Viroids- The smallest Pathogen

Viroids- The smallest pathogen

Viroid are small circular SS RNA molecules and the smallest pathogen with 246ntd in size 375ntds.
They cause a number of crop disease.
The extracellular form of virus is a naked RNA with no capsid.
The RNA molecule have no protein encoding genes show the viroid is fully dependent on host function for its replication.
The viroid RNA is single standard circle but another structure also takes place like ds molecule with closed ends.
It replicate in the host cell nucleus and replicate with the help of plant RNA polymerase .

Viroid disease

The result is a multimeric RNA consisting of many viroid unit joined end to end.

The viroid does contribute one function to it’s own replication because part of the viroid itself has ribosome activity.
The ribozyme activity is used for self cleavage of the multimeric which releases individual viroids.
Because of its structure the viroid are sufficiently stable to exist outside the host cell.
Because it lake a protein the viroid does not use a receptor to enter the host cell.
But the virus enters the plant through a wound as from insect or other mechanical damage.
Once it goes in the viroid moves from cell to cell through plasmodesmata.

Viroid cause a number of important plant disease which have severe agricultural impact.
Examples of some viroids are:-d
Coconut cadang- cadang viroid-246 ntd
Citrus exocortis viroid- 375 ntd
Patato spindle Tuber viroid- 359 ntd
Viroid infected plants can be symptom less or develop symptoms very mild or lethal depending upon the viroid.
Most severe symptoms are growth related.
No viroid diseases of animals and prokaryotes are known yet.

References