536 Biosynthesis and properties of the plant cell wall Wolf Dieter Reiter The characterization of cell wall mutants of Arabidopsis thaliana combined with biochemical approaches toward the purification and characterization of glycosyltransferases has led to significant advances in understanding cell wall synthesis and the properties of cell walls New insights have been gained into the formation of cellulose and the functions of the matrix polysaccharides rhamnogalacturonan II and xyloglucan Addresses Department of Molecular and Cell Biology University of Connecticut 75 North Eagleville Road Storrs Connecticut 06269 3125 USA e mail wdreiter uconnvm uconn edu Current Opinion in Plant Biology 2002 5 536 542 1369 5266 02 see front matter 2002 Elsevier Science Ltd All rights reserved DOI 10 1016 S1369 5266 02 00306 0 Abbreviations AtFUT1 Arabidopsis thaliana FUCOSYLTRANSFERASE1 AtXT1 Arabidopsis thaliana XYLOSYLTRANSFERASE1 CESA1 CELLULOSE SYNTHASE1 CSL CELLULOSE SYNTHASE LIKE cyt1 cytokinesis1 IRX2 IRREGULAR XYLEM2 KOR KORRIGAN mur2 murus2 RG I rhamnogalacturonan I rsw1 root swelling1 SCD sitosterol cellodextrins SG sitosterol glucoside Introduction The deposition and modification of cell wall material play essential roles during plant growth and development the responses of plants to the environment and the interactions of plants with symbionts and pathogens 1 As cell migrations do not contribute to the development of the plant body the planes of cell divisions and the ordered deposition of cell wall material ultimately determine the shapes of plant cells and organs Most photosynthetically fixed carbon is incorporated into cell wall polymers making plant cell walls the most abundant source of terrestrial biomass and renewable energy Cell wall material is also of great practical importance for human and animal nutrition and as a source of natural fibers for textiles and paper products For these reasons the study of cell wall synthesis is of considerable interest from both a basic and an applied point of view Two types of cell walls can be distinguished Primary walls are deposited during cell growth and need to be both mechanically stable and sufficiently extensible to permit cell expansion while avoiding the rupture of cells under their turgor pressure Primary cell walls consist mainly of polysaccharides that can be broadly classified as cellulose the cellulose binding hemicelluloses and pectins The latter two classes of cell wall components are often referred to as matrix polysaccharides These are synthesized within Golgi cisternae whereas cellulose is generated at the plasma membrane in the form of paracrystalline microfibrils Secondary cell walls are deposited after the cessation of cell growth and confer mechanical stability upon specialized cell types such as xylem elements and sclerenchyma cells These walls represent composites of cellulose and hemicelluloses and are often impregnated with lignins In addition to polysaccharides plant cell walls contain hundreds of different proteins Many of these proteins are considered to be structural proteins 2 whereas others participate in cell wall remodeling and turnover 3 This review focuses on recent advances in understanding the biosynthesis and function of plant cell wall polysaccharides with an emphasis on the model system Arabidopsis thaliana As the genome sequence of this small crucifer has recently been determined 4 the coding regions of all glycosyltransferases and other enzymes that are involved in cell wall synthesis and modification are available in public databases Now the challenge is to identify candidate genes for glycosyltransferases and other cellwall related proteins and to determine their function Strategies to accomplish these goals have been outlined in recent review articles 5 7 Because of space limitations advances in the characterization of cell wall proteins and lignification pathways are not included in this contribution and the reader is referred to recent reviews on these subjects 8 9 The synthesis of cellulose in higher plants In recent years substantial progress has been made in understanding the synthesis of cellulose It is a linear 1 4 D glucan that assembles into paracrystalline microfibrils each of which contains an estimated 36 parallel polysaccharide chains Cellulose synthesis occurs at rosette like structures that consist of six hexagonally arranged subunits that are embedded in the plasma membrane 10 As each rosette is believed to synthesize one microfibril some models propose that each of the six rosette subunits is composed of six 1 4 D glucan synthases each of which forms a single glucan molecule from cytoplasmic UDP D glucose 11 12 In this scenario 36 1 4 D glucan chains would emerge at the apoplastic side of the plasma membrane and would assemble into cellulose microfibrils in a process that may be aided by additional proteins such as KORRIGAN KOR see below The catalytic subunit of cellulose synthase is believed to be encoded by members of a multi gene family of transmembrane proteins that have sequence similarities to bacterial cellulose synthases such as acsA from Acetobacter xylinum 13 and celA from Agrobacterium tumefaciens 14 15 Biosynthesis and properties of the plant cell wall Reiter 537 Table 1 Mutants in the CESA and CSL genes of Arabidopsis Gene name Mutant Phenotype s CESA1 CESA3 CESA4 CESA6 rsw1 ixr1 irx5 ixr2 prc1 irx3 irx1 rat4 kojak Root swelling stunted growth rsw1 1 seedling lethality rsw1 2 Resistance to isoxaben stunted growth antisense plants Irregular structure of xylem elements Resistance to isoxaben Reduced length of roots and hypocotyls Irregular structure of xylem elements Irregular structure of xylem elements Resistance to root transformation by Agrobacterium Short and defective root hairs CESA7 CESA8 CSLA9 CSLD3 References 17 32 11 18 37 a 19 20 21 22 37 39 40 aS Turner personal communication Abbreviations prc1 procuste1 rat4 resistant to transformation by Agrobacterium tumefaciens4 The Arabidopsis genome harbors ten members of this gene family CELLULOSE SYNTHASE1 CESA1 through CESA10 all of which contain eight transmembrane domains a D D D QxxRW motif that is believed to be part of the active site and a putative zinc binding domain that may mediate protein protein interactions 16 Soon after the cloning of the temperature sensitive root swelling1 rsw1 allele of CESA1 in 1998 17 mutations in five additional CESA isoforms were identified by analyzing plants that had