[2020-12-11] Lignin is the compound that makes plant cell walls waterproof and rigid. This allows plants to stand upright and transport water within their stem. Bernadette Sztojka, PhD student in Hannele Tuominen’s group, studied how lignin is synthesised and deposited in the cell walls, a process called lignification. She showed that cooperation between neighbouring cells is important for lignification of water-transporting vessel elements and identified three new regulators involved in this process. Bernadette Sztojka successfully defended her PhD thesis at Umeå University yesterday.
You did already your master thesis at UPSC but on a different topic. Why did you choose to switch topic and study lignification during your PhD thesis?
The projects I worked on before in the group of Rishikesh Bhalerao were more cell biology-oriented, related to vesicular trafficking. That was already quite a shift because my original background is horticulture. For my PhD, I wanted to further explore new territories this is why I applied for this PhD position, focusing on cell wall and wood formation.
You identified and characterised three new regulators involved in lignification. What do these regulators have in common and why are they peculiar?
The first regulator that we chose to work on was PIRIN2. A gene which is closely related to PIRIN2 was identified earlier in a cell culture system and shown to play a potential role in lignification. That is why we decided to characterise this gene in Arabidopsis. To better understand the molecular function of PIRIN2, we looked for proteins that interact with PIRIN2. That is how we came across the other two molecular players we studied, and this allowed us to expand the known molecular network. One of these two proteins has the opposite effect on lignin biosynthesis than PIRIN2. While PIRIN2 is suppressing the synthesis of certain types of monolignols, which are the basic modules of lignin, this protein is promoting it. The other protein we identified has no direct effect on lignin content or composition but connects lignin biosynthesis to diurnal timing.
Why does lignin biosynthesis need diurnal timing?
There have been a few previous studies that suggested that lignin-biosynthetic genes are activated in a diurnal pattern but there was never any upstream regulator found that controls these genes. Lignin is a very big carbon sink that means that a lot of carbon is required for its biosynthesis. That is why lignin biosynthesis has to be coordinated with resource availability and that brings in the diurnal regulation to connect lignin biosynthesis with carbon fixation during photosynthesis.
Which of your results is the most fascinating for you?
For me the first project, characterizing PIRIN2 and the non-cellautonomous lignification, was the most fascinating. PIRIN2 is a negative regulator of lignification but in a cell-type specific way. It is affecting the lignin composition of the cell walls of its neighbouring cells where it is not expressed. It is located in cells next to xylem vessels which form the water transporting “pipes”. We believe that the vessels need a specific lignin composition to allow for efficient water transport and PIRIN2 makes sure that they acquire the correct composition by suppressing the biosynthesis of certain monolignols which may not be optimal for water transport. I find these types of cooperative processes between cells really fascinating.
What was the biggest challenge you faced during your PhD?
It was hard sometimes to give up on ideas. We might have had a hypothesis and not necessarily showed that the hypothesis was proven right or wrong, but we realized that we did not had the means or time to follow up on this hypothesis. My original project that made me interested in the PhD topic became a bit squeezed in time because some other projects which I was supposed to finish quickly took over like for example the PIRIN2 project. It worked out fine for me in the end, but sometimes it was challenging to accept that I had to let a part of my project go unfinished.
Do you think your results might lead to practical applications in future?
I think especially PIRIN2 could be interesting for the forest industry. By manipulating PIRIN2 it might be possible to generate woody biomass with different lignin composition. Plants often get really sick when the lignin content is modified genetically but this is not the case when PIRIN2 is mutated. The effect on the chemical composition is not very strong in PIRIN2 mutants but those changes can still be beneficial for industrial use and they do not compromise the plant’s fitness.
What are your plans for the future? Do you want to continue with research?
I would like to transition towards more applied research, combining my background in horticulture with my PhD experience in plant molecular biology. This could be either in academia or industry. I am still curious to explore new territories and would like to change a bit the subject again.
About the public defence:
The public defence took place on Thursday, 10th of December at Umeå University. Faculty opponent was Simon Hawkins from the Department of Biology, University of Lille, France. Bernadette Sztojka's supervisor was Hannele Tuominen. The dissertation was live broadcasted via Zoom.
Title of the thesis: New regulators of xylem lignification in Arabidopsis
Link to the thesis: http://umu.diva-portal.org/smash/record.jsf?pid=diva2%3A1500629
For more information, please contact:
Department of Plant Physiology
Umeå Plant Science Centre