Orchid Fleck Virus

''Orchid fleck dichorhavirus'', commonly called Orchid fleck virus (OFV), is a non-enveloped, segmented, single-stranded (ss) RNA negative-strand virus, transmitted by the false spider mite, ''Brevipalpus californicus''.
OFV causes necrotic and chlorotic lesions on the leaves of many genera in the family Orchidaceae.

Introduction

Orchid fleck virus, despite its presence worldwide, only affects a small spectrum of human life. Orchids are not used for food but rather serve mainly as ornamental decoration. Therefore, only about 2 to 3 scientific reports are written about OFV each year. The whole genome of OFV has been sequenced and its six main protein products have been sequenced as well.Kondo H, Maeda T, Shirako Y, Tamada T 2006. Orchid fleck virus is a rhabdovirus with an unusual bipartite genome. J Gen Virol 87:2413–2421.

Much is still not known about OFV including how exactly and why vector mites travel from orchid to orchid, and more host species of flowers are being discovered annually. The more information researchers discover, the more they realize there is still much left to learn. Many orchid enthusiasts are participating in "citizen science" by posting their OFV-related findings on international horticulture blogs and forums.

Virion morphology

OFV was first described as bacilliformDoi Y, Arai K, Yora K 1969
Distribution of bacilliform virus particles in Masaki mosaic disease and Cymbidium ring spot disease.
Ann Phytopathol Soc Jpn 35:388. but depending on the method of preparation, OFV can appear bullet-shaped or bacilliform. On average, OFV is 40 nm in diameter and between 100 and 150 nm long. Each viral particles is organized into a tight 25 turn helix, with a pitch of about 4.5 nm.Kondo H, Maeda T, Tamada T 2009
Identification and characterization of structural proteins of orchid fleck virus.
Arch Virol 154:37–45.

Genomics

OFV contains two ssRNA molecules, RNA1 and RNA2, of 6413 and 6001 base pairs, respectively. GenBank contains the whole sequenced genome of OFV. RNA1 (GenBank AB244417) codes for five proteins whereas RNA2 (GenBank AB244418) only codes for one. Both strands possess open reading frames (ORF), which are read in the negative sense.

RNA1
* ORF1: Nucleocapsid (N) protein
* ORF2: Phosphoprotein (P)
* ORF3: Proteins involved in viral cell-to-cell movement
* ORF4: Matrix (M) protein
* ORF5: Glycoprotein (G)

RNA2
* ORF6: Polymerase (L) protein (RNA-dependent RNA polymerase)

Pathophysiology

Chlorotic and necrotic flecks, spots, and/or ringspots, as well as yellow flecks or spots are all symptoms of an OFV infection. Studies have also shown that OFV may prevent the propagation of other viruses in an already OFV-infected plant.Peng DW, Zheng GH, Zheng ZZ, Tong QX, Ming YL 201
Orchid fleck virus: an unclassified bipartite, negative-sense RNA plant virus.
Archives of Virology.158(2):313-323.

Vector

The false spider mite, ''B. californicus'' serves as the major vector for OFV.Maeda T, Kondo H, Mitsuhata K, Tamada T 1998

In: Proceedings of the 7th Int Cong Plant Pathol, pp 13–18.
''Brevipalpus stages.Childers CC, Rodrigues JCV 2011
An overview of Brevipalpus mites (Acari: Tenuipalpidae) and the plant viruses they transmit
'' Zoosymposia '' 6:168–180. The protonymph, deutonymph, and adult stages can infect their host plants with OFV, whereas the larval stage is not infectious. Even after three weeks of incubation of an OFV-positive mite on an OFV-resistant plant, ''B. californicus'' proved to still be infectious, showing that OFV is persistent.Kondo H, Maeda T. & Tamada T 2003. Orchid fleck virus:Brevipalpus californicus mite transmission, biological properties and genome structure. Exp Appl Acarol 30, 215–223.

