Björn Sundberg - Wood Formation

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Our research is focused on the developmental and biosynthetic regulation of secondary xylem (wood) formation. Wood biomass, fibre morphology, and ultrastructure and chemistry of cell walls are all industrially important properties determined during the woodforming process. Specific targets of our research are the physiological function and downstream signalling of endogenous ethylene in wood formation, cellulose biosynthesis in wood fibres, and novel tools for wood phenotyping.
Sundberg_Bjorn_portrait

Intrinsic control of cambial growth and wood development along the stem assures an appropriate stem form, and regulates the formation of juvenile and mature wood with their different chemical/physical properties. Superimposed on this intrinsic control are environmental factors that also strongly influence wood development. The mechanical and gravitational loads imposed by wind and by leaning stimulate both diameter growth and tension wood (TW) formation in dicotyledonous angiosperms. In addition to a striking increase in cambial cell divisions, TW fibres form an additional inner, cellulose-rich, gelatinous secondary cell-wall layer.

We are using in vitro grown aspen trees as an experimental system to perform pharmacological experiments and for initial screening of wood properties. Picture modified from PNAS 2009 106: 5984-5989

Plant hormones are important signaling molecules by mediating in mediating intrinsic and environmental controls of wood formation and an enduring research interest of the group. Trees respond to mechanical and gravitational load with the production of ethylene, which is followed by stimulation of tension wood formation. We therefore can use tension wood induction to unravel the physiology and downstream signaling associated with endogenous ehtylene production. We have demonstrated that ethylene is an endogenous signal stimulating cambial cell division, and thus diameter growth, in Populus. The same screening system allowed us to unravel the potential role of endogenous ethylene in the regulation of biosynthesis and ultrastructure of cell walls. We found that 20 ethylene response factors (ERFs) in stem tissues. which are induced by ethylene. As part of our exploration of downstream signalling in ethylene responses, we have identified 20 major ethylene response factors (ERFs) induced by applied ethylene in stem tissues. Transgenic Populus plants over- expressing these transcription factors have been phenotyped, using the in vitro system, and candidate genes causing growth and cell wall phenotypes have been selected for in-depth study. We have also demonstrated that ethylene stimulates cambial growth and modifies secondary walls in the Arabidopsis hypocotyls, and are using this model system for genetic analysis of ERF function and hormonal cross-talk.

Tension wood is formed in leaning stems. The fiber cell wall (A) forms a normal S2 layer, but has an inner wall layer of almost pure cellulose (G), without lignin and hemicelluloses. Individual cellulose microfibrils are seen in B. Picture modified from Current Opinion Plant Biology 2008 11: 293-300 was generously provided by Prof. Geoffrey Daniel, SLU Uppsala.

During TW fibre development, the biosynthetic machinery for lignin and hemicellulose is switched off as the cells deposit a new cellulose-enriched inner gelatinous wall layer. We have used the poplar microarray and tangential crysectioning to visualize gene expression across this developmental switch, and thereby differentiate between genes involved in cellulose versus hemicellulose/lignin biosynthesis. Bioinformatic analysis, T-DNA mutant screens in Arabidopsis and previous knowledge were used to find key candidate genes in cellulose biosynthesis.
These genes include PttMAP20, PttCobra-like4, PttAGP-FLAs and unknown proteins, which are now undergoing functional analysis in Arabidopsis and Populus, using genetic, cell biology, biochemical and chemical approaches.
The Sundberg group has been instrumental in establishing the infrastructure for a state-of-the-art carbohydrate/cell wall analysis facility in the UPSC environment to support cell wall research. In addition, this unit is involved in wood chemotyping of the UPSC collection of transgenic trees, as well as spruce and poplar populations for association genetics studies. We are also incorporating additional technologies for wood analysis, including chemical imaging by FT-IR microscopy and GC-MS pyrolysis combined with chemometric tools.

Svensk sammanfattning

Key Publications

Uggla C, Moritz T, Sandberg G, Sundberg B (1996). Auxin as a positional signal in pattern formation in plants.Proc. Natl. Acad. Sci. USA 93: 9282-9286.

Hertzberg M, Aspeborg H, Schrader J, Andersson A, Erlandsson R, Blomqvist K, Bhalerao R, Uhlen M, Teeri T, Lundberg J, Sundberg B, Nilsson P, Sandberg G. (2001). A transcriptional roadmap to wood formation.Proc. Natl. Acad. Sci. USA 98: 14732-14737.

Andersson Gunnerås S, Mellerowicz EJ, Ohmiya Y, Nilsson P, Henrissat B, Love J, Moritz T. and Sundberg B. (2006). Biosynthesis of cellulose-enriched tension wood in Populus: global analysis of transcripts and metabolites identifies biochemical and developmental regulators in secondary wall biosynthesis. Plant J 45: 144-165

Rajangam AS, Kumar M, Aspeborg H, Guerriero G, Arvestad L, Pansri P, Brown CJ, Hober S, Blomqvist K, Divne C, Ezcurra I, Mellerowicz E, Sundberg B, Bolone V, Teeri TT (2008). MAP20, a microtubule-associated protein in the secondary cell walls of Populus tremula L. x tremuloides Michx is a target of the cellulose synthesis inhibitor, 2,6-dichlorobenzonitrile. Plant Physiology 148(3):1283-1294.

Love J, Bjorklund S, Vahala J, Hertzberg M, Kangasjärvi J, Sundberg B (2009). Ethylene is an endogenous stimulator of cell division in the cambial meristem of Populus. Proc. Natl. Acad. Sci. USA 106: 5984-5989.

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July 2010

Friday, July 30, 2010
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