Rice hoja blanca tenuivirus

RHBV is a single stranded, negative-sense RNA virus 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 performed with Tenuiviruses has yielded both single stranded and double stranded RNA.[1] Because it is unlikely that both single stranded and double stranded RNA's are encapsidated within the viral ribonucleoprotein, 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.[2]

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.[3] The particle morphology of Tenuiviruses is largely consistent across the genus, and RHBV is no exception. RHBV forms fine, thin, filamentous non-enveloped nucleocapsid particles that vary in length and range from 3 nanometers to 8 nanometers in width.[4] 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.[5]

RHBV is propagative, meaning it infects both host plants and its insect vector. Tagosodes orizicolus (changed from Sogatodes orizycola in 1993), is a species of planthopper, and the known common vector of RHBV.[7] It belongs to the family Delphacidae, as do the planthopper vectors of other Tenuiviruses. T. orizicolus feed in the phloem 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 is possible transovarially from infected females to their offspring[8] and from males to their progeny, although RHBV is the only Tenuivirus known to be paternally transmitted.[9] Transmission efficiency is variable between different populations of T. orizicolus, and can be selectively bred to 100% efficiency over controlled generations.[10]

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 host cell, in order to begin synthesis of viral mRNA.[11] However, there are some unique qualities possessed by RHBV. The NS3 protein encoded by RNA3 has been shown to suppress RNA silencing 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.[12] Also interesting to note is that NS4 is synthesized in large amounts in RHBV infected plant cells.[13] 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.[14] 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 is a suppressor of RNA silencing used by hosts for host defense.[15] Analogs exist in other Tenuiviruses.[15]