For the most part, this chapter is going to only talk about animal viruses. "But," you say, "What about those really cool carrot viruses or viruses that infect fungi or bacteria?" Well, calm down. We're getting to it.
Plant and fungal viruses are very similar to animal viruses; they fall in the same 6 types of genomes that we mentioned earlier (Virus Genomes section), they are either helical or icosahedral in structure, though very few have envelopes.
One of the largest difference between animal viruses and plant, bacterial, and fungal viruses (aside from their political differences) is that plants, bacteria, and fungi all have cell walls, which creates an extra barrier that viruses must cross through. Plant viruses get around this by being transmitted primarily through vectors. A vector is an organism, usually an arthropod, that carries a virus and infects a host by its biting the host. Some vectors of plant viruses include:
Ultimately, all these vectors do is damage plant cell walls that allow for entry of virus into plant cells. Viruses such as luteoviruses or geminiviruses are the most common types to be transmitted this way.
Alternatively, a virus can enter a cell through physical damage, such as when you tear blades of grass with a lawnmower, or when you punch rhododendrons. Entry through physical damage is called "mechanical transmission," though this is not necessarily because machines cause transmission. This is just a coincidence, and not another reason to destroy SkyNet (but seriously…destroy it). "Mechanically transmitted viruses" include bromoviruses and tobamoviruses.
FIGURE 6: Plasmodesmata structure
Once in the plant, the virus can transmit from cell-to-cell using channels called plasmodesmata
. These are small pores that plants use to communicate from cell to cell, like a subway, but cleaner. The center of Figure 6 is a cell wall, and the lines connecting one side to the next are endoplasmic reticulum organelles spanning the plasmodesmata. Viruses hijack the structure of plasmodesmata and make the channels bigger to allow for easier transmission.
Fungal viruses have the same problem as plant viruses, finding Mr. Right. They also have a problem of having a cell wall, though fungi do not have the nutritional value of plants, so vector transmission is uncommon, because nobody likes fungi, even though they're fun guys.*
The only evidence of fungal infection is through mechanical grafting or laboratory injection of viruses into fungi. In fact, the narnavirus family does not even produce a capsid protein, as its replication cycle is completely intracellular. It is spread through spores in mating or vertically from mother to daughter cells. Men, who needs them?
*We at Shmoop would like to apologize for that last joke. The writer
of that joke has been fired and we have replaced them with a
joke-writing cyborg we call PunBot3000.