Highly Branched Xylan Made by IRX14 and MUCI21 Links Mucilage to Arabidopsis Seeds

Abstract

ORCID IDs: 0000-0001-9105-014X (C.V.); 0000-0003-4576-6774 (M.H.-W.S.).All cells of terrestrial plants are fortified by walls composed of crystalline cellulose microfibrils and a variety of matrix polymers. Xylans are the second most abundant type of polysaccharides on Earth. Previous studies of Arabidopsis (Arabidopsis thaliana) irregular xylem (irx) mutants, with collapsed xylem vessels and dwarfed stature, highlighted the importance of this cell wall component and revealed multiple players required for its synthesis. Nevertheless, xylan elongation and substitution are complex processes that remain poorly understood. Recently, seed coat epidermal cells were shown to provide an excellent system for deciphering hemicellulose production. Using a coexpression and sequence-based strategy, we predicted several MUCILAGE-RELATED (MUCI) genes that encode glycosyltransferases (GTs) involved in the production of xylan. We now show that MUCI21, a member of an uncharacterized clade of the GT61 family, and IRX14 (GT43 protein) are essential for the synthesis of highly branched xylan in seed coat epidermal cells. Our results reveal that xylan is the most abundant xylose-rich component in Arabidopsis seed mucilage and is required to maintain its architecture. Characterization of muci21 and irx14 single and double mutants indicates that MUCI21 is a Golgi-localized protein that likely facilitates the addition of xylose residues directly to the xylan backbone. These unique branches seem to be necessary for pectin attachment to the seed surface, while the xylan backbone maintains cellulose distribution. Evaluation of muci21 and irx14 alongside mutants that disrupt other wall components suggests that mucilage adherence is maintained by complex interactions between several polymers: cellulose, xylans, pectins, and glycoproteins.

UUAT1 Is a Golgi-Localized UDP-Uronic Acid Transporter That Modulates the Polysaccharide Composition of Arabidopsis Seed Mucilage

“…The absence of UUAT1 also results in a reduction in Xyl in SM. However, this phenotype is different from the one observed for muci21 and irx14, xylosyltransferases mutants that show alterations in epidermal cell mucilage adhesion (Hu et al, 2016;Voiniciuc et al, 2015;Ralet et al, 2016). On the other hand, no Xyl (A) The uuat1-2 mutant displays early stem elongation when compared with the wild-type Col-0.…”

How does the cell wall ‘stick’ in the mucilage? A detailed microstructural analysis of the seed coat mucilaginous cell wall

“…Xylan branches are required to attach the pectins to the seed surface (Hu et al, 2016 a, b;Voiniciuc et al, 2015). Xylan is also necessary for maintaining the cellulose architecture in the mucilage (Voiniciuc et al, 2015b). Our studies revealed the probably specific xylan organization in Neopallasia pectinata mucilage as being chains of xylan arranged in longitudinal, individual fibrils (Fig.…”

“…This separation is derived from a number of immuno-and biochemical analyses of mucilage components which showed the presence of hemicelluloses such as arabinoxylan and xyloglucan typical of the PCW (Naran et al, 2008;Young et al, 2008;Arsovski et al, 2009). In the case of cellulose mucilage regarded as a modified SCW, the presence of xylan, typical of this type of cell wall was also shown (Ralet et al, 2016;Voiniciuc et al, 2015b;Xu et al, 2016 a, b).…”

“…Their key role is to strengthen the cell wall through additional interactions with cellulose within the mature wall (Caffal and Mohnen, 1990;Sheller and Ulskov, 2010;Busse-Wicher et al, 2014;Voiniciuc et al, 2015b).…”

KNS4/UPEX1: A Type II Arabinogalactan β-(1,3)-Galactosyltransferase Required for Pollen Exine Development

“…The interactions (covalent and/or noncovalent) between pectins and AGPs has been reported in numerous studies (Baldwin et al, 1993;Li et al, 1995;Jauh and Lord, 1996;Mollet et al, 2002;Majewska-Sawka and Rodriguez-Garcia, 2006) and structural analyses have provided multiple lines of evidence that AGPs are associated with pectins, especially RG-I (Immerzeel et al, 2006;Mohnen, 2008;Caffall and Mohnen, 2009) and more recently xylan (Tan et al, 2013;Voiniciuc et al, 2015). Moreover, SOS5/FLA4 (At3g46550), a fasciclinlike GPI-anchored AGP, was recently shown to be involved in the formation of either a pectin or a pectinhemicellulose network in seed mucilage (Griffiths et al, 2014).…”