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Published: 2020-12-14

Cells work together preparing cell walls for water transport

NEWS Bernadette Sztojka shows that cooperation between neighbouring cells is important for lignification of water-transporting vessel elements in plants and has identified three new regulators involved in this process. She successfully defended her thesis at Umeå University on Thursday 10 December.

Text: Anne Honsel

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 has studied how lignin is synthesised and deposited in the cell walls, a process called lignification.

The first regulator involved in lignification that Bernadette Sztojka chose to work on was PIRIN2. A gene that 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 she decided to characterise this gene in the plant Arabidopsis.

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. I find those types of cooperative processes between cells, like this non-cell-autonomous lignification process, fascinating”, says Bernadette Sztojka.

To better understand the molecular function of PIRIN2, she looked for proteins that interact with PIRIN2. That is how she came across the other two molecular players she studied, and this allowed her to expand the known molecular network. One of these two proteins has the opposite effect on the biosynthesis of the basic modules of lignin - the monolignols - than PIRIN2. While PIRIN2 is suppressing the synthesis of certain types of monolignols that may not be optimal for water transport, this protein is promoting it. The second protein she identified has no direct effect on lignin content or composition but connects lignin biosynthesis to diurnal timing.

There have been a few previous studies that suggest 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.

Bernadette Sztojka thinks that 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 of cell walls 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. 

Bernadette Sztojka already has plans for her future.

“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”. 

Read the whole thesis

Press photo. Credit: Anne Honsel


About the public defence:

On Thursday 10 December 2020, Bernadette Sztojka, Department of Plant Physiology at Umeå University, defended her thesis entitled: New regulators of xylem lignification in Arabidopsis

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.