Recent research on plant glycan structure and function has typically emphasized model plants such as Arabidopsis and plants of commercial importance. However, there is increasing interest in studying the glycans produced by plants from all the major orders of the Viridiplantae. Such studies, together with the availability of transcriptomic data for numerous green algae and land plants, have begun to reveal a rich diversity in glycan structures and insight into how some of these structures have changed during the evolution of the Viridiplantae. In this chapter, we provide an overview of the current knowledge of green plant glycan structures with an emphasis on the features that are unique to land plants.

Viridiplantae (green plants) are a clade of photosynthetic organisms that contain chlorophylls a and b, produce and store their photosynthetic products inside a double-membrane-bounded chloroplast, and have cell walls that typically contain cellulose. The Viridiplantae are comprised of two clades—the Chlorophyta and the Streptophyta. The Chlorophyta contain most of the organisms typically referred to as “green algae.” The term “algae” is also used for several other groups of photosynthetic eukaryotes, including diatoms and the red, brown, golden, and yellow-green algae. The Streptophyta comprise several other lineages that are also referred to as “green algae” and the land plants. Land plants include the liverworts, mosses, hornworts, lycopods, ferns, gymnosperms, and flowering plants.

PLANT GLYCAN DIVERSITY

Green plants synthesize diverse glycans that vary in their structural complexity and molecular size. Raffinose oligosaccharides (raffinose, stachyose, and verbascose) are almost ubiquitous in the plant kingdom and rank second only to sucrose in abundance as soluble carbohydrates. These oligosaccharides are derivatives of sucrose that contain one or more α-Gal residues. Plants also synthesize numerous low molecular weight glycoconjugates that contain either aromatic (e.g., phenolic glycosides) or aliphatic (glycolipids) aglycones.

Plant polysaccharides are linear or branched polymers composed of the same or different monosaccharides. For example, cellulose is composed of 1-4-linked β-D-Glc residues (Figure 24.1A), whereas the structurally complex plant cell wall pectic polysaccharide, referred to as rhamnogalacturonan II (RG-II), contains 12 different monosaccharides linked together by up to 21 different glycosidic linkages (Figure 24.2). Plant proteoglycans are structurally diverse glycans in which carbohydrate accounts for up to 90% of the molecule and is O-linked to the protein via hydroxyamino acids (Figure 24.3). Plant glycoproteins typically contain 15% or less of carbohydrate in the form of N-linked oligomannose, complex, hybrid, and paucimannose oligosaccharides (see Figure 24.4). Land plants also form O-GlcNAc-modified nuclear and cytosolic proteins (Chapter 19).

FIGURE 24.1.

Glycosyl sequences of cellulose and selected hemicelluloses present in plant cell walls.

FIGURE 24.2.

Schematic structure of pectin. The three main pectic polysaccharides are shown: rhamnogalacturonan I (RG-I), rhamnogalacturonan II (RG-II), and homogalacturonan (HG). The borate ester is formed between the apiosyl residue in side chain A of each RG-II (more...)

FIGURE 24.3.

Schematic structure of the proteoglycan referred to as arabinoxylan pectin arabinogalactan protein1 (APAP1).

FIGURE 24.4.

Types of N-glycans identified in plants.

NUCLEOTIDE SUGARS—THE BUILDING BLOCKS

Nucleotide sugars are the donors used for synthesis of glycoconjugates and glycosylated secondary metabolites (Chapter 5). In plants, the majority of these activated monosaccharides exist as their nucleotide-diphosphates (e.g., UDP-Glc), although at least one monosaccharide, Kdo, exists as its cytidine monophosphate derivative (CMP-Kdo). Nucleotide sugars are formed from the carbohydrate generated by photosynthesis, from the monosaccharides released by hydrolysis of sucrose and storage carbohydrates, and by recycling monosaccharides from glycans and the cell wall. Nucleotide sugars are also formed by interconverting preexisting activated monosaccharides. To date, 30 different nucleotide sugars and at least 100 genes encoding proteins involved in their formation and interconversion have been identified in plants.

