Viroids: Study with Video Lessons, Practice Problems & Examples

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

This video we're going to begin our lesson on viroids. First, we need to recall from some of our previous lesson videos that viroids, like viruses, are acellular obligate intracellular parasites. However, unlike viruses, which are made of proteins, nucleic acids, and sometimes lipids with enveloped viruses, viroids are only made of nucleic acid, not proteins, not lipids. More specifically, viroids are only made of a single short strand of RNA. This single short strand of RNA tends to form a closed ring, forming a ring structure. Viroids are only known to infect and cause disease in plant cells. Like plant viruses, these viroids need to enter the host plant cell via wound sites within the plant that can be caused by physical damage, like an insect causing physical damage. Viroids replicate using a host cell's RNA dependent RNA polymerase, an enzyme similar to the replicase of RNA viruses. Viroids do not encode for any proteins; instead, they act similarly to small interfering RNAs or siRNAs, capable of blocking the expression of plant genes, causing disease. If we look at our image below, we can get a better understanding of these viroids and viroid infections. Viroids consist of a single strand of RNA that is circular. In this case, you can see it creates the structure that you see here. The viroid can enter a plant cell via wounds. Here, you can see a healthy plant, but once it is infected with viroids, it can become an unhealthy plant with a disease caused by viroids. Notice that viroids do not get translated. Instead, they act similarly to siRNAs, where they block host mRNA translation, preventing gene expression of plant genes, leading to disease. The viroid siRNA generated from the viroid can block the host mRNA, preventing the translation of the host mRNA. As a result, these host proteins will not be expressed, which can lead to plant diseases. There are many questions about viroids that still exist. Continued research is ongoing every day to solve these issues and answer those questions about viroids. This here concludes our brief lesson on viroids and how they are acellular obligate intracellular parasites that are only made of RNA. I'll see you all in our next video.

Viroids characteristically are composed of:

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

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.

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.