How a virus gives back to its host

 

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Image by F_A (CC BY 2.0)

A study by UK scientists has shown that tomato plants infected with a virus are more attractive to bumblebees than healthy plants. Why would a plant virus want to change the behaviour of bumblebees?

The virus in question – cucumber mosaic virus (CMV) – can infect many different species of plant including tomatoes and a model plant called Arabidopsis thaliana. In tomatoes it causes many symptoms including yellowing, mottling, leaf distortion and can reduce the yield of seeds. As a result there is pressure for populations of plants to evolve better defences against the virus. Since CMV can only multiply within plant cells you might expect that, over time, CMV might become less common, but this doesn’t appear to be the case. One way the virus might be able to combat this problem is to compensate for the decrease in seed production in infected plants by encouraging pollinators, such as bumblebees, to visit the flowers.

Bumblebees fertilise tomato flowers by a process called buzz pollination, in which sounds produced by the bees shake the flowers to release pollen. Although tomato flowers can fertilise themselves without help from the bumblebees, buzz pollination makes the process more efficient and also leads to the transfer of pollen between flowers. Volatile compounds (molecules that easily become gases) released from the plants may help to guide the bees to the flowers. CMV infection can change the mix of volatile compounds that plants produce, but it was not clear whether this changes the behaviour of the bees.

Simon Groen, Sanjie Jiang, Alex Murphy, Nik Cunniffe et al. found that the bees are more attracted to the volatiles produced by CMV-infected tomato plants than those produced by healthy, uninfected plants. In the absence of buzz pollination, CMV-infected plants produce fewer seeds than healthy plants. However, mathematical modeling indicates that, in the “wild”, the bee’s preference for virus-infected flowers may help to compensate for this so that CMV-infected plants may produce more seeds than uninfected plants. Further experiments in A. thaliana suggest that molecules of micro ribonucleic acid (or miRNA for short) produced by the plants might regulate the mix of volatiles that plants produce.

These findings suggest that in some environments it may be in a virus’ interest to help its host plant by making the plant more attractive to bumblebees or other pollinators. Bumblebees are important pollinators for many crop plants so these findings may help us to develop new ways to increase crop yields in the future.

Reference: Groen SC, Jiang S, M, Murphy AM, Cunniffe N, Westwood JH, Davey MP, Bruce TJA,  Caulfield JA, Furzer O, Reed A, Robinson SI, Miller E, Davis CN, Pickett JA, Whitney HM,  Glover BJ, Carr JP. 2016. Virus Infection of Plants Alters Pollinator Preference: A Payback for Susceptible Hosts? PLOS Pathogens http://dx.doi.org/10.1371/journal.ppat.1005790