PLANT GLYCOSYLTRANSFERASES AND GLYCAN-MODIFYING ENZYMES

Plant genomes contain numerous genes encoding proteins involved in the synthesis and modification of glycans. These proteins are spread across many enzyme classes in the Carbohydrate-Active Enzymes (CAZy) database (Table 24.1). Many of these proteins may be involved in the formation and modification of the polysaccharide-rich cell wall. Indeed, the unicellular alga Ostreococcus tauri, which is one of the few plants that does not form a cell wall, has a much smaller number of genes predicted to be involved in glycan metabolism.

Table 24.1.

Estimated number of genes encoding proteins involved in the synthesis and modification of glycans in plants and humans

PLANT CELL WALLS

A substantial portion of the carbohydrate formed by photosynthesis is used to produce the polysaccharide-rich walls that surround plant cells. Primary and secondary cell walls are distinguished by their composition, architecture, and functions. A primary wall surrounds growing and dividing plant cells and nongrowing cells in the soft tissues of fruits and leaves. These walls are capable of controlled extension to allow the cell to grow and expand but yet are sufficiently strong to resist the cells internal turgor pressure. A much thicker and stronger secondary wall is often formed once a cell has ceased to grow. This wall is deposited between the plasma membrane and the primary wall and is composed of layers that differ in the orientation of their cellulose microfibrils. The secondary walls of vascular tissues involved in the movement of water and nutrients through the plant are further strengthened by the incorporation of lignin. The ability to form conducting tissues with lignified and rigid secondary walls was an indispensable event in the evolution of vascular land plants, as it facilitated the transport of water and nutrients and allowed extensive upright growth. Secondary cell walls account for most of the carbohydrate in plant biomass being considered as feedstock for the production of biofuels and value-added chemicals (Chapter 59).

PRIMARY CELL WALL GLYCANS

Primary cell walls are composites that resemble fiber-reinforced porous gels. The complex structures and functions of these walls result from the assembly and interactions of a limited number of structurally defined polysaccharides and proteoglycans. Wall structure and organization may change during cell division and development and in response to biotic and abiotic challenges by the differential synthesis and modification of the noncellulosic components or by the addition of new components.

Primary walls of land plants contain cellulose, hemicellulose, and pectin, in different proportions. They also contain structural proteins/proteoglycans, enzymes, phenolics, and minerals. Pectin and hemicellulose are present in approximately equal amounts in the so-called type I primary walls of gymnosperms, dicots, and nongraminaceous monocots, whereas hemicellulose is far more abundant than pectin in the type II walls of the grasses. Much less is known about the composition of the walls of avascular plants and green algae, although it is likely that the walls of these plants contain cellulose, pectin, and hemicellulose.

PLANT SECONDARY CELL WALL GLYCANS

The secondary walls of woody tissue and grasses are composed predominantly of cellulose, hemicellulose, and lignin. The inclusion of lignin results in a hydrophobic composite that is a major contributor to the structural characteristics of secondary walls.

Heteroxylans are the major hemicellulosic polysaccharide present in the secondary (lignified) cell walls of flowering plants. These heteroxylans are classified according to the type and abundance of the substituents on the 1-4-linked β-D-Xylp residues of the polysaccharide backbone. Glucuronoxylans (GX), which are major components in the secondary walls of woody and herbaceous eudicots, have a α-D-GlcA or MeGlcA substituent at O-2 (Figure 24.1C). Gymnosperm secondary walls contain arabinoglucuronoxylans (AGXs), which in addition to MeGlcA substituents, have Araf residues attached to O-3 of some of the backbone residues. The GAX in the secondary walls of grasses typically contain less Araf residues than their primary wall counterpart (Figure 24.1D). Ferulic or coumaric acids are often esterified to the Araf residues of GAX in grass primary and secondary cells walls.

