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Cuticle and Epicuticular wax
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Anatomy
National Coordinator: Prof. S.C. Bhatla, University of Delhi
Lesson: Cuticle and Epicuticular Wax
Lesson Developer: Dr. Monika Koul
Department/College: Hansraj College, University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, University of Delhi
Language Editor: Dr. Arun Kumar Maurya
Department/College: Botany Department, Dyal Singh College,
University of Delhi
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Institute of Lifelong Learning, University of Delhi
Table of Contents
Chapter: Cuticle and Epicuticular Wax
 Introduction
 Cuticle

•
Structure
•
Composition
•
Functions
Waxes
•
Chemical Composition
•
Quantification of Wax
•
Wax Biosynthesis
 Epicuticular Wax
•
Structure and Composition
•
Functions
•
Degradation of Epicuticular Wax
 Molecular Studies
 Summary
 Exercise/ Practice
 Glossary
 References/ Bibliography/ Further Reading
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Introduction
The cuticle is a superficial film or layer covering epidermal cells. It is derived from the latin
word cuticula meaning covering. Cuticle forms an extracellular hydrophobic layer that is
reported to play an important role in protecting the aerial parts of the plant from both biotic
and abiotic stress. It is found on leaves, stem, fruits and even floral organs (Fig.1). Cuticle
was discovered very early and its discovery dates back to palaeobiological timescale. Cuticle
resists decay for millions of years and is preserved over long duration. The oldest remnants
of plant cuticles were found in the fossils between the late Siluarian and the early Devonian
period. Remnants of cuticle from the vegetative parts of many primitive plants have been
found in soil sediments and other areas. Rhyniophytoids, a primitive group of land plants
are known to show depositions of cuticle on their sporangia. Palaeoecophysiology has
interpreted the simultaneous Persistence and presence of cuticle and stomata are
considered as evidence for the various types of adaptations taking place in plants in
response to the colonization on the land. The adaptations that are manifested at the
anatomical level are also taking place at various levels of organization.
Epidermis
Figure: T.S. of Nerium leaf showing cuticle
Source: http://botit.botany.wisc.edu/Anatomy/Glossary/images/nerium/cuticle-dt.jpg
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Figure 1: Diagrammatic representation of cross section of leaf showing waxy cuticle, present
on upper and lower epidermis.
Source: http://ap-bio-chs-plants.wikispaces.com/file/view/leaf.gif/140623741/leaf.gif
The nature of cuticular layer seems to be composite and heterogeneous as the
structure, thickness and chemical composition varies in different plant species, the plant
parts on which it is found and also on the environmental conditions where the plant is
found. Some anatomists consider it as a hyaline film. The thickness of which varies from a
few nanometers to micrometers. Studies have been carried out on ontogenetic development
of cuticle by morphologists and anatomists. The formation of this protective layer over the
surface of plants is considered an important evolutionary feature because it helps the plant
to retain the moisture levels in their tissues and help in coping up the water stress. As it is
highly recalcitrant material, it is seen as an interesting adaptive feature in terrestrial plants
that helps in negotiating a trade-off between water loss and uptake of water.
With the advent of newer techniques in plant histochemistry and microscopy,
numerous other roles of cuticle have been elucidated with experimental evidence. Some of
the recent studies in developmental biology and physiology reveal that cuticle has much
broader significance than just minimizing water loss. It is actually an interface between the
plant and the external environment. Cuticle plays an important role in various biotic
interactions. Deeper insights have gone into understanding of the structural and functional
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significance of this important layer and interesting revelations have been made. Studies
conducted by the scientists have clearly demonstrated that the cuticle is an important
structure in the plant that provides mechanical support and strength to the aerial organs.
The properties of the cuticle are amenable to changes and modifications and several studies
have been undertaken to study the response to changing environmental stimuli and it has
been deciphered that the cuticle properties change in response to internal and external
stimuli. Cuticular patterns, especially the surface topography has been used a tool for
identification since decades. Scanning electron micrographs show that these appear
different in different plants and can act as finger prints and are thus used as tool in plant
identification.
