Viroids: Videos & Practice Problems

Viroids are acellular obligate intracellular parasites composed solely of a single short strand of circular RNA, lacking proteins or lipids. They infect plant cells through wound sites, replicating using the host's RNA-dependent RNA polymerase. Unlike viruses, viroids do not encode proteins but can block plant gene expression, similar to small interfering RNAs (siRNAs), leading to plant diseases. Ongoing research aims to address unanswered questions about viroids and their mechanisms of pathogenicity.

Viroids

Viroids are unique infectious agents that, unlike viruses, consist solely of a single short strand of RNA, forming a closed ring structure. These acellular obligate intracellular parasites are known to infect only plant cells, causing various diseases. Viroids enter host plant cells primarily through wound sites, which can result from physical damage, such as that caused by insects.

Once inside the plant cell, viroids replicate using the host's RNA-dependent RNA polymerase, an enzyme similar to the replicase found in RNA viruses. Notably, viroids do not encode any proteins; instead, they function similarly to small interfering RNAs (siRNAs). This means that viroids can inhibit the expression of plant genes by blocking the translation of host mRNA. As a result, the absence of essential proteins can lead to disease symptoms in the infected plants.

In summary, viroids are circular RNA molecules that disrupt normal plant gene expression, leading to detrimental effects on plant health. Ongoing research continues to explore the complexities of viroids and their interactions with host plants, as many questions about their mechanisms and impacts remain unanswered.

Viroids characteristically are composed of:

• A. ssRNA.
• B. dsRNA.
• C. ssDNA.
• D. dsDNA.

Answer: (A)

Viroids are:

• A. Small fragments of naked dsDNA that infect plant cells.
• B. Small fragments of naked dsRNA that infect plant cells.
• C. Small fragments of naked ssRNA that infect plant cells.
• D. Small fragments of naked ssRNA that infect animal cells.
• E. Small fragments of naked ssDNA that infect animal cells.
• F. None of the above.

Answer: (C)

Here’s what students ask on this topic:

What are viroids and how do they differ from viruses?

Viroids are acellular obligate intracellular parasites composed solely of a single short strand of circular RNA. Unlike viruses, viroids lack proteins and lipids. Viruses are made of proteins, nucleic acids, and sometimes lipids (in the case of enveloped viruses). Viroids infect plant cells through wound sites and replicate using the host's RNA-dependent RNA polymerase. They do not encode proteins but can block plant gene expression, similar to small interfering RNAs (siRNAs), leading to plant diseases. In contrast, viruses can infect a wide range of hosts, including animals and humans, and often encode proteins necessary for their replication and infection processes.

How do viroids replicate within plant cells?

Viroids replicate within plant cells by utilizing the host's RNA-dependent RNA polymerase. This enzyme is similar to the replicase found in RNA viruses. The viroid RNA serves as a template for the synthesis of new RNA strands. Since viroids do not encode any proteins, they rely entirely on the host cell's machinery for replication. The replication process involves the synthesis of complementary RNA strands, which are then used to produce more circular viroid RNA molecules. This replication mechanism allows viroids to multiply within the host plant cells, leading to the disruption of normal cellular functions and causing disease.

What role do viroids play in blocking plant gene expression?

Viroids block plant gene expression by acting similarly to small interfering RNAs (siRNAs). Once inside the plant cell, the viroid RNA can be processed into smaller RNA fragments that interfere with the host's mRNA. These viroid-derived siRNAs bind to complementary sequences on the host mRNA, preventing its translation into proteins. By blocking the translation of specific mRNAs, viroids inhibit the expression of essential plant genes, leading to disrupted cellular functions and the development of disease symptoms. This mechanism of gene silencing is a key factor in the pathogenicity of viroids.

What are the common methods of viroid transmission in plants?

Viroids are commonly transmitted in plants through wound sites. These wounds can be caused by physical damage, such as insect bites, mechanical injury, or agricultural practices like pruning and grafting. Once a plant is wounded, viroids can enter the plant cells and initiate infection. Additionally, viroids can be spread through contaminated tools, seeds, and vegetative propagation materials. Insects that feed on plants can also act as vectors, carrying viroids from infected plants to healthy ones. Understanding these transmission methods is crucial for developing strategies to prevent and control viroid infections in crops.

What ongoing research is being conducted on viroids?

Ongoing research on viroids focuses on several key areas, including understanding their mechanisms of pathogenicity, exploring their interactions with host plants, and developing effective control strategies. Scientists are investigating how viroids interfere with plant gene expression and the specific pathways they target. Research is also being conducted to identify the host factors involved in viroid replication and movement within plants. Additionally, efforts are being made to develop diagnostic tools for early detection of viroid infections and to breed or engineer plants with resistance to viroids. This research aims to address unanswered questions and improve our ability to manage viroid-related diseases in agriculture.