Somewhat to my surprise, I have recently found myself very interested in plant viruses. This started a few years ago when I ate a most delicious variety of hot pepper that apparently is infected with a virus that gives the peppers white stripes. I’ve never really given much thought to plants and plant viruses before but as I began to look into their biology it seems that plant viruses have some terrific tricks up their sleeves (if you will) to aid in their transmission.
Plants aren’t walking around coughing on each other, so most of them depend on insects to come and bite the infected plant and carry the virus to the next host. But a plant that is infected isn’t very attractive to insects, since unhealthy plants aren’t as likely to be a valuable source of food. But viruses are masters of host manipulation. A recent study looked at Cucumber mosaic virus, and its ability to attract aphids to infected leaves. It seems that aphids dont like to spend much time on infected leaves, and they dont have to. The virus sticks to the aphid mouthparts quickly and easily and the aphid can then bring it to the next plant. But the aphids still have to be attracted to the leaves, even if they dont stay for long. So how does the virus attract the aphid to the plant? Researchers set up a special chamber with a leaf from an infected and a leaf from an uninfected plant. The leaves were not visible, but could be smelled by the aphids through wire mesh. Aphids released into the chamber were more attracted to the uninfected leaf. Analysis of volitile organic compounds being released from leaves showed that both infected and uninfected leaves release compounds that aphids can smell but infected leaves release much more. So even though the meal may not be as good, the strong smell brings the aphids to the table.

Its a common misconception among my students that simpler hosts like bacteria or single celled eukaryotes would host simpler (ie smaller) viruses, but that is certainly not the case.

Another giant virus that infects a protist has been identified and sequenced. Like its close relative Mimivirus, this new virus called Cafeteria roenbergensis virus (CroV) has a very large genome and has many genes not typically found in viruses. Before the discovery of Mimivirus, viruses were not known to encode proteins involved in protein translation. That was a function on which viruses were totally dependent on the host. However, these giant viruses seem to have their fingers in protein translation too, showing us yet another strategy in manipulating host processes. There is also block of genes that appear to be derived from bacteria. The host species, C. roebergensis, eats bacteria, so it would be interesting to know if the bacterial genes were the result of a horizontal gene transfer event from a preferred host food.

Before CroV, all giant viruses identified infect amoebas. CroV infects C. roenbergensis, a marine protist. So what is it about protists that makes them good hosts for such big viruses? Why haven’t we found giant viruses infecting other eukaryotes?
Could the explanation lie in the still murky evolutionary origin of viruses? Another recent paper attempts to put some viruses (nucleocytoplasmic large DNA viruses, including giant viruses, poxes and herpesviruses) into the tree of life along with bacteria, archaea and eukaryotes. Using genes common to all, they showed that these viruses have a very ancient evolutionary origin, probably right around the time of the appearance of eukaryotes. Were the ancestral viruses more cell-like and over time progressively lost genes?