Plant Cell Physiol 43 12 1407 1420 2002 JSPP 2002 Cellulose Biosynthesis in Plants from Genes to Rosettes Monika S Doblin1 2 Isaac Kurek Deborah Jacob Wilk and Deborah P Delmer University of California Davis CA 95616 U S A Modern techniques of gene cloning have identified the CesA genes as encoding the probable catalytic subunits of the plant CelS the cellulose synthase enzyme complex visualized in the plasma membrane as rosettes At least 10 CesA isoforms exist in Arabidopsis and have been shown by mutant analyses to play distinct role s in the cellulose synthesis process Functional specialization within this family includes differences in gene expression regulation and possibly catalytic function Current data points towards some CesA isoforms potentially being responsible for initiation or elongation of the recently identified sterol b glucoside primer within different cell types e g those undergoing either primary or secondary wall cellulose synthesis Different CesA isoforms may also play distinct roles within the rosette and there is some circumstantial evidence that CesA genes may encode the catalytic subunit of the mixed linkage glucan synthase or callose synthase Various other proteins such as the Korrigan endocellulase sucrose synthase cytoskeletal components Rac13 redox proteins and a lipid transfer protein have been implicated to be involved in synthesizing cellulose but apart from CesAs only Korrigan has been definitively linked with cellulose synthesis These proteins should prove valuable in identifying additional CelS components Keywords Arabidopsis thaliana Cellulose CesA Cotton Gossypium hirsutum Plant polysaccharide biosynthesis Abbreviations CalS callose synthase enzyme complex CD cellodextrin c di GMP cyclic diguanylic acid Csl cellulose synthaselike CelS cellulose synthase enzyme complex CesA formerly CelA cellulose synthase catalytic subunit CGAbp cellulose synthesis inhibitor CGA 325 615 binding protein CR P plant conserved region dpa days post anthesis DCB 2 6 dichlorobenzonitrile Glc glucose GT glycosyltransferase HVR hypervariable region Kor Korrigan endocellulase LTP lipid transfer protein Mt metallothionein MT microtubule SCD sterol cellodextrin SG sitosterolb glucoside SuSy sucrose synthase TC terminal complex TMH transmembrane helix UDP Glc uridine diphospho glucose Introduction Understanding the biosynthesis of wall polysaccharide 1 2 components has attracted considerable interest in light of the fundamental importance of these molecules not just to plant function but also to man Unfortunately very little is known of the mechanism s and regulation of the biosynthetic steps that control polysaccharide biosynthesis deposition and assembly or the interaction of these components to provide cells with a functional wall Furthermore manipulation of polysaccharide quantity and quality has been hampered by the lack of cloned genes for plant glycosyltransferases GTs As recently as 1995 not one single enzyme involved in plant cell wall biosynthesis had been purified to homogeneity nor had a single gene coding for such an enzyme been identified and cloned Fortunately since that time a number of GTs have been cloned using traditional biochemical or more modern in silico molecular and genetic techniques By far the most significant of these has been the cloning of the CesA genes of cotton and Arabidopsis presumed to encode catalytic subunits of cellulose synthase CelS the enzyme complex responsible for the synthesis of cellulose We refer to CelS as the entire synthase complex and will use the accepted term CesA when referring just to the catalytic subunits within that complex Identification of these genes has led to remarkable progress in the field and this review will focus on this recent work Due to space limitations we will not cover very interesting recent work on cellulose synthesis in bacteria Ausmees et al 1999 Ausmees et al 2001 Nakai et al 1999 R mling et al 2000 Zogaj et al 2001 The reader is also referred to a number of reviews that give the background on cellulose synthesis Brown 1996 Delmer 1999 Brown and Saxena 2000 Saxena and Brown 2000 Richmond and Somerville 2000 Dhugga 2001 However for the general reader a few words about our current understanding of the nature of the CelS complex may be helpful and are provided below Structures responsible for cellulose synthesis have been identified by electron microscopy in freeze fractured plasma membranes of many organisms Brown 1996 Kimura et al 1999a Linearly arranged terminal complexes TCs in single or multiple rows are observed in bacteria D discoideum and some algae or hexagonal structures with six fold symmetry termed rosettes are observed in mosses ferns algae and vascular plants Brown 1996 Delmer 1999 Tsekos 1999 Although TCs and rosettes reside in the plasma membrane Haigler and Corresponding author E mail msdoblin unimelb edu au Fax 61 3 9347 1071 Current address University of Melbourne School of Botany Royal Parade Parkville 3010 Victoria Australia 1407 1408 Cellulose biosynthesis in plants Fig 1 A model for the structure of the rosette A Six subunits possibly containing six CesA polypeptides interact to form a rosette a single CesA enzyme complex Each CesA polypeptide is shown to be involved in the synthesis of one b 1 4 glucan chain The CesA protein has eight predicted TMHs which could potentially form a pore in the plasma membrane through which the nascent chain is extruded into the wall Once the 36 chains emerge from the rosette they coalesce to form an elementary cellulose microfibril B In this modified rosette structure model of Scheible et al 2001 at least two types of CesA polypeptides a and b are required for spontaneous rosette assembly Two different types of a isoform can be distinguished a1 which interacts with two b isoforms only and a2 interacting with another a2 isoform and two b isoforms Brown 1986 revealed that during synthesis rosettes are assembled in the Golgi and then transported to the plasma membrane Biochemical studies indicate that the higher plant CelS complex is a large 500 kDa integral membrane multisubunit enzyme utilising uridine diphospho glucose UDP Glc as substrate Delmer 1999 Assumed to be included within each complex are a specific number of obligatory CesA catalytic subunits that utilize UDP Glc as substrate for glucan chain elongation as well as other components that may be involved in providing the substrate in initiating or terminating chain elongation or that