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  .

Viroids- The smallest Pathogen

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.

Viroids- The smallest Pathogen