Researchers at the Universities of Dundee and Aberdeen have made a breakthrough in understanding how the microbial spores which cause potato blight are so effective at infecting plants.

Phyophthora infestans is a highly destructive plant pathogen. It was the cause of the infamous Irish potato famine in the nineteenth century and remains to this day a significant global problem with associated costs estimated at $3billion USD (£1.8bn) around the world every year.

Key to the success of the pathogen is the dispersal of free-swimming cells called zoospores. Infection is spread through water by the release of these tiny spores but the mechanism by which they coordinate an attack on plants has been poorly understood, until now.

The researchers have found that as the spores clump together in water, ganging up to increase the chance of causing infection, they use two mechanisms to attract enough spores to attack plants, and they happen over two different timescales.

The study was carried out by researchers in the Division of Mathematics at Dundee and the School of Medical Sciences at Aberdeen. The results of their research are published in the Journal of Royal Society Interface.

Dr Fordyce Davidson, an expert in mathematical biology at the University of Dundee, said, "One zoospore on its own is unlikely to kill a plant but they have developed a swarming behaviour which makes them much more effective. Once there are enough zoospores gathered together they generate sufficient infection pressure to allow the pathogen to get into the plant and kill it.

"A poorly understood aspect of their behaviour is something called auto-aggregation, the spontaneous formation of large-scale patterns in cell density. Previously there were competing hypotheses that these patterns were formed by one of two distinct mechanisms. What we have shown is that both mechanisms – chemotaxis and bioconvection – are involved, each having a distinct, time separated role.

"This greater understanding of how this pathogen works to infect plants will in the longer term lead to advances in preventative treatment."

How these spores were able to carry out this relatively sophisticated behaviour had previously been a puzzle.

The researchers placed millions of zoospores into a petri dish to establish the patterns they formed. What they saw reminded them of mathematical models they'd seen before that were formed by chemical-sensing patterns.

"When we made computer chemical-sensing models we couldn't get the right number of spots to form in the dish on the right time scales,” said Dr Davidson. “We could get the pattern right, but the computer said it would take days, or we could get the timing right, but not the pattern. We thought we'd got it wrong until we realised perhaps it's the opposite way round - these early patterns were being formed by bio-convection.

"Bio-convection is a sort of swimming pattern we see in the zoospores. If you take a little cell, which looks like a coffee bean with a fatter bottom, then they'll swim upwards because of gravity. It's a very rapid process that works on the order of minutes. It sets up convective plumes, which are structures in the liquid pushing the cells to the top where they can group together."

The second mechanism was a form of chemical sensing. Similar to animals being attracted to pheromones, the zoospores are able to send chemical signals to draw in other zoospores.

“The chemical-sensing mechanism happens on the order of 4-5 hours. If you have lots of these plumes in a water drop formed by bio-convection, then the chemical sensing draws these plumes together until you get one super plume, to really drive in the infection,” added Dr Davidson.

“It is the combination of these two mechanisms that is driving the zoospores behaviour.”