The growth or defence dilemma
Young plants are often extra sensitive for insect damage. One well-placed bite and the plant is gone. Sap sucking insects also have a big impact. But it is not as easy as increasing its defence a study of Chinese researchers shows in Scientific Advances.
Not only are young plants extra sensitive, insects also prefer them over older plants. But how older they get, the more their defence is up, and the less herbivory takes place. The researchers decided to investigate how a plant is regulating this.
They studied how much of the defence hormones young and old Nicotiana benthamiana plants make. It turned oy that old plants make more of the hormone SA than young plants. The other defence hormone, JA, did not show an age dependent increase.
The growth hormone auxin puts a brake on age dependent defence
With this the researchers had a way to decipher hoe the plant is regulating age dependent defence. First the researchers found out via which pathway the plant produces SA. Coming to the conclusion that this goes via the PAL6 enzyme.
Subsequently the researchers analysed which gen regulator regulates the expression of PAL6, MYB42, and which gen regulators in turn activate this gene, ARFF18La/b. Thereby confirming each time if, when this gene is activated earlier in the life of the plant, the age dependent defence is also activated earlier, what turned out to be.
Now are ARF gen regulators regulated by the growth hormone auxin. Raising the question of the effect of auxin on the defence. When the researchers gave extra auxin to old plants, then their defence was turned down, they were more sensitive for sap sucking insects.
The defence hormone SA inhibits auxin
The question that remained for the researchers was how auxin managed this. To find out the researchers tested the influence of auxin on the activation of PAL6, MYB42, and ARF18La/b. They noticed that auxin did not hinder the activation of PAL6 by MYB42, or MYB42 by ARF18La/b. But that there was less MYB42 and ARF18La/b when there was more auxin. For ARF18La/b this, it turned out, it was not because the gene was less activated through auxin, there was nevertheless less ARF18La/b precent.
Auxin was thus responsible for the degradation of ARF18La/b mRNA. This often occurs via a miRNA who labels its target mRNA for degradation. After a search through the miRNA database, miR160c turned out to be a good match. In plants that produced more miR160c the there was no age dependent defence. While in plants who got at a young age less miR160c had also activated their age dependent defence earlier. In addition, it turned out that auxin directly turns on miR160c.
Now the researchers found out that auxin was the brake for age dependent defence, there remained one question. Why was a brake needed? To find out the researchers grew plants in the presence of SA. These plants it turned out remained small, because there was hardly any auxin precent. For young plants it is the choice between growth or defence.
Literature
Wen-Hao Han et al., Auxin-salicylic acid seesaw regulates the age-dependent balance between plant growth and herbivore defense.Sci. Adv.11,eadu5141(2025). https://www.science.org/doi/10.1126/sciadv.adu5141
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