Xylans Provide the Structural Driving Force for Mucilage Adhesion to the Arabidopsis Seed Coat

Abstract

Arabidopsis (Arabidopsis thaliana) seed coat epidermal cells produce large amounts of mucilage that is released upon imbibition. This mucilage is structured into two domains: an outer diffuse layer that can be easily removed by agitation and an inner layer that remains attached to the outer seed coat. Both layers are composed primarily of pectic rhamnogalacturonan I (RG-I), the inner layer also containing rays of cellulose that extend from the top of each columella. Perturbation in cellulosic ray formation has systematically been associated with a redistribution of pectic mucilage from the inner to the outer layer, in agreement with cellulose-pectin interactions, the nature of which remained unknown. Here, by analyzing the outer layer composition of a series of mutant alleles, a tight proportionality of xylose, galacturonic acid, and rhamnose was evidenced, except for mucilage modified5-1 (mum5-1; a mutant showing a redistribution of mucilage pectin from the inner adherent layer to the outer soluble one), for which the rhamnose-xylose ratio was increased drastically. Biochemical and in vitro binding assay data demonstrated that xylan chains are attached to RG-I chains and mediate the adsorption of mucilage to cellulose microfibrils. mum5-1 mucilage exhibited very weak adsorption to cellulose. MUM5 was identified as a putative xylosyl transferase recently characterized as MUCI21. Together, these findings suggest that the binding affinity of xylose ramifications on RG-I to a cellulose scaffold is one of the factors involved in the formation of the adherent mucilage layer.

Differences in glycosyltransferase family 61 accompany variation in seed coat mucilage composition inPlantagospp.

“…The majority of PoGT61 genes showed remarkably similar profiles to the PoIRX10_2 , PoIRX10_3 , UDP-xylose synthase ( PoUXS ) and UDP-arabinose mutase ( PoUAM ) genes (Fig. 7E) that are implicated in xylan biosynthesis (Rennie and Scheller, 2014). …”

“…These sidechains make for a more complex polysaccharide that can interact and form linkages with neighbouring polysaccharides, creating the framework that is essential for plant cell wall integrity (Cosgrove, 2005). Xylan biosynthesis occurs in the Golgi apparatus (Rennie and Scheller, 2014) where enzymes including glycosyltransferases (GTs) are required to catalyse glycosidic linkages between each monosaccharide moiety. Although a comprehensive molecular understanding is lacking, by analysing xylan deficient mutants, candidate genes involved in xylan biosynthesis have been identified (for review see Rennie & Scheller, 2014).…”

“…Xylan biosynthesis occurs in the Golgi apparatus (Rennie and Scheller, 2014) where enzymes including glycosyltransferases (GTs) are required to catalyse glycosidic linkages between each monosaccharide moiety. Although a comprehensive molecular understanding is lacking, by analysing xylan deficient mutants, candidate genes involved in xylan biosynthesis have been identified (for review see Rennie & Scheller, 2014). In eudicots, GT43 (IRX9, IRX9-L, IRX14 and IRX14-L) and GT47 (IRX10 and IRX10-L) enzymes have been implicated in synthesis of the backbone.…”

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.…”

Evolution of protein N-glycosylation process in Golgi apparatus which shapes diversity of protein N-glycan structures in plants, animals and fungi

“…Genes in Groups II and III are all derived from plant genomes: Group II genes consist of Klebsormidium flaccidum genes, while genes in Group III consist of genes from species spanning from lower to higher land plants (Supplementary Figure S14). In this group, although no genes were definitely identified enzymatically, AT3G10320 was demonstrated as a putative xylosyltransferase which was recently characterized as MUCI21, while the genes AT3G18170 and AT3G18180 are expressed highly in a heteroxylan containing mucilaginous tissues, which indicated that the genes in this group are related to mucilage production in terrestrial plants9495.…”