The secondary cell wall in higher plants consists mainly of cellulose, lignin, and xylan and is the major component of blomass in many species. The Arabidopsis thaliana Irregular xylem8 (Irx8) mutant is dwarfed and has a significant reduction in secondary cell wall thickness. IRX8 belongs to a subgroup of glycosyltranaferase family 8 called the GAUT1-related gena family, whose members include GAUT1, a homogalacturonan galacturonosyltransferase, and GAUT12 (IRX8). Here, we use comparative cell wall analyses to show that the irx8 mutant contains significantly reduced levals of xylan and homogalecturonen. Immunohiatochamical analyses confirmed that tha level of xylan was significantly reduced in the mutant. Structural fingerprinting of the cell wall polymers further revealed that irx8 is deficient in glucuronoxylan. To explore the biological function of IRX8, we crrossed irx8 with irx1 (affecting cellulosa synthase 8). The homozygous irx1 irx8 exhibited severely dwarfed Phanotypes, suggesting that IRX8 is essential for cell wall integrity during cellulose deficiency. Taken together, the data presented show that IRX8 affects the level of glucuronoxylan and homogalecturonan in higher plants and that IRX8 provides an important link between the xylan polymer and the secondary cell wall matrix and directly affects secondary cell wall integrity.