Fig mosaic virus
Fig mosaic virus is a segmented, negative sense, single-stranded RNA virus that is determined to be the causal agent of fig mosaic disease in fig plants, Ficus carica. It is a member of the genus Emaravirus and is transmitted mainly by the eriophyid mite Aceria ficus. FMV can cause a range of symptoms varying in severity, including leaf chlorosis, deformity, and mosaic or discoloration patterns, as well as premature fruit drop.
History
Fig mosaic disease was first described and suspected to be of viral origin in 1933 by Ira J. Condit and W.T.Horne. It was determined to be caused by a virus in 2009 by Jeewan Jyot Walia, Nida M. Salem, and Bryce W. Falk. The disease and associated virus have since been observed in Greece, Italy, Spain, Turkey, Syria, Tunisia, Algeria, Jordan, New Zealand, China, Great Britain, Puerto Rico, Australia, and the United States.Classification
FMV has been placed in the genus Emaravirus, which contains other single-stranded, negative-sense, segmented RNA plant viruses. Emaraviruses all contain four to six genome segments. They are also common in their mode of transmission, which is via various species of eriophyid mites by unknown mechanism as well as mechanical modes of transmission. Besides FMV, the genus Emaravirus includes European mountain ash ringspot-associated virus, Rose rosette virus, Raspberry leaf blotch virus, and other species.Genome Structure
The FMV genome consists of segmented negative-sense, single-stranded RNA. The genome has long been thought to have four segments, but recent findings have supported the existence of six RNA genome segments. Each segment has one open reading frame.The first segment, FMV vcRNA 1, is common to all viruses of genus Emaravirus and codes for the virus's 264 kDa RNA-dependent RNA polymerase, which has endonuclease activity near the N-terminus. The second segment, vcRNA2, encodes a 73 kDa putative glycoprotein. FMV vcRNA3 encodes a 35 kDa nucleocapsid protein. FMV vcRNA4 encodes a 40.5 kDa protein with function still unknown.
The two most recently discovered segments, RNA5 and RNA6, unlike the other four segments, are not highly conserved within the genus. Perhaps this is because they are truncated or defective, but the functions of the proteins they encode is still undetermined.
Movement between and into cells (FMV-p4)
Like all plant viruses, FMV encodes a movement protein to enable it to move between cells, usually by increasing the size exclusion limits of plasmodesmata to allow viral passage. FMV's MP is most likely p4, an inference that is supported by its localization in the plasma membrane but has not been definitively shown yet.Genome replication via cap snatching
FMV vcRNA 1 codes for RNA dependent RNA polymerase protein, which has endonuclease function at its N terminus and is packaged in the virion. Once the virion has entered a cell, RdRp binds to host mRNA and uses endonuclease activity to cleave the 5’ methylated cap for use as a primer for mRNA synthesis and as a way to hijack host translational machineryDMBs and LFPs
Infection with FMV results in distinct double-membrane bodies or particles, called DMBs or DMPs, 90–200 nm in diameter in the cytosol of infected parenchyma cells. These double membrane-bound bodies are surrounded by a fibril matrix and found only in leaves showing mosaic symptoms at a macro level, never in the uninfected tissue used as a control.Some cultivars have also shown long, flexuous, rod-shaped, virus-like particles in tissues showing signs of necrosis. These are also not seen in uninfected control tissues. Neither type has a determined function.