Epsilon Open Archive

Abstract

The hemicellulose glucuronoxylan (GX) is the second most abundant polysaccharide
after cellulose in the secondary cell walls of angiosperms and contributes significantly
to the properties of wood as raw materials. Despite this, very little is known about the
biosynthesis of hemicellulose. The thesis is based on studies performed in
Physcomitrella patens, Arabidopsis thaliana, and Populus plants with the aim to
increase the understanding of glucuronoxylan synthesis, and with main focus on the
role of the IRX10 gene family.

A number of genes have proven to be involved in GX biosynthesis in Arabidopsis,
namely IRREGULAR XYLEM (IRX) 9, IRX14, IRX8, FRAGILE FIBER (FRA) 8 and
PARVUS. This thesis presents two new genes, IRX10 and its close homolog IRX10-
LIKE (L), which can now be added to the group of presumed GX biosynthesis genes. In
addition, identification and characterisation of three homologs (IRX9-L, IRX14-L and
F8H) of previously identified GX synthesis genes is described.

Complementation experiments and morphological studies confirmed redundancy
between the IRX9 and IRX9-L, IRX14 and IRX14-L, and the FRA8 and F8H pairs of
genes in Arabidopsis. A bioinformatics based approach has led to the identification of
four putative homologs in Populus trichocarpa (PtGT47A-1, PtGT47A-2, PtGT47D-1,
and PtGT47D-4) of AtIRX10, and one putative homolog from Physcomitrella
(PpGT47D). Experiments further confirmed functional conservation between the
Arabidopsis (IRX10) and Populus GT47A proteins, and partial functional conservation
between the Populus GT47D, the Physcomitrella GT47D and Arabidopsis IRX10
proteins.

The work supports the existence of an Arabidopsis GX backbone biosynthesis
complex in which AtIRX10 and AtIRX10-L form an essential component together with
AtIRX9 and AtIRX14. It also suggests that parts of the GX biosynthesis machinery are
conserved across embryophytes. Finally, work presented in this thesis indicates that the
IRX10 gene family has gone through a subfunctionalisation event during the evolution.