COBRA-LIKE2, a Member of the Glycosylphosphatidylinositol-Anchored COBRA-LIKE Family, Plays a Role in Cellulose Deposition in Arabidopsis Seed Coat Mucilage Secretory Cells  ,    

Abstract

Differentiation of the maternally derived seed coat epidermal cells into mucilage secretory cells is a common adaptation in angiosperms. Recent studies identified cellulose as an important component of seed mucilage in various species. Cellulose is deposited as a set of rays that radiate from the seed upon mucilage extrusion, serving to anchor the pectic component of seed mucilage to the seed surface. Using transcriptome data encompassing the course of seed development, we identified COBRA-LIKE2 (COBL2), a member of the glycosylphosphatidylinositol-anchored COBRA-LIKE gene family in Arabidopsis (Arabidopsis thaliana), as coexpressed with other genes involved in cellulose deposition in mucilage secretory cells. Disruption of the COBL2 gene results in substantial reduction in the rays of cellulose present in seed mucilage, along with an increased solubility of the pectic component of the mucilage. Light birefringence demonstrates a substantial decrease in crystalline cellulose deposition into the cellulosic rays of the cobl2 mutants. Moreover, crystalline cellulose deposition into the radial cell walls and the columella appears substantially compromised, as demonstrated by scanning electron microscopy and in situ quantification of light birefringence. Overall, the cobl2 mutants display about 40% reduction in whole-seed crystalline cellulose content compared with the wild type. These data establish that COBL2 plays a role in the deposition of crystalline cellulose into various secondary cell wall structures during seed coat epidermal cell differentiation.

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

“…Disruption of the formation of the cellulosic rays in mucilage has systematically been associated with a redistribution of RG-I from the inner to the outer layer, in agreement with cellulose-pectin interactions (HarpazSaad et al, 2011;Mendu et al, 2011;Sullivan et al, 2011;Ben-Tov et al, 2015;Griffiths et al, 2015). Mechanisms through which such interactions could occur have been suggested to consist of noncovalent bonding between cellulose and RG-I side chains (Haughn and Western, 2012).…”

“…Two layers of mucilage are released from Arabidopsis seeds, an outer diffuse layer that can be easily removed by agitation and an inner layer that remains attached to the seed coat. Cellulose has been clearly demonstrated to mediate the adherence of inner mucilage RG-I to the seed surface (Harpaz-Saad et al, 2011;Mendu et al, 2011;Sullivan et al, 2011;Ben-Tov et al, 2015;Griffiths et al, 2015), but the exact mechanism remained to be defined. Determining the relationship between the outer and the inner layers should contribute to our understanding of the interactions involved, and it can be hypothesized that two key points are necessary for the partitioning of polysaccharides into two layers: (1) a correctly constructed cellulose scaffold; and (2) mucilage pectin with an appropriate structure and properties for both interaction with cellulose and hydration.…”

MUCILAGE-RELATED10 Produces Galactoglucomannan That Maintains Pectin and Cellulose Architecture in Arabidopsis Seed Mucilage

“…To tackle this problem, we conducted a reverse genetic screen for MUCI genes that has predicted many glycosyltransferases. Using eight gene baits in multiple coexpression tools, we generated a more comprehensive set of candidate genes for cell wall biosynthesis than previous approaches that used only one or two baits (Vasilevski et al, 2012;Ben-Tov et al, 2015). MUCI10, the first of these genes to be characterized in detail, encodes a putative a-1,6-galactosyltransferase related to the fenugreek TfGMGT enzyme that decorates mannan chains with t-Gal residues (Edwards et al, 1999).…”

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

“…The release of mucilage upon hydration depends on correct branching of RG I and on HG methylesterification (Dean et al, 2007;Macquet et al, 2007;Rautengarten et al, 2008;Arsovski et al, 2009;Saez-Aguayo et al, 2013;Voiniciuc et al, 2013). An adherent capsule, representing 35% of the total mucilage produced (Voiniciuc et al, 2015b), is anchored around each seed and is associated with ray-like structures synthesized in part by CELLULOSE SYNTHASE (CESA) proteins (Harpaz-Saad et al, 2011;Mendu et al, 2011;Sullivan et al, 2011;Griffiths et al, 2015) and potentially assembled by COBRA-LIKE2 (Ben-Tov et al, 2015). Two additional proteins, SALT-OVERLY SENSITIVE5 (SOS5; also annotated as FASCICLIN-LIKE ARABINOGALACTAN PROTEIN4 [FLA4]) and the receptor-like kinase FEI2 (named after the Chinese word for fat) are required for mucilage adherence, but their functions remain unclear (Harpaz-Saad et al, 2011;Griffiths et al, 2014).…”