Roots, suberin layers

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). 

Suberin is laid down in various internal locations in order to seal off specific regions of the plant (234). In the primary development of plant roots, suberin is deposited in the Casparian band of endodermal cells (45, 103, 162, 317, 364, 367, 369, 474, 483). The presence of suberin in the Casparian band of the endodermis of Sorghum bicolor was shown chemically by depolymerization (244). Suberized layers are also found in the mestome sheath and bundle sheath cell walls of grasses (52, 61, 108, 118, 171, 172, 331, 368), and these layers have been chemically characterized in Zea mays and Secale cereale (114, 146). 

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... 

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. 

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... 

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 " . 

Clarkson D T, Robards A W, Stephens J E, Stark M 1987 Suberin lamellae in the hypodermis of maize Zea mays) roots development and factors affecting the permeability of hypodermal layers. Plant Cell Environ 10 83-93... 

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