Mellerowicz next to aspen trees in the greenhouse

Wood cells accumulate the major proportion of terrestrial biomass in their thick cell walls, composed of cellulose, matrix polysaccharides and lignin. By interacting with cellulose and lignin, matrix polysaccharides affect mechanical and chemical properties of cell walls in developing and mature wood cells, which determines wood quality traits, such as fiber length, wood and fiber strength and durability, and properties important in industrial wood processing such as pulping and saccharification. Our research is aiming at elucidating these matrix functions in wood cells.

CAZymes involved in wood cell wall formation

Matrix polysaccharides are synthesized and modified by Carbohydrate Active Enzymes (CAZYmes) and by other enzymes acting on non-carbohydrate types of side chains. These enzymes reside either in the Golgi apparatus where they synthesize matrix polymers, or in cell walls where they modify them post-synthetically. We have been studying the function of some of these genes expressed during wood cell wall formation using Populus and Arabidopsis thaliana as model species.

Collage of three graphs showing matrix polysaccharide molecules projected on wood cross section on the left, the classification of CAZymes in the middle and a corresponding heat map on the right. Left: Matrix polysaccharides; middle and right: Almost 2000 CAZyme genes classified in super-families were identified in fully sequenced Populus genome. Over 60% of these genes show specific expression pattern during wood biosynthesis indicating their involvement in different wood cell differentiation stages.

Modification of xylan and other matrix polysaccharides in planta

To investigate the role of matrix polysaccharides in mediating cell wall properties we use both, native plant matrix biosynthetic/modifying genes, and microbial transgenes, to modify the structure of matrix polysaccharides in plant cell walls. The microbial enzymes having defined specificity to matrix components in wood, when targeted to cell walls and to developing woody tissues, are a powerful tool to modify specific chains of the matrix. This lets us reveal some of matrix intricate interactions among different wall components and their overall role during wood cell development.

Microscope image of fiber and vessle elements on the left and red coloured wood cells on the right.Left: Matrix modification affects wood cell expansion and thus regulates the size and shape of wood cells. Fibers and vessel elements have different ways of expansion; fibers elongate by intrusive tip growth but vessel elements do not elongate, but they radially expand by a combination of radial intrusive and symplastic growth. Accordingly, a highly regulated modification of matrix in developing wood cells is required to achieve these different outcomes. We found that pectin and xyloglucan modifications are needed for wood cell expansion. Right: Matrix is modified by cell wall-residing enzymes even in the dead cells. Activity of XET enzyme in wood cell walls visualized by the incorporation of the fluorescent substrate to cell walls of tension wood fibers four years after their biosynthesis.

Genetic improvement of aspen for bioprocessing

Aspen tree that is growing in a field trial. Field testing of trees with modified matrix reveals novel functions of matrix that are important in natural environment.

Some of the studied matrix modifications are beneficial for bioprocessing of plant biomass in biorefinery. We have generated many different types of transgenic lines with altered matrix polysaccharides, which had better wood properties based on greenhouse experiments. These lines are subsequently tested in field trials of short rotation. The field trials typically reveal new phenotypes, and are essential for tree improvement as well as for the better understanding of the implication of matrix modification.

Secondary wall integrity sensing?

Transgenic plants with modified cell wall polymers in secondary walls sometimes show increased growth and increased resistance to biotic stresses. The pathways leading to such outcomes are unknown, and we are testing a hypothesis that there exists a type of signaling of secondary cell wall integrity, which is responsible for the observed phenotypical changes. Interestingly, some effects of matrix deficiency are similar to effects of stem swaying. We are currently investigating of the molecular pathways involved in secondary wall integrity sensing and the sensing of stem strains share some common mechanisms.

Key publications

  • Derba-Maceluch M, Amini F, Donev EN, Pawar PM-A, Michaud L, Johansson U, Albrectsen BR and Mellerowicz EJ* (2020) Cell wall acetylation in hybrid aspen affects field performance, foliar phenolic composition and resistance to biological stress factors in a construct-dependent fashion. Frontiers Plant Sci 11:651.
  • Kushwah S, Banasiak A, Nishikubo N, Derba-Maceluch M, Majda M, Endo S, Kumar V, Gomez L, Gorzsás A, McQueen-Mason S, Braam J, Sundberg B, Mellerowicz EJ* (2020). Arabidopsis XTH4 and XTH9 contribute to wood cell expansion and secondary wall formation. Plant Physiology 182: 1946–1965.
  • Kumar V, Hainaut M, Delhomme N, Mannapperuma C, Street NR, Henrissat B, Mellerowicz EJ* (2019). Poplar Carbohydrate Active Enzymes - new annotation and functional analyses based on RNA expression data. Plant J, 99: 598-609.
  • Ratke C, Terebieniec BK, Winestrand S, Derba-Maceluch M, Grahn T, Schiffthaler B, Ulvcrona T, Özparpucu M, Rüggeberg M, Lundqvist SO, Street NR, Jönsson LJ, Mellerowicz EJ* (2018). Downregulating aspen xylan biosynthetic GT43 genes in developing wood stimulates growth via reprograming of transcriptome. New Phytologist 219: 230-245.
  • Leśniewska J, Öhman D, Krzesłowska M, Kushwah S, Barciszewska-Pacak M, Kleczkowski LA, Sundberg B, Moritz T, Mellerowicz EJ* (2017). Defense responses in aspens with altered PME activity reveal the hormonal inducers of tyloses. Plant Physiology 173:1409–1419.
  • Pawar P M-A, Derba-Maceluch M, Chong SL, Gandla ML, Bashar SS, Sparrman T, Ahvenainen P, Hedenström M, Özparpucu M, Rüggeberg M, Serimaa R, Lawoko M, Tenkanen M, Jönsson LJ, Mellerowicz EJ* (2017). In muro deacetylation of xylan increases lignin extractability and improves saccharification of aspen wood. Biotechnology for Biofuels 10:98.
  • Pawar P M-A, Ratke C, Balasubramanian VB, Chong SL, Gandla ML, Adriasola M, Sparrman T, Hedenström M, Szwaj K, Derba-Maceluch M, Ezcurra I, Tenkanen M, Jönsson LJ, Mellerowicz EJ* (2017). Downregulation of RWA genes in hybrid aspen affects xylan acetylation and wood processing properties. New Phytologist 214: 1491–1505.