Distinct Cell Wall Architectures in Seed Endosperms in Representatives of the Brassicaceae and Solanaceae      

Abstract

In some species, a crucial role has been demonstrated for the seed endosperm during germination. The endosperm has been shown to integrate environmental cues with hormonal networks that underpin dormancy and seed germination, a process that involves the action of cell wall remodeling enzymes (CWREs). Here, we examine the cell wall architectures of the endosperms of two related Brassicaceae, Arabidopsis (Arabidopsis thaliana) and the close relative Lepidium (Lepidium sativum), and that of the Solanaceous species, tobacco (Nicotiana tabacum). The Brassicaceae species have a similar cell wall architecture that is rich in pectic homogalacturonan, arabinan, and xyloglucan. Distinctive features of the tobacco endosperm that are absent in the Brassicaceae representatives are major tissue asymmetries in cell wall structural components that reflect the future site of radicle emergence and abundant heteromannan. Cell wall architecture of the micropylar endosperm of tobacco seeds has structural components similar to those seen in Arabidopsis and Lepidium endosperms. In situ and biomechanical analyses were used to study changes in endosperms during seed germination and suggest a role for mannan degradation in tobacco. In the case of the Brassicaceae representatives, the structurally homogeneous cell walls of the endosperm can be acted on by spatially regulated CWRE expression. Genetic manipulations of cell wall components present in the Arabidopsis seed endosperm demonstrate the impact of cell wall architectural changes on germination kinetics.

Branched Pectic Galactan in Phloem-Sieve-Element Cell Walls: Implications for Cell Mechanics

“…The role of RG-I glycans in cell walls is far from clear, but much evidence has accumulated relating these hypervariable domains of the pectic polysaccharides with generation of cell wall mechanical properties. 1,4-Galactan-rich domains can be associated with increased firmness of cell walls (McCartney et al, 2000), are not detected where extensive elastic properties may be required (Lee et al, 2012), and are often degraded during fruit softening processes (Brummell 2005). However, the micromechanical basis of galactanrich RG-I contributions to cell wall firmness are not known.…”

“…For light microscopy, sugar beet plants were collected at the eight-leaf stage (20°C, 16 h light: 8 h dark), and samples were taken from 20 mm below the widest diameter of the storage root. Indirect fluorescence immunolabeling procedures were carried out as described (Lee et al, 2012;Xue et al, 2013).…”

“…Plant materials were collected locally and prepared for anatomical investigation using wax-embedding (McCartney et al, 2005;Xue et al, 2013) or resinembedding (Lee et al, 2012) procedures. Cryosectioned samples of Miscanthus leaves (from around the second internode) were prepared via embedding tissue in optimal cutting temperature compound (OCT compound; VWR), and processed into 10 mm sections using a cryostat-microtome (Leica Microsystems), and stored at 280°C until use.…”

“…These are often structurally hypervariable and are important factors in cell wall biogenesis, cell extension, and cell function. Not all cell surfaces are the same in terms of precise structures of individual polysaccharides, and cell-wall glycan molecular configurations display developmental dynamics and cell type specificities (Burton et al, 2010;Knox 2008;Lee et al, 2012;Torode et al, 2016).…”

“…Rhamnogalacturonan-I (RG-I) is a hypervariable pectic polysaccharide with a rhamnosyl and galacturonosyl backbone with substitution by a complex array of side chains in which 1,4-galactosyl and or 1,5-arabinosyl residues often predominate (Yapo 2011;Lee et al, 2013). RG-I polysaccharides are widely distributed and studies in relation to cell functions have focused on roles in generation of cell wall mechanical properties such as cell wall firmness and cell wall elasticity (McCartney et al, 2000;Lee et al, 2012). Noncellulosic and nonpectic polysaccharides, often grouped as hemicelluloses, include xyloglucans, heteroxylans, heteromannan, and mixed-linkage glucans and these also display structural variations and in some cases cell and tissue specificities in occurrence (Burton et al, 2010;Scheller and Ulvskov 2010).…”