Anyone with a backyard garden—or a commercial-scale garden for that matter—lives in fear of the cucumber mosaic virus (CMV). The disease hits plants like tomatoes, cucumbers and peppers hard, not only stunting their growth, but also causing deformed leaves, yellow spots and streaks. There’s no treatment; once a plant is infected it’s done.
But the disease does more than kill the plants—it makes them attractive to bees, which flock to the infected vegetation. “You'd think the pollinators would prefer a healthy plant,” Beverley Glover, Director of the Cambridge University Botanic Garden says in a press release. “However, modeling suggested that if pollinators were biased towards diseased plants in the wild, this could short-circuit natural selection for disease resistance.”
To figure out what was going on, Glover and a team from Cambridge’s Virology and Molecular Plant Pathology group studied the relationship between CMV infected plants and bumblebees. Their results are published in the journal PLOS Pathogens.
The team grew tomato plants in a greenhouse then infected them with the virus. Plants naturally produce volatile organic compounds, some of which attract pollinators and others that repulse potential predators. What they found is that the virus changed the composition of the volatiles emitted by the tomato plant. When they released bumblebees into their greenhouses, the insects liked what they smelled. They headed to the infected plants first and spent more time spreading their pollen around.
“To my knowledge, this is the first evidence that virus infection can make plants more attractive to pollinators,” lead investigator John Carr tells Nenad Jarić Dauenhauer at New Scientist. “Viruses reprogram plant metabolism and we can speculate that by chance this resulted in some beneficial changes for bees.”
The virus short-circuits how the relationship between plants and pollinators normally works. Typically, pollinators choose healthy plants with plenty of flowers emitting attractive volatile compounds. This usually means the healthy plants produce more seed than their diseased neighbors. But CMV predisposes the bees to choose the disease susceptible plants, meaning those individuals produce more seeds of plants that are vulnerable to the virus. This allows the plant to reproduce and gives the virus plenty of disease-susceptible future hosts, a relationship called symbiotic mutualism.
“We would expect the plants susceptible to disease to suffer, but in making them more attractive to pollinators the virus gives these plants an advantage,” Carr says in the press release. “Our results suggest that the picture of a plant-pathogen arms race is more complex than previously thought, and in some cases we should think of viruses in a more positive way.”
In fact, Carr tells Dauenhauer that the newly discovered phenomenon could lead to better crop yields if researchers can figure out exactly how the virus modifies volatile compounds to attract more pollinators.