviridae:
    • Genera: Fijivirus (Fiji disease virus), Phytoreovirus (Wound tumor virus).
  • Partitiviridae:
    • Genera: Alphacryptovirus (White clover cryptic virus 1), Betacryptovirus (White clover cryptic virus 2).

B. Double-Stranded DNA (dsDNA) Viruses

Characteristics:

• Genome: Double-stranded DNA.

Examples:

• Family: Caulimoviridae
  • Genera: Caulimovirus (Cauliflower mosaic virus).

1. Single-Stranded DNA (ssDNA) Viruses

Characteristics:

• Genome: Single-stranded DNA.

Examples:

• Families:
  • Geminiviridae:
    • Genera: Mastrevirus (Maize streak virus), Begomovirus (Bean golden mosaic virus).
  • Circoviridae:
    • Genera: Nanovirus (Banana bunchy top virus).

Key Features of Plant Virus Classification

1. Genome Type:
   • RNA or DNA.
   • Single or double-stranded.
2. Order:
   • Three recognized orders: Mononegavirales, etc.
3. Family Grouping:
   • Families include genera with shared structural and functional characteristics.
4. Unassigned Genera:
   • Some genera remain unclassified.

Summary Table

Virus Infection and Synthesis

1. Entry into Plant Cells

Viruses cannot actively penetrate plant cells due to the rigid cell wall. They enter through:

• Mechanical Wounds: Caused by tools, environmental factors, or feeding by vectors like insects.
• Vector-Made Wounds: Created by insects, nematodes, mites, or other organisms that facilitate viral entry.
• Pollen Transmission: Viruses can infect ovules through infected pollen grains.

2. RNA Virus Replication

1. Uncoating: The viral RNA is released into the host cell.
2. Induction of Host Machinery:
   • Viral RNA induces the host cell to produce RNA polymerase specific for the virus.
3. Complementary RNA Synthesis:
   • Viral RNA serves as a template for the synthesis of complementary RNA strands.
4. Formation of Double-Stranded RNA:
   • Double-stranded RNA is formed, which separates to produce new viral RNA molecules.
5. Protein Synthesis:
   • New viral proteins are synthesized for assembly into complete virions.

3. DNA Virus Replication

1. Minichromosome Formation:
   • The viral double-stranded DNA (dsDNA) forms a minichromosome in the host nucleus.
2. Transcription:
   • The host machinery transcribes the viral DNA into single-stranded RNA (ssRNA).
3. Protein Synthesis:
   • Virus-coded proteins are synthesized in the cytoplasm.
4. Reverse Transcription:
   • ssRNA is reverse-transcribed into DNA to produce new viral genomes.

Spread of Viruses in Plants

1. Cell-to-Cell Movement

• Viruses spread locally through plasmodesmata, the microscopic channels connecting adjacent cells.
• Cytoplasmic Streaming facilitates the movement of viral particles.

2. Systemic Movement

• Viruses are transported long distances through the phloem and, rarely, through the xylem.
• Distribution can be limited in some cases, excluding apical meristems.

Transmission of Viruses

1. Vegetative Propagation

• Viruses spread through infected plant parts (e.g., tubers, cuttings, bulbs).
• Examples:
  • Potato mosaic virus
  • Leaf roll virus

2. Mechanical Transmission

• Infected sap is transferred via:
  • Contaminated tools, insects, or human contact.
• Examples:
  • Tobacco mosaic virus (TMV)
  • Potato virus X (PVX)

3. Seed Transmission

• Viruses infect seeds internally through:
  • Ovules or pollen.
• Examples:
  • Muskmelon mosaic virus
  • Tobacco ring spot virus

4. Pollen Transmission

• Viruses are transferred via infected pollen.
• Example:
  • Prunus necrotic ring spot virus in sour cherry.

5. Insect Transmission

• Major Vectors: Aphids, whiteflies, leafhoppers, mealybugs, thrips, beetles.
• Transmission Types:
  1. Stylet-Borne (Non-persistent): Viruses attach to the insect’s stylet and are transmitted briefly during feeding.
     • Example: Cucumber mosaic virus.
  2. Circulative (Persistent): Viruses circulate and propagate within the insect’s body before being transmitted.
     • Example: Tomato spotted wilt virus.
  3. Transovarial: Viruses pass through insect generations.
• Examples of Insect-Transmitted Viruses:

• • Potato virus Y: Transmitted by green peach aphid.
  • Rice stunt virus: Transmitted by leafhoppers.
  • Papaya leaf curl virus: Transmitted by whiteflies.

6. Mite Transmission

• Mites can vector viruses like:
  • Wheat streak mosaic virus.

7. Fungal Transmission

• Viruses are transmitted by certain fungal classes:
  • Chytridiomycetes
  • Plasmodiophoromycetes
  • Oomycetes
• Examples:
  • Lettuce big vein virus
  • Wheat soil-borne mosaic virus

8. Nematode Transmission

• Nematodes vector NEPO (nematode-transmitted polyhedral) and NETU (nematode-transmitted tubular) viruses.
• Examples:
  • Grapevine fan leaf virus

9. Dodder Transmission

• Parasitic plants like dodder create a bridge for virus transfer.
• Examples:
  • Cucumber mosaic virus
  • Tobacco mosaic virus

Symptoms of Viral Diseases

1. Common Symptoms

• Mosaic: Alternating light and dark green patches on leaves.
• Mottle: Uneven leaf color distribution, often milder than mosaic.
• Vein Clearing: Transparent veins due to chlorophyll loss.
• Vein Banding: Dark green bands along veins.

2. Distinct Symptoms

1. Ring Spots: Circular or concentric rings of discoloration.
   • Example: Tobacco ring spot virus.
2. Enations: Outgrowths on leaf surfaces.
   • Example: Pea enation mosaic virus.
3. Yellow Vein Mosaic: Yellowing along veins.
   • Example: Yellow vein mosaic virus in okra.
4. Leaf Curl: Upward or downward curling of leaves.
   • Example: Tomato leaf curl virus.
5. Stunting: Reduced growth of plants due to systemic infection.
6. Necrosis: Death of tissues in localized or systemic areas.
7. Chlorosis: Yellowing due to loss of chlorophyll.

3. Examples of Crop-Specific Symptoms

• Potato: Mosaic, leaf roll.
• Tomato: Yellowing, stunting.
• Okra: Yellow vein mosaic.
• Cucumber: Leaf curling, mosaic.
• Banana: Bunchy top.