Assistant Professor Dr. Adnan A. Lahuf
Agriculture College/University of Kerbala
Potato Mop-top virus,
a destructive virus should be prevented from entering Iraq
Assistant Professor Dr Adnan Abdaljeleel Lahuf
Plant Protection Department/Agriculture College/University of Kerbalal
Email:adnan.lahuf@uokerbala.edu.iq
There are plenty of devastating plant viruses affecting various plant hosts in Iraq. However, there are a lot have not entered to our country yet. Thus, the plant protector should pay attention on these important viruses in order to prevent them entering to our environment. One of these dangerous viruses is the Potato Mop top virus. it is known as the type species of the Pomovirus genus, Virgaviridae family. The particle morphology of PMTV is a tubular rod, with protein subunits arranged in a helix with a pitch of 2.4 to 2.5 nm around a hollow canal. Particles are 18-20 nm wide and found with two modal lengths either 100-150 or 250-300 nm (King et al., 2012).
The PMTV genome contains three positive-sense, single-stranded RNA species, approximately 6, 3-3.5 and 2.5-3 kb in size respectively (Torrance, 2008). The virus replicase protein is encoded on RNA1. It includes an open reading frame (ORF) which is interrupted by a stop codon (UGA) resulting in ORF1 (148 kDa). The ORF1 protein comprises methyltransferase (Mt) with helicase motifs (Hel). The sequence of RNA1 continues to encode a readthrough protein (RT) of 206 kDa that comprises the GDD RNA-dependent RNA polymerase (RdRp) motif. In RNA2, there are four ORFs. The first three overlaps and are known as the triple gene block (TGB) of movement proteins (Torrance et al., 2009). TGB1 is a 51 kDa protein containing deoxyribonucleotide triphosphate (dNTP) and helicase motifs. TGB1 is thought to bind genomic viral RNAs to enable their movement. TGB2 is a 13 kDa protein and TGB3 is a 21 kDa protein and both proteins have two transmembrane domains that associate with intercellular membranes, such as the endoplasmic reticulum (Lioliopoulou, 2002, Lahuf, 2015). As a result, the three TGB proteins act together to facilitate the movement of the virus in infected plants. The last ORF in RNA2 encodes a cysteine-rich protein (8 kDa) which is a virulence factor; besides, it has an important role in suppressing RNA silencing (VSRs; Lukhovitskaya et al., 2014). RNA3 contains an ORF that is interrupted by a UAG stop codon to encode a coat protein (CP; 19.7 kDa); the UAG stop codon can be suppressed to allow the production of the CPRT protein (67-91 kDa). The CPRT of PMTV is not required for systemic infection (Torrance et al., 2009). However, it is believed that the CPRT protein is necessary for PMTV transmission via S. subterranea in addition to its main function as a protector of genomic RNAs. The RNA3 is a variable size from 2315 to 3134 nts. This is because natural deletion mutations of 500-1000 nts occur in the RT domain (King et al., 2012).
The main economic damage caused by PMTV is spraing symptoms (brown arcs on potato flesh) on tubers of susceptible cultivars which lead to a serious quality problem. tubers with spraing symptoms are usually refused by food product manufacturers and supermarkets because they are unsuitable for chips and crisps, and thus undesirable for purchase. Some susceptible potato cultivars, PMTV might reduce productivity by up to 26%. (Latvala-Kilby et al., 2009).
The Andean region of South America is thought to be the origin of PMTV (Beuch et al., 2014). Nevertheless, symptoms of PMTV were first reported in Arran Pilot tubers in Northern Ireland before being reported by Todd (1965) in potato tubers grown in Scotland. One year later Calvert and Harrison (1966) were the first to study its impact, prevalence and transmission method and named it PMTV. Subsequently, PMTV was detected in many other European countries. For example, Santala et al. (2010) reviewed that the PMTV was recorded in the Netherlands, Norway, the Czech Republic, Sweden, Finland, Denmark and Switzerland. In Asia, PMTV has been discovered in China, Japan and Pakistan (Arif et al., 2014). It has also been observed in Canada and the United States of America (Xu et al., 2004), and in Costa Rica (Montero-Astúa et al., 2008). At present, PMTV occurs in many potato-producing regions in the world, particularly those in wet and cool climates.
