Replication cycles in plant viruses
Viral infections of plants can be spread by one of two principal pathways. Horizontaltransmission is the introduction of a virus from the outside, and typically involves insectvectors, which use their mouthparts to penetrate the cell wall and introduce the virus. This form of transmission can also occur by means of inanimate objects such as garden tools. In vertical transmission, the virus is passed from a plant to its offspring, either by asexual propagation or through infected seeds.
The majority of plant viruses discovered so far have an RNA genome, although DNA forms such as the caulimoviruses are also known. Replication is similar to that of animal viruses, depending on the nature of the viral genome. An infection only becomes significant if it spreads throughout the plant (a systemic infection). Viral particles do this by moving through the plasmodesmata, naturally occurring cytoplasmic strands linking adjacent plant cells.