A plant cuticle is a protecting film covering the outermost skin layer (
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...
) of
leaves
A leaf (: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, ...
, 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.
[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]
Description

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
A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...
generation of
hornworts, and in both sporophyte and
gametophyte
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 se ...
generations of
moss
Mosses are small, non-vascular plant, non-vascular flowerless plants in the taxonomic phylum, division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Wilhelm Philippe Schimper, Schimp. 1879) may also refer to the parent group bryo ...
es.
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.
Composition
The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with soluble
waxes.
Cutin, a
polyester polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
composed of inter-esterified
omega hydroxy acids which are cross-linked by
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
and
epoxide bonds, is the best-known structural component of the cuticular membrane.
[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] The cuticle can also contain a non-saponifiable
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
polymer known as
cutan.
[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] The cuticular membrane is impregnated with cuticular waxes
[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] and covered with
epicuticular wax
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 ...
es, which are mixtures of
hydrophobic
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
aliphatic compounds, hydrocarbons with chain lengths typically in the range C16 to C36.
[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]
Cuticular wax biosynthesis
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
In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...
.
Also present are other compounds in cuticular wax which are not VLCFA derivatives, such as
terpenoids,
flavonoids, and
sterols
A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
,
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,
and concludes with the extension of these chains in the
endoplasmic reticulum
The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
of
epidermal cells.
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.
Functions
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
''Nelumbo nucifera'', also known as the pink lotus, sacred lotus, Indian lotus, or simply lotus, is one of two extant taxon, extant species of aquatic plant in the Family (biology), family Nelumbonaceae. It is sometimes colloquially called a ...
'') 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 plant
Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...
s. In
angiosperm
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit ...
s 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".
Evolution
The plant cuticle is one of a series of
innovations, together with
stoma
In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...
ta,
xylem
Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...
and
phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...
and intercellular spaces in
stem and later
leaf
A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leav ...
mesophyll tissue, that
plants
Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...
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
stoma
In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...
tal guard cells, which regulate the rates of
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, c ...
and CO
2 exchange.
References
{{Botany
Plant anatomy
Plant physiology