Cassava brown streak virus

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Cassava brown streak virus
Virus classification Red Pencil Icon.png
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Pisuviricota
Class: Stelpaviricetes
Order: Patatavirales
Family: Potyviridae
Genus: Ipomovirus
Species:
Cassava brown streak virus

Cassava brown streak virus is a species of positive-strand RNA viruses in the genus Ipomovirus and family Potyviridae which infects plants. [1] Member viruses are unique in their induction of pinwheel, or scroll-shaped inclusion bodies in the cytoplasm of infected cells. Cylindrical inclusion bodies include aggregations of virus-encoded helicase proteins. [2] These inclusion bodies are thought to be sites of viral replication and assembly, making then an important factor in the viral lifecycle. [3] Viruses from both the species Cassava brown streak virus and Ugandan cassava brown streak virus (UCBSV), lead to the development of Cassava Brown Streak Disease (CBSD) within cassava plants. [4]

Contents

History

Cassava brown streak virus (CBSV) was first described in the former Tanganyika territory of East Africa, now Tanzania 70 years ago. [1] An endemic viral outbreak progressed throughout the Eastern African coastal cassava-growing areas from Southern Kenya, through Tanzania to the Zambezi river in Mozambique. [1] Cassava, a tuberous, starch-rich plant (Manihot esculenta) has remained a staple food crop among countries of sub-Saharan Africa for many years, and is a primary food source for many families. [4] Loss of such crops has a large effect on food sources and economic stability;however, the extent of the loss cannot be fully determined until the crop is harvested. Transmission is most commonly attributed to whiteflies (two whitefly species, Bemisia afer (Priesner & Hosny) and Bemisia tabaci (Gennadius)). [1] Disease spread can be rapid, resulting in incidence exceeding 50% in some coastal regions of Tanzania and Mozambique. [1] CBSV and other related virus strains are responsible for up to $100 million USD in losses each year in Africa due to crop destruction.

Virology

Structure

Potyvirus virion. Non-enveloped, helical symmetry. Potyvirus Virion (1).jpg
Potyvirus virion. Non-enveloped, helical symmetry.

CBSV is believed to be a non-enveloped virus with a capsid protein coat that is most similar to that of the ipomovirus, SPMMV (Sweet potato mild mottle virus). [5] Similar to SPMMV, CBSV was found to have larger than average coat proteins of approximately 45 kDa in size.

Genome

Potyvirus genome. Linear, single stranded (+)RNA. Potyvirus Genome (1).jpg
Potyvirus genome. Linear, single stranded (+)RNA.

As with all of the members of the Potyviridae family, the Cassava brown streak virus has a positive, single strand RNA genome. [4] Structurally, viruses in this family are linear, non-segmented, possess helical symmetry. [6] The CBSV genome was measured to be between 8,900 and 10,818 nucleotides in length. [4] This is characteristic of the average genome length among potyviridae which can be anywhere from 9.3-10.8 kb. [6] Genetic diversity on the nucleotide level among CBSV virus isolates can be anywhere from 79.3-95.5%, showing that the virus has greater genetic diversity than other similar potyviridae or CBSV strains. [4] The Potyvirus genome is monopartite, meaning the genome is made up of a single molecule of nucleic acid, not broken up into multiple pieces. [6]

Replication cycle

Upon viral entry into the host cell, the virus uncoats its outer protein shell and releases the viral RNA into the host cell's cytoplasm. CBSV is a single stranded RNA (ssRNA(+)) virus, therefore the virus must use the host cell machinery to translate the ssRNA(+) to a polyprotein which then can be processed by the viral proteases to produce RNA dependent RNA polymerase (RdRp) and other structural proteins. The viral replication happens in the cytoplasm, where a dsRNA is made from the genomic ssRNA(+). Next, the dsRNA is replicated to provide mRNA and new ssRNA(+) which can then either pass through the cycle again to be replicated, or be used later on. With this newly produced mRNA/ssRNA(+), the virus can then assemble in the cytoplasm. The protein P3N-PIPO, a movement protein, may help to initiate cell-to-cell movement and transfer. [6]

