A plant cuticle is a protecting film covering the epidermis of leaves, young shoots and other aerial plant organs without periderm. It consists of lipid and hydrocarbon polymers impregnated with wax, and is synthesized exclusively by the epidermal cells.
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 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. The cuticle can also contain a non-saponifiable hydrocarbon polymer known as Cutan. The cuticular membrane is impregnated with cuticular waxes and covered with epicuticular waxes, which are mixtures of hydrophobic aliphatic compounds, hydrocarbons with chain lengths typically in the range C16 to C36.
Cuticular wax biosynthesis
Current scientific understanding of cuticular wax is limited, yet considerable chemicals and pathways have been identified. 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. Also to note are other compounds in cuticular wax which are not VLCFA derivatives, such as terpenoids, flavonoids, and sterols, and thus have different synthesis pathways than that which is discussed here.
The first step of the cuticular wax biosynthesis pathway is the formation of VLCFAs, which occurs in the endoplasmic reticulum of epidermal cells. VLCFA synthesis begins with the de novo biosynthesis of C16 or C18 acyl chains, and concludes with the extension of these chains. An important catalyzer thought to be in this process is the fatty acid elongase (FAE) complex.
To form cuticular wax components, VLCFAs are modified through either two identified pathways, an acyl reduction pathway or a decarbonylation pathway. In the acyl reduction pathway, a reductase converts VLCFAs into primary alcohols, which can then be converted to wax esters through a wax synthase. In the decarbonylation pathway, aldehydes are produced and decarbonylated to form alkanes, and can be subsequently oxidized to form secondary alcohols and ketones. The wax biosynthesis pathway ends with the transportation of the wax components from the endoplasmic reticulum to the epidermal surface.
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. 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). The lotus effect has applications in biomimetic technical materials.
Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica 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 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. 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.
- Kolattukudy, PE (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stresses. In: Plant Cuticles. Ed. by G. Kerstiens, BIOS Scientific publishers Ltd., Oxford, pp 83-108
- Budke, J.M., Goffinet, B. and Jones, C.S. (2013). Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica. Annals of Botany doi:10.1093/aob/mct033
- Holloway, PJ (1982) The chemical constitution of plant cutins. In: Cutler, DF, Alvin, KL and Price, CE The Plant Cuticle. Academic Press, pp. 45-85
- Stark, RE and Tian, S (2006) The cutin biopolymer matrix. In: Riederer, M & Müller, C (2006) Biology of the Plant Cuticle. Blackwell Publishing
- Tegelaar, EW, et al. (1989) Scope and limitations of several pyrolysis methods in the structural elucidation of a macromolecular plant constituent in the leaf cuticle of Agave americana L., Journal of Analytical and Applied Pyrolysis, 15, 29-54
- Jetter, R, Kunst, L & Samuels, AL (2006) Composition of plant cuticular waxes. In: Riederer, M & Müller, C (2006) Biology of the Plant Cuticle. Blackwell Publishing, 145-181
- Baker, EA (1982) Chemistry and morphology of plant epicuticular waxes. In: Cutler, DF, Alvin, KL and Price, CE The Plant Cuticle. Academic Press, 139-165
- Yeats, Trevor H.; Rose, Jocelyn K.C. (September 2013). "The Formation and Function of Plant Cuticles". Plant Physiology. 163 (1): 5–20. doi:10.1104/pp.113.222737. ISSN 0032-0889. PMC 3762664. PMID 23893170.
- Kunst, L; Samuels, A. L (2003-01-01). "Biosynthesis and secretion of plant cuticular wax". Progress in Lipid Research. 42 (1): 51–80. doi:10.1016/S0163-7827(02)00045-0. ISSN 0163-7827. PMID 12467640.
- Suh, Mi Chung; Kim, Hae Jin; Kim, Hyojin; Go, Young Sam (2014-04-01). "Arabidopsis Cuticular Wax Biosynthesis Is Negatively Regulated by the DEWAX Gene Encoding an AP2/ERF-Type Transcription Factor". The Plant Cell. 26 (4): 1666–1680. doi:10.1105/tpc.114.123307. ISSN 1040-4651. PMC 4036578. PMID 24692420.
- Barthlott, W & Neinhuis, C (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1-8
- Freeman, S (2002) Biological Science. Prentice-Hall, Inc., New Jersey
- Raven, J.A. (1977) The evolution of vascular land plants in relation to supracellular transport processes. Advances in Botanical Research, 5, 153-219