A plant cuticle is a protecting film covering the outermost skin layer (epidermis) of leaves, young shoots and other aerial plant organs (aerial here meaning all plant parts not embedded in soil or other substrate) that have no periderm . The film consists of lipid and hydrocarbon polymers infused with wax, and is synthesized exclusively by the epidermal cells. [1]
The plant cuticle is a layer of lipid polymers impregnated with waxes that is present on the outer surfaces of the primary organs of all vascular land plants. It is also present in the sporophyte generation of hornworts, and in both sporophyte and gametophyte generations of mosses [2] The plant cuticle forms a coherent outer covering of the plant that can be isolated intact by treating plant tissue with enzymes such as pectinase and cellulase.
The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with soluble waxes. Cutin, a polyester polymer composed of inter-esterified omega hydroxy acids which are cross-linked by ester and epoxide bonds, is the best-known structural component of the cuticular membrane. [3] [4] The cuticle can also contain a non-saponifiable hydrocarbon polymer known as Cutan. [5] The cuticular membrane is impregnated with cuticular waxes [6] and covered with epicuticular waxes, which are mixtures of hydrophobic aliphatic compounds, hydrocarbons with chain lengths typically in the range C16 to C36. [7]
Cuticular wax is known to be largely composed of compounds which derive from very-long-chain fatty acids (VLCFAs), such as aldehydes, alcohols, alkanes, ketones, and esters. [8] [9] Also present are other compounds in cuticular wax which are not VLCFA derivatives, such as terpenoids, flavonoids, and sterols, [9] and thus have different synthetic pathways than those VLCFAs.
The first step of the biosynthesis pathway for the formation of cuticular VLCFAs, occurs with the de novo biosynthesis of C16 acyl chains (palmitate) by chloroplasts in the mesophyll, [1] and concludes with the extension of these chains in the endoplasmic reticulum of epidermal cells. [9] An important catalyzer thought to be in this process is the fatty acid elongase (FAE) complex. [8] [9] [10]
To form cuticular wax components, VLCFAs are modified through either two identified pathways, an acyl reduction pathway or a decarbonylation pathway. [9] In the acyl reduction pathway, a reductase converts VLCFAs into primary alcohols, which can then be converted to wax esters through a wax synthase. [9] [10] In the decarbonylation pathway, aldehydes are produced and decarbonylated to form alkanes, and can be subsequently oxidized to form secondary alcohols and ketones. [8] [9] [10] The wax biosynthesis pathway ends with the transportation of the wax components from the endoplasmic reticulum to the epidermal surface. [9]
The primary function of the plant cuticle is as a water permeability barrier that prevents evaporation of water from the epidermal surface, and also prevents external water and solutes from entering the tissues. [11] In addition to its function as a permeability barrier for water and other molecules (prevent water loss), the micro and nano-structure of the cuticle have specialised surface properties that prevent contamination of plant tissues with external water, dirt and microorganisms. Aerial organs of many plants, such as the leaves of the sacred lotus ( Nelumbo nucifera ) have ultra-hydrophobic and self-cleaning properties that have been described by Barthlott and Neinhuis (1997). [12] The lotus effect has applications in biomimetic technical materials.
Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica [2] and in the sporophytes of all vascular plants. In angiosperms the cuticle tends to be thicker on the top of the leaf (adaxial surface), but is not always thicker. The leaves of xerophytic plants adapted to drier climates have more equal cuticle thicknesses compared to those of mesophytic plants from wetter climates that do not have a high risk of dehydration from the under sides of their leaves.
"The waxy sheet of cuticle also functions in defense, forming a physical barrier that resists penetration by virus particles, bacterial cells, and the spores and growing filaments of fungi". [13]
The plant cuticle is one of a series of innovations, together with stomata, xylem and phloem and intercellular spaces in stem and later leaf mesophyll tissue, that plants evolved more than 450 million years ago during the transition between life in water and life on land. [11] Together, these features enabled upright plant shoots exploring aerial environments to conserve water by internalising the gas exchange surfaces, enclosing them in a waterproof membrane and providing a variable-aperture control mechanism, the stomatal guard cells, which regulate the rates of transpiration and CO2 exchange.
A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier. Another vital role of the cell wall is to help the cell withstand osmotic pressure and mechanical stress. While absent in many eukaryotes, including animals, cell walls are prevalent in other organisms such as fungi, algae and plants, and are commonly found in most prokaryotes, with the exception of mollicute bacteria.
Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.
A gametophyte is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The sporophyte can produce haploid spores by meiosis that on germination produce a new generation of gametophytes.
Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries, and in nanotechnology.
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low viscosity liquids. Waxes are insoluble in water but soluble in nonpolar organic solvents such as hexane, benzene and chloroform. Natural waxes of different types are produced by plants and animals and occur in petroleum.
Suberin is a lipophilic, complex polyester biopolymer of plants, composed of long-chain fatty acids called suberin acids and glycerol. Suberin, interconnected with cutins and lignins, also complex macromolecules, form a protective barrier in the epidermal and peridermal cell walls of higher plants. Suberins and lignins are considered covalently linked to lipids and carbohydrates, respectively, and lignin is covalently linked to suberin, and to a lesser extent, to cutin. Suberin is a major constituent of cork, and is named after the cork oak, Quercus suber. Its main function is as a barrier to movement of water and solutes.
Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants, the other being cutan. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, which assist in cuticle structure. Cutan, the other major cuticle polymer, is much more readily preserved in fossil records. Cutin consists of omega hydroxy acids and their derivatives, which are interlinked via ester bonds, forming a polyester polymer of indeterminate size.
An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable of punching through even Mylar.
The epidermis is a single layer of cells that covers the leaves, flowers, roots and stems of plants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulates gas exchange, secretes metabolic compounds, and absorbs water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such as potato tubers produce a secondary covering called the periderm that replaces the epidermis as the protective covering.
Epicuticular wax is a waxy coating which covers the outer surface of the plant cuticle in land plants. It may form a whitish film or bloom on leaves, fruits and other plant organs. Chemically, it consists of hydrophobic organic compounds, mainly straight-chain aliphatic hydrocarbons with or without a variety of substituted functional groups. The main functions of the epicuticular wax are to decrease surface wetting and moisture loss. Other functions include reflection of ultraviolet light, assisting in the formation of an ultra-hydrophobic and self-cleaning surface and acting as an anti-climb surface.
Lipid metabolism is the synthesis and degradation of lipids in cells, involving the breakdown and storage of fats for energy and the synthesis of structural and functional lipids, such as those involved in the construction of cell membranes. In animals, these fats are obtained from food and are synthesized by the liver. Lipogenesis is the process of synthesizing these fats. The majority of lipids found in the human body from ingesting food are triglycerides and cholesterol. Other types of lipids found in the body are fatty acids and membrane lipids. Lipid metabolism is often considered the digestion and absorption process of dietary fat; however, there are two sources of fats that organisms can use to obtain energy: from consumed dietary fats and from stored fat. Vertebrates use both sources of fat to produce energy for organs such as the heart to function. Since lipids are hydrophobic molecules, they need to be solubilized before their metabolism can begin. Lipid metabolism often begins with hydrolysis, which occurs with the help of various enzymes in the digestive system. Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals. The second step after the hydrolysis is the absorption of the fatty acids into the epithelial cells of the intestinal wall. In the epithelial cells, fatty acids are packaged and transported to the rest of the body.
The enzyme cutinase is a member of the hydrolase family. It catalyzes the following reaction:
A xerophyte is a species of plant that has adaptations to survive in an environment with little liquid water. Examples of xerophytes include cacti, pineapple and some gymnosperm plants. The morphology and physiology of xerophytes are adapted to conserve water during dry periods. Some species called resurrection plants can survive long periods of extreme dryness or desiccation of their tissues, during which their metabolic activity may effectively shut down. Plants with such morphological and physiological adaptations are said to be xeromorphic. Xerophytes such as cacti are capable of withstanding extended periods of dry conditions as they have deep-spreading roots and capacity to store water. Their waxy, thorny leaves prevent loss of moisture.
A cuticle, or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homologous, differing in their origin, structure, function, and chemical composition.
N-Acylphosphatidylethanolamines (NAPEs) are hormones released by the small intestine into the bloodstream when it processes fat. NAPEs travel to the hypothalamus in the brain and suppress appetite. This mechanism could be relevant for treating obesity.
CYP4F22 is a protein that in humans is encoded by the CYP4F22 gene.
Omega hydroxy acids are a class of naturally occurring straight-chain aliphatic organic acids n carbon atoms long with a carboxyl group at position 1, and a hydroxyl at terminal position n where n > 3. They are a subclass of hydroxycarboxylic acids. The C16 and C18 omega hydroxy acids 16-hydroxy palmitic acid and 18-hydroxy stearic acid are key monomers of cutin in the plant cuticle. The polymer cutin is formed by interesterification of omega hydroxy acids and derivatives of them that are substituted in mid-chain, such as 10,16-dihydroxy palmitic acid. Only the epidermal cells of plants synthesize cutin.
Russeting or russetting is an abnormality of fruit skin which manifests in russet-colored (brownish) patches that are rougher than healthy skin. It is a common feature in apples and pears. Russeting is typically an undesirable trait, which reduces the storage life of fruits and makes their appearance unattractive to consumers, although some cultivars, so-called russet apples, are appreciated for the feature.
Cuticle analysis, also known as fossil cuticle analysis and cuticular analysis, is an archaeobotanical method that uses plant cuticles to reconstruct the vegetation of past grassy environments. Cuticles comprise the protective layer of the skin, or epidermis, of leaves and blades of grass. They are made of cutin, a resilient substance that can preserve the shapes of underlying cells, a quality that aids in the identification of plants that are otherwise no longer visible in the archaeological record. This can inform archaeobotanists on the floral makeup of a past environment, even when surviving remains from the plants are limited. Plant cuticles have also been incorporated into other areas of archaeobotanical research based on their susceptibility to environmental factors such as pCO2 levels and stresses such as water deficit and sodium chloride exposure. Such research can help to reconstruct past environments and identify ecological events.
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