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A plant cuticle is a protective cover on the outer skin of leaves, young shoots, and other parts of plants that grow above the ground.^[1]

It is made of lipids and hydrocarbon polymers mixed with wax. The plant's skin cells make it.^[2] The plant cuticle can be removed from the rest of the plant by using enzymes like pectinase and cellulose.^[3]

The plant cuticle is one of the things that plants developed over 450 million years ago when they moved from life in water to life on land. Along with other features like stomata, xylem and phloem, and spaces between cells, the cuticle helps plants save water. It acts like a waterproof covering, protecting the places where gases are moved. The stomatal guard cells, a kind of control mechanism, manage how much water evaporates and how much carbon dioxide gets exchanged.^[4]

Structure

The cuticle is made up of a tough membrane filled with waxes. The main part of this membrane is a polyester polymer called cutin, made of special acids. Another component is a non-saponifiable hydrocarbon polymer called cutan. The membrane is loaded with waxes, and there are additional waxes on top of it, which are a mix of hydrophobic compounds. These compounds are often long chains of carbon atoms.^[5]

Cuticular wax mainly consists of compounds derived from very-long-chain fatty acids (VLCFAs), like aldehydes, alcohols, alkanes, ketones, and esters. There are also other compounds not derived from VLCFAs, like terpenoids, flavonoids, and sterols.^[6]

The process of making cuticular wax involves the creation of long acyl chains in chloroplasts, followed by extending these chains in the endoplasmic reticulum of epidermal cells. An important helper in this process is the fatty acid elongase (FAE) complex.^[7]^[8]

To create cuticular wax, VLCFAs undergo two main pathways: an acyl reduction pathway and a decarbonylation pathway. In the acyl reduction pathway, VLCFAs turn into primary alcohols, which can then become wax esters through a wax synthase. In the decarbonylation pathway, aldehydes change into alkanes and can further transform into secondary alcohols and ketones. The process ends with transporting the wax components from the endoplasmic reticulum to the surface of the epidermis.^[9]^[10]

In certain plants, especially in moss and vascular plants, having a protective cuticle from the parent plant helps the new plants survive better. In flowering plants, the cuticle is usually thicker on the upper side of the leaf, but not always. Plants in dry areas have more even cuticle thickness compared to plants in wet areas.^[11]

Besides preventing water loss, the cuticle also acts as a defence. It makes a barrier that stops viruses, bacteria, and fungi from getting in.^[12]