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Research overview

Sustainable food and energy supply to the world’s growing population is one of the biggest challenges of this century. The plant cell wall is a rich and renewable source of polysaccharides fed by sunlight, water and carbon dioxide from the air. Innovations to utilise this natural resource sustainably, based on our growing knowledge on plant cell walls composition, structure and assembly, will contribute to tackle these global challenges of our future.

The biosynthesis of cell wall polysaccharides involves the action of hundreds of different glycosyltransferases (GTs), the enzymes that catalyse the specific transfer of sugar moieties from activated nucleotide sugar donors to acceptor molecules, forming glycosidic bonds. Specific nucleotide-sugar transporters located in the Golgi membrane are also involved as suppliers of the monosaccharide (Figure 1). Over the years we have identified several glycosyltransferases involved in the biosynthesis of xylan, glucomannan and arabinogalactan and contributed greatly to our understanding of polysaccharide biosynthesis in the Golgi apparatus and detailed characterisation of polysaccharide structures. We mostly use a small flowering plant Arabidopsis thaliana as a model system for a dicot plant, but our research extends to monocots (the other class of angiosperms) and gymnosperms. We continue our research into this key area, as plenty of enzymes and processes involved in cell wall biosynthesis remain unknown.GolgiGTs schematicC small

Figure 1: A model of xylan polysaccharide biosynthesis in the Golgi, depicting glycosyltransferases involved in xylan backbone synthesis (IRX9, IRX10, IRX14) and addition of side chains (GUX, GT61). Additional enzymes responsible for further modification of xylan like acetylation (TBLs) and methylation of Glucuronic acid (GXMs) and some of Golgi transporter are shown too.

Polysaccharides synthesised in the Golgi apparatus are transported to the plasma membrane and secreted into the apoplast in order to form the cell wall. Cellulose, the most prevalent component of the plant cell wall, however, is synthesised by enzyme complexes in the plasma membrane and directly released into the apoplast. We have shown that Stello1/2, two putative Golgi glycosyltransferases, are important for correct assembly and trafficking of the cellulose synthase complex (Zhang et al., 2016). It is still largely unknown, how secretion and synthesis are coordinated and how the different polysaccharides assemble and interact with each other and with additional cell wall components in order to create the molecular architecture of specific cell walls, e.g. the primary flexible cell wall or the secondary rigid and recalcitrant plant cell wall.

Our research also aims to shed light on polysaccharide interaction in the plant cell wall and the understanding of underlying molecular mechanisms for plant cell wall assembly (Figure 2). Traditionally, plant cell walls are studied using biochemical techniques characterising individual components, allowing the characterisation of the composition and primary structure of the polysaccharides. In collaboration with Prof R. Dupree and Prof S. Brown, we now employ solid state Nuclear Magnetic Resonance (NMR) techniques to analyse polysaccharide structure and their interaction in the intact plant cell wall. This led to new discoveries, showing that the pattern of acetylation of xylan is fundamental in determining the pattern of glucuronic acid addition to the backbone (Grantham et al., 2017). Defects in this pattern affect the overall conformation of the xylan chain, leading to loss of xylan cellulose interaction.

Model from Simmons et al., 2016

Figure 2: A model of the interaction of xylan with cellulose in the plant cell wall (from Simmons et al., 2016)