Cuticle
Structure
Cuticle is made up of covalently linked macromolecular scaffold of insoluble polymers
like cutin, cutan biopolymer matrix and a variety of waxes. According to the studies carried
out by Von Mohl, the cuticle consists of two different layers - the cuticle proper (CP) which
is the outermost zone and the cuticular layer which is located in between the cuticle proper
and the outer cell wall. Cuticular layer is further divided into an external cuticular layer
(ECL) and an internal cuticular layer (ICL). Cuticle proper has been believed to be composed
of waxes and cutin/cutan, while polysaccharide material is additionally present in the
cuticular layer (Fig.2). Polysaccharides in different cuticle areas have been found in the
leaves of poplar, eucalyptus, and pear in recent studies.
Esau, Martin and Juniper (1954) recognized cuticle to be made of three distinct
layers – epicuticular wax, outer cuticularized layer and inner cutinized layer. In 1972,
Sergeant came up with new theory on cuticle and he distinguished cuticle on the basis of
development into primary and secondary cuticle.
The fine structure of cuticle is based on various attributes such as the persistence or
lack of lamellae, subdivision of cuticular layers and structure of cuticular membrane.
However, biochemistry of wax has not much relevance in the basic structure. Cuticle
structure has displayed lot of diversity in the plant kingdom. Difference in size, structure
and composition between the cuticles of earlier land ancestors – such as between
bryophytes (mosses and liverworts) and vascular plants, between one plant family and the
other as well as between plants adapted to different environmental conditions and habitats
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has been observed. The structure of cuticle also varies in cultivated plants, plants growing
in the wild and plants raised in in vitro conditions.
Histochemical studies also support that cuticle is composed of two sub layers:
1. Cuticle proper: It overlies the cuticular layer. It has less quantity of polysaccharides
but is rich in wax. The cuticle proper emerges on aerial plant organs at early stages
in developmental pathway especially during epidermal cell development. According
to Sargent proposed the cuticle can be called cuticle proper or the ‘primary cuticle,
as it is analogous to the primary cell wall in plants.
2. Cuticular layer: It is a cutin rich layer that is embedded with polysaccharides.
Figure 2: (a) Diagrammatic representation of cuticular layers; (b) Ultrastructure
details of leaf epidermis Arabidopsis.
Source: http://lipidlibrary.aocs.org/plantbio/polyesters/Figure1.jpg
According to the studies conducted by biochemists to find out chemical constituents, it
has been found that it is a three dimensional polymer of carbon16 and carbon18 hydroxy fatty
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acids that are cross linked by ester and other bonds. Hydroxycinnamic acids have also been
reported in many tissues in plant systems. Dicarboxylic acids have also been reported as
major monomers in some model plants such as Arabidopsis thailiana and Brassica napus.
Cutan, the other structural component is composed of cutin monomers linked by ether and
carbon-carbon bond. It is formed after ester-bond hydrolysis of cutin. Waxes present in the
cuticle include long chain alkanes as well as fatty acids, alcohols aldehydes and ketones.
Cuticular lipids are synthesized in endoplasmic reticulum and then transported from the site
of their synthesis to plasma membrane. From here, through the outer periclinal walls these
material reaches to the cuticle proper and form aggregates there. The deposition of cuticle
on leaf surface is dependent on complex metabolic network of enzymes and transporters.
For further details visit:
http://lipidlibrary.aocs.org/plantbio/polyesters/index.htm
Functions
The fine structure of cuticle has been attributed with numerous functions such as:
(1)Water Conservation
Terrestrial plants are always challenged to control water loss and maintain water
relations in the plant system. Therefore cells have to be less turgescent and maintain
equilibrium between transpiration loss and water uptake. The movement of water between
the two main compartments symplast and apoplast is controlled by plant cuticle. Water
transport across the cuticle is a carried out by a simple mechanism of diffusion. The process
of diffusion takes place along a gradient of the chemical potential of water.
