Viroids, Definition, Types, Characteristics and Diseases

Viroids are unique infectious agents that primarily infect plants, causing various diseases. Unlike viruses, viroids lack a protective protein coat and contain only a short strand of circular RNA. Understanding viroids is crucial for NEET biol ogy , especially when studying plant pathology and the diversity of infectious agents.

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Definition of Viroids

Discovery of Viroids

Structure of Viroids

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Characteristics of Viroids

• Viroids consist solely of RNA.
• They are extremely small and infect only plants.
• Viroids are among the smallest known infectious agents.
• They have a low molecular weight and a unique RNA structure.
• Viroids replicate within the host cell, causing changes that can eventually kill the plant.
• They are classified into two families: Pospiviroidae, which replicate in the nucleus, and Avsunviroidae, which replicate in the chloroplast.
• Viroids can move within plant cells through structures called plasmodesmata and travel long distances through the plant's phloem.

Types of Viroids

• Pospiviroidae : The Pospiviroidae family includes the type species Potato Spindle Tuber Viroid (PSTVd) and comprises five genera with over 24 species. This family is characterized by members that possess five major domains in their secondary structures and undergo asymmetric rolling-circle replication in the nucleus, typically lacking ribozyme activity. The Pospiviroidae family can infect a wider range of hosts , including both monocot and dicot plants . This classification highlights the diverse nature and infection patterns of viroids across different plant species.
• Avsunviroidae : The Avsunviroidae family features the type species Avocado Sunblotch Viroid (ASBVd) and consists of two genera with three total species. These viroids are known for their branched secondary structures and utilize a chloroplast-based symmetric rolling-circle replication mechanism, which includes ribozyme activity. Viroids from the Avsunviroidae family are restricted to infecting dicotyledonous plants, which can be either herbaceous or woody.

Viroid Diseases

Differences Between Viroids and Viruses

MCQs of Viroids

Q1. Which disease is caused by viroids?

1. Leaf curling disease
2. Potato spindle tuber disease
3. Dwarfing disease
4. All of these

Q2. In 1971 T.O. Diener discovered a new infectious agent that was smaller than viruses;

(i) it causes potato spindle tuber disease.

(ii) it is free RNA.

(iii) molecular wt. of RNA is low.

The above statements are assigned to;

1. viruses.
2. viroids.
3. virulent.
4. mycoplasma.

Q3.. Viroids differ from viruses in having;

1. DNA molecules with protein coat.
2. DNA molecules without protein coat.
3. RNA molecules with protein coat.
4. RNA molecules without protein coat.

Answers of MCQs of Viroids