Transmission

Potyviruses are known to be transmitted by mites and aphids. Past research has suggested that CBSV is transmitted by insects, specifically the whitefly. Scientists have used two specific species of whitefly, Bemisia afer (Priesner & Hosny) and B. tabaci (Gennadius), to experimentally test transmission rates. B. tabaci was found to be a more successful transmitter of the virus. Infection rates have been shown to correlate with the peak of whitefly population in the months of April and May. From 2002 to 2003, low numbers of whiteflies were observed in Tanzania, as well as little CBSD recorded. Likewise, in February and March 2004 when whitefly numbers soared, rapid spread of CBSD occurred. Laboratory studies, however, suggest that CBSV is transmitted poorly by B. tabaci and indicates high rates of infection in farming fields can be attributed to cutting-borne pathogens. [1]

Associated diseases

Cassava brown streak virus, as a potyvirus, promotes the development of disease and other ill effects within plant life, specifically in the woody Cassava shrub in particular. Both Cassava brown streak virus and the similar Ugandan cassava brown streak virus (UCBSV), lead to the development of Cassava Brown Streak Disease (CBSD) within Cassava plants. [4] It is also fairly common in infected plants to discover that they have been co-infected by both CBSV and UCBSV. [4] This disease is the main source responsible for the loss of Cassava crops in sub-Saharan Africa.

The disease manifests itself in many ways throughout the plant structure. The most common symptom of this disease is necrotic local lesions present in multiple places throughout the plant. In the storage tissues of the tuberous roots, the necrotic lesions appear as brown and dry spots on the plant. [7] Plants affected by CBSD may also develop symptoms in their leaves. CBSD is normally observed in mature or nearly mature leaves, yet does not affect expanding or immature leaves. These symptoms are often associated with the minor veins of leaves, including a type of feathery necrosis along the veins, as well as a blotchy yellow chlorosis. [1] Symptoms vary depending on severity of infection, and in the most severe cases the lesions may cause death of dormant axillary buds. Death of these buds is likely to cause death of the entire branch of the plant. [7]

Recent outbreaks

The most recent activity of this virus has not been expressly recorded in great detail. The most recent records of its activity show it continuing to cause widespread infection along areas of the eastern coast of Africa. These areas along the Indian Ocean coast of Africa include many of the areas where infections have persisted over time in sub-Saharan Africa such as Tanzania and Kenya. The most recent cases have shown a shift from the low altitude pattern of infection of areas below 1,000 meters above sea level. The most recent infections have branched into Uganda and the Democratic Republic of Congo where altitude levels are between 1,200 and 1,500 meters above sea level. [8] These new areas of infection were not previously considered to be at risk.

Prevention and antiviral strategies

Evidence has been collected that suggests that cross-protection against a diverse range of CBSD-causing virus isolates can be achieved through RNAi silencing. The conserved CP gene, which serves multiple functions in the CBSV life cycle, was used as the transgene of interest. Three distinct RNAi constructs were designed to target the FL, N-terminal, and C-terminal of the UCBSV-CP gene: the FL-CP construct exhibiting the highest level of protection. In conclusion, the study found a significant positive correlation between the presence of siRNAs and viral protection against UCBSV. A negative drawback to this antiviral approach is that RNAi is highly sequence specific. Viruses with more than 10% nucleotide diversity require the development of an RNAi construct with multiple fragments from different virus species. A construct made out of such large sections of genes could be very unstable during cloning and transformation procedures. However, the study included two transgenic plant lines in which remained completely immune to viral infection against six unique isolates of both UCBSV and CBSV. [2] RNAi FL-CP transgenic cassava lines also experienced 100% immunity when grafted with UCBSV infected cassava samples compared to nontransgenic control lines which became 100% infected with UCBSV over the 60-day period. [9] In the fields, it is common for asymptomatic plants to carry the virus and serve as a source of transmission with the help of whitefly vectors or stem cuttings. However, the study also collected RT-PCR data that suggested all asymptomatic plants also lacked detectable levels of UCBSV.

