Increasing the C6/C5 Sugar Ratio in Bioenergy Crops By Modulating Nucleotide Sugar Transporters and Glycosyltransferases

Biomass consists of about 30% of xylan, a polysaccharide composed of pentoses. Hexoses are more easily converted to biofuels and bioproducts, and therefore it is advantageous to develop plants with a higher ratio of C6 to C5 sugars in their cell walls. We have identified more than 20 nucleotide sugar transporters responsible for transporting substrates into the Golgi lumen, where they are used by glycosyltransferases for synthesis of polysaccharides. The transporters we have identified transport ten different nucleotide sugars, including UDP-Gal, UDP-Rha, UDP-Xyl, UDP-GlcNAc, UDP-Araf, GDP-Man, and GDP-Fuc. By modulating the expression of UDP-Gal and UDP-Xyl transporters we have achieved an improved C6/C5 ratio in plant cell walls. Likewise, we have achieved a higher C6/C5 ratio by overexpressing a number of glycosyltransferases for polysaccharides rich in C6 sugars, or inhibiting enzymes responsible for synthesis of polysaccharides rich in C5 sugars. The resulting plants are indistinguishable from the wild type under normal growth conditions. Changing the cell walls of plants may lead to altered environmental resilience, and we have therefore tested the drought tolerance of some of the engineered plants. Surprisingly, many of the plants show increased drought tolerance.