Rice hoja blanca virus (RHBV), Spanish for "white leaf rice virus", is a
plant virus
Plant viruses are viruses that have the potential to affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathoge ...
in the family ''
Phenuiviridae
''Phenuiviridae'' is a family of negative-strand RNA viruses in the order '' Hareavirales''. Ruminants, camels, humans, and mosquitoes serve as natural hosts.
Virology
Structure
Members of ''Phenuiviridae are'' enveloped viruses with hel ...
''. RHBV causes Hoja blanca disease (HBD), which affects the leaves of the rice plant ''
Oryza sativa
''Oryza sativa'', having the common name Asian cultivated rice, is the much more common of the two rice species cultivated as a cereal, the other species being ''Oryza glaberrima, O. glaberrima'', African rice. It was History of rice cultivation ...
'', stunting the growth of the plant or killing it altogether. RHBV is carried by an insect
vector
Vector most often refers to:
* Euclidean vector, a quantity with a magnitude and a direction
* Disease vector, an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematics a ...
, ''
Tagosodes orizicolus'', a type of
planthopper
A planthopper is any insect in the infraorder Fulgoromorpha, in the suborder Auchenorrhyncha, a group exceeding 12,500 described species worldwide. The name comes from their remarkable resemblance to leaves and other plants of their environment ...
. The virus is found in South America, Mexico, throughout Central America, the Caribbean region, and the southern United States. In South America, the disease is endemic to Colombia, Venezuela, Ecuador, Peru, Suriname, French Guiana and Guyana.
Virology
Viral classification
RHBV is a single stranded, negative-sense
RNA virus
An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...
of the genus ''
Tenuivirus'', derived from the Latin "''tenui"'', meaning thin or weak. This comes from the nature of ''Tenuiviruses'' to form thin, filamentous viral particles. Other viruses in the genus ''Tenuivirus'' include
maize stripe virus (MSV),
rice stripe virus (RSV), and
rice grassy stunt virus (RGSV). While RHBV and other tenuiviruses have single stranded RNA genomes, it is interesting to note that examination of non-denaturing
gel electrophoresis
Gel electrophoresis is an electrophoresis method for separation and analysis of biomacromolecules (DNA, RNA, proteins, etc.) and their fragments, based on their size and charge through a gel. It is used in clinical chemistry to separate ...
performed with tenuiviruses has yielded both single stranded and double stranded RNA. Because it is unlikely that both single stranded and double stranded RNA's are encapsidated within the viral
ribonucleoprotein
Nucleoproteins are proteins conjugated with nucleic acids (either DNA or RNA). Typical nucleoproteins include ribosomes, nucleosomes and viral nucleocapsid proteins.
Structures
Nucleoproteins tend to be positively charged, facilitating inter ...
, it is theorized that the double stranded RNA sometimes present in gel electrophoresis results comes from the annealing ''in vitro'' of single stranded RNA's that possess opposite polarities.
Viral structure
Previous to 1982, RHBV was theorized to be a part of the ''
Closterovirus'' group, due to the presence of 8-10 nanometer particles observed in the cells of plants affected by HBV. Such particles are often found in cells affected by closteroviruses. However, further research into the subject yielded results indicating that RHBV was more similar to viruses in the newly formed ''Tenuivirus'' group. The particle morphology of tenuiviruses is largely consistent across the genus, and RHBV is no exception. RHBV forms fine, thin, filamentous non-enveloped
nucleocapsid
A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or ma ...
particles that vary in length and range from 3
nanometer
330px, Different lengths as in respect to the Molecule">molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling
Despite the va ...
s to 8 nanometers in width. Examination of viral particles from infected plant cells indicates that they are sometimes configured in tight, heliocentric spiral formations. However, RHBV ribonucleoproteins tend to be mostly circular in formation, as are many other members of the genus ''Tenuivirus''.
Genome
RHBV possesses a tetrapartite genomic structure, meaning that the RHBV genome is composed of four separate RNA components, named RNA1-RNA4, and are ordered in decreasing base pair size.
