Edited by: Marisa Otegui, University of Wisconsin at Madison, USA
Reviewed by: Markus Pauly, University of California Berkeley, USA; Paul Knox, University of Leeds, UK
*Correspondence: Azeddine Driouich, Laboratoire “Glycobiologie et Matrice Extracellulaire Végétale” UPRES EA 4358, Institut Federatif de Recherche Multidisciplinaire sur les Peptides, Plate-forme de Recherche en Imagerie Cellulaire de Haute Normandie, Université de Rouen, Rue Tesnière, Bâtiment Henri Gadeau de Kerville, 76821. Mont Saint Aignan, Cedex, France. e-mail:
This article was submitted to Frontiers in Plant Cell Biology, a specialty of Frontiers in Plant Science.
This is an open-access article distributed under the terms of the
The Golgi apparatus of eukaryotic cells is known for its central role in the processing, sorting, and transport of proteins to intra- and extra-cellular compartments. In plants, it has the additional task of assembling and exporting the non-cellulosic polysaccharides of the cell wall matrix including pectin and hemicelluloses, which are important for plant development and protection. In this review, we focus on the biosynthesis of complex polysaccharides of the primary cell wall of eudicotyledonous plants. We present and discuss the compartmental organization of the Golgi stacks with regards to complex polysaccharide assembly and secretion using immuno-electron microscopy and specific antibodies recognizing various sugar epitopes. We also discuss the significance of the recently identified Golgi-localized glycosyltransferases responsible for the biosynthesis of xyloglucan (XyG) and pectin.
One of the most important functional properties of the plant Golgi apparatus is its ability to synthesize complex matrix polysaccharides of the cell wall. Unlike cellulose which is synthesized at the plasma membrane and glycoproteins whose protein backbones are generated in the endoplasmic reticulum, the cell wall matrix polysaccharides (pectin and hemicelluloses) are assembled exclusively in the Golgi cisternae and transported to the cell surface within Golgi-derived vesicles (Driouich et al.,
The synthesis of cell wall matrix polysaccharides occurs through the concerted action of hundreds of glycosyltransferases. These enzymes catalyze the transfer of a sugar residue from an activated nucleotide–sugar onto a specific acceptor. The activity of these enzymes depends, in turn upon nucleotide–sugar synthesizing/interconverting enzymes in the cytosol, and also on the nucleotide–sugar transporters necessary for sugar transport into the lumen of Golgi stacks and subsequent polymerization.
Because cell wall matrix polysaccharides exhibit a high structural complexity, their biosynthesis must be adequately organized and a certain degree of spatial organization/coordination must prevail within Golgi compartments, not only between glycosyltransferases themselves, but also between glycosyltransferases nucleotide–sugar transporters and nucleotide–sugar interconverting enzymes (Seifert,
Cell wall matrix polysaccharides are known to confer important functions to the cell wall in relation with many aspects of plant life including cell growth, morphogenesis and responses to abiotic, and biotic stresses. Plant cell walls are also an important source of raw materials for textiles, pulping and, potentially, for renewable biofuels or food production for humans and animals (Cosgrove,
Plants invest a large proportion of their genes (∼10%) in the biosynthesis and remodeling of the cell wall (Arabidopsis Genome Initiative,
Cell walls of flowering plants are highly diverse and heterogeneous (Popper et al.,
The primary wall of eudicotyledonous plants comprises cellulose microfibrils and a xyloglucan network embedded within a matrix of non-cellulosic polysaccharides and proteins (i.e., glycoproteins and proteoglycans). Four major types of non-cellulosic polysaccharides are found in the primary walls of plant cells (in taxa outside the gramineae), namely the neutral hemicellulosic polysaccharide xyloglucan (XyG) and three main pectic polysaccharides, homogalacturonan (HG), rhamnogalacturonan I and rhamnogalacturonan II (RG-I and RG-II; Carpita and Gibeaut,
XyG consists of a β-
The pectic matrix is structurally complex and heterogeneous. HG domains consist of α-
RG-II is the most structurally complex pectic polysaccharide discovered so far in plants and is of a relatively low molecular mass (5–10 Kda; Ridley et al.,
As for hemicellulosic polysaccharides, it is worth to note that glucurono(arabino)xylan (GAX) does exist in the primary cell walls of eudicotyledonous although at very limited amounts. It is however mostly present in the secondary walls of eudicotyledonous as well as in both the primary and secondary walls of grasses (see also Vogel,
In higher plants, the Golgi apparatus plays a fundamental role in “the birth” of the cell wall. During cytokinesis, a new cell wall is formed and starts to assemble with the transport of Golgi-derived secretory vesicles to the center of a dividing cell. Fusion of these vesicles gives rise to a thin membrane-bound structure, the cell plate, which undergoes an elaborate process of maturation leading to a fully functional cell wall (Staehelin and Hepler,
