BOTANY, VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. PHYSIOLOGY OF METABOLISM

6.17. Major Plant-Typical Polymers

Alongside low-molecular-weight Primary and secondary metabolites, plants synthesize polymeric Organic compounds. Some of these occur in all living Cells and are therefore not specific to plants, whereas others are absent from the animal kingdom (or occur only very rarely as exceptions). The principal polymers typical of plants include structural and Storage Polysaccharides, Lignin, cutin, and suberin, as well as several classes of storage Proteins. The following sections are dedicated to these plant-typical polymers, given their importance to the plant as a whole (as core polymers).

6.17.1. Polysaccharides

The basic structures of CARBOHYDRATES are presented in section 1.4 (see Figs. 1.18–1.20). Polymeric glycans (polysaccharides) function either as structural Materials or as reserves.

6.17.1.1. Structural Polysaccharides

Plant Cell Walls (both primary and secondary) contain a range of structural polysaccharides (along with structural proteins). Their proportions vary continuously (see 1.4.3, Fig. 2.64). In secondary cell walls, Cellulose is dominant (accounting for up to 90% of the organic matter), whereas in primary cell walls, although it provides mechanical strength, it constitutes only a small fraction (5–10%). Cellulose is also a constituent of oomycete cell walls and has been found in the tunic of tunicates (which synthesize it independently). Certain Bacteria are likewise capable of producing cellulose.

Cellulose molecules consist of numerous (over 15,000) linear β-D-glucopyranose units linked together by (β1—>4)-glycosidic bonds. Through intramolecular hydrogen bonding, these units aggregate into semicrystalline structures, elementary fibrils, and microfibrils (Figs. 2.65; 2.68). Cellulose Biosynthesis proceeds in a directed manner facilitated by cellulose synthase, an integral membrane protein of the Plasmalemma. During this process, several synthase subunits oligomerize into a rosette complex (see Fig. 2.67, A), and each individual subunit can transfer glucose units—supplied from the Cytoplasm by UDP-glucose (Fig. 6.130)—onto the growing cellulose molecule, thereby elongating the chain. The synthase extrudes the resulting cellulose molecule toward the apoplastic side of the membrane. It is hypothesized that each cellulose synthase monomer within the rosette complex synthesizes a single cellulose molecule; consequently, a single rosette complex simultaneously produces multiple cellulose molecules that subsequently assemble into fibrils. Active cellulose synthase complexes must "glide" along the cortical microtubules of the Cytoskeleton within the cytoplasmic plane. Thus, the arrangement of cortical microtubules presumably determines (and controls) the orientation of cellulose microfibrils within The Cell wall. Considerably less is known about The biosynthesis of other cell wall polysaccharides, such as hemicellulose and pectin (see 22.1.2).

Class="center">Fig. 6.130. Starch biosynthesis: A — Structure of activated glucose, shown as an acid glycoside between D-glucopyranose (here the α-anomer) and the phosphoric acid residue of a nucleoside diphosphate. ADP-Glucose serves as the substrate for starch biosynthesis, whereas UDP-glucose serves for sucrose and cellulose biosynthesis; B — starch synthase reaction; C — amylopectin is synthesized through the concerted action of starch synthase (chain elongation) and a branching enzyme, namely a transglycosylase; D — Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF starch. Amylose is generated from amylopectin via a "trimming" reaction (the isoamylase reaction). The illustrated amylose chains represent segments from the interior of the molecule, which is why the reducing ends are not depicted

Chitin, the primary structural component of The Cell wall in many Fungi, is a linear polymer of N-acetylglucosamine units linked by (β₁ —> 4)-glycosidic bonds (for structure, see Fig. 1.18, E); the donor of the N-acetylglucosamine units is UDP-N-acetylglucosamine (UDP-GlcNAc). Bacterial cell wall peptidoglycan (see 2.3.3, Fig. 2.98) can be formally regarded as a substituted chitin.