Detection of PMTV by visual inspection is unreliable since the symptoms of PMTV in tubers (spraing) are inconsistent and depend on cultivar. Also, similar symptoms can appear in tubers infected by TRV. This has led to a search to find sensitive methods of virus-specific detection of PMTV (Beuch et al., 2014). There are two types of the enzyme-linked immunosorbent assay (ELISA) employed to identify PMTV in infected leaves and tubers: the double-antibody sandwich (DAS-ELISA) and a triple-antibody sandwich (TAS-ELISA) which are serological detection methods that depend on the immuno-interaction between PMTV proteins, such as the coat protein, with specific antibodies (Santala et al., 2010). In addition, molecular diagnostic methods, based on the amplification of a specific part of the PMTV genome, have been used. These include reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative RT-PCR (qRT-PCR) which proved to be more precise and thus used for detection and quantification of gene expression (Mumford et. al., 2000). FLASH-PCR qualitative amplification, which is similar to qPCR principles, but less costly, has been used to detect PMTV (Ryazantsev and Zavriev, 2009). There are also other RT-PCR methods used for detection of PMTV, for example, RT-PCR microplate hybridisation (Nakayama et al., 2010) and the immune capture RT-PCR, that combine the serological and molecular diagnosis methods (Latvala-Kilby et al., 2009). A soil-bait method has been utilized to isolate and detect PMTV from infested soil by planting bait plants or indicator plants such as several species of Nicotiana genus which are sensitive to PMTV infection (Arif, 1995; Davey 2009). Systemic infection of indicator plants is more successful at low temperatures. It is important to examine indicator plants at temperatures between 5-15°C to gain reliable results (Santala et al., 2010). On manual inoculation of indicator plants, such as Chenopodium amaranticolor and N. tabacum cv. Xanthi-nc or Samsun NN, PMTV produces concentric necrotic ring spots or chlorotic local lesions during local, non-systemic infection. Furthermore, immunosorbent electron microscopy can be used to reveal PMTV virions, if the virus titre is sufficient (Roberts and Harrison, 1979).
Only dicotyledonous plants are reported as natural hosts for Pomoviruses. Most host species of PMTV belong to the Solanaceae family of which 26 species are infected; however, Chenopodiaceae is a second family that contains several species regarded as hosts of PMTV. Other host species include Tetragonia expansa (Harrison and Reavy, 2002). It has been also indicated that the PMTV host species might be the natural hosts of S. subterranea (the only vector of PMTV). PMTV is considered to be both soil and tuber-borne. The PMTV symptoms can be divided into two types: Foliar symptoms can appear as a result of secondary infection when infected tubers are cultivated and three common foliar symptoms are: (A) yellow mottling or blotching, mostly of lower leaves, (B) chlorotic V-form markings in leaflets and (C) excessive stunting of shortened stems, known as `mop-top’ (Harrison and Reavy, 2002). The second type of symptoms appears in tubers as a result of primary infection from the soil. These include (A) brown circles on the tuber surface and (B) lines of brown curves (spraing) inside tuber of susceptible potato cultivars, such as Arran Pilot and Nicola (Santala et al., 2010). In tubers of plants grown from infected tubers, cracks form that distort the tubers, leading to tuber malformation. Some cultivars may show superficial external raised rings without internal symptoms.
The most important transmission method of PMTV is the natural transmission from the soil by S. subterranea (the cause of potato powdery scab) compared with transmission from infected seed mother to daughter tubers (Davey, 2009).
Accordingly, the PMTV is more likely entering Iraq through importing potato tubers particularly from the infectious European countries and planting them during Spring season which is cold and wet particularly in North of Iraq. Thus, considerable attention should be paid to prevent this virus from reaching in the Iraqi environment.This can be through examination and certification of the imported tubers using molecular diagnostic methods, as well as monitoring and periodic inspection of the cultivated potato fields to destroy the diseased plants as soon as possible. This of course will participate in preventing the large spread of the infection in the Iraqi potato fields.
References
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