Aphids capable of ‘seeing’ bacteria

How can you protect yourself from harmful bacteria if your immune system is not up to scratch? The pea aphid (Acyrthosiphon pisum) has found an interesting solution, and perhaps one that could even be used against the pest itself.

A weak immune system

Aphids encounter all manner of bacteria on their host plant, and although the majority of these epiphytic bacteria pose little threat to the pea aphid, some may be harmful.

Those that are harmful include certain strains of Pseudomonas syringae, to which the pea aphid is particularly sensitive. The pea aphid lacks the immunity genes that it needs to be able to fight off these bacteria once they have entered its body, and so an infection is fatal.

But if the aphid has such a weak immune system, what can it do to survive the bacteria? It takes a good look around!

Pea aphid on alfalfa

Keen observers

A group of researchers at the Hendry Lab at Cornell University in the USA has discovered that pea aphids are in fact able to see pathogenic bacteria. Aphids may well be very small, but they certainly don’t possess microscope eyes and are unable to see individual bacteria. In fact, aphids can’t really see much at all.

But they do see what matters. Virulent strains of Pseudomonas syringae produce pyoverdine, a siderophore (iron-chelating substance), that appears fluorescent under ultraviolet light, which is abundant in daylight. A pea aphid’s eyes are sensitive to light of the exact wavelength emitted by virulent strains of Pseudomonas syringae, and so the insect avoids any leaves that it sees have this light.

Aphids are by no means microbiologists, closely studying bacteria and assessing risk; rather, like Hercule Poirot, they are keen observers, able to spot the fluorescence like a vital clue in an complex mystery.

Fluorescent control

This discovery offers interesting possibilities for the biological control of the pea aphid and other aphids with weak immune systems that have developed similar solutions. Biologists have the whole genome of the pea aphid, and of several other aphids, at their disposal, offering the ideal basis for comparison of the affected genes.

Spraying crops with virulent Pseudomonas syringae could kill aphids, who’d have nowhere to go, but that may not even be needed in the end. Those aphids that survive would still be chased out by the fluorescent bacteria — as soon as the aphid spots the bacteria, they stop sucking so as to minimise their risk of infection. In theory, therefore, simply spraying crops with pyoverdine alone ought to be sufficient.

From the lab to the grower

It will be some time before growers start to use fluorescence treatment to combat aphids. And when they do, for which aphids, crops and cultivation practices is fluorescence spraying likely to be successful? Are particular strains of bacteria more effective, or is pyoverdine alone sufficient? Does this substance break down too quickly, meaning that aphids can still find their way in? Does LED light, used in greenhouses, have an impact on the fluorescence and do the aphids still react the same way to the other light spectrum?

Many questions have still to be answered, but the initial steps have been taken. The researchers at Cornell University have provided a fantastic piece of fundamental research that other universities and industry can build upon.

Sources

Picture: Jpeccoud op Wikipedia, published under CC BY 3.0

Abstract of Visual Detection and Avoidance of Pathogenic Bacteria by Aphids. T.A. Hendry, R.A. Ligon, K.R. Besler, R.L. Fay, M.R. Smee, Curr Biol. 2018 Sep 20. pii: S0960-9822(18)31003-0. doi: 10.1016/j.cub.2018.07.073.

Aphids use sight to avoid deadly bacteria, could lead to pest control.
Science Daily, 27 september 2018

Acyrthosiphon pisum (Wikipedia article)

Siderophore (Wikipedia article)

AphidBase, genome database of the pea aphid and various other aphid species:

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