Related Research Articles

<i>Geminiviridae</i> Family of viruses

Geminiviridae is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: bright yellow mosaic, yellow mosaic, yellow mottle, leaf curling, stunting, streaks, reduced yields. They have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication. According to the Baltimore classification they are considered class II viruses. It is the largest known family of single stranded DNA viruses.

<i>Potyviridae</i> Family of viruses

Potyviridae is a family of positive-strand RNA viruses that encompasses more than 30% of known plant viruses, many of which are of great agricultural significance. The family has 12 genera and 235 species, three of which are unassigned to a genus.

Silverleaf whitefly Species of true bug

The silverleaf whitefly is one of several species of whitefly that are currently important agricultural pests. A review in 2011 concluded that the silverleaf whitefly is actually a species complex containing at least 40 morphologically indistinguishable species.

<i>Potyvirus</i> Genus of positive-strand RNA viruses in the family Potyviridae

Potyvirus is a genus of positive-strand RNA viruses in the family Potyviridae. Plants serve as natural hosts. The genus is named after member virus potato virus Y. Potyviruses account for about thirty percent of the currently known plant viruses. Like begomoviruses, members of this genus may cause significant losses in agricultural, pastoral, horticultural, and ornamental crops. More than 200 species of aphids spread potyviruses, and most are from the subfamily Aphidinae. The genus contains 190 species.

<i>Begomovirus</i> Genus of viruses

Begomovirus is a genus of viruses, in the family Geminiviridae. They are plant viruses that as a group have a very wide host range, infecting dicotyledonous plants. Worldwide they are responsible for a considerable amount of economic damage to many important crops such as tomatoes, beans, squash, cassava and cotton. There are 445 species in this genus.

Cassava mosaic virus Genus of viruses

Cassava mosaic virus is the common name used to refer to any of eleven different species of plant pathogenic virus in the genus Begomovirus. African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), and South African cassava mosaic virus (SACMV) are distinct species of circular single-stranded DNA viruses which are transmitted by whiteflies and primarily infect cassava plants; these have thus far only been reported from Africa. Related species of viruses are found in India and neighbouring islands, though cassava is cultivated in Latin America as well as Southeast Asia. Nine species of cassava-infecting geminiviruses have been identified between Africa and India based on genomic sequencing and phylogenetic analysis. This number is likely to grow due to a high rate of natural transformation associated with CMV.

Indian cassava mosaic virus(ICMV) is a plant pathogenic virus of the family Geminiviridae. It affects cassava in India and certain other countries. It is considered to be an invasive species.

<i>Wheat streak mosaic virus</i> Species of virus

Wheat streak mosaic virus (WSMV) is a plant pathogenic virus of the family Potyviridae that infects plants in the family Poaceae, especially wheat ; it is globally distributed and vectored by the wheat curl mite, particularly in regions where wheat is widely grown. First described in Nebraska in 1922, stunted growth and the eponymous “streaks” of yellowed, non-uniform discoloration are characteristic of WSMV infection. As it has been known to cause 100% crop mortality, WSMV is a subject of ongoing scientific research.

<i>Tobacco etch virus</i> Species of virus

Tobacco etch virus (TEV) is a plant virus in the genus Potyvirus and family Potyviridae. Like other members of the genus Potyvirus, TEV has a monopartite positive-sense, single-stranded RNA genome surrounded by a capsid made from a single viral encoded protein. The virus is a filamentous particle that measures about 730 nm in length. It is transmissible in a non-persistent manner by more than 10 species of aphids including Myzus persicae. It also is easily transmitted by mechanical means but is not known to be transmitted by seeds.

<i>Tobacco streak virus</i> Species of virus

Tobacco streak virus (TSV) is a plant pathogenic virus of the family Bromoviridae, in the genus Ilarvirus. It has a wide host range, with at least 200 susceptible species. TSV is generally more problematic in the tropics or warmer climates. TSV does not generally lead to epidemics, with the exception of sunflowers in India and Australia, and peanuts in India.

