Plant Viruses

While there are exceptions, the vast majority of plant viruses have a single- stranded linear, +RNAgenome with a capsid having helical or icosahedral symmetry. They have small genomes, encoding for only three to four proteins: (1) a helicase; (2) an RNAreplicase; (3) a cell-to-cell movement protein; and (4) a capsomere. The helicase is thought to be important in the unwinding and separation of the plus and minus RNAstrands. The replicase, an RNA-dependent RNA polymerase, is encoded in those viruses that are able to use the host's enzymes. The cell-to-cell movement protein facilitates the spread of the viral RNA through plant tissue. The capsomere is the protein subunit of the capsid.

Many plant viruses depend on insect vectors to infect plant cells. Tobacco mosaic virus is only dependent on mechanical damage to cell walls, which allows the virus to bind to the plasma membrane of host cells.