what is viroid disease

Full Answer

What is a viroid in microbiology?

Viroids are the smallest infectious pathogens made up of a short circular RNA without the protein coat. These pathogens affect only the plants and are therefore known as plant pathogens. Single-stranded circular RNA molecules, viroid vary in length from 246 to 463 nucleotides and are found only in plants.

What are the diseases caused by viroids?

These infectious diseases are spread by the propagation of seeds in plants by cutting, tubers, etc and also by mishandling the contaminated implements. The only disease that is caused by viroids in humans is Hepatitis- D.

What is the history of viroids?

History. Parallel to agriculture-directed studies, more basic scientific research elucidated many of viroids' physical, chemical, and macromolecular properties. Viroids were shown to consist of short stretches (a few hundred nucleobases) of single-stranded RNA and, unlike viruses, did not have a protein coat.

What is viviroid infection?

Viroid infections can be transmitted by aphids, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact.

How do viroids induce symptoms?

There has long been uncertainty over how viroids induce symptoms in plants without encoding any protein products within their sequences. Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid-like symptoms. This indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex. The viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs, and induction of degradation or inhibition of translation causes the classic viroid symptoms.

What is the potato spindle tuber called?

This viroid is now called Potato spindle tuber viroid, abbreviated PSTVd. Soon thereafter was discovered the Citrus exocortis viroid (CEVd), and together understanding of PSTVd and CEVd shaped the concept of the viroid. Although viroids are composed of nucleic acid, they do not code for any protein.

How does the viroid genome affect virulence?

First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants.

What is the mechanism of viroid reproduction?

The reproduction mechanism of a typical viroid. Leaf contact transmits the viroid. The viroid enters the cell via its plasmodesmata. RNA polymerase II catalyzes rolling-circle synthesis of new viroids.

How are viroids transmitted?

Viroid infections can be transmitted by aphids, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact.

What are viroids made of?

Viroids are small infectious pathogens. They are composed solely of a short strand of circular, single-stranded RNA. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms, and most cause diseases, whose respective economic importance to humans varies widely. The first discoveries ...

When was the first pathogen of a eukaryotic organism for which the complete molecular structure has?

determined the complete nucleotide sequence of potato spindle tuber viroid in 1978. PSTVd was the first pathogen of a eukaryotic organism for which the complete molecular structure has been established. Over thirty plant diseases have since been identified as viroid-, not virus-caused, as had been assumed.

What is the viroid of potato?

The viroid PSTVd was the first potato pathogen for which the whole genomic sequence was cloned and determined (Gross et al., 1978; Dickson et al., 1979 ). The genome of PSTVd consists of a rod-shaped, circular ssRNA molecule of 356–360 nucleotides that contains characteristic secondary structures with functional importance ( Flores et al., 1999 ). Viroids are the smallest known plant pathogens, but still many of their functions remain unknown. One of their most puzzling features is that they do not encode proteins but mediate their functions through direct viroid RNA–cellular factor interactions. Mutational analysis of PSTVd (cDNA) has revealed several genomic regions important for replication and symptom induction ( Qi and Ding, 2003; Wang et al., 2004; and references therein).

How is potato spindle tuber viroid transmitted?

Potato spindle tuber viroid is transmitted in true potato seed by contact, besides the infected seed tubers. However, there is evidence indicating that PSTVd RNA can get encapsidated in the particles of velvet tobacco mottle virus (genus Sobemovirus) ( Francki et al., 1986) and PLRV ( Querci et al., 1997) in plants co-infected with PSTVd and the virus. Heteroencapsidation of PSTVd in virus particles would allow the transmission of PSTVd with the virus vectors, which in turn has important epidemiological implications. It would seem worthwhile to re-address these questions as infectious cDNAs of PLRV are currently available and determinants for virion assembly are being elucidated, as discussed above.

What is the role of DCL4 in RNA silencing?

The preponderance of 21 nt vdsRNAs indicates that DCL4 is a major player in the host RNA silencing defense against viroids. DCL1 and DCL2, giving 21 nt and 22 nt vdsRNAs, respectively, also play a part in the process. When tobacco plants transformed with an infectious cDNA construct of PSTVd produced autonomously replicating viroid, the cDNA transgene insert became methylated ( Wassenegger et al., 1994 ). This suggests that DCL3 may have been involved.

How many nt vdsRNAs are in a pospiviroids?

