1Plant Cell Walls2All Plant Cells are surroundedby an extracellular matrix known as the Cell Wall• a polysaccharide-rich matrix that surrounds all plant cells• plays multiple roles in plant growth, development and defense responses• there are two types of wall: primary & secondary3Primary Wall• first wall laid down• surrounds growing cells • surrounds meristematic cells• cells in succulent tissues• found at the junction of cells and at the outer edges of secondary walls• composed of ~ 90% carbohydrate and 10% proteinSecondary walls• surround cells that differentiate to form specialized functions (i.e. wood cells, xylem cells) • have altered polysaccharide composition• often are lignified4from Katherine Esau, Anatomy of Seed Plants, 1977Cross section of Nelumbo nucifera petiole showing primary cell wall90% polysaccharide10% proteinPolysaccharides are the main components of the primary plant cell wall5Walls from round and elongated carrot suspension cultured cells.(Fast-freeze, deep-etch, rotary-shadowed replicas; McCann et al., 1993, J. Cell Science 106:1347)Three classes of polysaccharides make up the primary wallCelluloseHemicellulosePectin6Composition of primary cell walls of suspension-cultured sycamore cellsWall Component Mass % of Cell WallPectic polysaccharides 34Hemicellulose 24Cellulose 23Protein 19McNeil et al., 1979, Fortschritte der Chemie organischerNaturstoffe, Volume 37, 191.7Primary walls can be divided into two types:Type I primary walls(all flower plants except the grass family)CelluloseHemicellulose(xyloglucan)Pectin (~22-35%) (homogalacturonan, HGA; Rhamnogalacturonan I, RG-I; Rhamnogalacturonan II; RG-II)Type II primary walls (the grass family, Poaceae): CelluloseHemicellulose(glucuronoarabinoxylan)Pectin (~10%) (HGA, RG-I, RG-II)8Model of Primary Plant Cell WallCarpita & Gibeaut (1993) Plant J. 3: 1-309Malcolm O’Neill, CCRCSchematic Model of Plant Primary Cell Wall Synthesis10CELLULOSE• World’s most abundant biopolymer• Polymer of β1-,4-linked glucose• Individual glucan chains associate via H-bonds to form microfibrils that are largely crystalline.• Cellulose I (the type of cellulose found in nature), glucan chainsare aligned parallel to each other • Length of the glucan chains varies depending upon the organism from DP ~2000 to up to DP ~15,000• Size of microfibril also varies depending upon the organism and can range from the elementary fibril (~ 36 glucan chains) up to very large fibrils (> 200 chains) found in cellulosic algae • As plant cells mature from 1º to 2 º walls, cellulose can be found as associates of macrofibrils or bundles1112academic.brooklyn.cuny.edu/ biology/bio4fv/pag...13www.emc.maricopa.edu/.../ BIOBK/BioBookCHEM2.html www.emc.maricopa.edu/.../ BIOBK/BioBookCHEM2.html14•Cellulose gives tensile strength to the wall. •In planta the cellulose microfibrils complex with hemicellulosic polysaccharides such as xyloglucan. •The pattern of cellulose deposition in the wall determines the pattern of plant development. •Generally, cellulose deposition is transverse to the direction of cell elongation.•X-ray diffraction studies indicate that Cellulose I exists in a 2-fold ribbon-like helix 2(5.15) with 2 residues per turn, a residue distance of 5.15 Å, and is stabilized by a series of O3…05 H-bonds.1516Several organisms in addition to plants synthesize cellulose. These include several bacteria (e.g. Acetobacterxylinum and Agrobacterium tumefaciens), the slime mold (Dictyostelium discoideum) and the water mold (Saprolegnia).17Genes for plant cellulose synthase catalytic subunitwere identified in cotton based on deduced amino acid sequence homology to bacterial cellulose synthase(cesA). CesA belongs to multigene families in plants(i.e. Arabidopsis may have at least 17 members in the cesAgene family). Based on homology a “cesA-like superfamily has been identified. This family has four conserved motifs: U1, U2, U3, and U4 that are thought to be involved in substrate binding and/or catalysis.Doblin et al., 2002, Plant Cell Physiol. 43:1407This whole unit is the elementary fibril which contains 36 glucan chains18Figure 1 Freeze fracture replicas of rosettes associated with cellulose microfibrilbiogenesis. The rosettes after the fracture event exist in the leaflet of the plasma membrane bilayerthat is nearest the cytoplasm (the PF face). In the main micrograph, several rosettes are shown (threesurrounded by circles) in the plasma membrane of a differentiating tracheary element of Zinniaelegans; differentiating tracheary elements deposit abundant cellulose into patterned secondarywall thickenings. The inset shows one rosette at higher magnification and after high resolutionrotary shadowing at ultracold temperature with a minimum amount of platinum/carbon. (Mainmicrograph, 222,000 x; inset, 504,545 x; both micrographs courtesy of Mark J Grimson andCandace H Haigler, Department of Biological Sciences, Texas Tech University, Lubbock, Texas.)192021Figure 3 Hypothetical model for topology of one CeSA protein subunit in the plasma membraneof plants.The eight transmembranehelices are predicted to interact to form a pore through which the cellulose chain is secreted to the cell wall. The large central domain would fold in such a way as to bring together the conserved regions containing the 3-D residues and QXXRW motif that are believed to be important for substrate binding and catalysis. This would place the conserved (CR-P) and hypervariable (HVR) plant-specific regions also in the cytoplasm where they may serve to interact with other proteins. One CeSA subunit, such as the one shown here, must interact with other such subunits to form the synthase complex; it is not known how these interactions occur,although they might involve interactions between the transmembrane helices and/or some of the cytoplasmic domains.22from Delmer and Amor, 1995, Plant Cell 7:98723Doblin et al., 2002, Plant Cell Physiol. 43:140724Scanning electron micrograph of an untreated strand mechanically extracted from corn husk.Scanning electron micrograph of a partially purified fiber bundle from corn husks. “In their natural state, and before chemical extraction, fiber surfaces have waxes and other encrusting substances such as hemicellulose, lignin and pectin that form a thick outer layer to protect the cellulose inside.The presence of encrusting substances causes the fibers to have an irregular appearance. During fiber