Eudicot and gymnosperm secondary wall GX and AGX have a well-defined glycosyl sequence 1-4-β-D-Xylp-1-3-α-L-Rhap-1-2-α-D-GalpA-1-4-D-Xylp at their reducing end (Figure 24.1C). This sequence is required for normal xylan synthesis during secondary cell wall formation and may have a role in regulating the polymers chain length. This sequence is present at the reducing end of heteroxylans of all monocots except the grasses.

HEMICELLULOSE AND PECTIN BIOSYNTHESIS

Genes that encode polysaccharide biosynthetic enzymes, including many of those required for xyloglucan, glucuronoxylan, arabinoxylan, and cellulose synthesis and some of those required for pectin synthesis have been identified. This information, together with improved methods to generate recombinant plant glycosyltranferases (GTs) with high enzymatic activity is providing a framework for an increased understanding of how plant cell wall polysaccharides are synthesized. There is general consensus that cellulose is synthesized by GT complexes localized at the plasma membrane and that most pectins and hemicellulose are synthesized in the Golgi apparatus. Members of the cellulose synthase-like gene families, CSLF and CSLH are likely involved in 1-3- and 1-4-linked β-glucan biosynthesis. However, there is considerable debate about whether this polysaccharide is formed in the Golgi apparatus or at the plasma membrane.

Despite advances in understanding how polysaccharides are synthesized we still do not know how many of the wall polymers are synthesized by Golgi-localized multienzyme complexes or if they are assembled by GTs localized in different regions of the Golgi apparatus. We also do not understand how the newly synthesized polymers are assembled into a functional cell wall.

PLANTS PRODUCE PROTEOGLYCANS CONTAINING O-LINKED OLIGOSACCHARIDES AND O-LINKED POLYSACCHARIDES

Plants produce glycoproteins and proteoglycans that contain oligo- or polysaccharides that are linked to hydroxyproline (Hyp) and serine (Ser). Hyp is formed posttranslationally by endoplasmic reticulum (ER)-localized prolyl hydroxylases and is O-glycosylated in the ER and in the Golgi apparatus. The degree and type of Hyp glycosylation is determined to a large extent by the protein's primary sequence and the arrangement of Hyp residues. Hyp glycosylation is initiated by the addition of an Ara or a Gal residue. Contiguous Hyp residues are arabinosylated, whereas clustered but noncontiguous Hyp residues are galactosylated. Ser residues and occasionally threonine residues may also be O-glycosylated in these proteins.

Three structurally distinct plant proteoglycans containing glycosylated Hyp and Ser—the extensins, proline/hydroxyproline-rich proteoglycans, and arabinogalactan proteins—have been identified. Extensins are hydroxyproline-rich proteoglycans with Ser(Hyp)₄ repeat sequences and contain between 50% and 60% (w/w) glycan. Most of the carbohydrate exists as oligosaccharides containing one to four Ara residues linked to Hyp together with a small number of single Gal residues α-linked to Ser. The proline/hydroxyproline-rich proteoglycans, which contain from 3% to 70% (w/w) carbohydrate, are distinguished from the extensins by amino acid sequence. Both of these families of hydroxyproline-rich glycoproteins (HRGPs) likely have a structural role in the cell wall. The expression of genes involved in their synthesis is developmentally regulated and is often induced by wounding and fungal attack of plant tissues.

The glycopeptide signaling molecules PSY1, CLE2, and CLV3 contain arabinosylated hydroxyproline and have numerous roles in plant growth and development. A highly glycosylated Hyp-rich domain in which three or four Ara residues are attached to Hyp and a single Gal residue is linked to Ser is present in potato, tomato, and thorn-apple lectins.

Arabinogalactan proteins (AGPs) have a glycan content of up to 90% (w/w). Chains of between 30 and 150 monosaccharides are linked to the protein by Gal-O-Ser and Gal-O-Hyp linkages. These chains have a 1-3-linked β-Gal backbone that is extensively substituted at O-6 with side chains of 1-6-linked β-Gal. These side chains are terminated with Ara, GlcA, and Fuc residues. Some AGPs may contain homogalacturonan, RG-I and xylan covalently linked to the arabinogalactan (Figure 24.3) thereby forming a protein–hemicellulose–pectin complex referred to as APAP1. The location of this complex in the plant and its biological function remains to be determined.

