From bud burst to bud set: The different stages of the annual growth cycle in a juvenile aspen tree (Illustration: Domenique André).
Why are flowering genes important for non-flowering young aspen trees? PhD student Domenique André started to work in Ove Nilsson’s group because she was interested in flowering, but the research led her to studying the annual growth cycle. She showed that key genes that regulate flowering in other plants are crucial for proper bud set and burst in young aspen trees and they control growth during summer. Domenique André will defend her PhD thesis at SLU on Friday this week, 27th of August.
You came to UPSC as Bachelor student about ten years ago and did your master thesis at UPSC. How does it feel to finish your PhD now after all this time?
It is a weird feeling, like “the end of an era”. I have been here now for nine and a half years. UPSC is the place where I grew up academically and scientifically and it feels like my home in a way. And because of that, I also tried to give back as much as possible. For example, I was PhD representative of my department and attended the UPSC Board meetings. I also helped organising events and participated in various outreach activities like Soapbox Science and Fascination of Plants Day. I will miss UPSC for sure.
In your thesis, you studied the annual growth cycle in aspen trees concentrating on the molecular regulation. What interested you about this project?
I started in Ove Nilsson’s group during my master. At that time, I was mainly interested in flowering, and he was the only person working on it at UPSC. But then, Ove’s group started to shift focus from flowering towards the annual growth cycle because it turned out that the genes we were focussing on played a different role in aspen. So, I kind of slipped into this topic together with the group and the flowering topic moved more to the side. To be honest, it took me some time to understand how the annual growth cycle works because I had no previous experience with trees. Aspen trees look very different after they have gone through the first growth cycle and there were big changes between the start and the end of our experiments. That can be confusing. I tried to illustrate this in my thesis: why the growth cycle exists, what stages the tree goes through and how all this is regulated. I really enjoyed the time during my master thesis in Ove’s group and we both agreed that it would be cool if I could continue the project during my PhD and in the end, there is even a little bit about flowering included in my PhD thesis.
You focussed in your research on FLOWERING LOCUS T genes which are most known for their role in regulating flowering, but you studied their role in non-flowering juvenile aspen trees. What are your key findings?
The key findings are that the FLOWERING LOCUS T or FT genes have mainly functions unrelated to flowering but rather control the annual growth cycle in juvenile aspen trees. There are in total three FT genes in aspen. The first one, FT1, is important during winter and makes sure that the tree can wake up from its dormancy during spring. The other two, we call them FT2a and FT2b, are important during summer making sure that the tree grows. Whether they are involved in flowering at all in nature is currently not clear.
What makes it so difficult to prove that the FT genes are important to regulate flowering in aspen?
When we enhanced the gene activity of either of the three FT genes by mutation, the trees started to flower very early. Those flowers did not look normal and were sterile. This is the one flowering part in my thesis. But just because we could induce flowering with more FT, it does not mean that FT is important for flowering in nature. To prove that FT is indeed required, we would have to block FT activity and show that flowering is delayed or doesn’t occur at all. And we have indeed removed FT activity with the CRISPR-Cas9 technology, but the problem is that flowering takes too much time. A normal tree will flower only after 5 years (if you’re lucky), so how long would we have to wait to see it in our mutants? And given how important FT is for the annual growth cycle, it is unlikely that the mutated trees would survive for so long anyway.
Which of your results was most surprising for you?
We did not know that there were two FT2 genes when I started. When we wanted to block the FT2 gene activity using the CRISPR technology, it worked perfectly. We had no FT2 gene activity, but the trees looked absolutely normal. That was really surprising because we knew from mutant lines with reduced gene activity that even if you have only twenty percent of the expression left, you get a very significant effect and in our CRISPR lines there was nothing. It turned out then that there was a second copy of the FT2 gene and when we blocked the activity of both genes at the same time, we got a really, really strong effect. The trees were just around five centimetres tall and almost do not grow. They are alive and they can be propagated in tissue culture, but they grow very slow. This shows that the genes are very important for the trees.
Studying aspen can be demanding because it can be very time-consuming. Was this the biggest challenge you faced during your PhD?
Yes, indeed. People studying wood for example grow their trees in the greenhouse for about two to three months. For us, it takes ten months to do one growth cycle experiment. We simulate the seasons in a growth chamber by adjusting daylength and temperature so that we can see the different seasonal stages on the trees like bud set and bud burst. The most limiting factor during my PhD was the lack of space in growth chambers because I could only do one of these long-lasting experiments at once. I had no problem keeping myself busy with other things while waiting for the experiment to be done but it took very long to get results and move on with my research questions. It was also challenging to decide which experiment to focus on and to plan them very detailed to get as much out as possible, but I learned a lot and became more independent. Overall, I liked my time as a PhD student. I felt it was a nice and relaxed time and I got the freedom to do what I wanted.
What are your plans now? Would you like to stay longer at UPSC?
I will stay at UPSC until the end of the year to hopefully get our manuscripts out. After that I will see. I would like to stay in Umeå, and I like to work in the university environment. I am very interested in teaching and took basically all teaching courses that I was allowed to take as a PhD student. I would love to be a lecturer or head of a lab. I would like to learn new things and develop, but also do something I’m already good at. I’m sure I will find something fun.
About the public defence:
Domenique André will defend her thesis on Friday, 27th of August 2021 in P-O Bäckströms sal at SLU Umeå. Faculty opponent will be Professor Soraya Pelaz from the Centre for Research in Agricultural Genomics in Barcelona, Spain. Supervisor is Ove Nilsson. The dissertation will be live broadcasted via Zoom.
Title of the thesis: Molecular Regulation of the Annual Growth Cycle in Populus Trees
Link to the thesis: https://pub.epsilon.slu.se/24748/
For more information, please contact:
Photo: Rory Shelton
Domenique André
Department of Forest Genetics and Plant Physiology
Umeå Plant Science Centre
Swedish University of Agricultural Sciences
Email:
Twitter: @FloweringLocusT