536The characterization of cell wall mutants of Arabidopsisthaliana, combined with biochemical approaches toward thepurification and characterization of glycosyltransferases, hasled to significant advances in understanding cell wall synthesisand the properties of cell walls. New insights have been gainedinto the formation of cellulose and the functions of the matrixpolysaccharides rhamnogalacturonan-II and xyloglucan.AddressesDepartment of Molecular and Cell Biology, University of Connecticut,75 North Eagleville Road, Storrs, Connecticut 06269-3125, USA; e-mail: [email protected] Opinion in Plant Biology 2002, 5:536–5421369-5266/02/$ — see front matter© 2002 Elsevier Science Ltd. All rights reserved.DOI 10.1016/S1369-5266(02)00306-0AbbreviationsAtFUT1 Arabidopsis thaliana FUCOSYLTRANSFERASE1AtXT1 Arabidopsis thaliana XYLOSYLTRANSFERASE1CESA1 CELLULOSE SYNTHASE1CSL CELLULOSE SYNTHASE-LIKEcyt1 cytokinesis1IRX2 IRREGULAR XYLEM2KOR KORRIGANmur2 murus2RG-I rhamnogalacturonan-I rsw1 root swelling1SCD sitosterol-cellodextrinsSG sitosterol-β-glucosideIntroductionThe deposition and modification of cell wall material playessential roles during plant growth and development, theresponses of plants to the environment, and the inter-actions of plants with symbionts and pathogens [1]. As cellmigrations do not contribute to the development of theplant body, the planes of cell divisions and the ordereddeposition of cell wall material ultimately determine theshapes of plant cells and organs. Most photosyntheticallyfixed carbon is incorporated into cell wall polymers, making plant cell walls the most abundant source of terrestrial biomass and renewable energy. Cell wall materialis also of great practical importance for human and animalnutrition, and as a source of natural fibers for textiles andpaper products. For these reasons, the study of cell wallsynthesis is of considerable interest from both a basic andan applied point of view. Two types of cell walls can be distinguished. Primary wallsare deposited during cell growth, and need to be bothmechanically stable and sufficiently extensible to permitcell expansion while avoiding the rupture of cells undertheir turgor pressure. Primary cell walls consist mainly ofpolysaccharides that can be broadly classified as cellulose,the cellulose-binding hemicelluloses, and pectins. The latter two classes of cell wall components are often referredto as matrix polysaccharides. These are synthesized withinGolgi cisternae, whereas cellulose is generated at the plasmamembrane in the form of paracrystalline microfibrils.Secondary cell walls are deposited after the cessation ofcell growth and confer mechanical stability upon specializedcell types such as xylem elements and sclerenchyma cells.These walls represent composites of cellulose and hemi-celluloses, and are often impregnated with lignins. Inaddition to polysaccharides, plant cell walls contain hundreds of different proteins. Many of these proteins areconsidered to be ‘structural’ proteins [2], whereas othersparticipate in cell wall remodeling and turnover [3]. This review focuses on recent advances in understandingthe biosynthesis and function of plant cell wall polysaccha-rides, with an emphasis on the model system Arabidopsisthaliana. As the genome sequence of this small crucifer hasrecently been determined [4], the coding regions of all glycosyltransferases and other enzymes that are involvedin cell wall synthesis and modification are available in public databases. Now, the challenge is to identify candidate genes for glycosyltransferases and other cell-wall-related proteins, and to determine their function.Strategies to accomplish these goals have been outlined inrecent review articles [5–7]. Because of space limitations,advances in the characterization of cell wall proteins andlignification pathways are not included in this contribu-tion, and the reader is referred to recent reviews on thesesubjects [8,9].The synthesis of cellulose in higher plantsIn recent years, substantial progress has been made inunderstanding the synthesis of cellulose. It is a linear 1,4-β-D-glucan that assembles into paracrystalline microfibrils,each of which contains an estimated 36 parallel polysaccha-ride chains. Cellulose synthesis occurs at rosette-likestructures that consist of six hexagonally arranged subunitsthat are embedded in the plasma membrane [10]. As eachrosette is believed to synthesize one microfibril, somemodels propose that each of the six rosette subunits is com-posed of six 1,4-β-D-glucan synthases, each of which formsa single glucan molecule from cytoplasmic UDP-D-glucose[11••,12]. In this scenario, 36 1,4-β-D-glucan chains wouldemerge at the apoplastic side of the plasma membrane, andwould assemble into cellulose microfibrils in a process thatmay 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 tobacterial cellulose synthases, such as acsA from Acetobacterxylinum [13] and celA from Agrobacterium tumefaciens [14,15].Biosynthesis and properties of the plant cell wallWolf-Dieter ReiterBiosynthesis and properties of the plant cell wall Reiter 537The Arabidopsis genome harbors ten members of this genefamily (CELLULOSE SYNTHASE1 [CESA1] throughCESA10), all of which contain eight transmembranedomains, a D,D,D,QxxRW motif that is believed to be partof the active site, and a putative zinc-binding domain thatmay mediate protein–protein interactions [16]. Soon afterthe cloning of the temperature-sensitive root swelling1(rsw1) allele of CESA1 in 1998 [17], mutations in five addi-tional CESA isoforms were identified by analyzing plantsthat had defects in elongation growth, collapsed xylem elements or resistance to isoxaben, a herbicide that interfereswith cellulose synthesis during primary wall formation(Table 1). The phenotypes of these mutants and of CESA3antisense plants [18] indicate that the CESA1, CESA3, andCESA6 genes are involved in the synthesis of cellulose inthe primary cell wall [11••,17,18,19•,20], whereas theCESA4, CESA7,