Plant Cell Walls All Plant Cells are surrounded by 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 secondary 1 Primary 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 protein Secondary walls surround cells that differentiate to form specialized functions i e wood cells xylem cells have altered polysaccharide composition often are lignified Polysaccharides are the main components of the primary plant cell wall Cross section of Nelumbo nucifera petiole showing primary cell wall 90 polysaccharide 10 protein from Katherine Esau Anatomy of Seed Plants 1977 2 Three classes of polysaccharides make up the primary wall Cellulose Hemicellulose Pectin Walls from round and elongated carrot suspension cultured cells Fast freeze deep etch rotary shadowed replicas McCann et al 1993 J Cell Science 106 1347 Composition of primary cell walls of suspension cultured sycamore cells Wall Component Mass of Cell Wall Pectic polysaccharides 34 Hemicellulose 24 Cellulose 23 Protein 19 McNeil et al 1979 Fortschritte der Chemie organischer Naturstoffe Volume 37 191 3 Primary walls can be divided into two types Type I primary walls all flower plants except the grass family Cellulose Hemicellulose xyloglucan Pectin 22 35 homogalacturonan HGA Rhamnogalacturonan I RG I Rhamnogalacturonan II RG II Type II primary walls the grass family Poaceae Cellulose Hemicellulose glucuronoarabinoxylan Pectin 10 HGA RG I RG II Model of Primary Plant Cell Wall Carpita Gibeaut 1993 Plant J 3 1 30 4 Schematic Model of Plant Primary Cell Wall Synthesis Malcolm O Neill CCRC CELLULOSE World s most abundant biopolymer Polymer of 1 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 chains are 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 1 to 2 walls cellulose can be found as associates of macrofibrils or bundles 5 academic brooklyn cuny edu biology bio4fv pag 6 www emc maricopa edu BIOBK BioBookCHEM2 html www emc maricopa edu BIOBK BioBookCHEM2 html 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 and is stabilized by a series of O3 05 H bonds 7 Several organisms in addition to plants synthesize cellulose These include several bacteria e g Acetobacter xylinum and Agrobacterium tumefaciens the slime mold Dictyostelium discoideum and the water mold Saprolegnia 8 Doblin et al 2002 Plant Cell Physiol 43 1407 This whole unit is the elementary fibril which contains 36 glucan chains Genes for plant cellulose synthase catalytic subunit were 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 cesA gene family Based on homology a cesAlike 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 Figure 1 Freeze fracture replicas of rosettes associated with cellulose microfibril biogenesis The rosettes after the fracture event exist in the leaflet of the plasma membrane bilayer that is nearest the cytoplasm the PF face In the main micrograph several rosettes are shown three surrounded by circles in the plasma membrane of a differentiating tracheary element of Zinnia elegans differentiating tracheary elements deposit abundant cellulose into patterned secondary wall thickenings The inset shows one rosette at higher magnification and after high resolution rotary shadowing at ultracold temperature with a minimum amount of platinum carbon Main micrograph 222 000 x inset 504 545 x both micrographs courtesy of Mark J Grimson and Candace H Haigler Department of Biological Sciences Texas Tech University Lubbock Texas 9 10 Figure 3 Hypothetical model for topology of one CeSA protein subunit in the plasma membrane of plants The eight transmembrane helices 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 from Delmer and Amor 1995 Plant Cell 7 987 11 Doblin et al 2002 Plant Cell Physiol 43 1407 Scanning electron micrograph of an untreated strand mechanically extracted from corn husk Scanning electron micrograph of a partially purified fiber bundle from corn husks from Reddy and Yang 2005 Trends Biotechnol 23 22 7 Biofibers from agricultural byproducts for industrial applications 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 extraction most of the surface waxes and other noncellulosic substances are removed When agro based fibers are used