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Update on Mechanisms of Plant Cell Wall Biosynthesis: How Plants Make Cellulose and 1 Other (1/4)-B-D-Glycans

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Update on Mechanisms of Plant Cell Wall Biosynthesis: How Plants Make Cellulose and 1 Other (1/4)-B-D-Glycans UpdateonMechanismsofPlantCellWallBiosynthesis Update on Mechanisms of Plant Cell Wall Biosynthesis: How Plants Make Cellulose and 1 Other (1/4)-b-D-Glycans Nicholas C. Carpita* Department of Botany and Plant Pathology, and Bindley Bioscience Center, Purdue University, West Lafayette, Indiana 47907–2054 The discovery of a gene that encodes a cotton ences between plants with type I walls and those of the (Gossypium hirsutum) cellulose synthase (Pear et al., grasses with type II walls (Fig. 1A; Penning et al., 1996) revolutionized and invigorated the plant cell 2009). For CesA genes and certain Csl genes, establish- wall community to find the genes that encode the ment of specific function for the synthases they en- machinery of cell wall polysaccharide synthesis. The code comes from the analysis of mutants lacking a landscape was framed by the completion of the ge- particular function and, in some specific examples, nome sequence of Arabidopsis (Arabidopsis thaliana; by heterologous expression. However, genetic ap- Arabidopsis Genome Initiative, 2000), which gave a proaches alone do not inform us about the biological complete gene inventory for a model plant species, but mechanism of synthesis. The knowledge gained from one with many genes yet to be annotated for function. molecular genetic approaches now needs to be aug- An estimated 10% of the plant genome, about 2,500 mented by biochemical and cell biological approaches genes, is devoted to construction, dynamic architec- to achieve a greater understanding of proteins and ture, sensing functions, and metabolism of the plant their interactions within a synthase complex, their cell wall. Based largely on prior discoveries of function organization at membranes, and their dynamics. This in prokaryotic organisms, most of the tentatively an- Update focuses on the biochemical mechanisms of the notated genes are organized into gene families for synthesis of a single type of linkage, the (1/4)-b-D substrate generation, glycosyl transfer, targeting and linkage, in which one sugar is inverted nearly 180° trafficking, cell wall rearrangement, and modification with respect to each neighboring sugar in the chain. by hydrolases, esterases, and lyases (Yong et al., 2005; This linkage presents a unique steric problem for Penning et al., 2009). However, the biochemical activ- processive catalysis that all living organisms have ities of most enzymes involved in glycosyl transfer solved but we are still struggling to understand. within these families remain to be verified, and an This Update reviews our present state of knowledge additional 40% of the genome encodes genes whose of the biochemical mechanisms of polysaccharide functions are not known. As many of these proteins synthesis, including some classic discoveries, and contain secretory signal peptides (Arabidopsis Ge- presents an alternative hypothesis on the biochemical nome Initiative, 2000), it is reasonable to infer that mechanisms and organization of complexes involved some have roles in cell wall construction. in synthase reactions that yield (1/4)-b-D linkages. The Arabidopsis cellulose synthase/cellulose synthase- like (CesA/Csl) gene superfamily, which includes 10 CesA genes and 29 Csl genes in six distinct groups, was one CELLULOSE SYNTHESIS of the first large families to be described (Richmond and Somerville, 2000), and comparative analyses of a In flowering plants, cellulose is a para-crystalline reference dicot, Arabidopsis, with a reference grass, array of about two to three dozen (1/4)-b-D-glucan rice (Oryza sativa), revealed substantive differences in chains. Microfibrils of 36 glucan chains have a theo- family structures, adding two groups not seen in the retical diameter of 3.8 nm, but x-ray scattering and dicot genome (Hazen et al., 2002). Extension of these NMR spectroscopy indicate that some microfibril di- annotations to compare all cell wall-related gene fam- ameters could be as small as 2.4 nm, or about two ilies of the grasses with those of the dicots reveals dozen chains (Kennedy et al., 2007). The microfibrils some correlation of family structure with the differ- are synthesized at the plasma membrane by terminal complexes of six-membered “particle rosettes” that produce a single microfibril (Giddings et al., 1980; 1 This work supported by the Center for Direct Catalytic Conver- sion of Biomass to Biofuels, an Energy Frontier Research Center Mueller and Brown, 1980). Thus, each of the six funded by the U.S. Department of Energy, Office of Science, Office of components of the particle rosette is expected to Basic Energy Sciences (award no. DE–SC0000997). synthesize four to six of the glucan chains, and 24 to * E-mail [email protected]. 