Deep sequencing of the pospiviroids HSVd, GYSVd, and PLMVd reveals mainly 21–22 nt vdsRNAs together with some 24 nt vdsRNAs ( Bolduc et al., 2010; Navarro et al., 2010 ). The HSVd and GYSVd vdsRNAs are derived from hotspots primarily in the (−) strand with some from the (+) strand, and the three vdsRNAs map to the same hotspots ( Navarro et al., 2010 ). In contrast, the PLMVd sRNAs are equivalently distributed between the (+) and (−) strands with only some evidence of hotspots; some of vdsRNAs encompass the hammerhead self-cleavage site, indicating that the circular conformers are processed by the RNA-silencing machinery. There is a bias in the relative abundance of the type of 5′ residue with AU>GC ( Bolduc et al., 2010 ). Deep sequencing of the asunviroid PLMVd shows a prevalence of 21 nt (+) and (−) strand vdsRNAs over the 22 nt vdsRNA with hotspots along the (+) and (−) strand genome ( Di Serio et al., 2009 ). There is a bias in the 5′ residue, with U dominating in the (+) strand and C in the (−) strand of the 21 nt vdsRNA and C dominating in the 22 nt (+) strand vdsRNA. The hairpin involved in PLMVd replication (Chapter 5, Section 1, D), and which has a succession of small stems separated by symmetrical bulges reminiscent of precursor miRNA, is a substrate for Dicers ( Landry and Perreault, 2005 ). Because PLMVd replicates in plastids wherein RNA silencing has not been reported, Di Serio et al. (2009) suggested that the dicing occurs during cytoplasmic movement of the viroid dsRNA generated by host RDR, most probably RDR6 in concert with DCL4 processing.

Why is viroid secondary structure important?

The strong base pairing of viroid secondary structure exposes the molecule to the RNA silencing host defense system that targets dsRNA. There must be strong selection pressures to maintain the secondary structure and to find ways of avoiding this defense system. The secondary structure of a viroid RNA may have evolved to be the optimal compromise between the necessity to survive cellular degradation by RNA silencing or nucleases and the necessity to interact with cellular factors for replication and movement ( Ding, 2009 ).

What are the threats of viroids?

Viroid diseases pose a potential threat to agriculture, and several are of considerable economic importance. Ready transmission of PSTVd by vegetative propagation, foliar contact, and true seed or pollen continues to pose a serious threat to potato germplasm collections and breeding programs. Coconut cadang-cadang has killed over 30 million palms in the Philippines since it was first recognized in the early 1930s. While many viroids were first detected in ornamental or crop plants, most viroid diseases are thought to result from chance transfer from endemically infected wild species to susceptible cultivars. Several lines of circumstantial evidence are consistent with this hypothesis:

How do viroids denature?

These structures denature by cooperative melting (T m in 10 mM Na + at about 50 °C) to single stranded circles of about 100 nm contour length. Viroids can also form metastable structures with hairpins that may be functionally important. However, at least two viroids, peach latent mosaic viroid (PLMVd) and chrysanthemum chlorotic mottle viroid (CChMVd), do not follow this rule, and their RNAs clearly appear to adopt branched conformations. There is experimental support for the contention that PLMVd and CChMVd have unique conformations because unlike other viroids, they are insoluble in 2 M LiCl. Some viroids also contain elements of tertiary structure like kissing-loops. Sequences vary from 246 to 401 nt in length and are rich in G+C (53–60%), except avocado sunblotch viroid (ASBVd) which has 38% G+C.

Why are my viroid leaves curled?

Better-known viroid plant diseases include: The classic signs of viroid plant diseases, such as yellowing and curled leaves, are believed to be caused by the viroids paring their own RNA with that of the afflicted plant’s messenger RNA, interfering with proper translation.

How are viroids similar to viruses?

Unlike viruses, viroids consist of a single strand RNA molecule and lack a protective protein coat. They were discovered in the late 1960s, and since then we’ve been trying to determine just how viroids differ from viruses.

How to keep aphids off of plants?

Keep aphids off of your plants by encouraging insect predators, like ladybugs, and eliminating the use of potent insecticides. After all, those guys can respond much faster than you ever will to an aphid infestation.

What is a virod?

What Is A Viroid: Information About Viroid Diseases In Plants. There are a lot of little tiny creatures that go bump in the night, from fungal pathogens, to bacteria and viruses, most gardeners have at least a passing familiarity with the monsters that wait to destroy their gardens.

When were viroids discovered?

Unlike viruses, viroids consist of a single strand RNA molecule and lack a protective protein coat. They were discovered in the late 1960s, and since then we’ve been trying to determine just how viroids differ from viruses.

Can aphids transmit viruses?

