Arabidopsis GUX Proteins Are Glucuronyltransferases Responsible for the Addition of Glucuronic Acid Side Chains onto Xylan

Xylan, the second most abundant cell wall polysaccharide, is composed of a linear backbone of β-(1,4)-linked xylosyl residues that are often substituted with sugar side chains, such as glucuronic acid (GlcA) and methylglucuronic acid (MeGlcA). It has recently been shown that mutations of two Arabidopsis family GT8 genes, GUX1 and GUX2, affect the addition of GlcA and MeGlcA to xylan, but it is not known whether they encode glucuronyltransferases (GlcATs) or indirectly regulate the GlcAT activity. In this study, we performed biochemical and genetic analyses of three Arabidopsis GUX genes to determine their roles in the GlcA substitution of xylan and secondary wall deposition. The GUX1/2/3 genes were found to be expressed in interfascicular fibers and xylem cells, the two major types of secondary wall-containing cells that have abundant xylan. When expressed in tobacco BY2 cells, the GUX1/2/3 proteins exhibited an activity capable of transferring GlcA residues from the UDP-GlcA donor onto xylooligomer acceptors, demonstrating that these GUX proteins possess xylan GlcAT activity. Analyses of the single, double and triple gux mutants revealed that simultaneous mutations of all three GUX genes led to a complete loss of GlcA and MeGlcA side chains on xylan, indicating that all three GUX proteins are involved in the GlcA substitution of xylan. Furthermore, a complete loss of GlcA and MeGlcA side chains in the gux1/2/3 triple mutant resulted in reduced secondary wall thickening, collapsed vessel morphology and reduced plant growth. Together, our results provide biochemical and genetic evidence that GUX1/2/3 are GlcATs responsible for the GlcA substitution of xylan, which is essential for normal secondary wall deposition and plant development.