(2)Ion Conservation
For maintenance of ionic gradient in plant systems ions and polar solutes play an important
role. Cuticle, which is a highly resistant membrane, impedes the transport of ions from the
inside plant systems to external environment. Cuticle hinders uptake of some polar
substances from outer environment to inner cells. Transport of substances across the cuticle
is highly symmetric in nature and interesting revelations are being made regarding the
mechanism of transport.
(3)Gas Exchange Regulation
Cuticle limits the loss and uptake of gases and vapours across the plant and outside
environment when stomata get closed during the day time. Cuticle is considered as
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preferred pathway of exchange of various substances even under conditions when stomata
are open.
(4)Microbial Habitat
Presence of water on the plants leads to leaching of ions and polar organic solutes from the
plant cells that creates suitable conditions for the colonisation of potentially harmful
microbes such as phytopathogenic bacteria, parasitic fungi and insects. Cuticular surfaces of
aerial parts of the plants such as leaves, stem, petioles and fruits have water repellent
properties and repel water and other water-based solutions. Thus presence of cuticle
protects the aerial organs from getting attacked by the harmful microbes.
(5)Protection from UV radiation
Cuticle plays a major role in attenuation of ultraviolet radiation and protects the inner
tissues from harmful effects of UV radiations.
(6)Mechanical Strength
Cuticles attributes mechanical strength and gives support to various cellular structures
like cell walls in maintaining the structural integrity of plant tissues. Recent studies have
confirmed that cuticle plays a crucial role in development of plants. Studies carried out on
mutants have depicted that the plants with defective cuticles exhibited significant water loss
and various morphological abnormalities such as malformation of plant organs and fusion of
organs.
(7)As interface
Cuticle acts like an interface between the plants external and internal environment.
Various properties of the cuticle such as adhesion, host recognition and repellency protect
the aerial organs from harmful effects of particulate air pollutants, gases, suspended
particulate matter and other toxins that may affect the stomatal functioning.
Waxes
Chemical Composition
Waxes are complex substances composed of various chemical constituents. Wax
mixtures of many plant species are known to have number of aliphatic compounds. Mixtures
of primary n-alcohols, n-aldehydes, fatty acids as well as n-alkanes have also been
identified from the cuticular waxes. Esters of C16–C34 fatty acids and C20–C36 primary
alcohols have also been identified in the wax mixtures giving rise to homologous compounds
with chain lengths ranging from C36 to C70, and to dozens of isomers for each chain length.
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Besides the presence and availability of ubiquitous constituents, the wax mixtures of
individual plants may contain relatively high percentages of specific compounds or
compound classes or the both. Two or more secondary functional group of the taxon-specific
compounds may also be found. Cuticular wax constituents also indicate the presence of
scecondary alcohols, alkanediols and ketols. It has been found that the cuticular wax
mixtures of many plant species may contain triterpenoid constituents and these have been
detected using recent analytical techniques. Gas chromatography and Mass Spectroscopy
has been used in identification of cuticular triterpenoids and to identify and isolate isomers
with relatively small differences in structure and function. Fragmentation patterns and MS
peak intensities are less predictable for aromatic waxes than for aliphatic wax constituents.
Since the wax composition is difficult to analyse, comparison with authentic standards
available from the chemical industries is absolutely necessary. After MS interpretation,
comparison with authentic standards gives good results. Homologous series of very longchain fatty acids esterified with benzyl alcohol or phenylethyl alcohol as well as of alkyl
benzoates and other aromatic compounds have also been identified recently in cuticular
waxes. With newer analytical techniques available, there is plethora of information coming
in from various taxa on availability of new chemical molecules, conjugates and compounds
from the wax assemblies.
Quantification of Wax
Quantification of wax has started recently and the wax quantity is used as one of the
indicator parameters to study the response of plant systems to various biotic and abiotic
factors. Because, wax is generally present in minute quantities and extraction is a
cumbersome process, the quantification is being standardized. Waxes are usually quantified
in units of micrograms per square centimeter (mg/cm2).