Several AGPs are secreted into the cell wall whereas others are linked to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. Plant GPI anchors contain a phosphoceramide core. The glycan portion of the GPI anchor of pear cell AGP has the sequence α-D-Man1-2α-D-Man-1-6-α-D-Man-1-4-GlcN-inositol. At least 50% of the Man attached to the GlcN (see Chapter 12) is itself substituted at O-4 with a β-Gal, a feature that may be unique to plants. Many functions have been proposed for the AGPs including their participation in signaling, development, cell expansion, cell proliferation, and somatic embryogenesis.

THE N-LINKED GLYCANS OF PLANT GLYCOPROTEINS HAVE UNIQUE STRUCTURES

Many of the proteins that have passed through the plant secretory system contain N-linked oligomannose, complex, hybrid, or paucimannose-type glycans (Figure 24.4). The initial stages of the synthesis of these N-glycans, including the transfer of the oligosaccharide precursor from its dolichol derivative and the control of protein folding in the ER are comparable in plants and animals (Chapter 9). However, two modifications of N-glycans during passage through the Golgi are unique to plants.

Oligomannose-type N-glycans are often trimmed in the cis-Golgi and then modified in the medial-Golgi by N-GlcNAc transferase I (GnT-I) catalyzed addition of GlcNAc to the distal Man of the core. In reactions that are unique to plants, a β-Xyl is often added to O-2 of the core Man. In the trans-Golgi, α-Fuc may be added to O-3 of the GlcNAc residue that is itself linked to asparagine (Figure 24.5). The XylT and FucT that catalyze these reactions act independently of one another but do require at least one terminal GlcNAc residue for activity. The FucT is related to the Lewis FucT family, whereas the XylT is unrelated to other known GTs.

FIGURE 24.5.

Processing of N-glycans in the plant secretory system. Only those events that are unique to plants are shown in detail.

The xylosylated and fucosylated N-glycans are often trimmed by α-mannosidase II. A second GlcNAc may then be added by GnT-II. Some plant N-glycans do not undergo further mannose trimming and proceed through the Golgi as hybrid-type N-glycans. Complex and hybrid-type N-glycans may be further modified by the addition of Gal and Fuc in the trans-Golgi. Plant glycoproteins are either secreted from the cell or transported to the vacuoles. Many of the glycoproteins present in the vacuoles contain paucimannose type glycans, suggesting that they are trimmed by vacuolar glycosidases (Figure 24.5).

The presence of sialic acid in the N-glycans of plant glycoproteins was claimed but likely represented environmental contamination. Plants do have genes that encode proteins containing sequences similar to sialyltransferase motifs but their functions have not been established.

ALGAL GLYCANS

Only a few glycans of green algae have been studied in detail. For example, the cell wall of Chlamydomonas reinhardii is a crystalline lattice formed from hydroxyproline-rich glycoproteins. A sulfated polysaccharide composed of Kdo and GalA is the major glycan in the cell walls of Tetraselmis striata. Some, but not all, of the glycans present in the cell walls of land plants may also be present in the cell walls of some green algae. Several of the polysaccharides produced by red and brown algae are used in the food industry as gelling agents, stabilizers, thickeners, and emulsifiers. They are also used in paints, cosmetics, and in paper and as reagents for scientific research.

Agarose (agar) and carrageenan are sulfated galactans obtained from red seaweeds. The polysaccharide is comprised of the repeating disaccharide 3-β-D-Galp-1-4-3,6-anhydro-α-L-Galp-1 unit. Some of the D-Gal and L-Gal units are O-methylated. Pyruvate and sulfate groups may also present in small quantities. There are three main types of carrageenan—κ-carrageenan has one sulfate group per disaccharide, ι-carrageenan has two sulfates per disaccharide, and λ-carrageenan has three sulfates per disaccharide.

