Viroids

Key Points to Remember

• Viroids are tiny, circular RNA molecules.
• They infect plants only and lack a protein coat.
• They do not code for proteins but still replicate inside plant cells.
• They cause major crop diseases, spreading through tools, seeds, or pollen.
• Managed through field hygiene, disease-free seeds, and monitoring.

Keywords

Viroid, Circular RNA, Plant Disease, RNA Replication, Potato Spindle Tuber Viroid (PSTVd), Non-coding RNA.

Viroids – The Simplest Infectious Agents

Viroids are the smallest known infectious agents, made up of only a short, circular strand of RNA.
Unlike viruses, they do not have a protein coat and cannot make proteins.
They infect only plants, but even with their tiny size, they can cause serious plant diseases that reduce crop yield and affect global agriculture.

Definition

Viroids are small, circular, single-stranded RNA molecules that can replicate independently inside plant cells.
Size: Usually between 250–400 nucleotides long.
Unique Feature: Viroids cannot produce proteins and have no protective coat.
Hosts: Affect only plants.

Discovery

• Discovered by: Theodor O. Diener in 1971.
• First disease identified: Potato Spindle Tuber Disease caused by the Potato Spindle Tuber Viroid (PSTVd).
• This finding changed the scientific view that all infectious agents must contain DNA or proteins.

Structure of Viroids

• RNA Only: They are made entirely of RNA with no protein or lipid envelope.
• Circular RNA: The RNA strand is closed in a loop, giving it stability.
• Highly Base-Paired: Their RNA forms loops and stems, protecting them from damage.
• No Protein Coding: They do not make proteins but can replicate using the host’s enzymes.

How Viroids Infect Plants

Step 1: Entry

Viroids can enter plants through:

• Damaged tissues (cuts, tools, insect bites, or grafting)
• Infected seeds or pollen

Step 2: Movement in the Plant

Once inside, they move from cell to cell and reach the phloem, spreading throughout the plant.

Step 3: Replication

• Viroids use the plant’s RNA polymerase enzymes to make copies of themselves.
• They follow a rolling circle mechanism — creating long RNA strands that are cut and shaped into new circular viroids.

Step 4: Disruption of Host Functions

Viroids interfere with the plant’s gene expression and growth processes, causing visible disease symptoms.

Step 5: Symptoms

Common signs of viroid infection include:

• Twisted or deformed leaves
• Stunted growth
• Reduced yield
• Spindle-shaped tubers (as in infected potatoes)
• Flower or fruit discoloration

Major Viroid-Related Plant Diseases

Viroid	Host Plant	Disease
PSTVd	Potato, Tomato	Potato Spindle Tuber Disease
CEVd	Citrus plants	Citrus Exocortis
CSVd	Chrysanthemum	Chrysanthemum Stunt
ASSVd	Apple	Apple Scar Skin
PLMVd	Peach	Peach Latent Mosaic

Why Viroids Are Dangerous

• There is no effective cure or treatment.
• They spread easily through contaminated tools or seeds.
• They are hard to detect in early infection stages.
• Some plants may carry them silently without symptoms.
• They can cause serious losses in agriculture.

Detection Methods

Scientists use several lab techniques to identify viroid infections:

• RT-PCR (Reverse Transcription PCR): Detects and amplifies viroid RNA.
• Nucleic Acid Hybridization: Uses labeled probes to identify RNA sequences.
• Gel Electrophoresis: Separates RNA by size and structure.
• Bioassays: Observing infection symptoms in indicator plants.

Prevention and Control

• Use certified disease-free seeds and plants.
• Sterilize tools and equipment regularly.
• Remove and destroy infected plants early.
• Control insects that can transmit viroids (like aphids and thrips).
• Maintain field hygiene and proper crop rotation.

Unique Features of Viroids

• The simplest known infectious agents — RNA only, no proteins.
• Depend completely on the host cell’s enzymes for replication.
• Can cause major diseases despite their minimal structure.

Conclusion

Viroids may be extremely small, but they are powerful plant pathogens.
Their simple RNA structure, ability to replicate using host machinery, and resistance to normal treatments make them a fascinating subject in plant pathology.

By improving early detection and preventive farming practices, we can protect crops and maintain global food security.