THE SPIRIT OF PLANTS - UNDER INFLUENCE
Today's theme is somewhat irrelevant, but it seems interesting to me to classify it in this series.
It happens that plants do not have a normal behavior, nor even coherent, because they can be put under influence, they can show unnatural behaviors, and it appears that they can't avoid it, that they "act against their will".
An article recently published in the digital periodical The Conversation in its French version, and written by Véronique Brault, director of research in virology at INRA (French National Institute for Agronomic Research), with the participation of Simon Bourdin, master student in "Scientific Communication" at the University of Strasbourg, explains how certain viruses, in order to ensure their propagation, are able to interfere with the behavior of their hosts, and in this case, the plants in which they are found.
"These viruses that manipulate plants
October 12, 2018
Mice that are no longer afraid of cats, crustaceans that float on the surface of the water instead of sheltering under the rocks, ants that disguise themselves as berries to be eaten by birds, humans who emit odors attracting mosquitoes ... these unexpected behaviors, sometimes suicidal, seem almost fanciful.
They are, however, present in nature, and they all have one thing in common: they are the result of parasitic infections. Indeed, many so-called "manipulative" parasites are able to alter the behavior and even the physiognomy of their host to facilitate their survival and propagation.
Plants do not escape the rule. Viruses that parasitize them are often very ingenious to overcome their immobility, using for this purpose mobile intermediate organisms called vectors.
The virology unit of the INRA in Colmar has recently highlighted a case of manipulation of the plant Camelina sativa by Turnip Yellows Virus (TuYV). This virus has the particularity of being transported from plant to plant only by an aerial vector, the aphid. The virus spreads more effectively if aphids are attracted to infected plants before they succumb to the infection.
Make the plant more palatable
Researchers have shown that TuYV causes plants to emit odors that attract aphids. This same virus also changes the chemical composition of the plant, making it more palatable - in a word, more tasty - for the aphid. Having a more adapted food, the latter feeds more and ingests more viruses, a net benefit for the pathogen that can accumulate in the aphid and be more effectively propagated to other plants.
Examples abound of such "manipulations" of plants by viruses, and studies reveal that these alterations of the plant induced by the virus can vary according to the virus in question and especially according to their mode of transport by the vectors.
For example, TuYV is a "circulating" virus: for its efficient transmission, the aphid must land on the infected plant and feed for a long time to ingest the virus that travels through the sap. The pathogen is then carried with the sap in the digestive tract of the insect and then through the cells of the intestine, before joining the salivary glands, where it is reinjected to a new plant.
"Circulating" viruses have a strong interest in inducing changes in the plant to attract aphids and stimulate their diet.
Illustration showing the attraction of aphids by infected Camelins and not infected Arabidopsis. Véronique Brault/INRA
There is another category of so-called "non-circulating" plant viruses that don't pass through the insect cells but are retained in the oral tract at the stylet or esophagus of the vector.
In this group is, for example, the Cucumber Mosaic Virus (CMV) also transmitted by aphid. This virus causes infected plants to produce volatile substances that attract aphids but reduce the nutritional quality of infected plants, causing aphids to emigrate quickly.
These two contrasting phenomena are however perfectly adapted to the mode of transmission of CMV: the latter requires only brief punctures in the surface cells of the plant to be retained by the vector. The virus thus optimizes its transmission process by first encouraging the aphids to land on the infected plants and then encouraging them to leave these plants quickly after just tasting them.
More surprisingly, Rhopalosiphum padi aphids with Barley Yellow Dwarf Virus (BYDV) prefer non-infected plants, while non-infected aphids are attracted to infected plants (Ingwell et al., 2012).
This example demonstrates that the virus can, not only indirectly manipulate the plant to attract aphids, but also act directly on aphids to alter their behavior.
On the track of involved molecules
These observations raise many unresolved questions.
Why do not these examples of manipulation apply to all plants infected with the same virus? Why are there also variations in behavior according to the species of aphid considered even towards the same infected plant?
Current studies are therefore aimed at identifying in infected plants the molecules that are responsible for vector behavior changes, be they compounds that attract aphids or those that make them more appetizing. This identification would make it possible to envisage new control methods aimed at inhibiting the production of the molecules in question by infected plants, so that they no longer have this attractive power for aphids.
To date, the use of insecticides remains the best method for farmers to reduce vector populations and limit the losses associated with the viruses they carry.
A new control method based on the non-attraction or even repulsion of aphids for infected plants would reduce the spread of viruses in the field. To stop the dispersion of circulating viruses, it would also be possible to select plant varieties naturally producing little or no compounds responsible for the sustained ingestion of sap necessary for the acquisition of these viruses.
Understanding the fine mechanisms by which aphid-transmitted viruses manipulate their host plant to facilitate their spread represents a promising science front for the development of new control methods that are more respectful of human health and the environment. "