Alginic acid, a linear polysaccharide composed of 1-4-linked β-D-ManA and its C-5 epimer 1-4-linked α-L-GulA, is obtained from various species of brown seaweed. These monosaccharides are typically arranged in blocks of either ManA or GulA separated by regions comprised of 4-ManpA-1-4-GulpA-1 sequences.

Brown seaweeds produce polysaccharides that have potential in the treatment of diseases. Laminaran, a linear storage polysaccharide composed of 1-3- and 1-6-linked β-D-Glc residues. There are reports that laminaran has antiapoptotic and antitumor activities. Fucoidans are a group of sulfated polysaccharides isolated from several brown algae that have been reported to have anticoagulant, antitumor, antithrombosis, antiinflammatory, and antiviral properties. Fucoidans have a backbone of 1-3-linked α-Fuc that is substituted at O-2 with fucose and at O-4 with sulfate or fucose. Other fucoidans have backbones of alternating 1-3- and 1-4-linked α-Fuc residues.

PLANT GLYCOLIPIDS

Glycoglycerol lipids are the most abundant glycolipids in plants. Mono- and digalactosyldiacylglycerol have been identified in all plants, whereas tri- and tetragalactosyldiacylglycerol have a more restricted taxonomic distribution (Figure 24.6). The synthesis of these galactolipids is initiated by the formation of diacylglycerol in the ER membrane and the chloroplast membrane. Galactolipids formed in the ER membrane contain predominantly C16 fatty acids at the sn2 position and C18 fatty acids at the sn3 position. The chloroplast pathway produces C18 fatty acids at both positions. Each of these fatty acids is then desaturated to 16:3 or 18:3 acyl groups. Monogalactosyldiacylglycerol (MGDG) is synthesized by the transfer of Gal from UDP-Gal to diacylglycerol by an MGDG synthase. Digalactosyldiacylglycerol (DGDG) is formed from MGDG by the transfer of Gal from UDP-Gal by a DGDG synthase. These reactions occur primarily in the outer chloroplast membrane. The products are then transported to the inner membrane and the thylakoid membranes of the chloroplast. The presence and abundance of MGDG in the chloroplast thylakoid membrane is important for normal photosynthesis to occur. Sulfoquinovosyldiacylglycerol, which is formed from diacylglycerol, is also abundant in the thylakoid membrane and may also have a role in photosynthesis.

FIGURE 24.6.

The most abundant plant galactolipids.

Small amounts of MGDG and DGDG are present in the plasma membranes of cells, although the mechanism of galactolipid exchange among the membranes is not understood. Inositolphosphoceramides that contain GlcN and GlcA have been identified in plants, although their function remains to be determined. Glycosphingolipids containing Gal and Fuc (Chapter 11) are present in plant cell plasma membranes, but none have been fully characterized. No gangliosides have been identified in plants. However, there is evidence that Kdo-containing lipids with homology to bacterial lipid A are present in plant organelle membranes.

OTHER PLANT GLYCOCONJUGATES

Plants produce numerous phenolics, terpenes, steroids, and alkaloids that are collectively referred to as secondary metabolites. Many of these compounds are O-glycosylated or contain sugars linked via N, S, or C atoms. Glycosylated secondary metabolites often have important roles in a plants response to biotic and abiotic challenges and may also have value as pharmaceuticals.

In general, the addition of a single sugar or an oligosaccharide may increase water solubility, enhance chemical stability, or alter both chemical and biological activity. For example, the activity of several plant hormones may be regulated by converting them to their glucose esters or their glucosides. Digoxin and oleandrin are potent cardiac glycosides isolated from foxglove and oleander, respectively. Myrosinase-catalyzed cleavage of S-linked Glc from glucosinolates leads to the formation of pungent mustard oils when mustard and horseradish are damaged. The steviol glycosides, which are far sweeter than sucrose, are used as natural sugar substitutes. The bitter taste of citrus fruits is due to naringin, a glycosylated flavanoid.

