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Larvae living on leaves that are about to dry out can "bring them back to life" with the help of symbiotic bacteria

By: Yonat Ashchar and Noam Levithan

Imagine for a moment that you are the prey of a moth - a small caterpillar that lives inside a leaf and moves along it while eating. Your entire existence depends on this one leaf, which provides you with food and protection. But you are lucky to live on the leaf of a deciduous tree, and autumn is approaching. The leaf browns and turns yellow and its living tissues, full of sugars, other nutrients and moisture, dry out. What can the larvae do in such a situation? They are not able to come out and move to another leaf - not until they grow up, complete the reincarnation and become adults. "Their" leaf, on the other hand, will not be able to continue to finance them for much longer. To survive, the caterpillar must "perform a miracle" and bring its leaf back to life.

Wilfried Kaiser (Kaiser) and his colleagues from the Université Francois Rabelais in France studied the phenomenon of "green islands" - green spots of living tissue that appear on leaves that appear completely dry outside of these areas. It has long been known that the islands are flooded due to the presence of moth larvae, which thus gain a few more days of eating and growing. In a study published in the journal Proceedings of the Royal Society, the researchers tried to understand how the insects perform their resuscitation actions.

Their hypothesis was that this ability lies not in the larvae themselves, but in other creatures that live inside them - symbiotic bacteria, similar to the "friendly bacteria" that live in our intestines. There are species of bacteria that are able to induce growth in plants, partly with the help of a plant hormone called cytokinin, which is found in high levels in those green islands. Some bacteria use this ability to attack the plants themselves, and create exudates in which they develop. Cases are also known in which the activity of bacteria alone, without the involvement of insects, creates green islands very similar to those created by the larvae.

To check whether symbiotic bacteria are indeed involved in this case, the researchers examined moth larvae of the species Phyllonorycter blancardella, the life inside the leaves of apple trees. They found that they did contain bacteria, mostly Wolbachia bacteria, common in insects. The researchers gave the larvae an antibiotic that almost completely killed the bacteria. The results of the treatment were immediately apparent: the larvae raised on dry leaves failed to form the green islands, and almost all died before reaching maturity. When the leaves were tested, it was found that they did lack the high cytokinin levels found in the green islets. It is possible that the bacteria produce the hormone themselves and cause the larva to secrete it, since this bacteria has a gene that is important for the creation of cytokinin. It is also possible that substances produced by the bacteria encourage the plant itself to increase hormone levels - or both.

The research points to interactions between representatives of three different kingdoms: bacteria, animals and plants. Symbiosis between bacteria and animals is very common, of course - for example, bacteria living in the intestines of each of us provide us with essential nutrients. But this is the first time that cooperation between bacteria and animals has been documented, which together cause the manipulation of the plant in which they live - thus ensuring their survival.

The original article was published on Dr. Noam Levitan's blog SciPhile and in the Galileo magazine.

for further reading

The original article:

Kaiser, W., Huguet, E., Casas, J., Commin, C. & Giron, D. Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts. Proc. R. Soc. B 277, 2311-2319 (2010), first published online 31 March 2010. doi: 10.1098/rspb.2010.0214

On the topic of objects, see: Dror Bar-Nir, "Agrobacterium, from disease cause to genetic engineering", "Galileo" 119.

On bacteria and their effect on the sexual preference of flies: Noam Levithan and Yonat Ashchar,love from the belly", "Galileo" 148.