Most plant viruses are RNA viruses, and of these, plus-strand RNA viruses are most common. Tobacco mosaic virus (TMV; family Virgaviridae) is the best-studied plus-strand RNA plant virus. TMV virions are filamentous with coat proteins arranged in a helical pattern.
The TMV life cycle illustrates two new strategies to produce its needed proteins: subgenomic RNAs and readthrough. Subgenomic RNAs are mRNAs that are shorter than the genomic RNA. They can be synthesized in several ways, depending on the virus.
Usually, synthesis of subgenomic RNAs is dictated by inteRNAl transcription start sites, inteRNAl transcription termination sites, or both. Readthrough occurs at the level of translation. It occurs when the ribosome reaches a stop codon, ignores the stop signal, and continues translation.
Plant viruses, including TMV, usually enter the host through an abrasion or wound on the plant; biting insects are often involved in transmission of the virus. Following entry into its host, the TMV RNA genome is translated. Two proteins are produced, one smaller (about 125 kilodaltons; kD) and one larger (about 185 kD).
The larger protein is produced by readthrough of the stop codon at the end of the coding region for the 125 kD protein. The 185 kD protein has RNA-dependent RNA polymerase activity and functions as both a transcriptase and a replicase. Soon thereafter, TMV uses membranes of the endoplasmic reticulum (ER) to form a replication complex.
There, the RNA-dependent RNA polymerase synthesizes negative-strand RNA using the plus-strand genome as the template. It is not clear whether a double-stranded RF is created in vivo, but RFs have been observed in vitro. The negative-strand RNA serves as a template for synthesis of new genomes.
It also serves as the template for synthesis of subgenomic mRNAs that are translated into a variety of proteins needed by the virus. After the coat protein and RNA genome of TMV have been synthesized, they spontaneously assemble into complete TMV virions in a highly organized process (figure).
The elongation phase of tobacco mosaic virus nucleocapsid construction. The lengthening of the helical capsid through the addition of a protein disk to its end is shown in a sequence of four illustrations; line drawings depicting RNA behavior are included. The RNA genome inserts itself through the hole of an approaching disk and then binds to the groove in the disk as it locks into place at the end of the cylinder.
The protomers come together to form disks composed of two layers of protomers arranged in a helical spiral. Association of coat protein with TMV RNA begins at a specific assembly initiation site close to the 39 end of the genome.
The helical capsid grows by the addition of protomers, probably as disks, to the end of the rod. As the rod lengthens, the RNA passes through a channel in its center and forms a loop at the growing end. In this way, the RNA easily fits as a spiral into the interior of the helical capsid.
Multiplication of plant viruses within their host depends on the virus’s ability to spread throughout the plant. Viruses can move to new sites in the plant through the plant vasculature; usually they travel in the phloem.
They can also spread more locally in nonvascular tissue. Recall that plant cells have tough cell walls. Therefore, plant viruses move from cell to cell through plant cell plasmodesmata. These are slender bridges of cytoplasmic material, including extensions of the ER.
An RNAdependent RNA polymerase enters the host cell at the same time the negative-strand RNA genome enters. The genome serves as a template for synthesis of mRNA. Later in the infection, the negative-strand genome is used for plus-strand synthesis. These plus strands then act as templates for replication of the negative-strand genomes.
Plasmodesmata extend through holes in cell walls to join adjacent plant cells. Viral “movement proteins” are required for transfer through the plasmodesmata. TMV movement proteins are closely associated with ER membranes and so are near the TMV replication complex where viral genomes are being synthesized.
They bind the viral genomic RNA (vRNA) and also interact with components of the cytoskeleton. Therefore they facilitate the movement of vRNA from the replication complex to the plasmodesmata.
TMV movement proteins also cause the plasmodesmata to increase in diameter, allowing the vRNA to pass through to the adjacent cell. Several cytological changes can take place in TMV-infected cells.
These include microscopically visible intracellular inclusions that are similar to the replication complexes observed in animal cells infected with plus-strand viruses. Hexagonal crystals of almost pure TMV virions sometimes develop in TMV-infected cells.
In addition, host cell chloroplasts become abnormal and often degenerate, while new chloroplast synthesis is inhibited.