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Disease Cycle and Epidemiology

Transmission from plant to plant

TMV is very easily transmitted when an infected leaf rubs against a leaf of a healthy plant, by contaminated tools, and occasionally by workers whose hands become contaminated with TMV after smoking cigarettes. A wounded plant cell provides a site of entry for TMV. The virus can also contaminate seed coats, and the plants germinating from these seeds can become infected. TMV is extraordinarily stable. Purified TMV (Figure 6) has been reported to be infectious after 50 years storage in the laboratory at 4°C/40°F.

Figure 6

Replication

(Click here for the complete TMV replication cycle)TMV enters the plant cell through minor wounds. Once TMV enters the cell, the virus particles disassemble in an organized manner to expose the TMV RNA. The virus RNA is positive-sense, or "+ sense", and serves directly as a messenger RNA (mRNA) that is translated using host ribosomes. Translation of the replicase-associated proteins (RP) 126- and 183-kDa) begins within a few minutes of infection.

As soon as these proteins have been synthesized, the replicase associates with the 3' end of the + sense TMV RNA for the production of a negative sense, or "- sense", RNA. The - sense RNA is the template to produce both full-length genomic + sense RNA as well as the + sense subgenomic RNAs (sgRNAs) (Figure 8)

Figure 8

The sgRNAs are translated by the host ribosomes to produce the movement protein (MP) (30 kDa) and the coat protein (CP) (17.5 kDa). The coat protein then interacts with the newly synthesized + sense TMV RNA for assembly of progeny virions.

These virus particles are very stable and, at some point when the cells are broken or the leaf dries up, they are released to infect new plants. Alternatively, the + sense TMV RNA is wrapped in movement protein, and this complex can infect adjacent cells.

Movement in the infected plant

Click here for the complete TMV movement diagram.
TMV uses its movement protein to spread from cell-to-cell through plasmodesmata, which connect plant cells (figure 10). Normally, the plasmodesmata are too small for passage of intact TMV particles.

The movement protein (probably with the assistance of as yet unidentified host proteins) enlarges the plasmodesmatal openings so that TMV RNA can move to the adjacent cells, release the movement protein and host proteins, and initiate a new round of infection. As the virus moves from cell to cell, it eventually reaches the plant's vascular system (veins) for rapid systemic spread through the phloem to the roots and tips of the growing plant.

Epidemiology

The TMV disease cycle and its epidemiology are intimately related because the virus is completely dependent on the host for replication and spread. There is wide variation in disease incidence, depending on the time of disease onset in the field and on cropping practices. For example, a few plants could become infected early in the season, either from TMV on the seed coat or by workers contaminating plants. The disease could then spread rapidly throughout the field or greenhouse by TMV-infected plants contacting healthy plants, or by equipment or workers. TMV can also survive or overwinter in infected plant debris or perennial (weedy) hosts and, perhaps, in the soil. Agricultural practices, such as continuous cropping, have the potential to be a particular problem, especially in greenhouse facilities, where TMV inoculum may increase in more than one plant species.

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