When tomato plants are infected with a specific virus, it causes stunted growth and the production of small, poor-tasting fruit. Yet despite being so damaging, it is also incredibly persistent in the wild, and a new study published in PLOS Pathogens may have uncovered why.
It's been found that when the virus in question – confusingly named cucumber mosaic virus – infects a tomato plant, it can physically change the expression of the plant's genes that are responsible for the scent they emit. Now while it might be expected that the pollinating bumblebees that can pick up on these scents would avoid infected plants, it turns out that experiments show the opposite is true. Paradoxically, the bees actually prefer the infected plants over the healthy ones.
This in itself is slightly bizarre, considering the virus isn’t generally passed on by the bees, but is usually spread through aphids. So what is the advantage in manipulating the bumblebees' foraging behavior? It seems that the virus may be exploiting the bees to make sure that a larger proportion of the plants susceptible to the cucumber mosaic virus are pollinated and thus reproduce. This reduces the chance that the plants will develop resistance, as those infected are now more likely, rather than less, to pass their genes on.
“We were surprised that bees liked the smell of the plants infected with the virus – it made no sense,” explains Dr John Carr, who led the research, in a statement. “You’d think the pollinators would prefer a healthy plant. However, modelling suggested that if pollinators were biased towards diseased plants in the wild, this could short-circuit natural selection for disease resistance. The virus is rewarding disease-susceptible plants, and at the same time producing new hosts it can infect to prevent itself from going extinct. An example, perhaps, of what’s known as symbiotic mutualism.”
But it doesn’t stop there. While the researchers think that the bees may play a small role in spreading the virus via the plant's pollen, it may at the same time be compensating for the smaller yield of pollen produced by the plants and caused by the virus. Typically, infected plants produce smaller fruits with a lower yield, but the increase in visitations from bees may be making up for this. This amazingly complex plant-pathogen arms race shows that viruses can actually act in positive ways, and was revealed through eight years’ worth of experiments in controlled glass houses.
The results could have significant implications. Not only does it show how disease genes can persist in a population over a large number of generations, but it could also lead to ways to make commercial plants more attractive to pollinators.
