Plant cell walls Current Biology Herman Höfte, Aline Voxeur  Current Biology  Volume 27, Issue 17, Pages R865-R870 (September 2017) DOI: 10.1016/j.cub.2017.05.025 Copyright © 2017 Terms and Conditions

Figure 1 Cell-wall heterogeneity and diversity. Plants show a large variety in the shapes of cells and cell walls. (A) Puzzle-shaped epidermis cell; (B) cylindrical cell; (C) transfer cell in seed with polar primary cell wall outgrowths; (D) rapidly growing hypocotyl cell with thick multi-lamellated primary cell wall; (E) endodermis cell with lignified Casparian strip; (F) fibre cell with thick lignified secondary wall; (G) xylem vessel with lignified secondary wall containing oval pits. Primary cell wall (green), secondary wall (brown), nucleus (blue circle), cytosol (blue) and vacuole (white). Current Biology 2017 27, R865-R870DOI: (10.1016/j.cub.2017.05.025) Copyright © 2017 Terms and Conditions

Figure 2 Cell-wall macromolecules. Main primary cell-wall polysaccharides discussed in this primer. Upper panels, polysaccharides. Lower panel, monosaccharides. Current Biology 2017 27, R865-R870DOI: (10.1016/j.cub.2017.05.025) Copyright © 2017 Terms and Conditions

Figure 3 Macromolecule interactions in growing cell walls. The plasma membrane (light blue)–cell wall interface of a growing cell is shown. A cortical microtubule is attached to the cytoplasmic side of the plasma membrane. Cellulose microfibrils are being deposited by plasma membrane-embedded hexameric cellulose synthase complexes. A Golgi-derived secretory vesicle deposits matrix polysaccharides and wall-modifying enzymes to the cell surface. The inset shows a more detailed view of two 3-nm thick microfibrils with 18 glucan chains, the stacking of which creates polar (corresponding to the hydroxyl residues exposed to the surface, orange) and non-polar surfaces (corresponding to the hydrophobic sugar backbones exposed to the surface, blue). Heteroxylan domains, with substitutions on every second xylan residue, intercalate with Glc residues at the polar face of the microfibrils, with the substitutions (yellow dots) pointing away from the microfibril, thus preventing microfibril–microfibril interactions. Pectins interact extensively with cellulose through xylan, arabinan and galactan side chains of rhamnogalacturonan-I. Xyloglucan chains preferentially bind to the non-polar microfibril face and favor the formation of cellulose-xyloglucan-cellulose cross-links. Only a small number of such cross-links are formed in vivo. They are referred to as biomechanical hotspots since they are the targets of specific fungal endoglucanases and endogenous expansins that promote wall relaxation by uncoupling the mechanically coupled microfibril network. The bulk of the xyloglucans, however, are not linked to cellulose and contribute to the separation of the microfibrils. Pectin methyl esterases can de-methylesterify stretches of galacturonic acid residues of homogalacturonan, exposing negative charges at the polymer surface and allowing the formation of Ca2+-pectate crosslinks. Current Biology 2017 27, R865-R870DOI: (10.1016/j.cub.2017.05.025) Copyright © 2017 Terms and Conditions