Close homologs IRX9-L, IRX14-L, and IRX10-L have recently been proven to act as partially redundant genes. Mutating both genes of a pair (AtIRX9/AtIRX9-L, AtIRX10/AtIRX10-L, and AtIRX14/AtIRX14-L) results in severe growth defects compared with the relatively mild irregular xylem phenotype observed in single mutants. It was impossible to isolate xylan with an endo-xylanase or to detect with a xylan-specific antibody in stem material of the irx14 irx14-L and irx10 irx10-L double mutants emphasizing the essential role of these genes in xylan synthesis (Keppler and Showalter 2010; Lee et al. 2010; Wu et al. 2009, 2010).