New genes behind chemical defence in aspen identified

When Sara Rydman began studying biology, she was particularly interested in ecology and in understanding how different species interact with one another. Over time, however, she wanted to go deeper and explore the molecular mechanisms that explain why plants differ from each other.

“Plants are fascinating because they can tolerate so many different environments,” she says.

Sara Rydman became especially interested in how plants and insects influence each other. During her master’s project at the Umeå Plant Science Centre (UPSC), she worked on plant-insect interactions together with Benedicte Riber Albrectsen. Around the same time, she was asked whether she would like to continue with a PhD in Nathaniel Street’s research group who is studying natural variation.

I have mostly just followed my curiosity, and that is probably why I have never lost interest.

“It felt like the perfect match. Looking back, I have mostly just followed my curiosity, and that is probably why I have never lost interest.”

Chemical defence comes at a cost

In her research, Sara Rydman focused on salicinoids, a group of chemical compounds produced by willows, poplars, and aspen. Salicinoids play an important role in plant defence by helping trees protect themselves against herbivorous insects. Some salicinoids are also bioactive in humans and have anti-inflammatory properties. Research on these compounds eventually contributed to the development of acetylsalicylic acid, the active ingredient in aspirin.

For trees, however, the formation of salicinoids likely comes at a cost. A substantial portion of the carbon that trees take up is used to produce these defence compounds, which may affect how much energy remains available for growth. Understanding the balance between growth and defence is therefore central to understanding how trees allocate their resources.

Aspen is particularly well suited for studying salicinoids, as natural populations show large variation both in how much they produce and in which variants occur. Although many genes are involved in the formation of these compounds, only a few had previously been identified.

“By identifying the genes involved, we can better understand how salicinoid formation is linked to tree growth and resistance to insects,” Sara Rydman says.

Sara Rydman set up an experiment with herbivorous insects to test if they prefer eating on certain aspen variants more than others.

Image Sara Rydman

New genes identified using systems biology approaches

In her PhD project, Sara Rydman used a systems biology approach to study a natural population of Swedish aspen trees that differ in their salicinoid composition. By combining gene activity data with detailed chemical analyses and genetic data, the researchers were able to identify genetic patterns underlying variation in the trees’ chemical defence.

At the same time, they investigated how salicinoid levels vary between different parts of the tree. One clear result was that young leaves contain higher levels of salicinoids and show more active formation than older leaves.

Young leaves are especially vulnerable to insect attack and therefore have a greater need for chemical defence.

“That makes sense, because young leaves are especially vulnerable to insect attack and therefore have a greater need for chemical defence,” she says.

By integrating the different analyses, the researchers identified several new candidate genes involved in salicinoid formation. One of these genes was studied in more detail. When the activity of this gene was increased in aspen, salicinoid levels also increased. Its function was further confirmed in experiments using bacteria, showing that the protein acts directly on salicinoids.

“Taken together, this provided strong evidence that the gene plays a central role in the process.”

When results do not turn out as expected

Not all results, however, were what the researchers had anticipated. In one experiment with herbivorous insects, natural aspen variants with different salicinoid levels were used. The hypothesis was that trees with higher salicinoid levels would be less attractive to insects.

“To our surprise, we did not see any clear differences in insect damage, even though some of the trees consistently produce large amounts of salicinoids.”

The result was unexpected, but it also highlighted how complex plant-insect interactions really are. Chemical defence is clearly not controlled by a single compound alone.

“It was an important insight and we are now continuing to explore this furhter using genetically modified trees, which allow us to test the effects in a more detailed way.”

Conducting the herbivore experiment was also a major undertaking. Since similar experiments had not previously been carried out at UPSC, much of the work had to be built from scratch.

“I even took the insects home and took care of them all summer. They had to be fed and cleaned up after every day – it was almost like having pets.”

Next steps – a focus on bioinformatics

One of the biggest challenges in the project was handling the large datasets generated by the systems biology approach. Analysing and interpreting the results required strong skills in bioinformatics.

“Machine learning and AI can help identify patterns, but you still need to ask the right questions and make sure that each step of the analysis is carried out properly.”

Support from colleagues with bioinformatics expertise at UPSC and Umeå University proved crucial, she says.

In the beginning it was very challenging and not something I enjoyed at first, but over time bioinformatics has become what I find most rewarding.

Sara Rydman is now looking ahead. She has received a few offers, but nothing is official yet. Her hope is to remain in Umeå and continue working with bioinformatics.

“It’s like learning to read a completely new language. In the beginning it was very challenging and not something I enjoyed at first, but over time bioinformatics has become what I find most rewarding. That’s what I want to focus on next.”