PLANT MUTANTS PROVIDE CLUES TO GLYCAN FUNCTION

The availability of plant lines carrying mutations in specific genes has yielded considerable insight into glycan biosynthesis and function. Arabidopsis is widely used as a model dicot as it is easy to grow, has a short life cycle and its relatively small genome has been sequenced and extensively annotated. Numerous databases and resources are available for Arabidopsis, most notably TAIR (The Arabidopsis Information Resource), which provides access to a large number of plants carrying chemically induced or transfer DNA (T-DNA) insertion mutations in genes of both known and unknown function. Collections of other plant mutant lines including soybean and Brachypodium exist, but are not yet as well developed as TAIR.

Several Arabidopsis cell wall mutants were identified by screening 5200 chemically mutagenized Arabidopsis plants for changes in the glycosyl residue compositions of their cell walls. One of these mutants (mur1) lacks an isoform of GDP-Man-4,6-dehydrase (Chapter 6) and is deficient in Fuc. The mutant partially compensates for this deficiency by adding L-Gal, a homolog of Fuc (6-deoxy-L-Gal), to xyloglucan, RG-I, and RG-II. The addition of L-Gal to RG-II results in incomplete synthesis of the A side chain and a decrease in borate cross-linking of RG-II, which is likely responsible for the dwarf stature of mur1 plants. Two additional mutants (mur2 and mur3) identified in the same screen were found to be defective in xyloglucan side chain synthesis. Subsequently, all the other Arabidopsis genes encoding the GTs required for xyloglucan side chain formation have been identified. Some of the genes involved in xyloglucan synthesis in rice and tomato have also been identified and functionally characterized.

Genes involved in the synthesis of the GX backbone (IRX10 and IRX10-L), the addition of GlcA (GUX), the O-methylation of GlcA (GXMT1), and O-acetylation of the backbone (XOAT) have been identified and functionally characterized. There has also been progress in identifying grass genes and GTs involved in GAX synthesis. No genes involved in the synthesis of the Rha-containing reducing end sequence of secondary wall GX have yet been identified.

A family of genes that encode GTs involved in the synthesis of 1-4-linked α-galacturonans have been identified. Two of the encoded proteins, GAUT1 and GAUT7, exist as a complex that synthesizes homogalacturonan. Only one GT involved in RG-II synthesis has been identified. Similarly, few genes have been identified and shown to have a role in the synthesis of the backbone and side chains of RG-I.

Fucosyltransferases, glucuronosyltransferases, galactosyltransferases, and arabinosyltransferases involved in arabinogalactan and extensin biosynthesis have been identified. Plants carrying loss-of-function mutations in some of the genes encoding these GTs show growth defects and reduced fertility.

Plants carrying mutations at different steps along the N-glycosylation pathway have begun to provide insight into the role of protein glycosylation in plants. Mutants (dgl1) defective in the oligosaccharyltransferase complex have phenotypes that range from reduced cell elongation to embryonic lethality. However, a mutant (cgl) that lacks GnT-I activity has no discernible developmental or growth defects, even though it produces glycoproteins enriched in Man₅GlcNAc₂ but lacking complex N-glycans.

Plant mutants defective in O-GlcNAc modification of proteins show numerous changes in growth and development processes (Chapter 19). It is not known how this modification affects cellular processes as only a small number of O-GlcNAc modified proteins have been identified.

The plant-specific modifications of N-glycans result in glycoproteins that are often highly immunogenic and cause allergic responses in humans. The demonstration that complex N-glycans are not essential for plant growth initiated studies to engineer plant N-glycosylation pathways to produce glycoproteins that do not activate the mammalian immune system. Plants lacking the GTs that add Xyl and Fuc to N-linked glycans produce glycoproteins lacking immunogenic glyco-epitopes. Other glycosylation pathways involved in the addition of sialic acid and Gal must be introduced to fully “humanize” the glycoproteins if plants are to be used to produce recombinant therapeutic glycoproteins.

