Lily mottle virus (LMoV) is a plant virus of the Potyviridae virus family that causes asymptomatic to mild diseases of individual plant parts in plants of the lily family (Liliaceae). However, a frequently occurring simultaneous infection with other plant viruses, which on their own only cause moderate or no disease, can cause an entire plant to perish. This coinfection leads to considerable crop damage in lily cultivation and is therefore of great economic importance. Lily mottle virus is spread by aphids and in horticulture during vegetative propagation by splitting the lily bulb. LMoV was regarded as a synonym for a subtype of the Tulip breaking virus (TBV) that occurs in lilies, although since 2005 it has been classified as a closely related but independent virus species of the genus Potyvirus.
Discovery The symptoms of the plant disease caused by LMoV were already known in the 19th century. Yet it was not until 1944 that P. Brierley and F. F. Smith succeeded in proving a coinfection with two viruses as the cause through infection experiments on several tulip and lily species. They were able to detect the Lily symptomless virus (LSV, order Tymovirales: Betaflexiviridae: Carlavirus) in several lily species grown in the USA (Lilium auratum, L. speciosum, L. longiflorum), which showed streaky brightening (chlorosis) or individual necrotic spots on the leaves, which was always present simultaneously with the Cucumber mosaic virus (CMV) or the Lily mottle virus. They were also able to demonstrate that all three viruses are transmitted by aphids of the species Aphis gossypii.
Virus structure
Morphology Virus particles (virions) of the Lily mottle virus consist of a thread-like capsid with helical symmetry, in which a single-stranded RNA is packed as a genome; a viral envelope is not present. The capsid is 13 nm thick and about 740 nm long. The length of the capsid increases in the presence of divalent cations (especially calcium ions) in the preparation and decreases after binding by the addition of EDTA. The individual capsomeres that make up the capsid require a pitch of 3.4 nm for one helix turn. Compared to viruses with rigid rods and a comparable structure (e.g. the Tobacco mosaic virus- TMV), this duct height is relatively large and enables the LMoV capsid to be flexible and bendable. One turn requires 7.7 capsomeres, so that the entire capsid is composed of about 1700 capsomeres. The individual capsomeres consist of only one molecule of the LMoV capsid protein (CP, coat/capsid protein) with a length of 274 amino acids (33 kDa). The CP is folded several times in such a way that the N- and C-terminus point outwards. These outer ends of the capsid protein are very variable. The protruding N-terminus mainly determines the specific attachment to the host cell and enables the serological differentiation of different virus isolates. The highly conserved sections in the middle of the CP (216 amino acids) within the different members of the Potyviridae point inwards in the capsid and interact with the viral RNA.
The virions are stable against ethanol and lose their infectivity in the plant sap only after 10 minutes at 65-70 °C. The LMoV has a density of 1.31 g/ml in density gradient centrifugation (caesium chloride) and a sedimentation coefficient of 137 to 160 S.
Genome The genome of LMoV is a linear, single-stranded RNA with positive polarity [(+)ssRNA] and a length of 9644 nucleotides. A viral protein (VPg) is covalently bound to the 5'-end of the RNA. As with cellular messenger RNAs, a poly(A) tail of 20 to 160 adenosines is located at the 3' end of the viral genome. Between the two non-coding ends (NCR: non-coding region) is an open reading frame (ORF), which codes for a polyprotein of 3095 amino acids. This polyprotein is cleaved into the individual viral proteins by proteases during translation. An IRES structure was suspected in the 5'-NCR of potyviruses, as translation is initiated without a 5'-cap structure. LMoV does not have a cap structure, nor could an IRES be confirmed from sequence data. The VPg protein bound to the 5'-NCR possibly serves as a primer for the RNA polymerase to amplify the RNA. However, the VPg of other potyviruses also interacts directly with the translation initiation factors eIF4E and eIFiso4E. This could represent an as yet uncharacterized, Cap- and IRES-independent translation pathway.
Virus proteins and replication After infection, the virus enters the plant via the vascular bundle and is taken up by the cells through membrane vesicles (endocytosis). In the cytoplasm, the capsid disintegrates and the RNA is released. The viral RNA can also enter the cell very effectively via infected neighboring cells through cell contact sites (plasmodesmata). This direct transport of naked, infectious RNA is controlled by several viral proteins, including the so-called HC (helper component), which form a so-called movement complex. As with all (+)ssRNA viruses, the ingested RNA is first translated into protein at the ribosomes, as at least one copy of the viral RNA-dependent RNA polymerase is required for the replication of the RNA. After this has synthesized several copies of the viral RNA, the LMoV proteins are produced in large quantities. These accumulate at the synthesis sites of the viroplasm to form morphologically visible inclusion bodies. When infected with LMoV, these inclusion bodies have a characteristic, cylindrical to spiral-like shape in the cytoplasm; the virus protein that predominantly forms these cylinders is therefore also referred to as CI (cylindrical inclusion). Amorphous inclusion bodies are formed in the cell nucleus, which consist of two viral proteins NIa and NIb (nuclear inclusions). As the viral proteins are always formed in the same ratio during translation of the RNA and larger quantities of the capsid protein are required in comparison to other proteins, these proteins, which are not required in many copies, form inclusion bodies, are degraded or excreted from the cell.
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