The Golgi apparatus of plant cells is a dynamic and organized organelle consisting of a large number of small independent Golgi stacks that are randomly dispersed throughout the cytoplasm. At the confocal microscopy level, individual green fluorescent protein (GFP)-tagged Golgi stacks (around 1 μm in diameter) appear as round disks, small rings, or short lines depending on their orientation (Nebenführ et al.,
The
In contrast to the Golgi complex in mammalian cells that has a fixed location near the centrosomes, Golgi units in plants appear to move actively throughout the cytoplasm (Boevink et al.,
As in animal cells (Rabouille et al.,
The first studies implicating plant Golgi stacks in cell wall biogenesis date back to the 60 and 70 and involved cytochemical staining as well as autoradiographic experiments with radiolabeled sugars (Pickett-Heaps,
More recently, a proteomic method called LOPIT, for Localization of Organelle Proteins by Isotope Tagging, has been developed in order to determine the subcellular localization of membrane proteins in organelles of the secretory pathway such as Golgi stacks (Dunkley et al.,
In connection with XyG biosynthesis, it is remarkable to note that the structure of XyG present in isolated Golgi membranes has been investigated using oligosaccharide mass profiling (OLIMP) method (Obel et al.,
Progress toward understanding the compartmentalization of matrix cell wall polysaccharide biosynthesis has come from immuno-electron microscopical analyses with antibodies directed against specific sugar epitopes. In most cases, these immunolabeling studies have been performed on a variety of cells prepared by high pressure freezing, a cryofixation technique that is recognized as providing excellent preservation of Golgi stacks thereby allowing different cisternal subtypes to be easily distinguished (Staehelin et al.,
Quantitative immunolabeling experiments using antibodies recognizing either the XyG backbone (anti-XG antibodies: Moore et al.,
Another interesting approach to study sub-Golgi localization of glycosyltransferases is through development of
As for XyG synthesis, similar immunocytochemical studies using antibodies raised against pectin epitopes (including JIM7, anti-PGA/RG-I, and CCRCM2) has allowed a partial characterization of the assembly pathway of homogalacturonan (HG) and RG-I within Golgi cisternae (Zhang and Staehelin,
It is generally accepted that the transport of Golgi products, including glycoproteins and complex polysaccharides, to the cell surface occurs by bulk flow (Hadlington and Denecke,
The Golgi-mediated assembly of complex polysaccharides requires the action of a set of Golgi glycosyltransferases, in addition to nucleotide–sugar transporters and nucleotide–sugar interconversion enzymes (Keegstra and Raikhel,
XyG biosynthesis has long been an interesting, but a challenging, area of investigation. Biosynthesis of the XyG core is expected to require two different catalytic activities, a glucan synthase activity for the backbone and a xylosyltransferase activity adding xylosyl substitutions. Interestingly, Ray (
We have currently gained a better picture of XyG biosynthesis by identifying and characterizing some of the genes involved (Table
Gene |
Enzyme characteristics |
Arabidopsis mutant |
Notes |
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Designation | Code | CAZy | Activity | Localization | Designation | Phenotypes | Category | Reference | |
At3g28180 | GT2 | β-l,4-Glucan synthase | Golgi | None | None | Cocuron et al. ( |
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At3g62720 | GT34 | α-1,6-Xylosyltransferase | Golgi | 10% Reduction in XyG | Faik et al. ( |
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At4g02500 | GT34 | α-1,6-Xylosyltransferase | Golgi | 20% Reduction in XyG | Detectable amountof XyG | Xyloglucan | Cavalier and Keegstra ( |
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At2g20370 | GT47 | β-1,2-Galactosyltransferase | Golgi | Hypocotyls wall strength is 50% reduced, xyloglucan lacks fucogalactosyl sidechain | Madson et al. ( |
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At2g03220 | GT37 | α-1,2-Fucosyltransferase | Golgi | Hypocotyls wall strength is slightly reduced, xyloglucan lacks fucose substitution | Perrin et al. ( |
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Atlg70230 | GT | Golgi | Cell wall XyG extract is non-acetylated | Pena et al. ( |
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At2gl5390 | GT37 | α-1,2-Fucosyltransferase | Golgi | None | None | Arabinogalactan proteins | |||
Atlgl4080 | GT37 | α-1,2-Fucosyltransferase putative | Golgi | None | None | ||||
At3g25140 | GT8 | α-1,4-Galacturonosyltransferase | Predicted Golgi | Dwarf, reduced cell adhesion, 25% reduction in cell wall GalA content | Bouton et al. ( |
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At3g61130 | GT8 | α-1,4-Galacturonosyltransferase | Golgi | None | None | Sterling et al. ( |
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At4g01770 | GT77 | α-1,3-Xylosyltransferase | Golgi | RG-II from mutant (but not from WT) is an acceptor for a-l,3-xylosyltransferase acitvity | Egelund et al. ( |
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At4g01750 | GT77 | α-1,3-Xylosyltransferase | Golgi | RG-II from mutant (but not from WT) is an acceptor for a-l,3-xylosyltransferase acitvity | Egelund et al. ( |
