Blueberry mosaic associated ophiovirus

When a plant is initially infected with the virus the plant will often display a symptom-free period, meaning there is long latent period between viral infection and symptom appearance.[3] The virus can also become dormant in some areas of the plant and expressed in others, causing some leaves to display symptoms and others to be asymptomatic.

When a blueberry plant begins to show symptoms of infection by B1MaV a mild to severe mottle and mosaic patterns on foliage appears. The pattern is in the colors of yellow, yellow green, and pink. Usually symptoms appear on only a few leaves, but can appear on more of the plant in severe seasons.[2] The infection also causes late ripening of fruit, reduced yield of the crop, and poor quality of the berries, as the virus spreads throughout the plant and begins affecting cellular mechanisms.[3] The virus does not kill the plant, however, rather it just affects the quality and quantity of fruit produced by the plant.

The blueberry mosaic associated ophiovirus virion is described as a nonenveloped nucleocapsid. It is a naked filamentous nucleocapsid that can form circles making the open form of the virus or can form a pseudo-linear duplex form that forms the collapsed form of the virus.[5] The nucleocapsid is 3 nm in diameter and 700 to 2000 nm long, pseudo-linear duplex are about 9-10 nm in diameter. Ophiovirdae are composed of 3 to 4 RNA segments, with each segment being individually encapsulated. In the case of B1MaV, there are three RNA segments, and therefore each segment is encapsulated by a nucleocapsid protein. Each encapsulated structure contains a viral and viral complementary RNA strand (vRNA and vcRNA).[6] The population structure is highly conserved in B1MaV as a result of low genetic diversity among isolates of the virus.[7]

The genome is a linear segmented genome that is 11,467 nucleotides long and is composed of three negative single strand RNA sequences.[3][1] RNA one is the longest RNA sequence and contains two open reading frames (ORFs) that encode two proteins. The proteins encoded include a 23 kDa protein that has an unknown function and a 272 kDa RdRp (RNA-dependent RNA polymerase) replicase. The presence of these two ORFs in the largest RNA strand and the two ORF's having the same polarity is a key genomic feature of Ophioviridae. The RdRp encoded has five conserved motifs (A-E). The SDD sequence which is a characteristic sequence for segmented negative strand RNA viruses such as Ophiovirdae is found in motif C of the RdRp.[6] RNA 2 encodes for a 58 kDa movement protein that is thought to also be involved in the suppression of post transcriptional gene silencing.[3] RNA 3 encodes 50 kDa nucleocapsid protein.

The 5’ terminal sequences are not conserved between RNA transmissions, but all of the 5’ termini fold into conserved stem-loop structures. These structures are likely involved in packaging of the genomic RNAs into their capsid or in long-distance interactions for transcription and translation.[1] The 3’ terminal of all three RNAs are identical and conserved. The conserved nucleotide sequence is ‘AAUAUC’.[2]

Ophiovirdae have a G+C content is 32.3-39.8%.[8] The genome of Ophiovirdae is typically 11.3-12.5 kilobytes long.[6] These viral RNA sequences appear in higher concentrations in symptomatic leaves then in asymptomatic leaves of the same plant.[3]

There are several proteins produced by the viral mRNA in transcription and translation that affect the host cells processes, making a more suitable environment for viral replication and transcription. There is not much information known as to exactly what viral proteins interfere with the host cell processes.

One protein that is encoded on RNA 2 is the movement protein for B1MaV. This protein helps the virus travel between cells via plasmodesmata. It also is involved in suppression in post translational gene silencing of cellular genes, therefore preventing the silencing of some cellular genes causing consequent expression and the effects of these expressed genes.[3][9]  

The virus is transmissible via grafting, meaning that when an infected plant is joined with a noninfected plant to promote continued growth of the plant, the infected plant spreads the virus to the once noninfected plant.[3] Asymptomatic plants can serve as reservoirs for the pathogen that can in turn spread the disease to highly susceptible plants. The virus can further spread to new growing areas accidentally by plant nurseries since blueberries are reproduced asexually and asymptomatic blueberry plants may be used.[1] Ophiovirdae are also transmitted through the soil via fungi. This would allow B1MaV to survive in the spores of an infected root fungi leading to further spread.[1] Since the viral genome is three segmented negative RNA strands, there is the possibility for re-assortment of genetic segments between B1MaV and other segmented viruses infecting the same plant, leading to possible further transmission of the virus by other means.[7]