Analysis of the Arabidopsis IRX9/IRX9-L and IRX14/IRX14-L Pairs of Glycosyltransferase Genes Reveals Critical Contributions to Biosynthesis of the Hemicellulose Glucuronoxylan

International audienceThe hemicellulose glucuronoxylan (GX) is a major component of plant secondary cell walls. However, our understanding ofGX synthesis remains limited. Here, we identify and analyze two new genes from Arabidopsis (Arabidopsis thaliana),IRREGULAR XYLEM9-LIKE (IRX9-L) and IRX14-LIKE (IRX14-L) that encode glycosyltransferase family 43 members proposedto function during xylan backbone elongation. We place IRX9-L and IRX14-L in a genetic framework with six previouslydescribed glycosyltransferase genes (IRX9, IRX10, IRX10-L, IRX14, FRAGILE FIBER8 [FRA8], and FRA8 HOMOLOG [F8H]) andinvestigate their function in GX synthesis. Double-mutant analysis identifies IRX9-L and IRX14-L as functional homologs ofIRX9 and IRX14, respectively. Characterization of irx9 irx10 irx14 fra8 and irx9-L irx10-L irx14-L f8h quadruple mutants allowsdefinition of a set of genes comprising IRX9, IRX10, IRX14, and FRA8 that perform the main role in GX synthesis duringvegetative development. The IRX9-L, IRX10-L, IRX14-L, and F8H genes are able to partially substitute for their respectivehomologs and normally perform a minor function. The irx14 irx14-L double mutant virtually lacks xylan, whereas irx9 irx9-Land fra8 f8h double mutants form lowered amounts of GX displaying a greatly reduced degree of backbone polymerization.Our findings reveal two distinct sets of four genes each differentially contributing to GX biosynthesis