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At4g01220 | GT77 | Xylosyltransferase | Golgi | Defective for root and pollen tube growth | Fangel et al. ( |
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At2g35100 | GT47 | α-1,5-Arabinosyltransferase | Predicted Golgi | Cell wall composition altered, decrease in RG-I arabinose content | Pectins | Harholt et al. ( |
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At5g33290 | GT47 | β-1,3-Xylosyltransferase | Golgi | Jensen et al. ( |
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– | GT47 | Putative β-glucuronytransferase | Predicted Golgi | Callus harbored a cell-cell adhesion defect, and a reduced RG-II dimerisation ability | Iwai et al. ( |
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At3g55830 | GT64 | Unknown | Predicted Golgi | Reduced growth habit, defects in vascular formation and reduced cell-cell adhesion in hypocotyls | Singh et al. ( |
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Atlg78240 | – | Putative methyltransferase | Golgi | Dwarf, reduced cell adhesion, 50% reduction in HG content | Mouille et al. ( |
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At4g00740 | Putative methyltransferase | Golgi | No detectable phenotype | Miao et al. ( |
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At5g65810 | – | Putative methyltransferase | Golgi | Decrease HG methylesterification | Held et al. ( |
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Atlg64440 | UDP-glucose epimerase | Cytosolic and Golgi associated | Reduced root elongation rate, bulging of trichoblast cells, xyloglucan, and arabinogalactan galactosylation defects in roots | Andème-Onzighi et al. ( |
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At3g02230 | GT75 | Putative UDP-arabinopyranose mutase | Cytosolic and Golgi associated | Decrease in L-Ara, l development altered | Interconversion enzymes | Rautengarten et al. ( |
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At5gl5650 | GT75 | Putative UDP-arabinopyranose mutase | Cytosolic and Golgi associated | Decrease in L-Ara |
Later, the identification and characterization of the
An important contribution to understanding XyG biosynthesis was also made by the characterization of two α-1,6-xylosyltransferase activities required for XyG xylosylation (CAZy GT34). First, one xylosyltransferase activity (named AtXT1) was identified based on sequence homology with a previously identified α-1,6-galactosyltransferase from fenugreek (Edwards et al.,
Another enzyme involved in XyG biosynthesis is glucan synthase. While XyG glucan synthase activity has long been studied biochemically (discussed above), efforts to purify and ultimately characterize this enzyme have not been successful. A gene from the Cellulose Synthase-Like C family (
XyG biosynthesis does not only depend upon the cooperation between glycosyltransferase activities, but might also require close interaction between glycosyltransferases and nucleotide–sugar interconversion enzymes. The study on the
Although it is not the focus of this review, it is of interest to note that some glycosylhydrolases involved in XyG modification have been identified and characterized (Sampedro et al.,
Because there is a considerable diversity of monosaccharide units and glycosidic linkages making up pectic polysaccharides, it has been proposed that a minimum of 67 glycosyltransferases would be needed for pectin biosynthesis (Mohnen,
As for XyG biosynthesis, many biochemical studies have been devoted to the characterization of the enzymes involved in HG biosynthesis (Doong and Mohnen,
The
The HG backbone in
Four other genes involved in pectin biosynthesis, encoding a small glycosyltransferase family with four members named RG-II xylosyltransferase (RGXT1–4; CAZy GT77), have been described (Egelund et al.,
As compared to HG and RG-II biosynthesis, relatively little is known about the glycosyltransferases involved in RG-I biosynthesis and only one glycosyltransferase has been characterized so far. A reverse genetics approach with putative glycosyltransferases from the CAZy GT47 family led to the identification of the
The knowledge of XyG and pectin biosynthesis has progressed significantly over the past 10 years with respect to the identification of the enzymes involved in their biosynthesis, using functional genomics. The challenge now is, to determine how these players (and partners) cooperate, in a timely and probably spatially resolved manner, in order to achieve the coordinated and efficient synthesis of these polymers.
In plants, the only approach that has so far provided evidence for the compartmental organization of the Golgi with regards to complex polysaccharide biosynthesis is immunogold microscopy using antibodies raised against specific sugars of different polysaccharides (Follet-Gueye et al.,
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Very special thanks are due to Pr. A. Staehelin (who retired recently) for having passionately launched AD onto the path of plant Golgi research. The authors wish to thank Pr. S. Hawkins and Dr. J. Moore for their critical reading of the manuscript as well as Pr. B. H. Kang for providing Figure