Plant Viruses

While the genetic material of bacteriophages is DNA, the genetic molecule of plant viruses is most often RNA. Bacteriophages and plant viruses also replicate differently. Plant viruses generally do not rupture the cells in which they form; rather, they seep out through cell membranes and to neighboring cells.

More than one thousand plant diseases can be traced to viruses: tobacco, potato, sugar beet, peas, orange, elm, apples, cucumber, bean, cauliflower, tulips, and rice. It seems that freedom from viral infection is an uncommon condition in the plant kingdom.

Viral infections generally result in necrosis, yellowing, mottling, leaf spots, streaks, and wilting. Not all plant viruses have negative effects, however. The highly esteemed Rembrandt tulip, for example, owes its colorful streaks to a viral infection. A mosaic virus that damages rice plants has quite a different effect on jute (a plant used in making rope and burlap bags); it improves the growth of jute plants.

Viral transmission between plants can result from touching an infected plant and then touching a healthy plant. In nature, viruses are most commonly carried by insects (especially aphids and leaf hoppers) and nematodes. Some plant viruses multiply within the bodies of their insect carriers as well as in the plants themselves. It follows, then, that insect control is a proper means of combating viruses. Plants also may become infected by viruses through wounds.

People who raise potatoes know that attention must be paid to controlling viral diseases. It is inadvisable, for example, to use one’s own potatoes as seed pieces for successive crops, as gradual degeneration will result. Rather, seed potatoes should be obtained from other regions in order to minimize the hazard of an accumulation of viral diseases. Such viral diseases are transmitted by aphids. Weeds also can be carriers of viral diseases, and dahlias carry a virus that infects tomato plants.

While most plant viruses have RNA and several have DNA, no plant viruses have both. The tobacco mosaic virus is approximately 300 nanometers long, and its RNA molecule has 6,000 bases. These bases make up three genes. Other plant viruses contain as many as 200 genes. A first stage in the replication of plant viruses is the shedding of the protein coat. Recall that in bacteriophage, the protein remains outside of the cell. In other viruses, the protein may be shed inside the infected cell. The viral nucleic acids then direct the production of viral enzymes. These, in turn, direct the manufacture and assemblage of protein and the production of more viral RNA. The end result may be thousands of new virus particles in the cell.

Each virus particle is commonly organized into a twenty-sided figure, a shape called an icosahedron. RNA viruses have either a single strand of RNA (ssRNA) or a double strand (dsRNA). (DNA viruses can be classified in the same way.) Single-stranded RNA viruses can be further subdivided into two groups: plus strand or minus strand. Plus-stranded RNA acts directly to program replication; minus-stranded RNA acts as a template for the production of an m-RNA that then directs virus replication. Plus-stranded viruses can be further subdivided for classification purposes based on whether or not the virus particle has an envelope. Further details on classification of viruses can be found in the suggested readings.