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The hormones of trees | Max Planck company

The plant hormones of the poplars reinforce each other in the defense against pathogenic fungi

Contrary to previous hypotheses, the defense hormones salicylic acid and jasmonic acid do not always suppress each other in regulating the chemical defense of plants against pathogens. In trees, the interaction of both hormones can even increase endurance. A research group from the Max Planck Institute for Chemical Ecology in Jena came to this conclusion. The researchers found that poplars that have increased levels of salicylic acid also have higher levels of jasmonic acid. Plants with higher levels of both hormones are also more resistant to the rust fungus Larch-populin melamsporawithout negatively affecting growth. Knowledge of the positive interaction of these hormones, which are involved in plant defense, could help better protect poplars and other trees from pests and pathogens.

The job of plant hormones is to coordinate the growth and development of plants. In addition, they also control the plant’s immune response to pathogens such as fungi. So far there has been broad scientific consensus that the signaling pathways of the defense hormones salicylic acid and jasmonic acid work in opposite directions: if the plant produces more salicylic acid, this blocks the production of jasmonic acid and vice versa. Scientists discovered this in studies on the model plant Arabidopsis thaliana (Such cress) and many other annual plants are shown again and again. “Contrary to the assumption that salicylic acid and jasmonic acid act antagonistically, that is, against each other, we had already observed in our previous poplar studies that the concentrations of both hormones increase in response to infection. from pathogenic fungi. Therefore, the most important research question for us was to determine more precisely the interaction of these two defense hormones in the poplar, “explains Chhana Ullah, the first author of the publication, starting point of the present study.

To experimentally clarify how salicylic acid levels affect jasmonic acid formation, the scientists modified test plants for native black poplar (coloured people) genetically modified to produce higher levels of salicylic acid than control plants. In another experiment, they applied salicylic acid to the poplar leaves of genetically unmodified plants. “We manipulated the salicylic acid content in poplars by genetic modification and direct chemical application and then performed in-depth chemical analyzes of plants with and without fungal infection. This allowed us to separate the effects of salicylic acid from other factors and to demonstrate that it directly stimulates the production of jasmonic acid, “explains Chhana Ullah.

Salicylic acid stimulates the production of jasmonic acid

In plants that contained a lot of salicylic acid, higher concentrations of jasmonic acid were also detectable. Furthermore, these plants formed more antimicrobial agents, the so-called flavonoids, even when there was no infection with a pathogen. Further comparative studies with high salicylic acid plants and control plants, each with rust fungus Larch-populin melamspora they found that high levels of salicylic acid made poplars more resistant to attack by fungi.

Surprisingly, the increased resistance to fungi due to the increased defensive ability was not at the expense of plant growth, as had been observed with cress and other annual herbs. In the case of watercress, only salicylic acid or jasmonic acid controls the immune response, while the other hormone is suppressed. Higher levels of salicylic acid form when infested with pathogens that feed on living plant material. Jasmonic acid, on the other hand, is produced after being eaten by insects or pathogens that live on the tissues of dead plants. “The negative interaction between the two defense hormones in plants such as thale cress allows the plant to prioritize protection against a particular type of pest. Annual plants can benefit from this strategy because they lack the resources to defend themselves against. multiple enemies at once. This could also be the reason why thale cress restricts its growth when in defensive mode, “says Jonathan Gershenzon, head of the biochemistry department where the study was conducted.

More resources for defense

Unlike herbs like watercress, trees and other woody plants are less resource constrained under normal circumstances. Additionally, due to their long lifespan, trees are attacked simultaneously by many more different enemies, such as fungi and bacterial pathogens, leaf-feeding caterpillars and wood-destroying insects. They may have evolved to share the signaling pathways of salicylic acid and jasmonic acid for defense. The increased availability of resources in long-lived trees may also explain why high concentrations of salicylic acid do not affect plant growth in poplars.

The researchers were surprised that high levels of salicylic acid in poplars caused so-called pathogenesis-related genes (genes related to pathogenesis / PR) genes did not activate, although these are known to be established markers for the salicylic acid signaling pathway in the marsh wall. “However, we found that activation of PR genes was clearly detectable in poplars susceptible to rust fungus. Apparently, the activation of PR genes in poplar is not regulated by salicylic acid signals, but by another mechanism, ”explains Chhana Ullah.

Scientists led by Chhana Ullah have yet to find out exactly how the molecular mechanism of the positive interaction between salicylic acid and jasmonic acid works in poplar, as well as what role PR genes play in poplar and other woody plants. What is certain, however, is that basic knowledge about the positive interaction between salicylic acid and jasmonic acid in poplar and related trees could make an important contribution to better protect these plants from pest infestations and diseases, as Jonathan Gershenzon says. : “Poplars are very different due to their diversity Known in human use as the trees of people, hence the generic name People: the Latin term for people. Since poplars grow very quickly and have a high cellulose content, they are particularly important for the paper and pulp industry. But they are also interesting for energy use “.

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