Sadly, you can’t do much. So far, we’ve yet to develop an effective treatment, so vigilance is the only preventive. It’s unclear if aphids transmit these tiny pathogens, but because they readily transmit viruses, it’s generally accepted that they are a potential vector.

What are Viroids?

Viroids are infectious pathogens that affect only plants , therefore are also called as the plant pathogens. Structurally, viroids are smaller than viruses and possess circular strands of ribonucleic acids (RNA’s) with no protein coating. These entities hijack the cellular machinery present in plant cells to replicate new copies of itself. It primarily affects all forms of higher plants.

What is the disease that causes potato tubers to crack?

Most of the viroids are said to infect the plants, including coconut and the apple trees. The (PSTV) potato spindle tuber viroid causes significant crop damage to the potato yields causing the tubers to elongate and then crack. The other common type of viroid infection symptoms includes stunting and leaf epinasty.

What is the cause of hepatitis D?

Hepatitis- D is caused in humans by viroid like particles. The symptoms that are caused by the infection of viroid in plants include stunting of growth, stem necrosis, deformation of the leaves and fruits, and at last causing the death of the plant. Most of the viroids are said to infect the plants, including coconut and the apple trees.

What are the two families of viroids?

Viroids are mainly classified into two families namely Pospiviroidae- nuclear viroids and Avsunviroidae- chloroplast ic viroids. Viroids are said to move in an intracellular manner, cell to cell through the plasmodesmata, and a long-distance through the phloem.

What are the characteristics of viroids?

Some of the characteristic features of viroids are given below-. Viroids contain only RNA. These are known to be smaller in size and infect only the plants. These are among the smallest known agents causing infectious disease. Viroids are the species of nucleic acid with relatively low molecular weight and a unique structure.

What is the structure of a virod?

Viroids differ from the virus in structure and form. These consists of solely short strands of circular, and single-stranded RNA without the protein coats. The plants that are infected by viroids are responsible for the crop failures and also causes loss of millions of dollars in the agricultural revenue every year.

What diseases are caused by viroids?

These infectious diseases are spread by the propagation of seeds in plants by cutting, tubers, etc and also by mishandling the contaminated implements. Hepatitis- D is caused in humans by viroid like particles.

How are viruses transmitted?

Most viruses are transmitted by vegetative propagation of new plant material from infected mother plants. Many plant viruses and viroids are spread by physical contact or by tools. Some orchid viruses are spread when healthy plants come in contact with diseased ones. Some viruses are pollenborne (Cherry leaf roll virus, ...

What are the vectors of viruses?

Many viruses enter the host plant via the feeding activity of vectors that transmit the virus into plant cells. Insects, especially aphids, whiteflies, and leafhoppers, vector many viruses. Thrips vector Tomato spotted wilt virus and Impatiens necrotic spot virus. Mites, nematodes, and lower fungi also serve as vectors of a few viruses.

What are the symptoms of a virus?

Virus diseases are recognized by several characteristic symptoms. Light and dark green mosaic patterns, mottles, ringspots, distortion of leaves and other plant parts, vein clearing, and vein enations are some of the symptoms seen in the leaves. Deformed, yellow, stunted growth, or overall stunting are other possible symptoms.

What is the procedure for positive identification of a virus?

Positive identification of virus infection involves several procedures, including visualization of virus particles with the electron microscope, serological techniques such as ELISA (en zyme-linked immunosorbance assay), sap inoculations of indicator plants, budding and grafting to indicator plants, microscopic examination for inclusion bodies (aggregates of virus particles or virus-induced protein structures), RNA and DNA hybrid ization, polymerase chain reactions (PCR), and gel electrophoresis.

Why use disposable gloves in greenhouse?

Disinfection of pruning or propagation tools between cuts, or at least between different plant sets, varieties, or species, and the use of disposable gloves can help reduce the spread of virus diseases in a greenhouse operation .

How long should bleach be used to disinfect?

A solution of 1 part household bleach in 4 parts of water, applied for 5 minutes, acts as an effective disinfectant for virus-contaminated materials (tools, benches, etc.). Bleach solutions must be rinsed off with clean water to avoid toxicity to plants. (Note: Bleach treatments are corrosive to metal tools.)

What is the management of a virus?

Management. Control of virus diseases is a matter of prevention–the use of virus-free planting stock and resistant varieties. Once a plant is infected by a virus it usually remains infected for the life of the plant as there are no available pesticide treatments.

Overview

Taxonomy

Transmission and replication

RNA silencing

Retroviroids

RNA world hypothesis

Control

History