The mass of wax per area of
cuticle surface is estimated frequently by plant scientists. Corresponding values of wax have
can be directly compared between species and organs and data can be used for interpreting
various aspects. Values for the wax loads on leaf surface and other aerial organs also give
information about the thickness of the wax layer. Thickness of wax also offers selective
advantage to the plant organs suggesting that thickness essentially is an adaptive feature
for protection against various types of biotic as well as abiotic stress. Waxes can generally
dissolve in various organic solvents and not in polar solvents like that of water. It has been
interpreted that major wax constituents consist mainly of methylene units, causing fairly
constant densities of approximately 0.8–1.0 g/cm3 for the different compounds as well as
the resulting mixture. Extensive studies have been carried out to test the effects of light,
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temperature and air humidity on growth of aerial organs. Studies have been carried on
many plant species especially some model systems such as Brassica and Arabidopsis. In
some of these model systems growth conditions, availability of light, moisture and other
factors largely influenced the wax composition of the growing tissue. Increased temperature
and decreased relative humidity caused an increase in overall wax amounts. The
composition of wax was seen to be altered as relative percentages of alkanes, ketones,
aldehydes and primary alcohols observed and quantified showed variation in almost all the
plant systems studied by the scientists. Little effect on quantity as well as quality of wax
was observed in response to changes in irradiant light energy. Studies were also conducted
to see the effect of change in transpiration rates on wax quantity and quality and results
suggested under transpiration stress there is increased wax accumulation due to feedback
loop that operates in the plant systems. The role of light has also been elucidated in some
monocot plants. Studies carried out on Hordeum vulgare leaves depicted decrease in
average chain lengths of various wax compound classes.
Studies suggest that quantity and composition of wax constituents of many species
is under the genetic control. Evidences are coming up from various stidies carried out in
laboratory conditions that wax biosynthesis is essentially under the control of genes. Under
normal circumstances, wax properties in large number of species are largely controlled by
genetic programmes rather than by environmental factors. However, there is need of
further studies on various plant systems growing under controlled as well natural conditions
to reach at some formidable conclusions.
Wax Biosynthesis
Mixtures of (synthetic) long-chain aliphatic compounds, most of the methylene groups are
packed in a dense lattice with crystalline order wax biosynthesis have been revealed by
various investigations using various analytical techniques such as fourier transform infrared
spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction and
Differential Scanning Calorimetry (DSC). Saturated very long-chain fatty acids (VLCFAs;
Kolattukudy, 1966), with chain lengths between C16 and C18 are the precursors for the
biosynthesis of aliphatic cuticular wax components, such as primary and secondary alcohols,
aldehydes, alkanes, ketones and alkyl esters. Formation of VLCFAs is a complex process
that requires the coordinated activity of several enzyme complexes that are present at
various sites in
the cell. Different
cellular compartments (plastid, cytoplasm
endoplasmic reticulum) play an important role in wax biosynthesis.
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and
De novo fatty acid
synthesis of C16 and C18 acyl chains is carried out by the well-characterized soluble fatty acid
synthase complex (FAS) localized in the plastid stroma.
Proposed pathways for synthesis of Cutin
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This biosynthetic scheme represents one possible order of reactions for the synthesis of
cutin building blocks (possibly acyl glycerols). Acyl chains are activated to coenzyme-A by
LACS, acyl-CoAs are oxidized and then esterified to glycerol-3-phosphate by GPAT enzymes.
The site of polymerization and, thus, the nature of the molecules transported remain
unknown. Only ABC transporters have been associated with cutin synthesis; a membraneInstitute of Lifelong Learning, University of Delhi
bound LTP (LTPG1) is involved in cuticular wax export, and similar proteins may have a role
in cutin synthesis. Other possible transport mechanisms include oleophilic bodies, secretory
vesicles and PM-associated ER domains (not shown in the figure). Apoplastic movement of
monomers or oligomers might be facilitated by LTPs. Polyester synthase(s), required to link
monomers to produce high molecular weight polyesters, remain unknown, although an
extracellular lipase (BDG) has been proposed as a cutin synthase, and an acyltransferase
(DCR) has been suggested to form cutin oligomers in the cytoplasm. Known or putative
enzymes are indicated in blue. (Adapted from Pollard et al., 2008).
