Genetic identification and biochemical characterization of xylan backbone synthases in Arabidopsis and poplar
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Xylan is the second most abundant polysaccharide in plant cell walls. However, its biosynthesis including the initiation and elongation of xylosyl backbone, side chain addition, and sugar modification, is still poorly understood. In an effort to identify xylan biosynthetic enzymes, we first profiled a number of upregulated genes during the process of the secondary cell wall formation. PARVUS was specifically expressed in the fibers and vessels, and its encoding protein was localized in the endoplasmic reticulum. Mutant phenotypes and structure analysis of xylans from the parvus mutant stems clearly showed its involvement in the biosynthesis of reducing end sequence of the xylan. Recent genetic and molecular studies in Arabidopis have shown that two proteins in glycosyltransferase (GT) 43 family, IRX9 and IRX14, are golgi-localized GTs, genetic defects of which result in a xylan deficient phenotype and a significant reduction in xylan chain length. In addition, the in-vitro activity assay with stem microsomes revealed that irx9 and irx14 mutants possess substantially lower xylosyltransferase activity compared with the wild type, suggesting their possible roles in xylan backbone polymerization. Comprehensive genetic analysis of the four Arabidopsis GT43 members provides evidence that all four Arabidopsis GT43 members are involved in xylan biosynthesis and suggests that they form two functionally nonredundant groups essential for the normal elongation of xylan backbone. Multiple lines of evidence indicate that GTs involved in xylan biosynthesis are highly conserved between Arabidopsis and Poplar. Complementation studies revealed that PtrGT43 A/B/E are functional homologs of Arabidopsis IRX9, and PtrGT43 C/D are homologs of IRX14, forming two functionally nonredundant groups, which are required for the polymerization of xylan backbone. The studies presented here provide a wealth of genetic information that expands the knowledge of xylan biosynthesis.