6.17.1.2. Storage Polysaccharides

Aside from a few exceptions (such as sugarcane and sugar beet, which store sucrose in their vacuoles, as well as legumes

and Lamiaceae, which store sugars of the raffinose family, primarily stachyose), plants deposit reserve carbohydrates in the form of Water-insoluble polysaccharides, predominantly as starch within amyloplasts (see Fig. 2.90). Both components of starch—amylose and amylopectin (see Fig. 1.20)—are homoglycans containing α-D-glucopyranose as their sole structural unit. Unbranched, helix-prone amylose contains between 200 and 1,000 glucose molecules linked by (α1 —> 4)-glycosidic bonds, whereas amylopectin additionally features (α1 —> 6)-branch points (roughly 1 per 25 (α1 —> 4)-glycosidic bonds) and, with its 2,000 to 10,000 monomers, is vastly larger than amylose. The specific color reaction for starch—yielding a blue coloration with iodine/potassium iodide solution—relies on the inclusion of iodine molecules into the amylose helix. Glycogen, the principal storage carbohydrate of bacteria, Algae, and fungi, possesses a structure similar to that of amylopectin, but exhibits a higher degree of branching (approximately 1:14).

Starch synthesis is initiated by the synthesis of amylopectin. This process involves starch synthase and the branching enzyme (Q-enzyme). Starch synthase transfers α-D-glucopyranose from ADP-glucose to the non-reducing end of an (α1 —> 4)-glucan chain, thereby establishing an (α1—>4)-glycosidic bond. The branching enzyme operates as a transglycosylase that cleaves an oligomer of 5 to 7 glucose units from the non-reducing end of an (α1 —> 4)-glucan chain and re-attaches it internally via an (α1 —> 6)-glycosidic bond. Through the cooperative action of these two Enzymes, a branched amylopectin molecule grows outward from its reducing end (see Fig. 6.130). Amylose is subsequently formed from amylopectin when isoamylase cleaves the (α1 —> 6)-branches; this enzyme is also referred to as a "debranching" enzyme. The ratio of amylose to amylopectin (≈ 10–30% : 70–90%) is genetically determined and can consequently be modified through breeding as well as Introduction/32.html">Genetic Engineering.

Widely distributed, soluble Reserve Polysaccharides stored in vacuoles are known as Fructans. These heteroglycans contain, alongside β-D-fructofuranose, a single molecule of α-D-glucopyranose per molecule and are found, for instance, in members of the Asteraceae family (inulin and inulin-type fructans, see Fig. 1.20) as well as in Poaceae and other monocots (phlein and phlein-type fructans, see Fig. 1.20). It is presumed that fructans are synthesized within vacuoles using sucrose as a precursor.

Sucrose is transported into the vacuoles, while the glucose released during synthesis is shuttled back to the cytoplasm, where it is re-utilized in sucrose synthesis.

Starch mobilization occurs via either phosphorolysis or Hydrolysis (Fig. 6.131). Starch phosphorylase cleaves glucose-1-phosphate from the (α1 —> 4)-glucan chain, introducing a phosphate group directly into the glycosidic bond at its non-reducing end. Amylose can be degraded completely,

whereas amylopectin is degraded only as far as the branch points, yielding "limit dextrins."

Fig. 6.131. Phosphorolytic and hydrolytic Cleavage of starch. Both assimilatory and reserve starch are degraded through the concerted action of starch phosphorylase, amylases, and isoamylase. Phosphorolytic cleavage is more energetically favorable because the energy of the glycosidic bond is conserved within the phosphorylated sugar, glucose-1-phosphate (G-1-P). Hydrolytic degradation of reserve starch predominates in the endosperm of cereal grains

The hydrolytic breakdown of starch is catalyzed by amylases. Alpha-amylases are endoamylases that attack amylose and amylopectin molecules internally, capable of breaking down starch into the Disaccharides maltose (see Fig. 1.19) or isomaltose—Glcp(α1 —> 6)glcp—by bypassing the (α1 —> 6)-glycosidic bonds. In contrast to these ubiquitous amylases, β-amylases are found exclusively in plants. These exoamylases cleave maltose units from the non-reducing end of the chain and can completely degrade amylose, whereas amylopectin is broken down only as far as "limit dextrins." The (α —> 6)-glycosidic bonds of limit dextrins are cleaved by isoamylases, while the disaccharides maltose and isomaltose are split by maltases. Through this hydrolytic pathway, starch is completely degraded to glucose. Hydrolysis is less energetically favorable than phosphorolysis because the energy of the glycosidic bonds is simply lost in the process, rather than being conserved as in glucose-1-phosphate.