These components encode for the enzyme and various proteins that make up the virion. RNA1 is 8,999 base pairs long and is the first and largest segment of the genome, encoding for the viral
RNA-dependent RNA polymerase
RNA-dependent RNA polymerase (RdRp) or RNA replicase is an enzyme that catalyzes the self-replication, replication of RNA from an RNA template. Specifically, it catalyzes synthesis of the RNA strand Complementarity (molecular biology), compleme ...
, which is used by the virus to help synthesize a complementary RNA strand. RNA2 (3,620 base pairs) encodes for the first
nonstructural protein In virology, a nonstructural protein is a protein encoded by a virus but that is not part of the viral particle. They typically include the various enzymes and transcription factors the virus uses to replicate itself, such as a viral protease ( 3CL ...
(NS2) and a
membrane glycoprotein. RNA3 (2,299 base pairs) encodes for another nonstructural protein (NS3) and the nucleocapsid protein. RNA4 (1,998 base pairs) encodes for the major nonstructural protein and another minor nonstructural protein, NS4. RNA2-RNA4 use an
ambisense
In molecular biology and genetics, the sense of a nucleic acid molecule, particularly of a strand of DNA or RNA, refers to the nature of the roles of the strand and its complement in specifying a sequence of amino acids. Depending on the context, ...
coding strategy, meaning those RNA segments contain both negative sense and positive sense sections, in order to code for two proteins.
Replication cycle
In insects
RHBV is propagative, meaning it infects both
host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
* Host Island, in the Wilhelm Archipelago, Antarctica
People
* ...
plants and its insect
vector
Vector most often refers to:
* Euclidean vector, a quantity with a magnitude and a direction
* Disease vector, an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematics a ...
. ''
Tagosodes orizicolus'' (changed from ''Sogatodes orizycola'' in 1993)'','' is a species of planthopper, and the known common vector of RHBV.
It belongs to the family ''
Delphacidae
Delphacidae is a family of planthoppers containing about 2000 species, distributed worldwide. Delphacids are separated from other "hoppers" by the prominent spur on the Tibia (arthropod leg), tibia of the hindleg.
Diet and pest species
All speci ...
,'' as do the planthopper vectors of other tenuiviruses. ''T. orizicolus'' feed in the
phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...
of target plants, giving them the unique ability to infect plants without outright destroying plant tissue, which in turn promotes successful infection by RHBV. However, plants can be susceptible to "hopper burn", where excessive feeding cause an affected plant to dry up. Infection in ''T. orizicolus'' begins when a planthopper feeds on an infected plant. After the virus is acquired by the host, it will incubate for a period ranging for 4–31 days. The infected planthopper can then transmit the virus to new plants it feeds on.
Vertical transmission
Vertical transmission of symbionts is the transfer of a microbial symbiont from the parent directly to the offspring. Many metazoan species carry symbiotic bacteria which play a mutualistic, commensal, or parasitic role. A symbiont is acq ...
is possible transovarially from infected females to their offspring and from males to their progeny, although RHBV is the only tenuivirus known to be paternally transmitted. Transmission efficiency is variable between different populations of ''T. orizicolus'', and can be selectively bred to 100% efficiency over controlled generations.
In plants
Plant infection for RHBV is relatively standard for negative-sense single stranded RNA plant viruses, consisting of entering the cell, using an RNA-dependent RNA polymerase to convert to positive-sense RNA, and using the host's cellular machinery to produce viral proteins and genomes. Research shows that RHBV likely uses "
cap-snatching", a technique where the virus cleaves and uses the 5' cap of the mRNA of the host cell, in order to begin synthesis of viral mRNA. However, there are some unique qualities possessed by RHBV. The NS3 protein encoded by RNA3 has been shown to suppress
RNA silencing
RNA silencing or RNA interference refers to a family of gene silencing effects by which gene expression is negatively regulated by non-coding RNAs such as microRNAs. RNA silencing may also be defined as sequence-specific regulation of gene expressi ...
in both rice and insect vectors, contributing to the successful propagation of the virus. Research shows that RNA3 interferes with the cell's siRNA's by binding to them, which in turn suppresses their ability to assist in RNA silencing. Also interesting to note is that NS4 is synthesized in large amounts in RHBV infected plant cells. In the rice stripe virus genome, research shows that it is likely that RSV NS4 is a movement protein that accumulates adjacent to and inside of cell walls and helps move viruses or infectious material between plant cells. Due to similar accumulation of NS4 in RHBV infected cells, it is possible that RHBV NS4 serves a similar purpose, although this has not yet been explicitly confirmed by experimentation.