* Gene/enzymes catalyzing these steps have not been identified.
Abbreviations: ABC, ATP binding cassette transporter; BDG, BODYGUARD; CYP, cytochrome
P450 monooxygenase; DCR, DEFECTIVE IN CUTICULAR RIDGES; EH, epoxide hydrolase;
FAH, Fatty acyl ω-hydroxylase; FAIH, Fatty acyl in-chain hydroxylase; G3P, glycerol-3phosphate; GPAT, glycerol 3-phosphate acyltransferase; HFADH, ω-hydroxy fatty acyl
dehydrogenase; HFAE, ω-hydroxy fatty acyl epoxygenase; LACS, long-chain acyl-coA
synthetase; LTP, lipid transfer proteins; LTPG, glycosylphosphatidyl-inositol (GPI)-anchored
protein; OFADH, ω-oxo fatty acyl dehydrogenase; PO, peroxygenase; PM, plasma
membrane; PS, polyester synthase.
Source: http://lipidlibrary.aocs.org/plantbio/polyesters/index.htm
Epicuticular Wax
Epicuticular wax (EW) is a form of complex wax deposition that often forms a visible
waxy layer or crust on various plant surfaces (Fig.3). For many years scientists were of the
opinion that the amount of epicuticular wax deposited on plant surfaces is dependent on
intrinsic attributes of the plant systems as well as on interplay between the various
environmental factors that include both biotic and abiotic factors. Different standard
scientific methods are used to isolate the wax and wax depositions are examined through
light and electron microscopy. Over the last few years, their properties, structure and
chemistry and composition are well studied. It has been substantiated by various studies
that Epicuticular wax composition is similar in majority of land plants.
Epicuticular wax found in bryophytes and pteridophytes contain similar compounds
to those of gymnosperms and flowering plants. This suggests that waxes have an
evolutionary significance. The wax is deposited on the surface of the cuticle proper or is
deposited within the cutin. The wax deposited on the surface is referred to as epicuticular
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wax and that deposited in cutin is known as intracuticular wax.
In many plant species
epiculticular wax is uniformly spread on the surface layers and in many other plants
epicuticular wax is deposited in the form of aggregates or crystals.
Leaves have glossy
appearance due to presence of epicuticular wax films. Wax crystals deposited on surface
layers of certain plant species have dull glossy appearance due to presence of wax crystals.
Fruits of grapes and stem of sugarcane owe their glossiness to the presence of wax.
Scanning and transmission electron microscopy has been used to understand
surface topography and internal organization of wax. Now-a-days, confocal microscopic
techniques are also being used to provide deep insights into various aspects and fine
structure of cuticle, wax and epicuticular wax in many plant species. Ultrastructural details
of the EW reveal that the wax structure of species is both complex and varied. On the
leaves and fruits of grapes, the epicuticular wax layer is developed sufficiently to scatter
light. The wax structures of sugarcane (Saccharum officinarum) are sufficiently large that
these can be easily identified under powerful hand lens or light microscope. In many
species, epicuticular wax is studied in detail using electron microscopy and interesting
observations have been made on the role of organelles in epicuticular wax biosynthesis.
The quantity of epicuticular wax may be as little as 1 μg/cm2 on leaves of some
temperate annuals, whereas in plants growing in tropical, sub-tropical and boreal habitats
the epicuticular wax is very thick. Thick epicuticular wax crusts weighing several milligrams
per square centimeter have been mechanically harvested by thrashing the dried leaves of
Fan Palm, Copernica cerifera, the source of carnauba wax, and the ouricuri palm Syagrus
coronata.