Hosts

OFV is able to naturally infect around 50 different species in 31 genera, all belonging to the family Orchidaceae. 25 other species from 11 non-orchid families have been infected through sap transmission or artificial viral inoculation.

Life cycle

Studies have not shown whether or not OFV actually replicates within ''B. californicus'' but electron microscopy has revealed an intricate viral life cycle within the host cells.

Viral ssRNA is replicated and transcribed into mRNA in the host cell's nucleus. Viral mRNA is then exported out of the nucleus into the cytoplasm where it is translated into viral protein by the host's ribosomes. The viral proteins then reenter the nucleus where they aggregate into a viroplasm. There, the various viral structural proteins assemble with both strands of ssRNA to form complete OFV particles. These particles often cluster in between the inner and outer nuclear membranes, causing visible projections which often evaginate into cytoplasmic vesicles. Electron microscopy has revealed clusters of viral particles positioned perpendicular to the inner nuclear membrane, the endoplasmic reticulum, as well as the aforementioned cytoplasmic vesicles, forming distinctive “spoked wheel”
structures.

Effects on fitness

Infected orchids don't bloom as well as healthy ones, affecting efficacy of pollination and fertilization. Also, the orchids that do bloom look lifeless making them less attractive on the cut flower market.

Epidemiology

Cases of orchid fleck virus or OF-like viruses have been reported in Australia, Brazil, China, Columbia, Costa Rica, Denmark, Germany, Japan, Korea, South Africa, and the United States, i.e. every continent except for Antarctica.

Due to the fact that viruses depend on their host cell for replication, OFV cannot be cultured independently. However, two non-orchid indicator hosts (plants used in research that show characteristic symptoms of specific viral infections) ''C. quinoa'' and ''T. expansa'' are commonly used for viral inoculation and isolation.

There are no known pathogens of OFV itself but its vector, ''B. californicus'' has a symbiotic relationship with bacteria of the genus ''Cardinium''. The symbiont is the cause of the mites' thelytokous method of reproduction (where females are produced from unfertilized eggs) and the explanation for the absence of male ''B. californicus'' mites.Groot TVM & Breeuwer JAJ 2006. Cardinium symbionts induce haploid thelytoky in most clones of three closely related Brevipalpusspecies. Experimental and Applied Acarology, 39, 257–271.

Diagnosis

Thin tissue samples from plants with visible symptoms of OFV can undergo:
* Electron microscopy to visualize virions and complete viral particles
* Serological analysis to identify and isolate specific viral proteins
* Reverse Transcription-Polymerase Chain Reaction to identify viral RNA

Prevention

Methods for preventing the spread of OFV among separate plants:Liu L, Lin ZK, Guo Y 2010. Progress on molecule biology of phalaenopsis virus and the corresponding prevention measures. Anhui Agric Sci Bull 16(21–23):126.
* Ensure seedlings are virus-free
* Improve quarantine measures
* Eliminate sources of infection (mites or other infected plants)
* Ensure proper environment for cultivation
* Work towards developing OFV-resistant plants through genetic engineering

Treatment

Once a plant is infected with OFV, it is unclear whether pruning visibly infected tissue will cure the plant of the virus. It is also unclear whether infected plants produce seeds containing viral particles.

History

The earliest recorded work concerning Orchid Fleck Virus was published in Japan in 1969. OFV was reported as concentrations of short, rod-like particles in chlorotic lesions on the leaves of boat orchids of the genus '' Cymbidium''.

Human relevance

Orchids and other tropical flowers are extremely important to the agricultural economy of many Southeast Asian countries. The ''Vanda'' 'Miss Joaquim' orchid hybrid is Singapore's national flower. According to the Orchid Society, the world market for orchids is worth more than $1 billion with Japan and the United States leading the way in 2010. In the US in 2005 produced around $144 million worth of orchids.U.S. Department of Agriculture. 2006a. Floriculture crops 2005 summary. Agricultural Statistics Board, Washington D.C.

References

{{Taxonbar, from=Q16986942

Orchid diseases
Mononegavirales
Rhabdoviridae