In many Tissues, starch is degraded both phosphorolytically and hydrolytically; this also applies to the nighttime breakdown of assimilatory starch. Hydrolytic starch degradation predominantly takes place in the endosperm during the germination of starchy cereal grains. Here, the synthesis of α-amylases is induced in the aleurone layer in response to a hormonal signal (gibberellin—see Fig. 7.55) originating from the embryo.

6.17.2. Lignin

Along with cellulose, lignin is a major constituent of wood. Quantitatively, it is the most abundant organic polymer in nature after cellulose (biosynthetic production is about 2 • 10¹² tonnes per year, compared with 2 • 10¹¹ tonnes for cellulose). During lignification, lignin is polymerized within the cellulosic framework of the secondary cell walls. Lignin, cellulose, and other cell wall components become covalently linked in the process. Lignin polymerization is a free-radical reaction in which enzymes control radical generation, but not their subsequent coupling. Consequently, lignin has a stochastic composition and forms a massive, highly rigid polymer that withstands compression from all directions. Combined with the tensile strength of cellulose, it imparts exceptional mechanical strength to wood (sharing a similar structural "architecture" with reinforced concrete).

The monomeric Building Blocks of lignin are phenylpropanoids, specifically cinnamyl alcohols (Fig. 6.132), derived from cinnamic acid derivatives (see Fig. 6.115) via the reduction of a carboxyl group. This reduction begins with the activation of precursors—p-coumaric, ferulic, and sinapic acids—into their respective coenzyme A thioesters (see Fig. 6.117). Subsequent cleavage of coenzyme A by cinnamoyl-CoA reductase converts them into cinnamyl aldehydes, which are then reduced to cinnamyl alcohols by cinnamyl Alcohol dehydrogenase (CAD); NADPH + H⁺ serves as the reducing agent for both enzymes. The substrate Specificity of CAD likely determines the monomer composition of lignin in different plant species. The angiosperm enzyme reduces all three cinnam aldehydes, whereas sinapaldehyde is a poor substrate for gymnosperm CAD. The lignin of pteridophytes and gymnosperms is characterized by a predominance of coniferyl alcohol and only minor amounts of the other two alcohols. In dicotyledonous angiosperms, coniferyl and sinapyl alcohols occur in nearly equal amounts, with p-coumaryl alcohol present only in trace quantities. The lignin of monocots (especially grasses) contains a substantial proportion of p-coumaryl alcohol alongside the other two components; thus, the characteristic methoxyl group (—OCH₃) content of these components serves as a key parameter for identifying the botanical origin of lignin.

Different tissues within the same plant—such as periderm versus wood, or late (summer) versus early (spring) wood—can also exhibit variations in lignin composition. For instance, the methoxyl content is higher in late oak wood than in early wood.

Dehydrogenative free-radical polymerization of lignin takes place extracellularly. Cinnamyl alcohol precursors, which can be stored in a soluble form as β-glucosides—glucocoumaryl alcohol, coniferin, and syringin (see Fig. 6.132)—are secreted into the cell wall region, where the free alcohols are released by β-glucosidase. Radical formation is mediated by cell-wall peroxidases using H₂O₂ as a cosubstrate (Fig. 6.133). The synthesized lignin, whose schematic structure is shown in Fig. 6.134, consists of monomeric units linked together by a variety of bonds. This reflects the numerous Resonance structures of the generated radicals (see Fig. 6.133). Randomly distributed carbonyl residues within the lignin polymer (shown in Fig. 6.134) are responsible for the characteristic red coloration produced when lignin reacts with phloroglucinol and Hydrochloric acid (due to hemiacetal formation between carbonyl residues and phenolic hydroxyl groups).

Fig. 6.132. Activation and reduction of cinnamic acids, which serve as lignin precursors. Cinnamyl alcohols are exported from The Cell as β-D-glucopyranosides.

Fig. 6.133. Radical formation in lignin precursors via The oxidation of cinnamyl alcohols (e.g., coniferyl alcohol) by cell-wall-bound peroxidases. Unpaired electrons are indicated by dots.

