Pieter Nibbering and his PhD supervisor Totte Niittylä after the successful defence (photo: Anne Honsel)
[2020-10-30] Plant cell walls need to be flexible and at the same time rigid to give stability but also allow growth. Glycoproteins, proteins with attached carbohydrates, are only a minor component of the cell wall which consists mainly of cellulose, hemicellulose, pectin and lignin. Nevertheless, modifications of such glycoproteins can have a strong impact on the properties of the plant cell wall. This was shown by Pieter Nibbering, PhD student in Totte Niittylä’s group, who studied cell wall formation in the herbal plant Arabidopsis and in poplar. Pieter Nibbering successfully defend his PhD thesis at the Swedish University of Agricultural Sciences on Friday, 30th of October.
Your thesis focuses on plant cell wall formation in Arabidopsis and aspen. What was interesting you about doing research on cell walls?
I find cell walls very interesting because they so complex. It is very challenging to work with them and I wanted to take on this new challenge. Another reason why I decided for this PhD project was that you cannot just use one or two methods when studying cell walls but need to use many different tools like e.g. molecular cloning, chemistry and protein expression to come closer to an answer of your questions. This diversity was also very interesting for me because it allowed me to learn a lot of different techniques.
You worked with certain glycoproteins that are lesser known components of the cell wall. Why are these glycoproteins important?
That is actually the question we tried to answer during my PhD. We do not know yet why they are important. The difficulty is that approximately 1200 proteins in Arabidopsis are predicted to have this kind of carbohydrates called arabinogalactan glycans attached - not only those ones we worked with. The attached glycans fulfil many different functions. They can e.g. stabilize the protein to make sure that the protein can perform the right function, they can interact directly with the cell wall or with cell wall components or they can bind salts for instance calcium and be involved in signal transduction. For our experiments, we used mutants in which the biosynthesis of some glycoproteins is impaired meaning that the glycans are not added properly to the proteins. The mutants clearly showed growth defects and/or defects in stress response and this let us conclude that the impaired glycoproteins are important for normal growth and stress responses.
Which of your results is the most fascinating for you?
Some of our mutants only showed an obvious difference to our control plants when they were grown under stressful conditions. This is rather what you expect if you modify minor components of the cell wall. However, the growth of other mutants was strongly affected also under optimal growth conditions. This was really unexpected for me because I did not think that a modification of a protein that is adding glycans to some of the 1200 predicted glycoproteins has such a severe effect on the cell wall. We still do not fully understand this result, but we have some ideas and are hopefully close to figure out what the cause is.
What was the biggest challenge you faced during your PhD?
The biggest challenge in my whole PhD was to transfer those proteins to bacteria or tobacco which allows to multiply and isolate these proteins and do functional studies. They need a very specific environment to be active which made the full procedure very challenging. We are still struggling with this, but I will continue on optimizing the process in the next couple of weeks.
Do you think your results might lead to practical applications in future?
We still need to do more research before we come closer to practical applications. When we understand better how modifications of the glycoproteins affect cell wall properties, we might be able to alter the interaction between cell wall components and like this change the properties of the cell wall. We could for example make cellulose or hemicellulose easier to extract which could be interesting for the pulp and paper industry or make the cell walls a bit more bendable or allow that it can expand more. Beside of the mutant studies with Arabidopsis, we also performed a bioinformatic analysis where we predicted which proteins in poplar are glycoproteins and where these proteins are expressed in the wood. Also this study does not result in direct applications but it is groundwork for future research in poplar and closely related species there is the potential for possible future applications.
Do you plan to continue doing research on cell wall or do you have already other plans for your future?
My plan is to go back to the Netherlands, and I am currently searching a postdoc there. It is quite difficult now during the Corona crisis because not many positions open up. I would like to continue doing research on cell walls. There is still a lot to uncover. Unfortunately, not many research groups in the Netherlands are working on cell walls. That is why I am also thinking to write my own postdoc proposal focusing on the role of the cell wall in abiotic stress responses in Arabidopsis but also in crops. I have never worked with crops before and I would like to start with that. On the long run, I would like to do research in a breeding company and working with crops hopefully will help me to come closer to that.
About the public defence:
The public defence took place on Friday, 30th of October at SLU in Umeå. Faculty opponent was Grégory Mouille, Institut Jean-Pierre Bourgin, UMR 1318 INRAE-AgroParisTech, France. Pieter Nibbering's supervisor is Totte Niittylä. The dissertation was live broadcasted on SLU Play: https://play.slu.se.
Title of the thesis: The role and synthesis of β1,3-galactans in plant cell wall formation
Link to the thesis: https://pub.epsilon.slu.se/17817/
For more information, please contact:
Pieter Nibbering
Department of Forest Genetics and Plant Physiology
Umeå Plant Science Centre
Swedish University of Agricultural Sciences (SLU)
Email: