Plant Viruses
In contrast to bacteriophages and archaea viruses, over 75% of plant viruses have RNA genomes; typically, less than a dozen genes. As such, they can be quite small; sometimes less than half the size of a bacteriophage. They need to be.
Unlike animals, the cell walls of plants are an effective barrier to viral infection. A virus cannot penetrate a plant’s outer defense, illustrating another aspect of the astonishing savvy behind the evolution of flora.
Plant viruses must rely on another invasive agent, often an insect, to open a path by eating away at a plant. Roundworms and soil-borne protozoa deliver viruses via plant roots.
Once inside, viruses sneak throughout a plant by passage through plasmodesmata: the microscopic channels between plant cells.
A plant virus depends entirely upon the invasive vector it associates with. Such a virus can only infect the plants that its insect agent feeds on.
As most plant viruses are RNA, plants evolved a defense using RNA interference (RNAi), which disables the plant virus by chopping it up. For DNA viruses, a plant employs RNAi to methylate the viral DNA, thereby gumming its works. Viruses can counteract RNA silencing by expressing potent RNAi suppressor proteins.
Some viruses cooperate with one another to infect a plant. The tomato spotted wilt virus and iris yellow spot virus help each other tackle a tomato by dramatically changing their genetic expressions.
Virtually all plants fall victim to a viral infection some time during their lives. Some are infected as seeds – a tough way to sprout into life, though common. 20% of plant virus transmissions are from one generation to the next.
To cope with plant defenses, the RNA viruses that infect plants have extremely high mutation rates. A jump to animals is not especially difficult for these versatile viruses, especially animals that come into regular contact with plants.
The tobacco ringspot virus managed the leap from the tobacco plant to the mites that pester pollinating honeybees, and then to honeybees themselves. Such species-jumping multiplies the ways that a virus can disperse.
