Saccharide analysis of onion outer epidermal walls

Abstract

Background Epidermal cell walls have special structural and biological roles in the life of the plant. Typically they are multi-ply structures encrusted with waxes and cutin which protect the plant from dehydration and pathogen attack. These characteristics may also reduce chemical and enzymatic deconstruction of the wall for sugar analysis and conversion to biofuels. We have assessed the saccharide composition of the outer epidermal wall of onion scales with different analytical methods. This wall is a particularly useful model for cell wall imaging and mechanics. Results Epidermal walls were depolymerized by acidic methanolysis combined with 2M trifluoracetic acid hydrolysis and the resultant sugars were analyzed by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Total sugar yields based on wall dry weight were low (53%). Removal of waxes with chloroform increased the sugar yields to 73% and enzymatic digestion did not improve these yields. Analysis by gas chromatography/mass spectrometry (GC/MS) of per-O-trimethylsilyl (TMS) derivatives of the sugar methyl glycosides produced by acidic methanolysis gave a high yield for galacturonic acid (GalA) but glucose (Glc) was severely reduced. In a complementary fashion, GC/MS analysis of methyl alditols produced by permethylation gave substantial yields for glucose and other neutral sugars, but GalA was severely reduced. Analysis of the walls by 13C solid-state NMR confirmed and extended these results and revealed 15% lipid content after chloroform extraction (potentially cutin and unextractable waxes). Conclusions Although exact values vary with the analytical method, our best estimate is that polysaccharide in the outer epidermal wall of onion scales is comprised of homogalacturonan (~ 50%), cellulose (~ 20%), galactan (~ 10%), xyloglucan (~ 10%) and smaller amounts of other polysaccharides. Low yields of specific monosaccharides by some methods may be exaggerated in epidermal walls impregnated with waxes and cutin and call for cautious interpretation of the results.

Bimodally oriented cellulose fibers and reticulated homogalacturonan networks - A direct visualization of Allium cepa primary cell walls

“…We sought to identify the chemical nature of the meshing. According to the literature, pectic homogalacturonans (HGs) make up 50% of the primary onion cell wall 32 . In the most recent models of the interactions between the different components of the primary cell wall, pectins are thought to surround and tether the cellulose fibers in a calcium-dependent manner 8,10,23 .…”

Building an extensible cell wall

“…The model was based on the wall obtained by peeling the abaxial epidermis of onion scales. This polylamellate wall has been extensively studied by AFM ( Zhang et al, 2014 , 2016 , 2017 ), various spectroscopic methods ( Kafle et al, 2017 ; Huang et al, 2018 ; Ye et al, 2018 , 2020 ) polysaccharide analysis ( Wilson et al, 2021 ) and enzyme digestions combined with mechanical assays ( Zhang et al, 2019 ; Wang et al, 2020 ), providing many details for building the model. A lamella is distinguished largely by its CMFs which are deposited in a common orientation, forming a reticulated network of CMF bundles ( Ding et al, 2014 ; Zhang et al, 2014 , 2016 ; Song et al, 2020 ; Figure 1 ).…”

Recalcitrance of Corn Straw Significantly Correlated to the Heterogeneity in Physicochemical Properties of Cell Wall

“…Epidermic cells with special structure encrusted with waxes and cutin mainly envelop leaves, rind and sheath. The cuticle is overlaid by wax deposits that play a role in resistance to microbes and insects, and water retention in the nature [21]. Fibers and dermal tissues together provided rigidity, which was signi cantly resistant to enzyme adsorption.…”