Replication cycles in plant viruses

Viral infections of plants can be spread by one of two principal pathways. Horizontal transmission is the introduction of a virus from the outside, and typically involves insect vectors, 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 (see above) 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

Figure 10.16 Replication in retroviruses. Reverse transcriptase makes a DNA copy of the single-stranded RNA retroviral genome. This is integrated into the host genome and is transcribed by the cellular machinery. Messenger RNA passes out to the ribosomes, where translation into viral coat proteins and more reverse transcriptase occurs. Retrovirus packaging takes place outside the nucleus. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Figure 10.16 Replication in retroviruses. Reverse transcriptase makes a DNA copy of the single-stranded RNA retroviral genome. This is integrated into the host genome and is transcribed by the cellular machinery. Messenger RNA passes out to the ribosomes, where translation into viral coat proteins and more reverse transcriptase occurs. Retrovirus packaging takes place outside the nucleus. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services particles do this by moving through the plasmodesmata, naturally occurring cytoplasmic strands linking adjacent plant cells.

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