Glycolipids are important for chloroplast development and for photosynthesis. The mgd1 mutant, which contains 50% of the normal amounts of MGDG, is deficient in chlorophyll production and has altered chloroplast ultrastructure. Additional evidence for the role of MGDG has been obtained using galvestine-1 or (2-oxobenzo[d]imidazol-3-yl) piperidine-1-carboxylate, a chemical inhibitor of monogalactosyldiacylglycerol synthases, which has been shown to impair chloroplast development in Arabidopsis.

ACKNOWLEDGMENTS

The authors appreciate helpful comments and suggestions from Corinna Landig and Robert Townley.

FURTHER READING

• Painter T. 1983. Algal polysaccharides. In The polysaccharides (ed. Aspinall G, editor. ), pp. 195–285. Academic, New York.
• Pérez S, Mazeau K, du Penhoat CH. 2000. The three-dimensional structures of the pectic polysaccharides. Plant Physiol Biochem 38: 37–55.
• Gachon CM, Langlois-Meurinne M, Saindrenan P. 2005. Plant secondary metabolism glycosyltransferases: The emerging functional analysis. Trends Plant Sci 10: 542–549. [PubMed: 16214386]
• Hölzl G, Dörmann P. 2007. Structure and function of glycoglycerolipids in plants and bacteria. Prog Lipid Res 46: 225–243. [PubMed: 17599463]
• Albersheim P, Darvill A, Roberts K, Sederoff R, Staehelin A. 2010. Plant cell walls. From chemistry to biology. Garland Science, New York.
• Gomord V, Fitchette A-C, Menu-Bouaouiche L, Saint-Jore-Dupas C, Plasson C, Michaud D, Faye L. 2010. Plant-specific glycosylation patterns in the context of therapeutic protein production. Plant Biotechnol J 8: 564–587. [PubMed: 20233335]
• Bar-Peled M, O'Neill MA. 2011. Plant nucleotide sugar formation, interconversion, and salvage by sugar recycling. Annu Rev Plant Biol 62: 127–155. [PubMed: 21370975]
• Kieliszewski MJ, Lamport D, Tan L, Cannon M. 2011. Hydroxyproline-rich glycoproteins: Form and function. Annu Plant Rev 41: 321–342.
• Popper ZA, Michel G, Hervé C, Domozych DS, Willats WGT, Tuohy MG, Kloareg B, Stengel DB. 2011. Evolution and diversity of plant cell walls: From algae to flowering plants. Annu Rev Plant Biol 62: 567–590. [PubMed: 21351878]
• Atmodjo MA, Hao Z, Mohnen D. 2013. Evolving views of pectin biosynthesis. Annu Rev Plant Biol 64: 747–779. [PubMed: 23451775]
• Pauly M, Gille S, Liu L, Mansoori N, de Souza A, Schultink A, Xiong G. 2013. Hemicellulose biosynthesis. Planta 238: 627–642. [PubMed: 23801299]
• Cosgrove DJ. 2014. Re-constructing our models of cellulose and primary cell wall assembly. Curr Opin Plant Biol 22: 122–131. [PMC free article: PMC4293254] [PubMed: 25460077]
• Knoch E, Dilokpimol A, Geshi N. 2014. Arabinogalactan proteins: Focus on carbohydrate active enzymes. Frontiers Plant Sci 5: 198. [PMC free article: PMC4052742] [PubMed: 24966860]
• Matsubayashi Y. 2014. Posttranslationally modified small-peptide signals in plants. Annu Rev Plant Biol 65: 385–413. [PubMed: 24779997]
• McNamara JT, Morgan JL, Zimmer J. 2015. A molecular description of cellulose biosynthesis. Annu Rev Biochem 84: 895–921. [PMC free article: PMC4710354] [PubMed: 26034894]