36 chains are then assembled into a functional micro- www.plantphysiol.org/cgi/doi/10.1104/pp.110.163360 fibril (Doblin et al., 2002). In freeze fracture, the par- Ò Plant Physiology , January 2011, Vol. 155, pp. 171–184, www.plantphysiol.org Ó 2010 American Society of Plant Biologists 171 Carpita estimated to be 50 nm wide and extend 35 nm into the cytoplasm (Bowling and Brown, 2008), a feature that has escaped consideration in many published models of the rosette structure (Fig. 2, B and C). Cellulose synthase is an ancient enzyme (Nobles et al., 2001), and cellulose synthase genes in green algae are homologous to those of flowering plants (Roberts et al., 2002). The deduced amino acid sequences of CesAs share regions of similarity with the bacterial CesA proteins, namely the four catalytic motifs con- taining the D, DxD, D, Q/RxxRW that are highly conserved among those that synthesize several kinds of (1/4)-b-D-glycans (Saxena et al., 1995). The higher plant CesA genes are predicted to encode polypeptides of about 110 kD, each with a large, cytoplasmic N-terminal region containing zinc-finger (ZnF) domains, and eight membrane spans sandwiching the four U motifs of the catalytic domain (Fig. 1B; Delmer, 1999). Further evidence for the functions of plant CesA genes in cellulose synthesis came from Arabidopsis mutants of three of the CesA genes involved in pri- mary wall synthesis: the temperature-sensitive radial Figure 1. The CesA/Csl gene superfamily. A, Of the 10 Arabidopsis swelling mutant rsw1 (AtCesA1; Arioli et al., 1998), CesA genes, at least three are coexpressed during primary wall forma- the dwarf-hypocotyl procuste mutant prc1 (AtCesA6; tion, and mutations in each of them, AtCesA1 (RSW1; At4g32410), Fagard et al., 2000), and a stunted root phenotype AtCesA6 (PRC1; At5g64740), and AtCesA3 (CEV1 and ELI1; with altered jasmonate and ethylene signaling (cev1) At5g05170), result in cellulose deficiencies, indicating that each is and ectopic lignification (eli1) mutant alleles (AtCesA3; essential for cellulose synthesis. The irx mutants AtCesA8 (IRX1; Ellis and Turner, 2001; Can˜o-Delgado et al., 2003). At4g18780), AtCesA7 (IRX3; At5g17420), and AtCesA4 (IRX5; Despite coexpression in the same cells and an expec- At5g44030) are deficient in cellulose synthesis specifically in secondary tation of redundancy, cellulose synthesis is impaired in walls. Seven additional subgroups were identified that are the likely / each mutant. The same was observed with the irregular synthases for noncellulosic polysaccharides with backbones of (1 4)- xylem mutants irx1, irx3, and irx5, which display a b-D-glycans. Whereas the CesA genes of Arabidopsis, rice, and maize appear to be orthologous, the Csl genes are divergent between dicots phenotype of collapsed mature xylem cells as a result and grasses, species that make two distinct kinds of walls. From mutants of lowered cellulose content during secondary cell and heterologous expression studies, members of the CslA group encode wall deposition (Taylor et al., 2000, 2003). A widely the synthases of (gluco)mannans, members of the CslC group are likely accepted hypothesis is that the AtCesA1, AtCesA3, to encode the glucan backbone of xyloglucans, and the rice- and maize- and AtCesA6 proteins assemble to function in primary only members of CslH and CslF encode the synthases of the mixed- wall cellulose synthesis, while the AtCesA4, AtCesA7, linkage (1/3),(1/4)-b-D-glucans found only in grasses (after Penning and AtCesA8 proteins assemble to make secondary et al., 2009). B, Domain model and class-specific regions (CSRs) for wall cellulose (Fig. 1A), with each member of the trio three CesAs known to function in primary cell wall cellulose synthesis. performing a nonredundant function in the complex Two ZnF domains (in yellow) are found in the N terminus before the first (Taylor, 2008). Lack of one CesA prevents incorpora- membrane-spanning domain (in blue). Eight transmembrane helices, two upstream and six downstream of the catalytic domain, are predicted tion of the other two into the plasma membrane to interact to form a channel through which a single b-glucan chain is (Gardiner et al., 2003). However, at least some of the secreted to the cell wall. The large central catalytic domain contains four subunits are potentially interchangeable, as inferred highly conserved “U motifs” of D, DxD, D, and QxxRW, important for by the dominant-negative inhibition of growth and substrate binding and catalysis. Once thought to be a hypervariable primary wall thickness caused by constitutive expres- region (Pear et al., 1996), the class-specific regions are conserved among sion of a mutated fra5 (irx3 allele) transgene (Zhong orthologs of the same subclade and vary in the number of upstream et al., 2003) and by the semi-dominant-negative phe- conserved Cys residues, the number of consecutive basic amino acids, notype observed in the heterozygous AtCesA3 mutant Lys and Arg, and the number of consecutive acidic amino acids, Asp and (Daras et al., 2009). AtCesA1 is essential for cellulose Glu, downstream from the basic residues (after Carpita and Vergara, synthesis (Beeckman et al., 2002), whereas knockouts 1998; Vergara and Carpita, 2001). of AtCesA3 (Ellis and Turner, 2001; Can˜o-Delgado et al., 2003) and AtCes6 (Fagard et al., 2000) result in ticle rosettes, found only on the P-face of the mem- partially impaired synthesis but not in total inhibition.
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