THERE you are, stood in a green grocers poring over your favourite variety of apple. Suddenly you catch the scent of something heavenly; a smell not unlike the apple you have in your hand, only better. You abandon your apple and follow the scent to the next aisle where you find more apples of the same variety. They smell superior to the others. You pick one up and are compelled to take a bite; on doing so you realise something – it’s pretty bloody awful. You put down the unpalatable apple and move on to alternative apples.
I could be describing a situation reminiscent of the selectively bred, brightly coloured, sweet smelling fruits that line our supermarket shelves; those that in fact taste like tasteless facsimiles of the original spots-and-all varieties. In this situation we are being manipulated by the supermarkets, but in nature it may be viruses doing the manipulating.
Viruses are parasites, making use of infected host cells to replicate more virus. Of course, it isn’t enough just to replicate, viruses also need to spread to new cells, and new hosts. Plant viruses are often carried from plant to plant by insects; the insects become known in this context as ‘vectors’. The study of the biology of insect vectors is, as you may imagine, fundamentally important to understanding the transmission of a whole range of parasites (viral, bacterial and protozoan) between plants, or between humans and animals. Of particular interest is how parasites, such as viruses, manipulate their insect vectors by altering the physical properties of the host they infect.
A Penn State based group, headed by Mark Mescher, have been using Cucumber Mosaic Virus (CMV), a known generalist plant pathogen, to study the effect it has on the interaction between cultivated squash plants and aphids (sap sucking bugs). The results of this study are reported by Kerry Mauck et al. in a recent paper.
They show that CMV-infected plants have elevated volatile (readily dispersing in air) emissions that attract aphid vectors. This in itself is not a revelation; the authors cite two well documented examples of this phenomenon, from Potato leaf roll virus (PLRV) and Barley yellow dwarf virus (BYDV), where infected plants release volatiles that attract aphids. However, these other viruses employ a different method of transmission to CMV, and the main thrust of this paper is to identify how the mode of transmission modifies the host-insect interaction.