Suberin layers

Cutin, Suberin, and Wax. Cells exposed to air or the soil generally are covered by a cutin or suberin layer coated with wax. These polymeric sheets provide a physical barrier to penetration by some microorganisms and they also prevent leakage of water and nutrients from the cytoplasm to the plant surface. These structures have been shown to be important for resistance to microbial infection in other plants (31), but they have received limited attention in cotton. 

Since in most cases the resistance is caused by factors other than alkaloids, such as differences in cell wall structure, accumulation of phenolic compounds, or an ability to form suberin layers around the infection point, it seems that alkaloids have never played an important role in protecting plants against fungal and bacterial infections. Most alkaloids are without effect on the development of adapted, as well as foreign, fungi in experiments in vitro. Therefore, it seems justified to assume that in cases in which an alkaloid-rich plant is not infected by a certain parasite, the reason for its immunity is not the presence of alkaloids. 

Suberized layers in plant roots have been ultrastructurally characterized in detail (457). The characteristic lamellar appearance has been observed in the en-dodermal cell walls of the roots of several plants (286, 309, 381, 403, 495). Similarly, suberin layers have been observed in hypodermal and epidermal cells of plant roots (77, 78, 122, 174, 334, 365, 366, 380, 498). Lamellar suberin has also been ultrastructurally characterized at various internal locations in the plant such as the bundle sheath of grasses (51, 118, 173, 331, 332), the chalazal region of the seedcoat (111), and in the cell wall of crystal idioblasts (441, 479, 480) (Fig. 6.4.3). 

Juglans regia LM Thick suberin layer presents strong diffusion barrier 18... 

The conquest of the land by plants necessitated the development of a coating, the cuticle, that would reduce water loss. Suberin and cutin vary in their proportion of fatty acids, fatty alcohols, hydroxyfatty acids, and dicarboxylic acids. The cuticle is synthesized and excreted by the epidermis of aerial portions of the plant, such as the primary stems, leaves, flower organs, and fruits. The two major hydrophobic layers that contribute to the cuticle are composed of phenolic molecules combined with lipid polymers. Cutin is a polymer found in the outer cell wall of the epidermis, which is... 

Cork cells Tabular with all walls suberized occur in thick layers on the outer surfaces of older stems and roots Secrete a fatty substance, suberin, into the walls, suberin renders cork cells waterproof and helps protect the tissues beneath... 

Many cell walls have layers in the outer regions of the wall that carry lipid material. These are cutin, suberin, and waxes (67). How these are transported to the outside of the cell wall is not known. Pores have not been found, nor has a volatile lipid solvent been detected that would carry the lipid through the hydrophilic wall. 

Suberin is a composite of polymeric phenylpropanoids and ester-linked long chain fatty acids and alcohols and consists of a hydrophobic layer attached to the cell walls of roots, bark and the vascular system (8,10). The phenylpropanoid portion of suberin purportedly has a lignin-like structure to which both aliphatic domains and hydroxycinnamic acids are esterified. 

It therefore follows that when isolated lignins (and suberins) are examined and subsequent structural representations are proposed, critical information on native structure has already been lost, e.g., as regards the extent of polymer modification during removal from the cell wall, and the effect of mixing polymers from the various cell wall layers from which they originated. For these reasons, all current representations of native lignin (and suberin) structure should be viewed with caution until such questions are satisfactorily resolved. 

Surface lipids of plants. The thick cuticle (Fig. 1-6) that covers the outer surfaces of green plants consists largely of waxes and other lipids but also contains a complex polymeric matrix of cutin (stems and leaves) or suberin (roots and wound surfaces).135/135a Plant waxes commonly have C10 - C30 chains in both acid and alcohol components. Methyl branches are frequently present. A major function of the waxes is to inhibit evaporation of water and to protect the outer cell layer. In addition, the methyl branched components may inhibit enzymatic breakdown by microbes. Free fatty acids, free alcohols, aldehydes, ketones, 13-dike tones, and alkanes are also present in plant surface waxes. Chain lengths are usually C20 - C35.136 Hydrocarbon formation can occur in other parts of a plant as well as in the cuticle. Thus, normal heptane constitutes up to 98% of the volatile portion of the turpentine of Pin us jeffreyi.81... 

Suberins. The cork cells in the outer bark contain polyestolides or su-berins. The suberin content in the outer layer of the cork oak bark (cork) is especially high and amounts to 20-40% in the periderm of birch bark. Polyestolides are complicated polymers composed of co-hydroxy monobasic acids which are linked together by ester bonds. In addition, they contain a,/3-dibasic acids esterified with bifunctional alcohols (diols) as well as ferulic and sinapic acid moieties. The chain lengths vary but suberins are enriched with molecules having 16 and 18 carbon atoms. There are also double bonds and hydroxyl groups through which ester and ether cross-links are possible. The outer layer of the epidermis contains so-called cutin, which is heavily branched and has a structure similar to suberin. 

Endodermis of one layer of endodermal cells whose walls are extremely thickened through the infiltration of suberin and lignin. 

Cotton strains and species with green fiber have lamellar layers (up to 26) of suberin and wax deposited alternately with cellulose during formation of secondary walls in the epidermal cells, including fiber cells, of the seed coat. Seeds of cotton with green lint are less permeable to water than those with white lint, indicating that suberin may be involved in regulating water uptake by seeds. 

Sediments of higher plant remains like seeds, pollen, cuticle, bark, and wood remains, indicated that their fossilization was comprised of highly resistant biopolymers like cutan, suberin, and lignin. In most cases only the seed coats are found in the geological record, because the outer layers of seeds contain resistant compounds to protect the genetic material against physical and chemical processes such as temperature and humidity changes and bacterial... 

Cutin. Structural component of the outer lipophilic protective layer (cuticle) of the aerial parts of plants, especially leaves. Suberin serves similar functions in roots and bark. C. is a natural polyester, formed enzymatically from hydroxyfatty acids with 16 and 18 C atoms. o+Hydroxy- and dihydroxyfatty acids, e.g., 10,16-dihydroxypalmitic acid, as well as epoxy- and oxofatty acids, and a,o>-dicarboxylic acids are the main components of cutin. Cutinases (C.-cleaving enzymes) occur especially in pollen and in plant-pathogenic fungi, e.g., Fusarium solani (while rot in potatoes). 

Suberin, suberic acid. Suberin (Latin suber=cork) in plants forms the lipophilic protective layer on the outer dermal tissue (cork) of trunks and branches as well as roots and other underground plant parts (see cutin). S. is also formed in plant tissues during wound healing. S. is a mixture of polyesters, the main components are a,w-dicarboxylic acids, e.g., docosane-dioic acid (see Japan wax) and tetracosanedioic acid (C24H46O4, Mr 398.63, mp. 127.1 °C), tt>- hydroxy fatty acids, fatty acids, and fatty alcohols as well as phenolic compounds. 

Suberin impregnates the walls of cork cells and in layers covers bark, tubers, roots, wound peridorm and bundle sheaths of monocotyledons to provide a protective mantle impervious to liquids and gases. The associated waxes are not as well studied as those from cutin but do exhibit certain periodicities. The hydrocarbons have a broader distribution of chain lengths than the cuticular material, a predominance of shorter carbon chains and a higher proportion of even-length chains. In addition, no single alkane predominates usually several are present in similar proportions " . 

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