Fig. 6.134. Schematic model of spruce lignin according to Freudenberg, illustrating potential linkage patterns between monomer units. The molecule should be visualized in three dimensions. The aryl-ether bond between the β-carbon of the side chain and the aromatic ring of an adjacent unit (indicated by arrows) represents a vulnerable site during fungal degradation of lignin. The histochemical detection of lignin using acidic phloroglucinol is based on hemiacetal formation (see Fig. 1.18) upon reaction with carbonyl groups in the lignin polymer.

Lignification of the cell wall proceeds in three stages:

✵ Deposition of lignin in the cell corners and middle lamella following the completion of pectin accumulation in the primary wall (see 2.2.7.4);

✵ Gradual lignification of the S2 layer of the secondary wall (see Fig. 2.74, B);

✵ Major lignification following the assembly of cellulose microfibrils in the S3 layer.

The lignin composition varies across these three zones.

The degradation of lignin is primarily driven by white-rot fungi. It is an overall aerobic, energy-demanding process that proceeds very slowly. Key players include the ligninolytic oxygenase ("ligninase"), an O₂- and H₂O₂-dependent heme-containing peroxidase that primarily catalyzes the oxidative cleavage of C—C bonds, alongside a depolymerizing enzyme that mainly cleaves aromatic ether bonds (Fig. 6.134). The remarkably low rate of this process (evidenced by fallen logs persisting in forests for years) demonstrates that lignin—owing to its random composition, diverse bonding patterns, and energy-poor yet highly stable aromatic core—serves as an exceptionally effective structural barrier against microbial invasion.

6.17.3. Cutin and Suberin

Cutin and suberin are related lipophilic Biopolymers. Impermeable to gases and water, they form protective barriers that shield plants from pathogen penetration. Biosynthetically, they are derived from Fatty acids, specifically palmitic and stearic acids.

Cutin, along with cell-wall glycans, is the primary constituent of the plant cuticle, whose outer surface is coated with a layer of wax. Waxes are complex monoesters formed from long-chain Fatty Acids and long-chain monohydroxyalkanes, which tend to form lamellar layers rather than cross-linked polymers. Cutin, by contrast, is a polyester composed of heavily hydroxylated fatty acids—predominantly 10,16-dihydroxystearic and 9,10,16-trihydroxystearic acids—with Phenolic Compounds incorporated as minor auxiliary components.

The fatty acid constituents of suberin originate from stearic acid. They are synthesized from very long-chain fatty acids (up to C₃₀), very long-chain omega-hydroxyalkanes (up to C₂₀), and dicarboxylic acids. These compounds are esterified with one another, and particularly with the aliphatic hydroxyl groups of cinnamyl alcohols (mainly p-coumaryl alcohol). Phenylpropanoid units couple in a manner similar to lignin; thus, suberin can be viewed as a lignin-like core whose free aliphatic hydroxyl groups are esterified with very long-chain acyl components. Suberin, alongside lignin, occurs in the Casparian strips of ROOT endodermis (see 6.3.3) and, together with waxes, acts as a major structural component of the cell walls in corked tissues (see 3.2.2.2).

6.17.4. Storage Proteins

Like Carbohydrates and Lipids, proteins serve as essential storage reserves in plants. Storage proteins are predominantly localized in the seed endosperm (e.g., in the aleurone layer of cereal grains), in storage cotyledons (e.g., in legumes), as well as in vegetative storage Organs (such as roots and tubers) and stem storage tissues (e.g., phloem parenchyma and cambium). In terms of amino acid Composition and Structure, storage proteins generally differ markedly from enzymatic and structural proteins, with numerous molecular variants often occurring even within a single species. A Brief Overview of the main types follows.

Based on their solubility, cereal storage proteins are classified into prolamins (soluble in 60–80% aqueous alcohol) and glutelins (soluble in dilute acids or alkalis); however, these groups share a common evolutionary origin (and are now often collectively referred to as prolamins) and consist of mixtures of diverse subunits partially cross-linked by disulfide bridges. The biosynthesis of these subunits, as well as their initial aggregation, takes place in The Endoplasmic reticulum (ER). The resulting protein bodies destined for storage are pinched off from the rough ER as membrane-bounded, protein-dense vesicles.

The prolamins include gliadin and glutenin in wheat and rye; their presence in flour largely determines the baking quality of these cereals.