The RHBV NS3
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
is a suppressor of
RNA silencing
RNA silencing or RNA interference refers to a family of gene silencing effects by which gene expression is negatively regulated by non-coding RNAs such as microRNAs. RNA silencing may also be defined as sequence-specific regulation of gene expressi ...
used by hosts for
host defense.
Analogs exist in other tenuiviruses.
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RHBV symptoms
The symptoms caused by RHBV are seen across the whole spectrum of the infected plant, and can be different depending on the specific rice strain and the age of the plant upon infection. Plant tissues that are already mature upon infection remain symptomless, while immature tissues display a variety of symptoms. Approximately 4 days after infection, cream-colored spots ranging in size from begin to appear on immature leaves. Eventually, infected leaves suffer
chlorosis
In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...
and the spots turn white as the leaves mature, giving the virus its namesake. As time goes on, chlorotic spots increase and form stripes, with entire leaves eventually succumbing to infection and becoming completely chlorotic. RHBV infection is systemic to the plant, and subsequent leaves will emerge displaying either heavy stripes or complete chlorosis.
Tillers infected by RHBV will be stunted compared to normal, non-infected tillers. The
panicle
In botany, a panicle is a much-branched inflorescence. (softcover ). Some authors distinguish it from a compound spike inflorescence, by requiring that the flowers (and fruit) be pedicellate (having a single stem per flower). The branches of a p ...
s of infected tillers may be sterile, and often have malformed or discolored grains. Root size and number will reduce, with the roots eventually turning brown and dying completely.
RHBV can also cause symptoms in its insect vector, infecting various organs and glands in the planthopper. These infections can have mild to severe results on the host, leading to organ failure, respiratory and digestive track failure, and muscle atrophy Such infections can possibly reduce the fertility or longevity of an infected female.
History and impact
RHBV was first described biologically in the 1980s, although it had been destroying rice crops in the Americas for almost half a century before scientists were able to understand it and identify it as a viral agent. The first hint that HBD was caused by a virus came from the cyclical nature of its
epidemics
An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of Host (biology), hosts in a given population within a short period of time. For example ...
and their association with the planthopper ''T. orizicolis''. Agricultural records indicate that the disease was first observed in the
Cauca Valley area of Colombia in 1935. Within approximately 3 decades, RHBV and the associated HBD were present in various South American countries and had crossed the Caribbean sea to Cuba and the southern Florida peninsula. It has since been affecting rice crops in many countries located throughout both tropical and subtropical America, generating moderate to severe yield losses. In 1965, yield losses in Cuba and Venezuela ranged between 25-50%. In 1981, serious epidemics in the countries of Colombia, Ecuador, and Venezuela occurred, causing yield losses of up to 100% in the worse cases.
Similarities to other viruses
RHBV is most closely related to
Echinocloa hoja blanca virus (EHBV), another member of the ''Tenuivirus'' genus.
Proteins in both the RHBV genome and the EHBV genome have been shown to be antigenically related and to possess a high degree of similarity. However, the two viruses have separate insect vectors, with EHBV infecting ''Tagosodes cubanus'' and RHBV infecting ''T. orizicolis''. Nucleotide sequence similarity is around 80-85% in coding regions, and only 46-53% in non-coding regions.
References
External links
ICTVdB - The Universal Virus Database: Rice hoja blanca virus
{{DEFAULTSORT:Rice Hoja Blanca Virus
Tenuiviruses
Viral plant pathogens and diseases