Plant Polysaccharides BCMB 8020 overview of plant glycobiology plant cell walls types of plant cell wall polysaccharides structure and biosynthesis RG II a curious case in plant glycobiology special functions of plant polysaccharides biofuel green plants constitute about half of the living matter on earth plants synthesize many of the same types of oligosaccharides that are found in animals but also produce a wide variety of unique sugar chains many of the earliest studies on carbohydrates were done in plants General Organization and Features of a Plant Cell vacuole lysosome storage granule symplast shared cytoplasm plant cells were first described by Robert Hooke who noted that slices of cork looked like cells in a monastery under his early microscope Plants form two types of cell walls a primary cell walls that differ in composition and function Primary cell wall highly flexible found in growing plant cells polysaccharide rich 80 90 polysaccharide 10 20 protein Secondary cell wall thicker and stronger contains majority of plant biomass lignin a macromolecule of cross linked phenolic structures is a major component of the secondary wall polysaccharides are also present but in relatively lower proportion and altered content Primary cell wall is the extracellular matrix of plants contains many types of polysaccharides main polysaccharides are cellulose hemicellulose pectins plant cells also contain several structural glycoproteins that are rich in hydroxyprolines and heavily modified by serine and threoninelinked oligosaccharides O linked extensins proline hydroxyproline rich glycoproteins arabinogalactans Cellulose structure and biosynthesis cellulose is a long linear polysaccharide consisting of 1 4 linked glucose residues the most abundant biopolymer in nature forms a crystalline microfiber via extensive hydrogen bonding between individual cellulose polysaccharides biosynthesis of cellulose occurs at the plasma membrane using cytoplasmic UDP glucose generated from sucrose cellulose provides tensile strength to plant cell matrix similar to role of collagen fibers in animal cells Cellulose biosynthesis involves nearly 40 different CESA proteins at the plasma membrane and is functionally tied to microtubules rosette Hemicellulose structure and biosynthesis hemicellulose consists of 1 4 linked glucose residues like cellulose that are substituted with other sugars xyloglucan is the predominant hemicellulose others include glucuronoxylan arabinoxylan glucomannan and galactomannan coat and cross link cellulose fibers in the primary cell wall as cells increase in volume H bonds that link cellulose and hemicellulose loosen allowing the internal osmotic pressure of the cell to push apart the cellulose microfibers this process is critical for plant cell growth and is helped by enzymatic cleavage of xyloglucan chains Pectins structure and biosynthesis pectins are complex acidic polysaccharides that contain 1 4 linked D galactosyluronic acid residues they resemble glycosaminoglycan GAG chains pectins that have been characterized include homogalacturonan 65 of plant pectins substituted galacturonans like apiogalacturonan and xylogalacturonan and rhamnogalacturonan I and II embedded within the cellulose hemicellulose network pectins provide hydration and additional strength to the primary wall pectins can be modified by methyl esterases extent of methylation can determine porosity and stiffness of cell wall Determination of pectic polysaccharide structure requires the full range of available analytical techniques Rhamnogalacturonan I RG I structure and biosynthesis RG I consists of a 4 D GalpA 1 2 L Rhap backbone GalA residues are not usually substituted in RG I Rhamnogalacturonan II RG II structure and biosynthesis RG II consists of a 1 4 D GalpA backbone 2 2 9 8 RG II is substituted with four highly complex and conserved side chains that contain 12 different sugars in more than 20 different linkages RG II is a major polysaccharide component of red wine 1 liter of red wine may contain between 100 and 150mg of RG II white wine typically contains 20 30 mg of RG II per liter differences result form the different processing used to make these wines white wines are made by fermenting grape juice which contain little of the cell wall but red wines are made by fermenting grape berry pulp the cell wall accounts for a large portion of the pulp and the RG II is solubilized during the fermentation process RG II is extremely resistant to known microbial polysaccharide degrading enzymes and thus is not utilized as a carbon source during fermentation Why do higher plants invest so much energy and effort in producing such a complicated structure in their wall first clue came from observations that RG II exists as boroncross linked dimer between two apiose residues on the side chain A of an RG II monomer two stereoisomers of borate linked RG IIs Formation of the borate ester cross link Changes in boron content affect plant growth Bor1 mutant mutation in a boron transporter WT and bor1 RG II have similar glycosyl residue compositions but bor1 has much lower levels of cross linked monomers boron and other metals are important components of plant cell walls and play a role in cross linking pectin does this prove that RG II is also important for cell growth Does the absence of RG II dimers cause growth defects to address this question different mutants in Arabidopsis were studied including the pleiotropic mutant mur1 mur1 mutant identified by screening hydrolysates of chemically modified plants characterized by fragile cell walls and dwarfed size found to be deficient in GDP mannose 4 6 dehydrase enzyme responsible for GDP fucose synthesis Problem fucose is not only found in RG II xyloglucans and glycan side chains of many proteins contain fucose Which altered polysaccharides are responsible for the growth defect in the mur1 mutant another mutant cgl1entirely lacks fucose in its glycoproteins but surprisingly does not show any phenotypes mur2 mutants synthesize xyloglucans with only 2 of normal fucose But also have no growth defect effects on RG II in mur1 Arabidopsis mutants with altered RG II structure cont L galactose is substituted in place of fucose this leads to reduced amounts of RG II dimer less stable dimers and a slower rate of dimer formation mutant can be partially rescued by watering plants in fucose or borate provide evidence for crucial role of RG II in plant growth and the necessity of boron