Submerged and amphibious plants do not have any traces of epicuticular wax
suggesting this as an adaptive feature. The amount of wax varies on abaxial and adaxial
leaf surfaces. Myriophyllum spp., Potamogeton crispus and some obligate submerged
aquatic and amphibious plants have little or no epicuticular wax. Lactuca sativa, B. vulgaris
and N. tabacum and some other mesophytic species have no visible wax. Rumex obtusifolia,
Stellaria media and Myosotis arvensis and many other weed species do not have visible wax
loads and wax is almost absent making them easy to wet with herbicide sprays so that
these can be eradicated easily.
Chemical investigations of epicuticular waxes suggest that these are derivatives of nacyl alkanes with chain lengths in the range C16–C35. The hydrocarbon chains may have
substituted groups in terminal (fatty acids, primary alcohols, aldehydes) or mid-chain
positions (β-diketones, secondary alcohols). Epicuticular waxes are generally not soluble in
non-polar solvents barring few exceptions such as estolides of gymnosperm waxes,
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polymeric aldehydes in the waxes of S. officinarum and Nepenthes plant waxes. Otherwise,
the constituents of plant epicuticular waxes are freely soluble in non-polar solvents such as
hexane, benzene, chloroform and diethyl ether and insoluble in water.
Structure and Composition
Amelunxen et al. (1967) gave an interesting classification of wax by substantiating
through evidences (images) obtained using scanning electron microscopic. According to this
classification, the epicuticular wax is present mostly in the form of:
i.
Grains (granules that can be elongated, flattened or granular)
ii. Rods and filaments (that can be interwoven)
iii. Plates/scales (vary in shape, composition and spatial pattern)
iv. Films/crusts (uniform or undulated patterns)
v. Fluids and greasy (intermediate and semi-solid)
The composition of leaf epicuticular wax also varies from one species to another and also in
the plants found in various environmental conditions. Cuticular waxes is a general term for
complex mixture of homologue series of long chain aliphatic like alkanes, alcohols,
aldehydes, fatty acids and esters along with varying proportions of cyclic compounds like
pentacyclic triterpenoids and hydroxycinnamic acid derivatives. It has been reported that
epicuticular crystals of various other plant species also consist of triterpenoids.
Functions
Leaf epicuticular wax plays a very important role in acting as a interface between plant
interior and external environment or as physical barrier between the plant pathogens and
the leaf tissue. Following functions have been attributed to epicuticular wax:
(a) Epicuticular wax offers protection to the plants. Fungal pathogens that cause rust
diseases must penetrate the wax lining present on the stomatal chamber before
entering into the stomata. Wax is hydrophobic in nature; hence absence or low
availability of moisture content on leaf surface does not allow the germination of
fungal spores on the surface. Inhibition of spore germination reduces the probability
of attack and attributes resistance to the plant.
(b) Crystallization pattern of wax present on the leaves gives additional resistance to
various insect and fungal pathogens.
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(c) Water repellency also called as ‘Lotus Effect’ or self-cleaning surface properties for
the removal of dust, spores, and other foreign matter require a hydrophobic microrelief on the surface layers and that is provided by the epicuticular wax as observed
in aquatic plants such as Nelumbo nucifera.
(d) Plant organs may also be protected against walking insect herbivores, or serve to
catch insect prey in some carnivorous plants, through the action of epicuticular wax.
(e) Due to the presence of epicuticular wax crystals, the cuticle surface become slippery
so that insect feet is not attached to surface and are thus deterred to stay on plants.
The resulting non-adhesive surfaces of aerial organs and offer protection to the
plants.
(f) Epiculticular wax reflects the solar radiation falling on the aerial parts and attenuates
the short wave radiation. Thus it plays an important role in reducing the heat load
via transpiration. This also plays an important function in saving water loss in plants
on account of transpiration. There is clear cut evidence that epicuticular wax also
protects the plants from the frost damage.