Most storage proteins in other plant species belong to the globulins. Unlike albumins, globulins are insoluble in distilled water but dissolve in dilute salt solutions, from which they can be precipitated at higher ionic strengths (e.g., in semi-saturated ammonium sulfate). Globulins include legumins and vicilins, which are the major storage proteins in legumes. Legumins are hexameric complexes whose monomers consist of heterodimers comprising an α-chain and a β-chain covalently linked by Disulfide Bonds. Vicilins are trimers whose monomers consist of a single polypeptide chain with an Amino Acid Sequence homologous to that of legumins; unlike legumins, however, vicilins are glycosylated. Globulin biosynthesis occurs in the ER, from where the storage proteins are transported to storage vacuoles via the Golgi apparatus (where glycosylation reactions take place if needed). Finally, the protein-storing vacuoles fragment into membrane-bound protein bodies. Legume seeds can accumulate storage proteins up to 40% of their dry weight.

The Amino Acid Composition of storage proteins is, in most cases, suboptimal for Human Nutrition. For instance, legume storage Proteins are very low in Methionine, whereas cereal prolamins largely lack Lysine and are also deficient in Tryptophan and Threonine. Consuming an exclusively plant-based diet high in cereal grains can lead to severe malnutrition, particularly in children, because The Human Body cannot synthesize the aforementioned Amino Acids on its own. Genetic engineering techniques have successfully brought The amino acid composition of seed storage proteins into closer alignment with human nutritional requirements.

The mobilization of storage proteins, such as during seed germination, occurs via hydrolysis involving various proteinases. Endopeptidases cleave peptide bonds within the protein molecule, while exopeptidases act from the ends: Carboxypeptidases from the carboxy terminus and aminopeptidases from the amino terminus. The Cleavage of disulfide bridges, at least in cereals, likely involves reduced thioredoxin.

Protein Hydrolysis products—amino acids—are either utilized in Protein Synthesis (for example, to meet the demand for enzymes during seed germination) or undergo further degradation if they are not required for protein synthesis (such as those amino acids that happen to be structurally present in storage proteins in amounts greater than the average requirements of protein synthesis). Consequently, they are converted into 2-ketoacids by transaminases. The amino nitrogen is transferred to other 2-ketoacids, such as 2-ketoglutaric acid, yielding glutamate (see 6.6.1). In most cases, the resulting 2-ketoacids, through several enzyme-regulated steps, become intermediates of glycolytic degradation or the nitrate cycle (Krebs cycle). Naturally, the Catabolism of individual amino acids is much better understood in bacteria than in plants (Fig. 6.135).

Fig. 6.135. Points of entry of amino acid carbon skeletons into Pyruvate synthesis and the citrate cycle (Krebs cycle), as identified through experiments with bacteria

Some storage proteins, especially in seeds, additionally serve as a defense against animal consumption. Among these are Lectins. These are sugar-binding proteins or Glycoproteins often found in high concentrations in seeds, particularly legumes. Lectins bind specifically to particular sugar residues, even within glycoproteins or polysaccharides. This property accounts for the characteristic agglutination of erythrocytes used for their detection. An older term for lectins is phytohemagglutinins. In the intestine, lectins bind to surface glycoproteins and cause functional disorders in the digestive tract. Examples of lectins include the well-studied concanavalin A from the jack bean Canavalia ensiformis and trifolin from Trifolium repens, which, like other surface lectins on legume roots, is believed to play a role in the specific binding of mycorrhizae during the establishment of root-nodule Symbiosis.

Another group of storage proteins with a defensive function are proteinase inhibitors, which occur in the storage organs of many plants as well as in major staple foods (e.g., legume seeds, potatoes). They primarily inhibit proteinases of animal or bacterial origin and most likely play a role in protecting the plant against herbivores and pathogens. Consequently, potatoes and legume seeds are safe for consumption only when cooked (due to thermal Denaturation of the proteins). Along with the constitutive proteins classified as storage proteins, many plants also synthesize induced proteinases upon demand (e.g., in response to herbivory) (see 9.4.1). Toxic storage proteins include ricin from Ricinus communis and amylase inhibitors from Phaseolus species. Ricin inactivates the 60S subunit of eukaryotic Ribosomes.