Engineering polysaccharides in plants

Both historically and contemporarily, plant cell walls have contributed a wealth of natural resources to humankind, and, largely speaking, this utility is indebted to their complement of diverse carbohydrate polymers, the polysaccharides. Our research is concerned with a particularly interesting and abundant polysaccharide called xylan that is thought to have roles in wall strength and digestibility in plants, as well as in human health following consumption of vegetable foods. Xylan possesses a linear backbone of polymerised xylose monomers, but its properties are influenced by length and augmented by an assortment of short branches that 'decorate' the backbone and differ in structure and spacing. We are therefore interested in engineering the backbone length and the nature of this decoration as a means to producing novel materials and products from cell wall material.

However, we currently have very little understanding of how the backbone polymer of xylan is made. This represents an obstacle to xylan engineering because we cannot currently explain how the backbone comes to be decorated in a precise pattern, which may require co-ordination between backbone synthesis and decoration. It is currently thought that three enzymes (putatively forming a 'xylan synthase complex'), rather than one, are needed for backbone synthesis alone, and understanding why such co-operation is necessary will help us determine whether it is linked to regulation of decoration. We would also like to know why separate sets of enzymes have evolved for backbone synthesis of xylan in the growing primary and the thickened secondary plant cell walls, and whether this is significant in determining xylan properties.

In this project, we aim to determine the structure and mechanism of the xylan synthase protein complex. Using synthetic biology tools, we will employ systematic gene cloning and protein expression approaches in producing the enzymes of interest. Use of this engineering approach will facilitate screening of activities of enzymes and enzyme mutants. Ultimately we aim to resolve the atomic structure of the xylan synthase complex by cryo-electron microscopy. The information will conceivably permit modulation of xylan decoration and backbone length and thus allow directed engineering of the plant cell wall.