Abstract
During differentiation, the Arabidopsis seed coat epidermal cells synthesize and secrete large quantities of pectinaceous mucilage into the apoplast, which is then released to encapsulate the seed upon imbibition. In this study, we showed that mutation in Irregular Xylem 14 (IRX14) led to a mucilage cohesiveness defect due to a reduced xylan content. Expression of IRX14 was detected specifically in the seed coat epidermal cells, reaching peak expression at 13 days post-anthesis (DPA) when the accumulation of mucilage polysaccharides has ceased. Sectioning of the irx14-1 seed coat revealed no visible structural change in mucilage secretory cell morphology. Although the total amount of mucilage was comparable with the wild type (WT), the partition between water-soluble and adherent layers was significantly altered in irx14-1, with redistribution from the adherent layer to the water-soluble layer. The monosaccharide composition analysis revealed that xylose content was significantly reduced in irx14-1 water-soluble and adherent mucilage compared with the WT. The macromolecular characteristics of the water-soluble mucilage were modified in irx14-1 with a loss of the larger polymeric components. In accordance, glycome profiling and dot immunoblotting of seed mucilage using antibodies specific for rhamnogalacturonan I (RG I) and xylan confirmed the ultra-structural alterations in the irx14-1 mucilage. Meanwhile, the crystalline cellulose content was reduced in the irx14-1 mucilage. These results demonstrated that IRX14 was required for the biosynthesis of seed mucilage xylan, which plays an essential role in maintaining mucilage architecture potentially through altering the crystallization and organization of cellulose.
Highlights
In myxospermous species such as Arabidopsis thaliana, seed coat mucilage secretory cells (MSCs) synthesize and deposit large amounts of pectinaceous mucilage in the apoplast (Western et al, 2000; Windsor et al, 2000)
These results demonstrated that Irregular Xylem 14 (IRX14) was required for the biosynthesis of seed mucilage xylan, which plays an essential role in maintaining mucilage architecture potentially through altering the crystallization and organization of cellulose
Apart from these reported genes, we found that several more genes involved in xylan biosynthesis (e.g. IRX14, IRX14L, IRX9, and IRX9L, etc) were significantly up-regulated during seed coat development, suggesting a potential role of these genes in mucilage xylan biosynthesis
Summary
In myxospermous species such as Arabidopsis thaliana, seed coat mucilage secretory cells (MSCs) synthesize and deposit large amounts of pectinaceous mucilage in the apoplast (Western et al, 2000; Windsor et al, 2000). A secondary cell wall is synthesized and deposited, forming a volcano-shaped columella in the centre of the cell (Western et al, 2000; Windsor et al, 2000). The epidermal cells have the central columella connected to reinforced radial cell walls and surrounded by doughnut-shaped mucilage under a primary cell wall. The MSCs of Arabidopsis have been presented as an ideal model system for the study of the biosynthesis and modification of cell wall components (Arsovski et al, 2010; Haughn and Western, 2012; Western, 2012; Francoz et al, 2015; Voiniciuc et al, 2015b;)
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