The key difference between viroid and virusoid is that viroid is a small infectious agent composed only of a single-stranded RNA while virusoid is a type of infectious circular single-stranded RNA which requires a helper virus to infect the cell.
Viroid and virusoid are two types of infectious particles composed of single-stranded RNA. They are different from viruses since they do not possess a protein coat. However, similar to viruses, they are unable to self replicate. Therefore, they need a host cell to reproduce. Moreover, viroids and virusoids are the smallest subviral replicons.
CONTENTS
1. Overview and Key Difference
2. What is a Viroid
3. What is a Virusoid
4. Similarities Between Viroid and Virusoid
5. Side by Side Comparison – Viroid vs Virusoid in Tabular Form
6. Summary
What is a Viroid?
Viroid is an infectious RNA particle composed of a single-stranded circular RNA. It is a few hundred base pairs long. Viroids often exist as pairs. They were first discovered and named by plant pathologist Theodor O. Diener in 1971. Potato Spindle Tuber Viroid (PsTVd) was the first viroid identified; up to now thirty-three species of viroids have been identified. Viroids do not contain a protein capsid. They are just RNA molecules which are infectious. Since viroids are RNA particles, they can be destroyed by ribonucleases. The size of the viroid is smaller than a typical virus particle. Viroids also need a host cell for multiplication.
Figure 01: Viroid
Viroids do not cause human diseases. They only infect higher plants. Potato spindle tuber disease and chrysanthemum stunt disease are two diseases caused by viroids. Moreover, viroids are responsible for crop failures and the loss of millions of money in agriculture annually. Potato, cucumber, tomato, chrysanthemums, avocado and coconut palms are frequently subject to viroid infections. Viroid infections are transmitted by cross-contamination followed by mechanical damage of the plant. Some viroid infections are transmitted by aphids and leaf to leaf contact.
What is a Virusoid?
Virusoid is a small pathogenic circular single-stranded RNAs similar to viroids. However, virusoid requires a helper virus to replicate and establish an infection. J.W. Randles and coworkers discovered the virusoids in 1981. Virusoids also possess a few hundreds of base pairs similar to viroids. Moreover, virusoids are considered as a specific group of satellite RNAs. Human hepatitis D virus is a virusoid. Barley yellow dwarf virus satellite RNA is another viruoid, and its helper virus is Luleovirm. Tobacco ringspot virus satellite RNA and its helper virus Nepovirus is another example of a virusoid.
Figure 02: Virusoid
Virusoid replicates in the cytoplasm of the host cell using an RNA-dependent RNA polymerase. But, it does not interfere with the replication of their helper virus.
What are the Similarities Between Viroid and Virusoid?
- Viroids and virusoids are circular single-stranded RNA molecules that are infectious.
- They are unable to self-replicate.
- They do not possess protein capsids.
- Moreover, they do not code for proteins.
- Both viroid and virusoid are smaller than viruses.
- Furthermore, both are non-living disease agents.
- Both viroid and virusoid can infect commercially important agricultural crops.
What is the Difference Between Viroid and Virusoid?
Both viroid and virusoid are single-stranded, circular RNAs that lack a protein capsid. The key difference between viroid and virusoid is that viroid does not require a helper virus to establish an infection while virusoid requires a helper virus to establish an infection in the host. Moreover, viroid replication takes place in the nucleus of the host while virusoid replication takes place in the cytoplasm of the host cell by utilizing the transcription and processing machinery encoded in part by their helper viruses and in part by their hosts. So, this is another difference between viroid and virusoid. Furthermore, viroids are not encapsidated while virusoids are encapsidated by their helper virus coat proteins.
The below infographic summarizes the difference between viroid and virusoid.
Summary – Viroid vs Virusoid
Viroid and virusoid are two types of pathogenic RNA composed of single-stranded RNA molecules that are a few hundred base pairs long. They do not possess a protein capsid. Both are not encoding any protein, and they replicate via the rolling-circle mechanism. However, virusoid requires a helper virus to establish an infection, unlike viroid. Thus, this is the key difference between viroid and virusoid.
Reference:
1. “Microbiology.” Lumen, Available here.
2. “Viroid: Definition & Structure.” Study.com, Available here.
Image Courtesy:
1. “PSTviroid” Par Author: Jakub Friedl (user kyknos) — Source file with layers (gimp xcf file) (CC BY-SA 3.0) via Commons Wikimedia
2. “BYDV SS” By Rfam – Rfam, Public Domain) via Commons Wikimedia