Hosts for Tomatoe Mosaic Virus include tobacco, tomato, and other solanaceous plants. TMV enters the plant cell through minor wounds. The virus can also contaminate seed coats, and the plants germinating from these seeds can become infected. 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) than begins. 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 RNA. The - sense RNA is the template to produce both full-length genomic + sense RNA as well as the + sense subgenomic RNAs (sgRNAs). The sgRNAs are translated by the host ribosomes to produce the movement protein (MP) (30 kDa) and the coat protein (CP). 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 a movement protein, and this can infect adjacent cells. TMV uses its movement protein to spread from cell-to-cell through plasmodesmata, which connect plant cells. Normally, the plasmodesmata are too small for passage of intact TMV particles. The movement protein (probably with some unknown assistance) 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.
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