Tomato yellow leaf curl virus (TYLCV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. TYLCV causes the most destructive disease of tomato, and it can be found in tropical and subtropical regions causing severe economic losses. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci, commonly known as the silverleaf whitefly or the sweet potato whitefly. The primary host for TYLCV is the tomato plant, and other plant hosts where TYLCV infection has been found include eggplants, potatoes, tobacco, beans, and peppers. Due to the rapid spread of TYLCV in the last few decades, there is an increased focus in research trying to understand and control this damaging pathogen. Some interesting findings include virus being sexually transmitted from infected males to non-infected females, and an evidence that TYLCV is transovarially transmitted to offspring for two generations.

Sweet potato feathery mottle virus (SPFMV) is a member of the genus Potyvirus in the family Potyviridae. It is most widely recognized as one of the most regularly occurring causal agents of sweet potato viral disease (SPVD) and is currently observed in every continent except Antarctica. The number of locations where it is found is still increasing; generally, it is assumed that the virus is present wherever its host is. The virus has four strains that are found in varying parts of the world.

Commelina mosaic virus (CoMV) is a plant pathogenic virus in the genus Potyvirus and the virus family Potyviridae. Like other members of the Potyvirus genus, CoMV is a monopartite strand of positive-sense, single-stranded RNA surrounded by a capsid made for a single viral encoded protein. The virus is a filamentous particle that measures about 707-808 nm in length. This virus is transmitted by two species of aphids, Myzus persicae and Aphis gossypii, and by mechanical inoculation.

Cassava brown streak virus disease (CBSD) is a damaging disease of cassava plants, and is especially troublesome in East Africa. It was first identified in 1936 in Tanzania, and has spread to other coastal areas of East Africa, from Kenya to Mozambique. Recently, it was found that two distinct viruses are responsible for the disease: cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Both have (+)ss RNA genomes, belong to the genus Ipomovirus in the family Potyviridae, and produce generally similar symptoms in infected plants. Root rot renders the cassava tuber inedible, resulting in severe loss of economic value; therefore, current research focuses on achieving cultivars that do not develop the necrotic rot. This disease is considered to be the biggest threat to food security in coastal East Africa and around the eastern lakes.

Ipomovirus is a genus of positive-strand RNA viruses in the family Potyviridae. Member viruses infect plants and are transmitted by whiteflies. The name of the genus is derived from Ipomoea – the generic name of sweet potato. There are seven species in this genus.

<i>Watermelon mosaic virus</i> Species of virus

Watermelon mosaic virus (WMV) also known as Marrow mosaic virus, Melon mosaic virus, and until recently Watermelon mosaic virus type 2 (WMV-2), is a plant pathogenic virus that causes viral infection in many different plants. The virus itself is referred to as Watermelon Mosaic Virus II or WMV-2 and is an isolate of the U.S. WMV-2 is a ssRNA positive strand virus that is part of the Potyviridae or Potyvirus clade. Like all RNA viruses, it contains a protein capsid which protects the inner viral RNA. First described on squash in Florida, WMV arose from a unique recombination of genetic material contributed by Soybean mosaic virus (SMV) and Bean common mosaic virus (BCMV) along with Peanut Stripe virus (PSV).

Chilli leaf curl virus(ChiLCV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. ChiLCV causes severe disease especially in pepper, but also affects other crops such as tomato. It can be found in tropical and subtropical regions primarily in India, but has also been detected in countries such as Indonesia and Sri Lanka. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci. The primary host for ChiLCV are several Capsicum spp., but host species also include tomato and amaranth. ChiLCV has been responsible for several epidemics and causes severe economic losses. It is the focus of research trying to understand the genetic basis of resistance. Currently, a few sources of resistance have been discovered and used to breed resistant varieties.

Papaya leaf curl virus(PaLCuV) is a DNA virus from the genus Begomovirus and the family Geminiviridae. PaLCuV causes severe disease in papaya, but can sometimes infect other crops such as tobacco or tomato. It can be found in tropical and subtropical regions primarily in India, but closely related species have also been detected in countries such as China, Malaysia, Nigeria and South Korea. This virus is transmitted by an insect vector from the family Aleyrodidae and order Hemiptera, the whitefly Bemisia tabaci. PaLCuV has been responsible for several epidemics and causes severe economic losses. Because of the broad diversity of these viruses, their characterization and control remains difficult.

