VIRUSES
Definition: A virus is a submicroscopic, obligate parasite that can multiply only inside living host cells and is composed mainly of nucleic acid (DNA or RNA) and a protein coat.
- The word virus means “poison” in Latin.
- They cannot grow on artificial media.
- They lack cell wall, nucleus, cytoplasm, and organelles.
- All plant viruses contain either RNA or DNA, but never both.
General Characteristics of Plant Viruses
|
Feature |
Description |
|
Nature |
Acellular, obligate intracellular parasites |
|
Size |
Very small (20–300 nm) |
|
Structure |
Nucleic acid + Protein coat (capsid) = Nucleocapsid |
|
Nucleic Acid |
Either ssRNA (most common) or dsDNA |
|
Enzyme system |
Absent (depend on host metabolism) |
|
Multiplication |
Only inside living host cells |
|
Transmission |
Mainly through vectors (aphids, whiteflies, etc.) |
|
Cultivation |
Cannot be grown on artificial media; only in living tissues |
|
Shape |
Helical, Polyhedral, Rod-shaped, or Spherical |
Structure of a Typical Virus (Example: TMV)
- Tobacco Mosaic Virus (TMV) is rod-shaped (300 × 18 nm).
- Contains single-stranded RNA (ssRNA) and protein coat made of 2,130 capsomeres.
- TMV was first crystallized by Stanley (1935).
🧠 Remember: TMV → RNA virus → Rod shape → Infects tobacco, tomato, and other Solanaceous plants.
Classification of Plant Viruses (Based on Nucleic Acid)
|
Type |
Example |
Notes |
|
RNA Viruses (≈78%) |
TMV, Potato Virus Y (PVY), Cucumber Mosaic Virus (CMV) |
Most common |
|
DNA Viruses |
Cauliflower Mosaic Virus (dsDNA), Gemini Virus (ssDNA) |
Few in number |
Transmission of Plant Viruses
|
Mode |
Examples |
Remarks |
|
Mechanical / Sap |
TMV |
Through wounds or contact |
|
Seed transmission |
Cowpea mosaic virus |
Infected seed embryo |
|
Pollen transmission |
Alfalfa mosaic virus |
Pollen carries virus |
|
Vegetative propagation |
Sugarcane mosaic virus |
Through cuttings, tubers |
|
Vector transmission |
Aphids, whiteflies, leafhoppers |
Most common (≈80% of viruses) |
|
Soil transmission |
Tobacco rattle virus |
Via nematodes or fungi |
Common Virus Vectors
|
Vector |
Example of Virus Transmitted |
|
Aphids |
Potato leaf roll virus, Cucumber mosaic virus |
|
Whiteflies (Bemisia tabaci) |
Tomato yellow leaf curl virus |
|
Leafhoppers / Planthoppers |
Rice tungro virus |
|
Thrips |
Tomato spotted wilt virus |
|
Nematodes |
Tobacco rattle virus |
Important Virus-Related Facts
|
Question |
Answer |
|
Father of Virology |
M.W. Beijerinck (1898) |
|
First discovered virus |
Tobacco Mosaic Virus (TMV) |
|
Crystallized virus |
TMV by Stanley (1935) |
|
Virus-free plants obtained by |
Meristem culture |
|
Double-stranded DNA virus |
Cauliflower Mosaic Virus (CaMV) |
|
Single-stranded DNA virus |
Gemini virus (Goodman, 1971) |
|
Virus detected by |
ELISA test |
|
Prions |
Infectious proteinaceous particles (no nucleic acid) |
|
Viroid |
Infectious circular RNA (T.O. Diener, 1971) |
|
Example of Viroid disease |
Potato spindle tuber disease |
|
Satellite virus discovered by |
Kassanis (1962) |
|
Vector of Potato leaf roll virus |
Aphids |
|
Shape of TMV |
Rod-shaped |
|
Shape of Gemini virus |
Twin (Geminate) shaped |
