Many viruses are transmitted between plants in the saliva of sap-sucking insects. But this is a risky business — billions of virus particles never make it. So, some viruses have evolved elaborate mechanisms to optimize their chances. The tomato yellow leaf curl (TYLC) virus, for example, is transmitted by whiteflies (Bemisia tabaci) in a way that involves passing through the insect's body. Reporting in Virology (256, 75-84; 1999), Shai Morin et al. show that, en route to the plant, the virus takes advantage of bacteria carried by the whitefly.

When a whitefly feeds from a plant infected with the TYLC virus (see diagram), virus particles (red) pass through the insect's gut wall and into the fluid (haemolymph) in its body cavity. The virus probably does not replicate in the insect, but, once inside, some viral particles reach the salivary glands (S). Here they are transported into the insect's saliva, from where they can infect another plant.

This strange mode of transmission has its advantages. Most other plant-infecting viruses that use insect vectors are carried for only a few hours or days in the insect's mouthparts or throat. Their particles are shed when the insect moults its exoskeleton. Whiteflies that acquire the TYLC virus, however, often remain infectious for their entire life, increasing the odds of the virus being transmitted to another plant.

But passage through the insect is hazardous. Enzymes in the haemolymph degrade the virus particles, and this is where the bacteria help. Buchnera bacteria, which are found only in certain insects, live within specialized insect cells known as mycetocytes (yellow). These bacteria produce large quantities of a protein called GroEL (blue), some of which leaks into the haemolymph. Although we don't know why GroEL is released, Morin et al. show that the virus particles avoid degradation by interacting with this protein. GroEL is a chaperonin — it binds to other proteins and helps them to fold. So it could change the conformation of the viral proteins, preventing them from being degraded.

Another kind of virus uses almost exactly the same survival trick. Potato leafroll virus is transmitted by aphids. As with TYLC virus, its particles pass through the insect's body, without replicating, from the gut to the salivary glands. Aphids also carry Buchnera bacteria, and the GroEL produced by these bacteria protects the viral particles. But whereas the potato leafroll virus has an RNA genome, TYLC virus has a DNA one, and their gene sequences and particles are very different. So these viruses have clearly converged on the same strategy independently.