In this article we will discuss about Viroids. After reading this article you will learn about: 1. Discovery of Viroids 2. Structure of Viroids 3. Multiplication.
- Discovery of Viroids
- Structure of Viroids
- Multiplication of Viroids
1. Discovery of Viroids:
Viroids were first discovered and given this name by Theodor Otto Diener (1971), a plant pathologist working at Agricultural Research Centre in Maryland. The first viroid to be identified was the Potato spindle Tuber Viroid (PsTVd). At present-33 species have been identified.
Viroids are known to cause different plant diseases. They move within a plant, probably in association with the host proteins via the phloem vascular channels and plasmodemata (cell contact points).
Viroids are transmitted by mechanical method, vegetative propagation and through pollens and seeds for e.g., Chrysanthemum stunt viroid (CSVd) is transmitted by vegetative propagation. Citrus exocortis viroid (CEVd), Hop stunt viroid (HSVd) is transmitted by mechanical method.
Avocado sun-blotch Viroid (ASBVd) is transmitted by seeds and pollens. Plot spindle tuber viroid (PSTVd) is transmitted at low frequency in a non-persistent manner by the aphid Macrosiphum euphorbiae. The only human disease known to be caused by a viroid is hepatitis D; in this case the viroid is enclosed in a hepatitis B virus capsule.
Viroids are classified in two groups: Avsunviroids e.g., Avocado Sun-blotch viroid and pospiviroids e.g., Potato spindle tuber viroid. Avsunviroids replicates in chloroplasts while pospiviroids replicates in nucleus and nucleolus.
2. Structure of Viroids:
Structure of viroid was first shown directly by electron microscope, Viroid’s are small, circular, single stranded RNA molecules. They consist a short stretch (a few hundred nucleobase) of highly complementary circular single stranded RNA without protein coat with molecular weight between 1,07,000 and 1,27,000. Viroid’s are 240 to 380 nucleotides long and all of them have dumb-bell structures (Fig. 1).
The smallest viroid is 220 nucleobase ScRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV) is reported.
H. J. Cross (1979) sequenced the nucleotide sequence of the Potato spindle Tuber Virus (PSTV). It consists of 359 ribonucleotides and is characterized by numerous intermolecular base pairings that lends ability in the structure. Structurally, the pospiviroid and Avsunviroid are also different. The pospiviroid has been divided into file structural/functional domains (Fig. 1).
1. Conserved central domain.
2. Pathogenic domain
3. Variable domain
4. Left terminal domain, and
5. Right terminal domain.
The structural domains are related to specific functions. The conserved central domain, mainly the upper strand is involved with cleavage and ligation of RNA. The left and right domain still waits for the function. Members of the avsunviroid group are different from the structure described above. They lack a conserved central Domain (CCR) and possess a ribozyme activity (a ribozyme is a catalytic RNA molecule, in the case RNA cleavage in the ribozymic activity).
3. Multiplication of Viroids:
Viroid RNA does not code for any protein. They replicate automatically and spread in the plant by recruiting host proteins. Three enzymatic activities are required for viroid replication: RNA pol. II, RNAase and RNA ligase.
The two groups of viroids replicate by two different methods:
Avsunviroids replicates via symmetric rolling circular mechanism (Fig. 2). The positive circular RNA strand of viroid serves as a template to make a large, multimetric -ve strand. Probably it is done by the activity of RNA pol II enzyme.
Avsunviroid replication of the long ─RNA is self-cleaved by the associated ribozyme activity. RNA circularize to form a -ve circle. A second rolling circle event makes a long +ve strand which is again cleaved by ribozyme activity.
The short viroid RNA is then ligated to the circular form. Pospiviroids with an asymmetric pathway make +RNA from the long linear molecule. A host RNA Ase activity cleaves the + strand into unit viroid length. The molecule is then cleaved to form a circular viroid.