<i>Sweet potato leaf curl virus</i> Species of virus

Sweet potato leaf curl virus is commonly abbreviated SPLCV. Select isolates are referred to as SPLCV followed by an abbreviation of where they were isolated. For example, the Brazilian isolate is referred to as SPLCV-Br.

Carrot virus Y (CarVY) is a (+)ss-RNA virus that affects crops of the carrot family (Apiaceae), such as carrots, anise, chervil, coriander, cumin, dill and parsnip. Carrots are the only known crop to be infected in the field. Infection by the virus leads to deformed roots and discolored or mottled leaves. The virus is spread through insect vectors, and is currently only found in Australia.

References

  1. 1 2 3 4 5 6 7 Maruthi, MN; Hillocks, RJ; Mtunda, K; Raya, MD; Muhanna, M; Kiozia, H; et al. (2005). "Transmission of Cassava brown streak virus by Bemisia tabaci (Gennadius)". Journal of Phytopathology. 153 (5): 307–12. doi:10.1111/j.1439-0434.2005.00974.x.
  2. 1 2 Patil, BL; Ogwok, E; Wagaba, H; Mohammed, IU; Yadav, JS; Bagewadi, B; et al. (2011). "RNAi-mediated resistance to diverse isolates belonging to two virus species involved in Cassava brown streak disease". Mol Plant Pathol. 12 (1): 31–41. doi:10.1111/j.1364-3703.2010.00650.x. PMC   6640250 . PMID   21118347.
  3. ↑ Parker, JSL; Broering, TJ; Kim, J; Higgins, DE; Nibert, ML (2002). "Reovirus core protein mu2 determines the filamentous morphology of viral inclusion bodies by interacting with and stabilizing microtubules". J Virol. 76 (9): 4483–96. doi:10.1128/jvi.76.9.4483-4496.2002. PMC   155082 . PMID   11932414.
  4. 1 2 3 4 5 6 7 Ndunguru, J.; Sseruwagi, P.; Tairo, F.; Stomeo, F.; Maina, S.; Djinkeng, A.; Boykin, L. M. (2015). "Analyses of Twelve New Whole Genome Sequences of Cassava Brown Streak Viruses and Ugandan Cassava Brown Streak Viruses from East Africa: Diversity, Supercomputing and Evidence for Further Speciation". PLOS ONE. 10 (10): e0139321. Bibcode:2015PLoSO..1039321N. doi: 10.1371/journal.pone.0139321 . PMC   4595453 . PMID   26439260.
  5. ↑ Monger, WA; Seal, S; Isaac, AM; Foster, GD (2001). "Molecular characterization of the Cassava brown streak virus coat protein". Plant Pathol. 50 (4): 527–34. doi: 10.1046/j.1365-3059.2001.00589.x .
  6. 1 2 3 4 ViralZone: Potyvirus. (n.d.). Retrieved from http://viralzone.expasy.org/all_by_species/50.html
  7. 1 2 Kaweesi T, Kawuki R, Kyaligonza V, Baguma Y, Tusiime G, Ferguson ME. Field evaluation of selected cassava genotypes for cassava brown streak disease based on symptom expression and virus load. Virol J [Internet]. 2014;11(1):216. Available from: http://www.virologyj.com/content/11/1/216
  8. ↑ Mohammed, IU; Abarshi, MM; Muli, B; Hillocks, RJ; Maruthi, MN (2012). "The symptom and genetic diversity of cassava brown streak viruses infecting cassava in East Africa". Adv Virol. 2012 (795697): 1–10. doi: 10.1155/2012/795697 . PMC   3290829 . PMID   22454639.
  9. ↑ Yadav, JS; Ogwok, E; Wagaba, H; Patil, BL; Bagewadi, B; Alicai, T; et al. (2011). "RNAi-mediated resistance to Cassava brown streak Uganda virus in transgenic cassava". Mol Plant Pathol. 12 (7): 677–87. doi:10.1111/j.1364-3703.2010.00700.x. PMC   6640337 . PMID   21726367.
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