The formation and function of plant cuticles

Abstract

The plant cuticle is an extracellular hydrophobic layer that covers the aerial epidermis of all land plants, providing protection against desiccation and external environmental stresses. The past decade has seen considerable progress in assembling models for the biosynthesis of its two major components, the polymer cutin and cuticular waxes. Most recently, two breakthroughs in the long-sought molecular bases of alkane formation and polyester synthesis have allowed construction of nearly complete biosynthetic pathways for both waxes and cutin. Concurrently, a complex regulatory network controlling the synthesis of the cuticle is emerging. It has also become clear that the physiological role of the cuticle extends well beyond its primary function as a transpiration barrier, playing important roles in processes ranging from development to interaction with microbes. Here, we review recent progress in the biochemistry and molecular biology of cuticle synthesis and function and highlight some of the major questions that will drive future research in this field.

Figure 1.

Plant cuticle structure. A, Schematic diagram highlighting the major structural features of the cuticle and underlying epidermal cell layer (not drawn to scale). B, Scanning electron micrograph image of an Arabidopsis leaf epidermis and overlying cuticle, seen in cross section. Bar = 5 μm. (Image courtesy of Dr. Lacey Samuels.) C, Transmission electron micrograph image of an Arabidopsis stem epidermal cell wall and cuticle. Bar = 500 nm. (Image courtesy of Dr. Christiane Nawrath.) D, Light microscopy image showing the cuticle of a mature green-stage tomato fruit stained with Sudan Red and the polysaccharide cell walls stained with Alcian Blue. Bar = 50 μm.

Figure 2.

Cutin and wax biosynthetic pathways. Genes (blue text) are described in the review. Red text denotes compound classes that are typically observed in cuticular wax mixtures.

Figure 3.

Typical cutin monomers and polymeric structure. A, Some typical C16 and C18 fatty acid-derived cutin monomers. From top to bottom: 10,16-dihydroxyhexadecanoic acid, 16-hydroxyhexadecanoic acid, 9,10-epoxyoctadecanoic acid, 9,10,18-trihydroxyoctadecanoic acid, and octadeca-cis-6,cis-9-diene-1,18-dioate, the major cutin monomer of Arabidopsis stems and leaves. B, Linear and branched domains made possible by different ester linkages of 10,16-dihydroxyhexadecanoic acid, depicted schematically as indicated.

Figure 4.

Regulation of cuticle biosynthesis. A summary of the interaction of environmental factors and regulatory genes that are known to influence cutin or wax biosynthesis is shown.