Degradation of Epicuticular Wax
Epicuticular waxes degradation is a common phenomenon and is being studied by
scientists all across the world. Degradation of this takes place in plants through various
ways. There has been widespread concern that pollutants play a major role in accelerating
degradation of the epicuticular waxes of temperate trees such as conifers. It has been found
in expanded leaves of beech (Fagus sylvatica), that about 35% of the epicuticular waxes are
susceptible to atmospheric photo-oxidation and there is degradation of aldehydes and
decline in amount of alkanoic acids in the leaves. However, certain investigations reveal that
common constituents of epicuticular waxes are very resistant to chemical change in vivo
during the normal lifetime of plant organs. Therefore, extensive research is required to be
carried out to reach firm conclusions.
Molecular Studies
It is imperative to undertake extensive studies to understand the understanding the
dynamic nature of cuticle and epiculticular wax properties. Literature suggests that we still
know a little about these structures. Many gaps in knowledge prevail
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regarding
understanding of wax deposition in response to environmental variables. The experiments
conducted on understanding of genetic basis of wax biosynthesis on large number of land
plants suggest that wax biosynthesis is under transcription control. However, transcription
factors regulating the process have still to be identified.
Model systems such as Arabidopsis thaliana and Brassica are now being studied to
decode the various attributes of wax biosynthesis and elucidate the pathway involved in
secretion. The genes involved in the various steps of functional cuticle formation have been
identified. Genes that are apparently involved in the differentiation of epidermis-related
tissues, transcriptional control, cuticle biogenesis and export of cuticle-related lipids have
also been identified using marker systems. The plant genome presumably contains a family
of genes involved in the biogenesis and export of cuticle-related lipids. Although, no clarity
at present about the genetic and biochemical relationships between the regulators and the
genes themselves appear to be directly involved in cuticle biogenesis. Further efforts using
genetic materials is required to reveal the causal relationship between loss of specific
constituents or physiological alteration of cuticle and cellular interactions as well as the
molecular mechanisms regulating cuticle biogenesis. Transcriptional factors regulating the
expression of the metabolic enzymes responsible for cuticle biogenesis have been emerging
and elucidated. Cuticle biogenesis is tightly linked with the differentiation of the protoderm
and the epidermis during development of organs. Elucidation of target genes as well as the
studies of regulation mechanisms for the expression and activation of these transcription
factors would help our understanding of cuticle biogenesis during the plant life cycle. The
production of epicuticular wax involves various genes that are predominantly expressed in
epidermis-related tissues. Transcripts of the CER2, CER5, CER6/POP1 and WAX2/YRE/PEL6
genes are have been studied and are preferentially detected in the L1 layer of the shoot
apical meristem, protodermal cells of organ primordia and epidermal cells. It is evident from
the present studies that mutations in these genes result in modification and reductions in
wax content in the surface layers of plants.
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Summary
Epidermal features such as cuticle and epicuticular wax are important features of
aerial organs of the terrestrial plants. Enormous information is now available on the
structural and ultrastructural details of the leaf cuticle and epicuticular wax. Analytical
techniques, scanning and transmission electron microscopy has helped in understanding of
newer aspects of these structures. The biosynthetic pathways have also been elucidated
using molecular models and cellular details. The functional significance of these micromorphological structures have now been elucidated. It is now understood that cuticle and
leaf epicuticular wax is a blue print that plays an important adaptive and protective role in
plants against various kinds of biotic and abiotic stress. Molecular genetic studies is being
carried out to understand the role of transcription factors involved in biosynthesis of the
chemical substances that constitute the major proportion of these structures.
Exercises
1
Cuticle is considered an evolutionary adaptation to terrestrial life. However, many
aquatic plants have glossy cuticles. Explain the role played by cuticle in these plants?
2
What are the main constituents of leaf epicuticular wax? Enumerate some functions
attributed to leaf epicuticular wax.
3
Draw an illustrated diagram showing various layers of cuticle.
4
Name some eminent scientists who have worked on these structures?
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5
Enumerate the recent techniques used in understanding of cuticle and other related
structures?
6
Scanning electron microscopy is an essential tool to study cuticle. Justify the statement.
