Cuticle

The cuticle is also known as cuticula which is a tough and flexible outer covering of an organism. In zoology, the cuticle found in invertebrates is a multilayered structure that is present on the outer layer of the epidermis. In botany, a cuticle is a protective layer present on the epidermal cells of the leaves. By combining this information, we can say that cuticle is a term used to describe the outer layer of tissue. The cuticle is the outermost layer of an organism that is exposed with the environment. In different types of invertebrates, the dead, noncellular cuticle is determined by the epidermis and the layers of it may contain pigments and chitin, in humans the cuticle is the epidermis. While, in different bigger plants, the cuticle is a water-resistive protective layer covering the epidermal cells of leaves along with other parts and limiting water loss. The plant cuticle is one of the innovations that are found along with the xylem, phloem, and stomata. With these features, the aerial plant’s environment has explored that to conserve water the gas exchange surfaces are internalized. These surfaces are enclosed with a waterproof membrane. 

Let us see what cuticles are in plants and their function? 

What is Cuticle in Plants?

The cuticle found in plants provides a covering to the external epidermis of leaves, young shoots, and the other parts of the plant without any periderm. These are the lipid polymers that are soaked with the help of waxes. These are coated on the outer surface of the organs of the vascular plants present on the land. These also can be found in the hornworts sporophyte generation and the sporophyte and gametophyte generation of the mosses. The cuticle plant forms an inherent outer protective layer where these can be isolated with the help of treating the plant tissue with some of the enzymes such as pectinase and cellulase. The cuticle contains cutin which is a waxy, water-repellent substance allied to suberin, that is found in the cell walls of corky tissues. Cutin is especially identified on various fruits like apple, nectarine, and cherry, which can be buffed to a high gloss. Copernicia cerifera, a Brazilian palm consists of carnauba wax that is derived from the cuticles of the leaves.

Cuticle Structure

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The cuticle consists of an insoluble cuticular membrane that is soaked and covered with soluble waxes. The cuticle is a polyester polymer that is composed of the interesterified omega hydroxy groups of acids. These have cross-linked bonds of ester and epoxides. It is known as the structural component of the cuticular membrane. The cuticular membranes are soaked in the cuticular waxes and these are further covered with epicuticular waxes. These are a mixture of hydrophobic aliphatic compounds where the chain lengths are in a range of C16 to C36. 

These cuticular waxes are composed of compounds that derive from the VLCFAs (Very Long Chain Fatty Acids) . These long chains are derived from aldehydes, alcohols, alkanes, esters, and ketones. There are some other compounds that are not derivatives of VLCFAs that including sterols, flavonoids, and terpenoids. Due to the presence of different derivatives the synthetic pathway is different from VLCFAs. The first step is the formation of the biosynthesis pathway of the cuticular VLCFAs. The de novo biosynthesis of the acyl chains of C16  occurs due to the presence of chloroplasts in the mesophyll. These are concluded with the extensions that are made to the endoplasmic reticulum present in the epidermal cells. FAE (Fatty Acid Elongase) complex is the important catalyst that is present in the process. 

VLCFAs are modified by two different pathways to form cuticular wax components. One is the decarbonylation pathway and the other is the acyl reduction pathway. In the acyl reduction pathway, an enzyme called reductase is used to convert VLCFAs to form primary alcohols. These alcohols are further converted to form wax esters with the help of an enzyme called wax synthase. In the case of the decarbonylation pathway, the aldehydes are produced and these are decarbonylated to form the alkanes. These alkanes are further oxidized to form secondary alcohols and ketones. This biosynthesis pathway ends by transporting the wax components from the endoplasmic reticulum to the epidermal surface. 

Plant cuticles are the type of structures that are made of a macromolecular scaffold of cutin and a variety of organic solvent-soluble lipids that are generally called as waxes, together with polysaccharides. The plant cuticle has been described to be an independent structure, apart from the polysaccharide cell wall underneath, but the two structures are physically associated and have some overlapping functions. Considering the cuticle as a specialized lipid-modification of the cell wall, just as lignification is a common modification of secondary cell walls in plants is not wrong at all. So the cuticle waxes are either deposited within the cuticle, which is called intracuticular wax, or deposited on the surface as epicuticular wax crystals, or films, these epicuticular waxes confer distinct macroscopic surface properties to the plant surface and are in charge of the shiny display of many leaves and fruits, while epicuticular wax crystals account for the dull, glaucous appearance and serve as the interface of the plant with the external environment. These waxes are polar in nature, provide a massive degree of roughness, and are efficient barriers to transpirational loss. The structure and composition of cuticles in all the plants are very complex and can differentiate widely among plant species and within plant species in various organ and developmental stages. This differentiation can be identified in the typical range of thickness (1–10 μm) and quantity (100 – 1000 μg cm−2) of the deposited cuticle.

Cuticle Leaf - Function

In all types of plants, the cuticle is the layer of wax and Cutin that covers the outermost surfaces of a plant. The cuticle is widely discharged by the epidermis and helps in preventing water loss and infection by parasites. The primary function of the cuticle acts as a permeability barrier in plants to prevent the evaporation of the water from the outer epidermal surface. Along with this, it prevents the entering of the water molecules and the solutes from the external environment. The micro and nanostructures of the cuticle not only act as a water permeability barrier in the plants but also helps to prevent the contamination of the tissues with dirt, external water, and microorganisms. In some of the plants such as the leaves of the lotus, the aerial organs have self-cleaning properties and these are ultra hydrophobic. This lotus effect is used in the application of biomimetic materials. 

In the moss such as Funaria hygrometrica and in the sporophytes of the vascular plants the cuticle provides the offspring fitness due to the dehydration protection of the maternal cuticle. In the case of angiosperms, the cuticle is said to be thicker on the top layer of the leaf. The xerophytic plants present in a dry climate, in these types of plants the cuticle on the leaves are thicker than compared the mesophytic plants present in wetter climates. Due to the presence of thicker cuticles, the risk of dehydration will be less. 

These waxy cuticles also play an important role in the defence, it forms a physical barrier that acts as a resistance to the virus or bacterial cells, spores, and the growing filaments of the fungi.

In all different animals, the cuticle is a noncellular, hardened, or membranous protective covering of many invertebrates that is the transparent membrane that covers annelids. The cuticle is also the outermost layer of the skin covering the exterior body surface of vertebrates.

Conclusion

We have got to know the multiple roles played by cuticles in the interaction with pathogens. According to some of the research, much evidence has been found in relation to the cuticle layer of plant-pathogen interaction. The vascular plants in the aerial environment have got the surfaces and to minimize the evaporation of the water these surfaces are internalized. And these are covered with a waterproof membrane. But the cuticle is found absent on the surface of the roots because the cuticle layer will not allow the water and solute molecules to enter inside the outer layer but the roots are involved in the transport of water and mineral nutrients, thus the cuticle is found absent on the surface of roots.