7
Enlist some major differences between cuticular and epicuticular waxes.
8
Draw a schematic diagram illustrating the wax biosynthesis pathway in angiosperms.
Glossary
Cuticle: Protective film covering all aerial parts of the plant.
Cutinase: Enzyme that catalyses the hydrolysis of the cutin, to allow penetration by
phytopathogenic fungi.
Herbivory: A type of predation in which animals/organisms consume autotrophs such as
plants, algae, and photosynthesizing bacteria.
Wax: A substance that is less greasy harder, brittle and contains chemicals of high
molecular weight.
Abaxial Surface: Located away from the central axis.
Adaxial Surface: located on the side facing towards the axis. Upper exposed surface.
Histochemistry: Method used for identification of chemical substances in plant cells and
tissues using certain stains or dyes.
Guard cells: These are a pair of cells that surround a stoma and form a stomatal complex
with subsidiary cells. Guard cells play an important role in opening and closing of stomata.
Cutin: Material present in walls of some plant forming cuticle, which covers the epidermis.
Lipophilic: substances having affinity for fats
Hydrophilic: Substances that attract water
Hydrophobic: Substances that repel water.
Transpiration: The process of evaporation of water into the atmosphere from the leaves
and stems of plants.
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Stomata: minute aperture structures on plants found typically on the outer leaf skin layer,
also known as the epidermis.
Protoderm: It is primary meristematic tissue that forms a layer around the young embryo
in plants. It is an embryonic cell layer that gives rise to epidermis at maturity in various
organs of the plant. It is often protective in function.
Scanning Electron Microscope: A microscope used to view the surface topography of the
material without looking into internal details. Electrons are source of illumination.
Confocal microscopy: An advanced technique of microscopy where laser beam is used as
a source of illumination and is concentrated on an object through a pin-hole to have better
resolution and details of the cell without any glare or artifacts.
Model System/Model organism: Model system or a model organism is any such
organism that has been well understood by scientists, has been used as an experimental
system, has short life cycle and can be used to study and answer questions related to
development, reproduction etc…Arabiospis, Brassica are some common model systems in
plants and Drosophila is a model system used in animal sciences.
References
Barthlott, W. and Frolich, D. (1983) Micromorphology and orientation patterns of
epicuticular wax crystalloids – a new systematic feature for the classification of
monocotyledons, Plant Systematics and Evolution, 142, 171–185.
Barthlott, W., Neinhuis, C., Cutler, D. et al. (1998) Classification and terminology of plant
epicuticular waxes, Botanical Journal of the Linnean Society, 126, 237–260.
Kolattukudy, P.E. (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to
environmental stresses, in Plant Cuticles: An Integrated Functional Approach (ed. G.
Kerstiens), BIOS Scientific Publishers Ltd., Oxford, pp. 83–108.
Shepherd, T., Robertson, G.W., Griffiths, D.W., Birch, A.N.E. and Duncan, G. (1995) Effects
of
environment
on
the
composition
of
epicuticular
wax
from
Phytochemistry, 40, 407–417.
Institute of Lifelong Learning, University of Delhi
kale
and
swede,
Suggested Readings
1.
Cutler, D.F., Alvin, K.L. and Price, C.E. (1982). The Plant Cuticle, Academic Press,
London.
2.
Biology of plant cuticle / edited by Markus Riederer & Caroline Muller. 2006 by
Blackwell Publishing Ltd.
3.
Markus Riederer. "Introduction: Biology of the Plant Cuticle", Biology of the Plant
Cuticle.
4.
Reinhard Jetter. "Composition of Plant Cuticular Waxes", Biology of the Plant Cuticle.
5.
Buschhaus, C., and R. Jetter. "Composition differences between epicuticular and
intracuticular wax substructures: How do plants seal their epidermal surfaces?", Journal of
Experimental Botany, 2011.
Web Links
•
http://lipidlibrary.aocs.org/plantbio/polyesters/index.htm
•
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1310549/
Institute of Lifelong Learning, University of Delhi